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Practical Cathodes for Sodium‐Ion Batteries: Who Will Take The Crown?

Advanced Energy Materials

August 2023
13(37)

DOI:10.1002/aenm.202301975

Authors:

Xinghui Liang

Hanyang University

Yang-Kook Sun

Hanyang University

In recent decades, sodium‐ion batteries (SIBs) have received increasing attention because they offer cost and safety advantages and avoid the challenges related to limited lithium/cobalt/nickel resources and environmental pollution. Because the sodium storage performance and production cost of SIBs are dominated by the cathode performance, developing cathode materials with large‐scale production capacity is the key to achieving commercial applications of SIBs. Therefore, developing host materials with high energy density, long cycling life, low production cost, and high chemical/environmental stability is crucial for implementing advanced SIBs. Among the developed cathode materials for SIBs, O3‐type sodiated transition‐metal oxides have attracted extensive attention owing to their simple synthesis methods, high theoretical specific capacity, and sufficient Na content. However, the relatively large Na‐ion radius leads to sluggish diffusion kinetics and inevitable complex phase transitions during the deintercalation/intercalation process, resulting in poor rate capability and cycling stability. Therefore, this review comprehensively summarizes the research progress and modification strategies for O3‐type cathodes, including the component design, surface modification, and optimization of synthesis methods. This work aims to guide the development of commercial layered oxides and provide technical support for the next generation of energy‐storage systems.

a) Number of publications on SIBs and cathode materials for SIBs. b) Proportion of publications on various cathode materials between 2013 and 2023 (data collected using Web of Science, May 16, 2023). c) Typical crystal structures of layered oxides (O3‐type), polyanionic compounds (NASICON), and PBAs (cubic phase structure). d) Key properties of layered oxide, polyanionic compounds, and Prussian blue analogs.

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a) Average operating voltage, specific capacity, and energy density of three representative cathodes. b) Cycle life of three kinds of cathodes.

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Schematic diagram of the main challenges and optimization strategies for O3‐type layered oxide cathode materials.

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a) Ion‐diffusion energy barriers in the P2 and O3 phase structures. Reproduced with permission.[⁸⁸] Copyright 2017, Wiley‐VCH GmbH. b) In situ XRD pattern of Na[Ni0.5Mn0.5]O2 during the initial charging process. c) Cross‐sectional scanning electron microscopy (SEM) images of Na[Ni0.5Mn0.5]O2 under different charging states. Reproduced with permission.[¹⁰⁷] Copyright 2020, Wiley‐VCH GmbH. d) Schematic diagram of the structural degradation and crack formation of Na0.8Mg0.2Fe0.4Mn0.4O2 due to exposure to air. Reproduced with permission.[¹⁰⁸] Copyright 2021, the Royal Society of Chemistry. e) Schematic diagram of chemically induced surface reconstruction. Reproduced with permission.[¹⁰⁹] Copyright 2018, Wiley‐VCH GmbH.

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a) Charge/discharge curves and phase‐transition diagram of NaCrO2. b) Schematic diagram of the synthesis of C‐coated plate‐like NaCrO2 cathodes. Reproduced with permission.[¹³⁶] Copyright 2015, the Royal Society of Chemistry. c) Transmission electron microscopy (TEM) images of nanowire‐like samples. Reproduced with permission.[¹³⁹] Copyright 2019 American Chemical Society. d) Operando XRD patterns of the original (right) and Sb‐doped (left) cathode materials. Reproduced with permission.[¹⁴⁰] Copyright 2022 Elsevier.

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REVIEW

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Practical Cathodes for Sodium-Ion Batteries: Who Will Take

The Crown?

Xinghui Liang, Jang-Yeon Hwang, and Yang-Kook Sun*

In recent decades, sodium-ion batteries (SIBs) have received increasing

attention because they oﬀer cost and safety advantages and avoid the

challenges related to limited lithium/cobalt/nickel resources and

environmental pollution. Because the sodium storage performance and

production cost of SIBs are dominated by the cathode performance,

developing cathode materials with large-scale production capacity is the key

to achieving commercial applications of SIBs. Therefore, developing host

materials with high energy density, long cycling life, low production cost, and

high chemical/environmental stability is crucial for implementing advanced

SIBs. Among the developed cathode materials for SIBs, O3-type sodiated

transition-metal oxides have attracted extensive attention owing to their

simple synthesis methods, high theoretical speciﬁc capacity, and suﬃcient Na

content. However, the relatively large Na-ion radius leads to sluggish diﬀusion

kinetics and inevitable complex phase transitions during the

deintercalation/intercalation process, resulting in poor rate capability and

cycling stability. Therefore, this review comprehensively summarizes the

research progress and modiﬁcation strategies for O3-type cathodes, including

the component design, surface modiﬁcation, and optimization of synthesis

methods. This work aims to guide the development of commercial layered

oxides and provide technical support for the next generation of energy-storage

systems.

1. Introduction

At present, lithium-ion batteries (LIBs) successfully dominate

the market of portable electronic devices, and their applica-

tion is gradually expanding into the ﬁeld of electric vehicles.[]

Nevertheless, the limited and uneven distribution of lithium

(Li), nickel (Ni), and cobalt (Co) worldwide has raised concerns

about the sustainable development of this battery technology.

Sodium-ion batteries (SIB) are considered the most appropriate

X. Liang, J.-Y. Hwang, Y.-K. Sun

Department of Energy Engineering

Hanyang University

Seoul 04763, Republic of Korea

E-mail: yksun@hanyang.ac.kr

J.-Y. Hwang, Y.-K.Sun

Department of Battery Engineering

Hanyang University

Seoul 04763, Republic of Korea

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/aenm.202301975

DOI: 10.1002/aenm.202301975

alternative to LIBs because of their abun-

dant raw materials, potentially low cost,

and similar intercalation chemistry.[,]

In fact, the development of both SIBs and

LIBs originated in the s. However,

with the discovery of the Li-storage mech-

anism of graphite anodes in the s,

research on SIBs stagnated because

sodium (Na) has a large ionic radius and

cannot be eectively inserted/extracted

from graphite anodes. During this pe-

riod, high-temperature Na batteries (ZE-

BRA cells) using molten Na as the anode

and ceramic as the separator developed

rapidly, but the high operating tempera-

ture (– °C) led to issues such as

low energy eciency and corrosion. With

the development of hard-carbon (HC)

anodes and rising concerns about Li re-

sources and the supply chain of Li/Co/Ni

raw materials, SIB development has

accelerated since  (Figure 1a).

Similar to Li technology, an SIB com-

prises a cathode, anode, separator, and

electrolyte.[] Beneﬁting from the low

cost of cathodes and the possibility of re-

placing the expensive copper (Cu) foils

used as anode current collectors with

inexpensive aluminum (Al) foils, the total cost of SIBs is reported

to be at least % lower than that of LIBs.[] Due to the relatively

high standard electrochemical potential of Na metal (−. V

versus standard hydrogen electrode (SHE) for Na; −. V ver-

sus SHE for Li), Na-ion full cells can be fully discharged to  V

for storage without structural degradation, which improves their

safety during operation and transportation. However, since the

energy density of SIBs is generally lower than that of LIBs, the

real unit energy cost of the two technologies is almost equal

($. per W h for SIBs and $. per W h for LIB).[] Admit-

tedly, it would be unfair to directly compare Li technology, which

has been commercialized since , with Na technology, which

is still in the prototype stage. Based on various advanced elec-

trode materials developed in recent years, the energy density

and cycle life of SIBs are expected to be further improved.[, ]

For instance, Faradion Limited pointed out that the produc-

tion cost of their second-generation cathode (Faradion’s Gen 

material: mixed O-Na.Ni. Mn.Mg. Ti. Oand P-

Na.Ni.Mn. Mg.Ti.O)//HC Na-ion pouch cells will

be –% lower than that of the current state-of-the-art

LiFePO/graphite Li-ion cells.[] Therefore, considering that

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Figure 1. a) Number of publications on SIBs and cathode materials for SIBs. b) Proportion of publications on various cathode materials between

2013 and 2023 (data collected using Web of Science, May 16, 2023). c) Typical crystal structures of layered oxides (O3-type), polyanionic compounds

(NASICON), and PBAs (cubic phase structure). d) Key properties of layered oxide, polyanionic compounds, and Prussian blue analogs.

large-scale energy storage systems and moderate-range electric

vehicles require long cycling life and high safety, cost-eective

SIBs have received renewed attention in recent years.[]

To accelerate the commercialization of SIBs, several startup

companies, including Faradion Limited, HiNa, CATL, Altris,

and Tiamat, have made signiﬁcant progress.[] In most scale-

up and demonstration cases, prototype Na-ion pouch cells are

typically assembled using an HC anode, but the optimal cath-

ode material is yet to be identiﬁed. CATL (China), Natron En-

ergy (USA), and Novasis Technologies (USA) chose Prussian

Blue analogs as the development direction for SIBs; Faradion

Limited (UK) developed a series of doped layered oxide cath-

odes (NaaNi(−x−y−z)MnxMgyTizO); Tiamat (France) produced

cylindrical SIBs with a NaV(PO)F(NVPF) cathode; and

HiNa (China) fabricated a  kWh battery device in Liyang

city based on a Nax[Cu,Fe,Mn]Ocathode.[,–] The practi-

cal large-format prototype SIBs shows similar electrochemi-

cal performance to the early commercial LIBs. For example,

a  A h prismatic NaCrO//HC full cell can deliver a cell

energy density of  W h kg−(calculated based on all cell

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components), with a capacity retention of % after  cy-

cles at  A.[] Na.Cu.Ni.Fe.Mn. Ti. O//HC[] and

NaNi. Zn.Mn. Ti. O//HC[] Na-ion full cells are able to

deliver practical energy densities of  and  W h kg−in  A h

cylindrical cells (-type cells), respectively. The  A h pouch

cell can be realized by coupling HC as anode and Faradion’s Gen

 material as cathode, with a high cell energy density of  W

hkg

−and an ultra-long service life of  cycles.[] Last year,

a  A h pouch cell (anode-free Na battery) was developed, uti-

lizing Na[Cu/Ni/ Fe/ Mn/]Oas the cathode and Al foil as

the anode, exhibiting a very high cell energy density of  W h

kg−.[] Although the heavy steel casing of prismatic and cylindri-

cal cells reduces the practical energy density, the sti metallic cas-

ing and safety valve enhance the safety of the battery. Considering

that SIBs are mainly used for stationary energy storage systems

rather than portable electronic devices, prismatic and cylindrical-

type SIBs with higher safety may be more suitable for commer-

cial applications. Therefore, developing advanced cathode mate-

rials and constructing practical full cells are crucial for achieving

commercial applications of SIBs. This review paper speciﬁcally

focuses on the challenges, solutions, and future development di-

rections encountered by SIBs in large-scale applications.

2. Characterization and Selection of Suitable

Cathode Materials

The ideal cathode material should possess the advantages of

high capacity, suitable operating voltage (outputting high en-

ergy density without decomposing the electrolyte), high power

density, sucient electronic/ionic conductivity, and high chemi-

cal/environmental stability. To ensure the stability and safety of

SIBs, commercial cathode materials should also have the advan-

tages of low environmental impact, easy preparation, abundant

raw materials, and high thermal stability.[] To date, researchers

have developed various types of cathode materials, including

Prussian Blue analogs (PBAs), polyanionic compounds, organic

compounds, and sodiated transition metal oxides (Figure b).[,]

Although the advantages of structural diversity, high theoretical

capacity, and low environmental impact of organic compounds

have attracted the attention of researchers, their large-scale ap-

plications are hindered by slow reaction kinetics and the dissolu-

tion of active materials.[] In addition, based on the current de-

velopment path of SIB production companies in the ﬁeld of cath-

ode materials, it is reasonable to believe that the ﬁrst commercial

cathode materials will probably be PBAs, polyanionic polymers,

and/or layered oxide materials.[] Therefore, the basic principles

of these three potential inorganic cathode materials are discussed

below to critically evaluate the development direction of SIB com-

mercialization.

2.1. Polyanionic Compounds

Beneﬁting from the stable framework structure and strong in-

ductive eects, polyanionic compounds are considered promis-

ing cathode materials for advanced SIBs. Polyanionic com-

pounds are covalently linked through (XO)n−(X =sulfur

(S), phosphorus (P), silicon (Si), and other elements) groups

and generally provide three-dimensional (D) Na-ion diusion

channels (Figure c).[] Currently, common research systems

include phosphates (NaFePO,Na

V(PO),NaVOPO

), py-

rophosphates (NaMPO,Na

MPO,Na

M(PO)PO)(M=

manganese (Mn), Co, Ni, iron (Fe), vanadium (V) and oth-

ers), ﬂuorophosphates (Na(VOx)(PO)F−x,Na

MPOF), sul-

fates (NaFe(SO),Na

+xM−x(SO),NaMSO

F), and silicates

(NaMnSiO,Na

FeSiO).[] Compared with layered oxides, a D

framework structure can eectively alleviate the structural rear-

rangement and inhibit the dissolution of oxygen (O) during the

Na+insertion/extraction process, thereby enabling good cycla-

bility and thermal stability.[] However, these cathode materials

possess inherently low electrical conductivity due to their unique

structure. Taking NaV(PO)(NVP) as an example, since O

atoms are shared in the adjacent VOoctahedra and POtetrahe-

dron, electron transfer follows the V–O–P–O–V pattern instead

of the faster V–O–V pattern.[] Therefore, to improve the intrin-

sically low electronic conductivity, conductive carbon (C) coat-

ings, nanostructure designs, and elemental doping have been

proposed.

As a typical Na superionic conductor (NASICON)-type cath-

ode, NVP has attracted widespread attention from researchers

due to its high structural/thermal stabilities and ion conduc-

tivity. NVP cathodes with a rhombohedral structure can pro-

vide a theoretical capacity of  mA h g−and exhibit a ﬂat

charge/discharge plateau at . V (versus Na+/Na), correspond-

ing to the redox pairs of V+/V+.[] Although NVP has many ad-

vantages as a cathode material, its inherently low electronic con-

ductivity (<−Scm

−) limits its application potential. Gener-

ally, highly conductive C-layer coatings are considered the most

eective materials to improve the electrochemical performance

of NVP. Therefore, amorphous-C, graphite-like C, porous C,

S/nitrogen (N)/P/boron (B)-doped C, carbon nanotubes (CNTs),

and graphene-modiﬁed NVP have been evaluated. The use of

these materials resulted in enhanced cycling stability and rate

properties, because the introduced C-coating not only increases

the electronic conductivity but also acts as a buer layer to pre-

vent damage to the active materials.[, ] In addition, shorten-

ing the ion-transport path by changing the microstructure of the

cathode material is an eective method to improve the rate per-

formance and long-term cycling stability of NVP. Hence, a se-

ries of cathodes with nanoﬁber, micro-ﬂower, and microsphere

morphologies has been reported, which exhibited ultra-high cy-

cle lives (over  cycles).[, ] Moreover, heteroatom doping

(with Al, Mn, magnesium (Mg), zinc (Zn), molybdenum (Mo),

niobium (Nb), lanthanum (La), and other elements) has been

proven to eectively enhance the electrochemical performance of

NVP, as the doping agent expands the Na-ion diusion channel,

produces additional vacancies, and reduces the ionic migration

barrier.[] Through the eorts of researchers, modiﬁed NVP ma-

terials have exhibited ultra-long cycling stability of over  

cycles and ultra-high rate capability of above  C (although

some researchers criticize this excessively high rate as meaning-

less in practice), which is even better than the performance of

most reported cathode materials for LIBs.[, ] Notably, owing to

the diculty in activating V+/V+redox pairs, NVP-based cath-

odes can only deliver a limited energy density, and the high con-

tent of expensive and toxic V limits its potential for large-scale

production.

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Since the introduction of strong electronegative groups (such

as ﬂuorine ion (F−)) can increase the inductive eect, ﬂuorinated

phosphates with relatively high operating voltage and energy

density are attractive cathode candidates for advanced SIBs.[]

Swoyer et al. ﬁrst reported the NaVPOF cathode with a tetrag-

onal structure (space group I4/mmm), which exhibited a spe-

ciﬁc capacity of  mA h g−and an average discharge voltage of

. V.[] The C-coated monoclinic NaVPOF(spacegroupC2/c)

prepared by a solution-based approach exhibited a speciﬁc capac-

ity of  mA h g−and excellent cycling stability ( cycles).[]

An in situ X-ray diraction (XRD) study of the phase evolution

and reaction kinetics of tetragonal and monoclinic NaVPOF

showed that, unlike the monoclinic structure that undergoes a

two-phase reaction, the tetragonal NaVPOF undergoes a single-

phase reaction during the charge/discharge process, resulting

in reduced structural damage and improved electrochemical

performance.[] Although the optimized NaVPOF exhibits im-

proved Na storage performance, its actual capacity is lower than

the theoretical value. It was found that single-phase NaVPOFis

dicult to achieve by solid-phase synthesis (NaF:VPO=:),

indicating that the crystal structure of NaVPOF requires fur-

ther investigation.[] As an important member of the V-based

ﬂuorophosphate family, Na(VO−xPO)F+x ( ≤x≤) has re-

ceived extensive attention. A typical NVPF consists of VOFbi-

octahedral and POtetrahedron units and exhibits a high aver-

age operating voltage (≈. V) and stable structure.[] Croguen-

nec et al. re-evaluated the crystal structure of NVPF and found

that the structural dierences in the reported literature may come

from ﬂuctuations in the ratio of F and O.[] Synchrotron radia-

tion diraction studies revealed a small amount of orthorhom-

bic distortion in NVPF, implying that the appropriate crystal

space group should be Amam rather than the commonly assigned

P42/mnm. To overcome the inherently low electronic conduc-

tivity of ﬂuorophosphates, researchers successfully encapsulated

NVPF in a C-matrix using various methods (hydrothermal, elec-

trospinning, spray-drying, sol-gel, and ball-milling methods), re-

sulting in unique morphologies with short Na-ion diusion paths

that accelerate electron transport.[] For example, an N-doped C-

coated nanocube-like NVPF cathode material was produced by a

surfactant-assisted solid-state method, which exhibited excellent

rate capability (. mA h g−at  C) and long-cycle stability

(.% capacity retention after  cycles).[] Furthermore, a C-

free high-entropy cathode (NaV.(Ca,Mg,Al,Cr,Mn). (PO)F)

material was developed, which enhanced the electronic conduc-

tivity, reduced ionic diusion barriers, and increased the average

discharge capacity (. V) compared to the original NVPF.[] As

a result, this high-entropy cathode exhibited higher energy den-

sity (from . to . W h kg−) and demonstrated ultra-long

cycling stability (maintaining .% of the initial capacity after

 cycles). It should be noted that harmful F-containing gases

generated during the synthesis process may limit the application

of ﬂuorophosphate-based cathodes.

To reduce the content of V and increase the working volt-

age, metal-ion substitution was proposed. For example, a series

of NASICON-structured cathode materials, NaxMV(PO)(M =

Fe, Mn, Ni), were developed.[] Among them, NaMnV(PO)

(NMVP) with high operating voltage (. and . V plateaus)

emerged as a promising cathode candidate and displayed higher

energy density and lower cost than NVP. However, the rate per-

formance and long-term cycling stability of NMVP are limited

by its low electronic conductivity and the Jahn–Taller (JT) dis-

tortion of Mn+, which can be improved through C-coating,

atomic doping, or nanostructuring. For example, a graphene

aerogel-coated NMVP cathode material with a high energy den-

sity of  W h kg−and an ultra-long cycling life of over

 cycles was achieved.[] Furthermore, an N-doped C-coated

bayberry-like NMVP cathode was fabricated by ball-milling.[]

The conductive C-coating and hierarchical nanostructure pro-

moted ion/electron migration, and the optimized cathode exhib-

ited a high reversible capacity (. mA h g−) and a long cy-

cling life (.% capacity retention after  cycles). In addi-

tion, V-free phosphates have attracted attention owing to their

low cost. These include Mn-based cathode materials with high

potentials (≈. V for Mn+/Mn+;≈. V for Mn+/Mn+), such

as NaMnTi(PO)(NMTP), NaMnZr(PO),Na

MnAl(PO),

and NaMnCr(PO).[] Based on the charge compensation of

the Mn+/+/+redox reaction, these cathode materials typically

exhibit limited reversible capacity. For example, NMTP exhib-

ited a low discharge capacity of  mA h g−within a volt-

age window of .–. V and poor cycling stability (the dis-

charge capacity dropped to  mA h g−after  cycles).[] Al-

though a C-coating can improve the reversible capacity (e.g.,

 mA h g−) and energy density (e.g.,  W h kg−) of NMTP,

its service life is lower than that of most V-based polyanionic com-

pounds (.% capacity retention after  cycles at C).[] Al-

though reducing the discharge cuto voltage of NMTP can ac-

tivate the titanium (Ti+/Ti+) redox couple at ≈. V to provide

a high theoretical capacity of  mA h g−, its abnormal initial

Coulombic eciency (ICE) and low-voltage plateau lead to low

practicability.[]

Given the higher thermal stability of pyrophosphate (PO)an-

ions than phosphate anions, structurally stable pyrophosphates

are another strong contender for high-performance cathode ma-

terials. Yamada’s group ﬁrst reported a novel cathode material for

SIBs, namely NaFePOwith a triclinic structure, which exhib-

ited an average operating voltage of  V and a reversible capac-

ity of  mA h g−.[] Dierential scanning calorimetry results

showed that the desodiated NaFePOmaintained structural in-

tegrity up to  °C, demonstrating its good thermal stability.[]

A novel Mn-based sodium pyrophosphate (NaMnPO) cathode

was shown to provide a reversible capacity of  mA h g−and an

average discharge voltage of ≈. V.[] Despite pyrophosphate

cathode materials exhibiting high structural/thermal stabilities,

the low speciﬁc capacity caused by heavy anionic groups may hin-

der their commercial application. Therefore, mixed phosphates

consisting of phosphate (PO)−and pyrophosphate (PO)−an-

ions have received increasing attention. For example, potentially

low-cost NaFe(PO)POand NaMn(PO)POhave been

presented, which exhibited high reversible capacities ( and

 mA h g−, respectively) and high average discharge voltages

(. and . V, respectively).[ ,] Although both cathode mate-

rials exhibit high energy density, their low electronic conductiv-

ity, JT distortion, and dissolution of active materials result in un-

satisfactory cycling stability, which requires further optimization.

In conclusion, polyanionic compounds with high structural sta-

bility are considered as potential commercial cathode materials,

but further optimization of the component design and synthe-

sis methods is needed to reduce production costs (reducing the

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content of expensive V) and increase the volumetric energy den-

sity (C-matrix and nanostructure).

2.2. Prussian Blue Analogs

Prussian blue analogs (PBAs) have been widely studied be-

cause of their facile preparation methods and potentially low

cost.[] PBAs are generally represented by the formula of

AxM[M’(CN)]y□−y·nHO(≤x≤; y <), where □repre-

sents the vacancies occupied by coordinating HO, A represents

alkali metal ions (Li, Na, potassium (K)), and M/M’ is Fe, Mn,

Ni, Co, Zn, or chromium (Cr).[] Among them, hexacyanofer-

rates (Fe(CN), HCFs) have become the most attractive subgroup

due to their abundant resources and high redox potential. In the

case of Na-based materials, the content of Na ions in the unit

cell aects the color (blue, green, or white) and crystal struc-

ture (cubic, rhombohedral, or monoclinic phase) of the result-

ing cathode material, leading to dierent electrochemical perfor-

mances. Fully sodiated Prussian white is considered the most

suitable cathode material because this Na-rich phase structure

is conducive to the commercial application of Na full cells.[]

Compared with sodiated transition metal oxides and phosphates,

PBAs with larger interstitial A sites show higher ionic diusion

coecients (−to −cms−), allowing reversible electro-

chemical reactions of large Na ions.[] According to the redox re-

action, PBAs can be divided into single-electron transfer types (M

=Zn, Ni, Cu, Cr, and M’ =Fe, Co, Mn) and double-electron trans-

fer types (M and M’ =Fe, Co, Mn). PBAs with single-electron re-

actions exhibit negligible lattice parameter changes (almost zero

strain) during repeated charging/discharging processes, provid-

ing long-term cycling stability.[] However, the low theoretical

speciﬁc capacity of  mA h g−limits their application poten-

tial. PBAs with two-electron reactions exhibited a high theoret-

ical capacity of ≈ mA h g−; for example, Na.Fe[Fe(CN)]

with a rhombohedral structure provided a reversible capacity of

 mA h g−and an average operating voltage of . V.[] Due

to lattice distortion, the monoclinic phase undergoes a complex

phase transformation (from monoclinic to cubic, then tetragonal

phase), resulting in poor cycling stability and low Coulombic e-

ciency. Because the cubic structure with high symmetry is main-

tained during the electrochemical process, the development of

cubic-phase PBAs with high Na contents can eectively increase

the structural stability of the modiﬁed cathode. Accordingly, cu-

bic Na.Mn[Fe(CN)].·.HO was synthesized using a high

concentration of trisodium citrate as a chelating agent at  °C.[]

The cubic phase is typically present in PBAs with low Na content,

as interstitial water enters the crystal lattice when the Na-ion con-

tent increases, resulting in the formation of monoclinic or rhom-

bohedral structures with higher amounts of interstitial water.[]

Therefore, compared to the monoclinic MmHCF, the resulting

cubic MnHCF with slightly higher interstitial water and slightly

lower Na contents sacriﬁced some reversible capacity (from .

to . mA h g−).[] However, its high structural reversibility

during the electrochemical process provided excellent rate prop-

erties (. mA h g−at  mA g−) and cycling stability (%

capacity retention after  cycles at  mA g−). Optimizing the

composition of active/inactive metal ions to balance the trade-o

between capacity and structural stability is another eective strat-

egy to enhance the Na-storage performance of PBAs. The struc-

tural stability and ionic diusion of Fe/Mn-based PBAs can be

improved by introducing inactive elements. However, excessive

substitution results in capacity loss (e.g., Zn, Cu, Ni) or higher

production costs (e.g., Co).[] For example, high concentrations

of Zn substitution lead to the transition of Na.Fe[Fe(CN)]from

the rhombohedral to the cubic phase.[] Due to the improved

structural reversibility, the cycling stability of the optimized cath-

ode (Na.Zn.Fe.[Fe(CN)]) was greatly improved (.% ca-

pacity retention after  cycles at  mA g−), but at the cost of

some capacity (. mA h g−for FeHCF and . mA h g−for

ZnFeHCF). Another study showed that a small amount (. per

unit) of Zn-ion substitution can maintain the monoclinic struc-

ture of NaxFe[Fe(CN)].[] Due to the reduced ionic diusion bar-

rier and enhanced activity of low-spin Fe+, the modiﬁed cathode

exhibited high reversible capacity ( mA h g−) and cycling sta-

bility (≈% capacity retention after  cycles at  A g−). In ad-

dition, single/dual element-doped and high-entropy PBAs have

been shown to improve the Na-storage performance, indicating

that appropriate compositional design can improve the perfor-

mance of PBAs to some extent.[, ]

Another key factor limiting the electrochemical performance

of PBAs is the presence of vacancies and defects in the frame-

work structure.[] During PBA fabrication by the precipita-

tion method, the fast precipitation reaction inevitably introduces

M’(CN)defects in the crystal structure. These defects are sponta-

neously occupied by water molecules to form coordinated water.

The vacancies and coordinated water in the structure can cause

lattice distortion and structural damage during cycling, leading

to the accumulation of capacity loss. In addition, the dissociated

crystal water undergoes signiﬁcant side reactions with the elec-

trolyte and releases large amounts of gas. This poses a huge safety

concern for pouch-type full cells.[] Therefore, it is necessary to

put more eort into developing PBAs with high crystallinity, high

Na content, fewer defects, and minimal interstitial water. The

crystal water in the unit cell of PBAs mainly comprises adsorbed,

interstitial, and coordinated water. Adsorbed and interstitial wa-

ter can be easily removed through common drying procedures.

However, the elimination of strongly bound coordinated water re-

quires higher drying temperatures, which can damage the crys-

tal structure.[] Thus, the development direction for PBAs is

to minimize the presence of defects without damaging the in-

tegrity of the crystal structure. It was previously demonstrated

that rhombohedral NaMnFe(CN)·zHO, from which intersti-

tial water can be removed through vacuum drying ( °C), ex-

hibits a high reversible capacity of  mA h g−and a combined

charging/discharging plateau (about . V).[] PBAs with low in-

terstitial water were synthesized on a large scale using a sim-

ple heat-treatment method (dried in an argon (Ar) atmosphere

at  °C for  h).[] In situ high-temperature synchrotron XRD

analysis revealed a new triangular structure in the resulting cath-

ode, which activated the low-spin Fe+/+redox reaction, resulting

in excellent cycling stability (.% capacity retention after 

cycles at  C).

To further reduce the amounts of vacancies and inter-

stitial water in the crystal structure, it has been proposed

to use high-concentration Na reaction solutions or chelating

agent/surfactant-assisted coprecipitation methods. Increasing

the concentration of Na ions in the reaction solution allows more

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Na ions to ﬁll the interstitial sites preferentially and is an eec-

tive way to reduce the amount of coordinated water. The lattice

parameters of the PB cathode increase with increasing NaCl con-

centration, indicating that more Na-ions enter the crystal unit.[]

Thermogravimetric analysis results showed that by reducing the

precipitation reaction rate, the content of interstitial water de-

creased from . to .% (although it remained at a high level),

indicating that this could be an eective way to synthesize per-

fect crystals.[] Due to the rapid nucleation and crystal growth

processes, it is dicult to synthesize PBAs with controllable size

and few defects. Therefore, researchers proposed introducing

chelating agents that bond with metal ions to reduce the co-

ordination reaction rate.[] For example, citric acid and citrate

salts, commonly used as chelating agents, not only reduce va-

cancies and interstitial water but also serve as reducing agents

to prevent the oxidation of Fe+, thereby increasing the Na con-

tent in the crystal lattice. Hence, Mn-PBA cathodes with high

Na content and few vacancies were synthesized using saturated

trisodium citrate, and regular spherical microparticles (. μm)

were obtained after aging at  °C for  h.[] The modiﬁed

cathode with a smooth surface and high tap density exhibited a

high reversible capacity of  mA h g−and good cycling sta-

bility (the capacity retention increased from  to .% after

 cycles at  mA g−). Because high synthesis temperatures

may increase the risk of defect formation, an ice-assisted synthe-

sis method was proposed to reduce the crystal growth rate.[]

The results of inductively coupled plasma and element analy-

sis showed that defects in the ﬁnal product reduced from 

to %. In addition, other surfactants (such as polyvinylpyrroli-

done) and chelating agents (including ascorbic acid, acetic acid,

and sodium ethylene diamine tetraacetate) were evaluated to im-

prove the crystallinity of PBAs.[] Wang et al. proposed the prin-

ciple of balanced coordination to design PBAs with less intersti-

tial water and fewer transition-metal ion vacancies.[] A modi-

ﬁed cathode (Na.Fe[Fe(CN)].·. HO) with .% intersti-

tial water was synthesized using sodium carboxymethylcellulose

as a suitable chelating agent. It exhibited high reversible capac-

ity ( mA h g−) and ultra-stable cycling performance (capac-

ity retention over % after  cycles). Moreover, the addi-

tion of immiscible organic solvents to water to create a salt-in-

water-in-oil environment was successfully applied to the synthe-

sis of high-crystallinity PBA materials.[, ] Na–x FeFe(CN)was

synthesized with a low crystal water content (.%) by a low-

temperature solvothermal method with a mixture of water and

ethylene glycol.[] By adjusting the solvent ratio and reaction

time, the size of monodispersed cubic particles was controlled

( μm), and the optimized sample exhibited high reversible ca-

pacity ( mA h g−) and excellent cycling stability (.% ca-

pacity retention after  cycles at  A g−).

The particle size is another important factor for commercial

cathode materials, as it aects the capacity, volumetric energy

density, and productivity. Due to the rapid precipitation reaction

rate, PBA materials typically exhibit a nanocubic morphology

with a large speciﬁc surface area that facilitates the activation

of charge-transfer reactions, thereby increasing the reversible

capacity.[,] However, developing nanoscale PBA materials is

impractical because of their low crystallinity and insucient tap

density. From a practical perspective, micro-sized PBAs with low

surface area and high tap density can inhibit electrolyte decom-

position and side reactions, thereby extending the service life of

the full cell. Therefore, synthesizing PBAs with fewer defects

by reducing the coordination reaction rate can increase particle

size and enhance their applicability.[,,] High-entropy single-

crystal PBAs ( μm) were prepared by a modiﬁed coprecipitation

method to reduce the number of grain boundaries to suppress

surface side reactions and increase the chemical/thermal stabil-

ity by the use of micro-sized particles.[] The optimized cathode

exhibited excellent electrochemical performance, with a high ca-

pacity retention rate of .% after  cycles at  mA g−.

Microspherical PB was assembled from cubic primary particles

by spray drying and then coated with reduced graphene oxide to

improve its ionic conductivity.[] The low content of coordinated

water and improved electronic conductivity resulted in an opti-

mized cathode with a high reversible capacity of . mA h g−

(activation of low-spin Fe) and excellent cycling stability (.%

capacity retention after  cycles). In conclusion, PBAs that can

be synthesized at low temperatures have good cycling stability

and cost advantages, but issues related to their limited thermal

stability, electronic conductivity, and tap density, and excessive

defects need to be resolved.

2.3. Layered Oxides

Sodiated transition-metal oxides (NaxTMO;x≤; where TM is

a transition metal such as Ni, Co, Mn, Fe, Cu, V, or Cr) are con-

sidered promising cathode materials for SIBs because of their

high speciﬁc capacity, easy synthesis, and good electrochemical

performance.[] According to the coordination environment of

Na ions (trigonal prismatic site or octahedral site) and the num-

ber of TMOlayers in a repeating stacking unit, Na layered ox-

ides are mainly classiﬁed into P (ABBA oxide ion stacking) and

O (ABCABC oxide ion stacking) types.[] The P-phase cath-

ode materials generally exhibit excellent cycling stability over a

narrow voltage window (e.g., .–. V versus Na+/Na) but suf-

fer from rapid capacity fading over a wider voltage range (e.g.,

.–. V).[] The instability of the P-type cathode over a wide

potential window mainly originates from the severe phase transi-

tion (P to O structure) at ≈. V and the dissolution of TM ions

at low voltages. More importantly, due to the low Na content of

P-type cathode materials, they generally exhibit low ﬁrst-charge

capacity and abnormal ICE, greatly hindering the large-scale ap-

plication of Na full cells.[] Therefore, some P-type cathode ma-

terials with Na vacancies exhibit a promising high energy den-

sity of ≈ W h kg−in Na half-cell conﬁgurations.[] Neverthe-

less, only ≈% of this is retained in the Na-ion full cell due to

the initial Na deﬁciency.[] Although a pre-sodiation treatment

can compensate for the irreversible capacity loss during the ﬁrst

charge, this step increases the manufacturing cost of the full cell,

reducing the potential technical advantages of SIBs.[] There-

fore, considering the success of layered oxide cathodes in LIBs,

O-type cathodes that can be paired with an HC anode to fab-

ricate practical Na-ion full cells with high speciﬁc capacity and

long cycling life are considered the most promising candidate

for commercialization.[] The challenges and corresponding so-

lutions for O-type cathode materials are discussed in the next

section.

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Figure 2. a) Average operating voltage, speciﬁc capacity, and energy density of three representative cathodes. b) Cycle life of three kinds of cathodes.

2.4. Commercialization Potential

Owing to their cost and safety advantages, SIBs have attracted

attention from both academia and industry. However, there is a

gap between these two ﬁelds needs to be bridged through further

research eorts. Therefore, the commercialization prospects of

the candidate cathode materials mentioned above are discussed

considering their electrochemical performance, cost, safety, and

environmental impact. Figure 2 shows the capacity, gravimetric

energy density, and cycling stability of dierent cathode materi-

als. The gravimetric energy density of layered oxides is higher

than that of PBAs and polyanionic compounds (Figure a). More

importantly, the volumetric energy density of layered oxides is

much higher than that of polyanionic compounds and PBAs

(low atomic packing density).[] The energy density of the cath-

ode material has a signiﬁcant impact on the unit energy cost of

Na-ion full cells because more active materials and electrolytes

are required to compensate for a low energy density. Conversely,

polyanionic compounds exhibit outstanding cycling stability ow-

ing to their stable framework structure (Figure b). Fortunately,

it is feasible to optimize the electrochemical performance of the

cathode materials by speciﬁc strategies to meet practical require-

ments. For example, the structural/air stability of layered oxides

can be improved by elemental substitution, surface modiﬁcation,

and structural optimization strategies; the reversible capacity and

cycling stability of PBAs can be enhanced by optimizing the

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composition, reducing defects, and increasing the particle size;

the inherently low electronic conductivity of polyanionic com-

pounds can be overcome by surface C-coating and nanostructur-

ing.

Although the Na-ion storage performance of the modiﬁed cath-

ode can be improved through component optimization, the com-

position of transition metals greatly aects the price of the ﬁnal

product, so it is necessary to consider a balance between cost and

performance.[] Reducing the manufacturing cost of the cathode

is the key to the successful commercialization of secondary bat-

teries, especially for SIBs with potential low cost as the selling

point. Although the cost of Na is particularly low, its content in

cathode materials is limited (i.e., especially for polyanionic com-

pounds with large molecular weight); therefore, the manufactur-

ing cost of the cathode is more related to the raw material cost of

the cations in the framework. Introducing expensive metal ions

such as Co, Li, and rare-earth elements greatly increase the pro-

duction cost of the cathode materials, making them unpopular

in SIB systems. Therefore, it is necessary to reduce or even elim-

inate the usage of expensive metals (generally, to below %).

Due to the widespread use of inexpensive Fe/Mn ions, PBA cath-

odes exhibit the lowest production cost (when evaluated only on

raw material costs). For layered oxides, although the cost of Ni

is higher than that of Mn and Fe, the average operating voltage

and energy density of Ni-based layered oxides are relatively high,

resulting in a signiﬁcant advantages in energy cost (lower than

LiFePO) for layered oxides composed of a small amount of Ni

(/) and other non-precious metals (such as Mn, Mg, Fe, Al,

Cu).[] In contrast, V-based polyanionic compounds are the most

mature technology and exhibit satisfactory cycling stability and

rate performance in SIBs. Nevertheless, the usage of expensive

and toxic V limits their potential for large-scale application. To

this end, researchers have developed low-cost (Fe/Mn-based) or

high-energy density (pyrophosphates, ﬂuorophosphates) polyan-

ionic compounds to reduce energy costs. However, their Na stor-

age performance needs further optimization. In terms of pro-

cessing cost, unlike polyanionic compounds and layered oxides

that need to be calcined at high temperature to obtain the ideal

crystal structure, PBA cathodes that can be synthesized by copre-

cipitation without further processing once again show unparal-

leled cost advantages.[] Although the synthesis of layered oxides

through coprecipitation method increases the processing cost of

the cathode material to a certain extent, the regular morphology

increases the tap density, and the scale eect reduces the actual

production cost, making the processing cost of layered oxides and

polyanionic polymers acceptable. On the other hand, it is neces-

sary to develop newer electrode slurry combinations to simulta-

neously reduce costs and improve electrochemical performance.

In particular, solvent-free casting technology (dry method) have

been proposed to increase the energy density as well as reduce the

cost of electrode processing even though it was still early stage.[]

Considering that the energy density of SIBs is inevitably lower

than that of LIBs, optimizing the active materials and synthesis

process to further reduce the cost of SIBs is the key to its indus-

trialization.

Scaling up the production capacity is another important chal-

lenge faced by commercial cathode materials and is related to

the synthesis methods. The coprecipitation process suitable for

synthesizing PBAs and layered oxide cathodes oers signiﬁcant

advantages in batch consistency and large-scale preparation po-

tential. This is because it involves the homogenization of the liq-

uid phase, which has been fully demonstrated in LIBs systems.[]

However, synthesizing PBAs still needs further optimization,

as the rapid precipitation reaction rate results in materials with

small particle sizes and numerous defects.[] Polyanionic com-

pounds are synthesized by solid-state, hydrothermal, sol-gel, elec-

trospinning, and spray-drying methods, among which the solid-

state method is widely used because of its simple operation, low

cost, and few by-products.[] However, solid-state methods are

typically used in research studies for synthesizing small amounts

of products, making it necessary to consider the consistency of

the cathode materials when these methods are up-scaled.

The environmental impact of commercial cathode materials

is another important factor. The by-products produced during

the synthesis of layered oxides mainly include ammonium salt,

water, and CO, which have little harmful impact on the en-

vironment. However, the synthesis of PBAs generates a large

amount of wastewater containing high concentrations of Na salts,

chelating agents/surfactants, and highly toxic cyanide (CN−),

which could result in serious environmental problems.[] There-

fore, the ecient elimination or reuse of this wastewater is an-

other challenge for the commercialization of PBAs. Polyanionic

compounds containing toxic V are harmful to the environment,

while ﬂuorophosphates with high energy density release toxic hy-

drogen ﬂuoride (HF) gas during high-temperature calcination.

Therefore, polyanionic compounds seem to have a higher envi-

ronmental impact than layered oxides.

Finally, safety is crucial for the commercialization of SIBs.

Considering that the thermal runaway of SIBs is usually triggered

by material degradation, it is necessary to evaluate the thermal

stability of the cathode, especially under abusive operating con-

ditions such as overcharging, fast charging, and ultra-high tem-

perature cycling. Beneﬁting from their strong covalent bonds,

polyanionic compounds exhibit high thermal stability.[] Layered

oxides generate free Ounder high voltage or ultra-high temper-

ature (– °C), accompanied by heat release, thus exhibiting

moderate thermal stability.[] In the case of PBAs, in addition

to the side reactions caused by interstitial water, the cyanide re-

leased during their decomposition above  °C also reacts with

the electrolyte and releases heat, demonstrating the lowest safety

of the three types of cathode materials.[] In summary, layered

oxides are assumed to have the highest overall commercialization

potential because of their favorable electrochemical performance,

cost, scalability, environmental impact, and safety (Figure d).

3. O3-tye Layered Oxide Cathodes

As a Na reservoir, O-type cathodes are considered suitable can-

didates for advanced SIBs because of their high capacity and

potentially low production costs. However, there are still some

scientiﬁc issues and signiﬁcant drawbacks that need to be ad-

dressed before they can be commercialized. Therefore, this re-

view summarizes the main challenges of O-type layered oxide

cathode materials for SIBs and outlines the current mainstream

optimization strategies (composition, structure, and interface) to

bridge the gap between academia and industry (Figure 3).

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Figure 3. Schematic diagram of the main challenges and optimization strategies for O3-type layered oxide cathode materials.

3.1. Challenges of O3-Type Na-TM-O Cathodes

3.1.1. Sluggish Na-Ion Diﬀusion Kinetics

The Na/vacancy ordering caused by the electrostatic repulsion

of Na ions, JT distortions, and the charge ordering of TM ions

results in a multi-step voltage plateau and a reduced ionic dif-

fusion coecient.[, ] Due to the strong in-plane repulsion of

Na+–Na+, the transformation of Na/vacancy ordering is more

signiﬁcant than that of Li/vacancy ordering.[] In addition, com-

pared with the P-type cathode with an open ion-diusion path,

because the direct hop from adjacent octahedral sites in the O-

type layered system requires high activation energy, Na ions pref-

erentially enter face-shared interstitial tetrahedral coordination

sites temporarily. Both theoretical simulations and experimental

measurements have revealed that in Na/Fe/ Mn/ Ocathodes,

the Na ion diusion coecient of the O-type structure is lower

than that of the P phase structure (Figure 4a), which greatly hin-

ders the rate capability.[] In addition, the changes in the stacking

sequence caused by electrochemical creep aect the Na-ion mi-

gration mechanism.[] A systematic investigation of the eect of

Co substitution on the electrochemical performance and struc-

tural evolution of the O-NaFeOsystem showed that the intro-

duction of Co ions maintains the P-type phase structure dur-

ing the wide operating voltage window, which is the key factor to

achieving the high rate capability of O-NaFe/Co/O.[] How-

ever, the detrimental phase transition at high potential causes ad-

verse Na migration behavior, which needs to be optimized by ion

doping or multiphase structural design.[, ] For example, ﬁrst-

principles calculations showed that Mg-ion doping in NaCoO

causes charge disproportionation and the formation of electron

holes, thus signiﬁcantly reducing the diusion barrier and im-

proving the rate capability.[] In addition, strategies such as par-

ticle nanosizing and morphological control have been proposed

to improve the diusion kinetics of Na ions. Because the char-

acteristic time for diusion (𝜏) is proportional to the square of

the diusion length (L;𝜏=L/D,whereDis the diusion coe-

cient of Na ions), adjusting the morphology to shorten Lis one of

the eective methods to enhance the rate properties of cathode

materials.[]

3.1.2. Deleterious Phase Transitions

A ﬁrst-principles study showed that the O-type Na layered oxides

have no thermodynamic driving force to transform into spinel

structures because the relatively large size of Na ions leads to an

increase in the TM migration barrier.[] For instance, NaxTMO

oxides were synthesized from spinel LiMnOby electrochemi-

cal Li/Na ion exchange. It was demonstrated that the insertion

of Na ions into the spinel phase introduces lattice strain and fur-

ther transformation into a new monoclinic layered structure.[]

These ﬁndings show that the Na-ion system can compete with the

Li-ion system, providing an optimistic perspective for developing

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b) In situ XRD pattern of Na[Ni0.5Mn0.5 ]O2during the initial charging process. c) Cross-sectional scanning electron microscopy (SEM) images of

GmbH.

cathode materials with a stable structure and high capacity.[]

However, the extraction of Na ions induces the sliding of the

TMOlayers at a relatively low potential to minimize electrostatic

repulsion and the Na-ion diusion barrier, which may modify the

coordination environment of Na.[] In addition, the JT eect of

high-spin TM (Mn(III), Ni(III), Fe(IV), and Cu(II)) induces local

structural distortion and lattice mismatch. Speciﬁcally, the octa-

hedral MnOcomplex contains high-spin Mn(III) with the asym-

metric occupation state of the egorbital (t2g3eg1), which implies

that the overall dorbital does not match with the octahedral Oh

symmetry, resulting in the instability of Mn(III) cations.[] To

stabilize trivalent Mn cations, the octahedral unit deforms with

the elongation of two longitudinal Mn–O bonds and the shrink-

age of the other four horizontal Mn–O bonds, which leads to

the lattice deformation from a hexagonal phase to an orthogonal

structure.[] Although ﬁrst-principles calculations showed that

the O’ and P’ structures with JT distortion are dynamically

stable, such geometric deformation causes structural degrada-

tion and deﬁcient electrochemical performance.[ ] As shown in

Figure b, extracting Na ions from a typical O-NaNi. Mn. O

cathode leads to the reduction of the screening eect between

the O−anion planes and further induces the lattice distortion

and phase transition of Ohexagonal to O’monoclinic to Phexagonal to

P’monoclinic to P’hexagonal to O’hexagonal.[ –] Such multi-step

phase transitions have been widely observed in many O-type

cathode materials.[, ] A multi-step phase transition that re-

quires additional energy consumption is usually accompanied by

large volume changes and collapse of the crystal structure, result-

ing in large polarization and poor cycling stability. A novel O-

type Na cathode (Na−xLix[(Mn.Co.Ni.). □.]O−y)was

synthesized from Li-rich layered oxides through Li/Na electro-

chemical exchange.[ ] The introduction of TM and O vacancies

induces the reversible migration of TM ions between the TM slab

and Na layer, thus increasing the kinetic barrier of the O-P

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phase transition, which maintains the O-type structure during

the electrochemical process (.–. V). However, considering

that the electrochemical Li/Na exchange method is not suitable

for commercial application, it is necessary to further explore the

reaction mechanism of O-type Na layered oxides and propose

eective strategies to inhibit adverse phase transitions.

3.1.3. Mechanical Failure

The phase transformation that occurs during electrochemi-

cal processes is also accompanied by the anisotropic shrink-

age/expansion of the unit cell, resulting in the accumulation of

residual stress along grain boundaries.[ ] It has been shown that

electrochemically induced mechanical degradation is responsible

for the capacity degradation of intercalation-layer cathodes.[ ]

First, the Na concentration gradient formed during the electro-

chemical process leads to non-uniform lattice deformation. The

internal tension caused by the expansion of the c-parameter leads

to delamination cracks, while the compression caused by the con-

traction of the a-axis induces bending and kinking. These lattice

defects accumulate during cycling, resulting in slow reaction ki-

netics and degradation of the active materials. For instance, the

phase transition from hexagonal P’ to hexagonal O’ above . V

was shown to be the main reason for the rapid capacity decay

of O-NaNi.Mn.Ocathodes.[ ] It was proven that the phase

transformation accompanied by the anisotropic changes in the

lattice parameters led to the inhomogeneous accumulation of in-

ternal stress on the secondary particles and the formation of mi-

crocracks extending to the surface (Figure c). Although the in-

tegrity of secondary particles can be restored during the discharge

process, the repeated opening and closing of cracks cause perma-

nent damage to the cathode materials. The microcracks through

the secondary particles accelerate the penetration of the corro-

sive electrolyte, resulting in the continuous depletion of the elec-

trolyte and chemical damage (formation of electrochemical in-

active NiO-like rock-salt phase). Although the cycling stability of

Na layered oxides can be improved to some extent by limiting

the voltage window, this constrains the average operating voltage

and reversible capacity of SIBs.[ ] Fortunately, it is possible to

solve this problem by delaying/inhibiting the structural transfor-

mation by elemental substitution and relieving the mechanical

stress by optimizing the microstructure.

3.1.4. Surface Residual Species and Surface Degradation

The air sensitivity of layered oxides, especially sodiated TM ox-

ides with large interlayer spacing, imparts strict requirements

for the production, storage, and transportation of cathode ma-

terials, which poses a huge obstacle to the large-scale application

of SIBs.[ ] When the layered oxides are exposed to humid air,

for even a short time, some of the active Na migrates to the sur-

face and reacts with HO/COto generate residual alkali species,

such as NaOH, NaHCO,orNa

CO.[ ] The loss of active Na

accompanying the oxidation of TM ions results in the sliding

of the TMOlayer, resulting in irreversible structural changes

and signiﬁcant capacity loss. An in-situ mass spectrometry study

showed that the exposure of the O-Na.Mg.Fe.Mn.Ocath-

ode to air triggers the formation of spinel- and rock salt-like struc-

tures, accompanied by the evolution of oxygen gas (O).[ ] The

gas pressure caused by Orelease forms internal cracks and elec-

trochemical failure (Figure d). In addition, water molecules are

easily inserted into the Na layer and form a hydrated phase after

Na extraction, which greatly hinders the ionic diusion channel

and increases the interfacial resistance.[ ] This problem is made

worse by the electronic/ionic insulating residual alkali species

that reduce the electronic conductivity of the cathode material

and trigger the dehydroﬂuorination reaction and cross-linking

of the polyvinylidene ﬂuoride binder, leading to slurry gelation,

particle agglomeration, uneven coating, and corrosion of the cur-

rent collector, gas generation.[] Moreover, the irreversible loss

of the active material due to corrosion by HF (as a reaction by-

product) cannot be ignored. Therefore, the air stability of layered

oxides is crucial for the large-scale application of SIBs. A fea-

sible ethanol-washing method was proposed to remove surface

residues, thus improving the electrochemical performance of O-

NaNi. Mn.O.[ ] Although this post-treatment method signif-

icantly reduces the surface impurities on the cathode material, it

increases the number of production steps and waste-treatment

costs. Hence, it is necessary to further optimize the structure of

layered oxides to fundamentally improve their air stability.

Researchers have proposed surface engineering and elemental

doping strategies to improve the environmental stability of lay-

ered cathodes. A protective coating can be introduced to act as a

physical barrier for sensitive cathode materials to prevent direct

contact between the active material and humid air. A phosphate-

coating strategy was proposed to improve the air stability of O-

NaNi. Fe. Mn.O(capacity loss of  mA h g−after exposure

to humid air for  days).[ ] In addition, reducing the interlayer

spacing of the Na-layer and enhancing the Na–O bonds to prevent

the insertion of HO/COmolecules is another eective strategy

for improving environmental stability. The introduction of appro-

priate Na vacancies (Na.Li.Ni.Fe.Mn. O)wasshown

to enhance the oxidation resistance of TM ions and lower the dif-

fusion energy barrier, thereby simultaneously improving air sta-

bility and reaction kinetics.[ ] The structural and air stability of

Na[Ni. Mn.]Owas enhanced by calcium (Ca) doping, as the

strong Ca–O bond inhibits the Na+/HO+exchange.[]

During electrochemical processes, especially at high voltage,

the cathode material and electrolyte undergo parasitic oxidation

reactions, which result in the continuous consumption of the

electrolyte and reconstruction of the surface crystal structure.[]

It is generally believed that the relatively large radius of Na

ions makes the migration of TMs in the Na system kinetically

unfavorable.[] However, it has been shown that when the lay-

ered oxide cathode is in a highly charged (desodiated) state, metal

ions in the TMOlayer can migrate into the Na layer, especially

Cr and Fe.[] It was pointed out that there was no Cr migration

before the transformation from O-NaCrOto P’-Na.CrO,

but further removal of Na (charging over . V) triggers the

phase transition from layered to rock-salt phase (P’-Na.CrO

to O-Na𝛿CrOto rock-salt CrO).[ ] The migration of Fe-ions

from the TMOslab to the Na layer was also observed at the

atomic scale.[ ] During desodiation, Fe ions are activated into

the JT-active high-spin Fe+state to provide structural deforma-

tion to accommodate Fe migration and reduce the energy bar-

rier of Fe penetrating the O-triangle, resulting in inevitable Fe

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migration.[, ] Although the migration of TM ions is partially

reversible, it comes at the cost of irreversible capacity loss and

high polarization. Considering that Fe in octahedral sites prefer-

entially migrates to Na octahedrons through tetrahedron sites in

the Na layer, the absence of tetrahedral sites in the P-type struc-

ture may prevent TM migration to some extent.[, ] However,

the formation of Z-or OP-phase structures by stacking O- and

P-type structures provides the necessary Fe migration environ-

ment under deep desodiation. Therefore, inhibiting structural

deformation without reducing the concentration of active ions

is the developmental goal for advanced layered oxides.[ ] Re-

cently, it was proposed that the random extraction of Na and the

subsequent formation of the so-called Na-free layer at high po-

tential are the main reasons for TM migration.[ ] A ruthenium

(Ru)/Ti co-doping strategy was further developed to achieve a se-

lective desodiation reaction through the pre-oxidation of active

4d Ru, thus inhibiting the formation of a Na-free layer. More-

over, this irreversible TM migration results in the formation of

a random and uneven reconstructed layer composed of spinel

and/or rock-salt phases on the surface of the cathode material,

accompanied by oxygen loss.[ ] The anisotropic reconstructed

layer blocks ion diusion and forms edge dislocations with the

ordered Na-ion diusion layer, greatly aecting the Na-ion trans-

port kinetics. Furthermore, the dissolution of TM ions (for exam-

ple, the disproportionation reaction of Mn+→Mn++Mn+)

and the accompanying nano-cracks are also responsible for ca-

pacity losses.[ ] The dissolution of TMs results in the collapse

of the layered structure, accompanied by the migration and depo-

sition of these metals on the anode, which act as catalysts for elec-

trolyte decomposition.[ ] Therefore, the described above surface

chemical process is the key reason for the inferior electrochemi-

cal performance of layered oxides (Figure e).

To date, crystal structural design, elemental substitution, mor-

phological control, and surface engineering have been demon-

strated as eective strategies to improve the structural stability,

reaction kinetics, and thermal/environmental stability of layered-

oxide cathode materials.[, ] Therefore, this section discusses

feasible development directions for O-type cathodes based on

published research work and the requirements of commercial

SIBs.

3.2. Potential O3-Type Layered-oxide Cathodes

Notably, despite the many similarities between Li and Na tech-

nologies, SIBs cannot be developed by simply using the Na coun-

terpart of LIB cathode materials. For example, the similar ionic

radii of Li and Fe (rLi/rFe =.) easily lead to cation mixing,

resulting in the formation of a cubic rock-salt 𝛼-LiFeOphase

rather than an electrochemically active layered structure. Surpris-

ingly, 𝛼-NaFeO(rNa /rFe =., hexagonal layered structure) ex-

hibits suitable Na storage activity with a distinct charge/discharge

plateau around . V.[ ] Similarly, LiCrOis considered an elec-

trochemical inactive cathode material because oxidized Cr+ions

irreversibly migrate to the Li layer and occupy interstitial tetra-

hedral sites, thereby blocking ionic diusion channels.[ ] How-

ever, the mismatched ionic radius and tetrahedron height result

in Cr remaining in the TMOslab within a narrow voltage win-

dow (below . V), so the NaCrOcathode provides a discharge

capacity of ≈ mA h g−and an average operating voltage of  V.

In , Cu was introduced into a Ni/Mn-based cathode mate-

rial (Na.[Cu.Fe.Mn.]O), with the electrochemical activity

of the Cu+/Cu+redox couple evaluated using X-ray absorption

near-edge spectroscopy (XANES) analysis.[ ] The incorporation

of active Cu ions into the TMOlayer simultaneously improves

the structural stability, reaction kinetics, and air stability, provid-

ing more opportunities for the large-scale application of layered

oxides. It should be noted that, unlike LIBs with a high-Ni and

Co-free cathode, the optimal compositions of the cathode materi-

als for SIBs have not yet been determined.[ ] Based on the high

voltage provided by Fe and Cr, high capacity provided by Ni, high

electronic conductivity provided by Co, and good structural sta-

bility provided by Mn and Cu, researchers have developed a se-

ries of single (NaCrO), binary (Mn/Ni and Mn/Fe-based oxides),

ternary (MnNiCo, NiMnFe and FeMnCu systems), and multi-

metal oxides.[ ]

3.2.1. NaCrO2

O-NaCrOcathodes with excellent thermal and cycling sta-

bility have received widespread attention from researchers.[ ]

Ex-situ XRD results revealed that NaCrOundergoes a simple

phase transition from hexagonal O to monoclinic O and ﬁ-

nally to monoclinic P during desodiation (.–. V), endow-

ing it with excellent structural reversibility (Figure 5a).[ ] Large-

grained NaCrOparticles were synthesized by directly calcining

NaCrO·HO at  °C.[ ] Reducing the speciﬁc surface area

to minimize side reactions and exposing the () plane to im-

prove the ionic diusion kinetics resulted in a cathode with ex-

cellent cycling stability (capacity retention of .% after  cy-

cles at  C). In addition, due to the low oxidation state of Cr in the

layered structure, NaCrOneeds to be sintered in a reducing or

inert atmosphere, which provides the possibility of applying a C-

coating in situ. Fu et al. ﬁrst synthesized a C-coated NaCrOcath-

ode via a simple solid-state reaction, which delivered a capacity re-

tention of % after  cycles.[ ] Hexagonal plate-like NaCrO

was synthesized by emulsion drying method, followed by C-

coating with pitch as the raw material (Figure b).[] The highly

crystalline C-layer was evenly coated on the surface of the active

material, which endowed the obtained cathode with high ionic

conductivity (−Scm

−) and thermal stability. Cathodes based

on NaCrOnanowires were obtained by electrospinning.[] This

unique D porous network composed of nanocrystalline sub-

units provides a fast diusion channel for Na ions while reduc-

ing the diusion resistance of ionic species, thereby improving

the electrochemical kinetics and enhancing structural stability

(Figure c). Therefore, the obtained cathode material exhibited

ultra-high rate capability (≈. mA h g−at  C) and a wide op-

erating temperature range (− to  °C). In addition, elemen-

tal substitution strategies have been proposed to enhance the

crystal stability of layered frameworks, thereby suppressing high-

pressure phase transitions and increasing reversible capacity.[]

It was proposed that the introduction of an antimony ion, Sb+

(more than . mol), with a ﬁxed oxidation state and high elec-

tronegativity, would lead to thermodynamically unfavorable Cr

migration.[ ] Therefore, the optimized O-Na.Cr.Sb. O

cathode undergoes a reversible O–P phase transition over a

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Figure 5. a) Charge/discharge curves and phase-transition diagram of NaCrO2. b) Schematic diagram of the synthesis of C-coated plate-like NaCrO2

wide voltage range of .–. V without forming the irreversible

O’ phase (Figure d), resulting in a high reversible capacity

(. mA h g−) and long-term cycling stability (.% capac-

ity retention after  cycles at  C). Although NaCrOexhibits

excellent electrochemical performance, its toxicity and high pro-

duction cost indicate that it is not the preferred choice for com-

mercial applications.

Other single-metal oxides have signiﬁcant drawbacks that hin-

der their commercialization, such as the complicated structural

evolution of NaNiO, JT distortion of NaMnO, high cost of

NaCoO, and Fe migration in NaFeO.[] To alleviate the inher-

ent drawbacks of single-metal oxides, the general research strat-

egy toward developing ideal cathode materials is to design multi-

metal oxides with synergistic eects to enhance structural stabil-

ity and alleviate phase transitions.

3.2.2. NaNi0.5Mn0.5 O2

Ni/Mn-based oxides are the most extensively studied layered

cathode materials because the Ni+/Ni+redox couple provides

high voltage and reversible capacity, while the JT-inactive Ni+

and Mn+form a robust framework structure. NaNi.Mn.O

is a classic O-type cathode material, which delivers a high re-

versible capacity of . mA h g−with .% ICE within a

potential window of .–. V.[ ] However, the speciﬁc capac-

ity of this material drops rapidly due to severe phase transi-

tions and mechanical stress accumulated during cycling. There-

fore, compositional modiﬁcation has been proposed to improve

the cycling performance and rate capability of O-type cathodes.

Heterogeneous-atom substitution provides the following ben-

eﬁts: ) increases the binding energy between TM and O to

improve the structural reversibility and reduce the ionic diu-

sion barrier;[,] ) adjusts the interlayer spacing of the Na

layer to improve ion-diusion kinetics and/or air stability;[, ]

and ) suppresses the JT eect of Mn+through charge com-

pensation (such as Mg+,Zn

+,Cu

+,andLi

+).[– ] For ex-

ample, it was proposed that Al doping can shorten the bond

length of TM–O and expand the interlayer spacing of the Na

layer by increasing the TM–O bond energy, thereby improv-

ing the cycling reversibility and ionic diusion kinetics of

the resulting cathode (NaAl.Ni. Mn.O).[  ] The modiﬁed

NaNi. Mn.Ocathodes doped with Zr,[ ] Ti,[ ] tin (Sn),[]

and Sb[ ] also demonstrated similar results. A novel O–

Na.Li. Ni. Mn.Owcathode material was synthesized by

electrochemical Na/Li-ion exchange, which provided an ultra-

high reversible capacity of  mA h g−and a total energy den-

sity of  W h g−within a voltage window of .–. V.[  ] In

addition, the introduction of non-toxic Ca+into the Na-layer has

been proposed.[, ] Due to the strong bonding between Ca and

O ions, which alleviates the sliding of the TMOslab and the

loss of Na/O, the resulting cathode (Na.Ca. [Ni.Mn. ]O)

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Figure 6. Contour plots of in-situ XRD patterns of a) Sn-doped (4.0 V high-cutoﬀ voltage) and b) Mg/Ti co-substituted (4.5 V high-cutoﬀ voltage)

2021, the Royal Society of Chemistry. In situ XRD patterns of c) NaNi0.5Sn0.5O2(2.8–4.0 and 2.8–4.2 V in the initial and second cycles, respectively) and

2023, Elsevier.

exhibited enhanced high-voltage stability and environmen-

tal/thermal stability.[] To date, little research has been con-

ducted on Na-site substitution. However, other potential dopants

such as Mg, Zn, Zr, Li, K, and Bi provide the possibility of fur-

ther extending the cycling life of layered-oxide cathodes.[– ] It

should be noted that the substitution on Na sites may result in

more initial capacity loss than the substitution of TMOlayers,

and the narrowed ion-diusion channel adversely aects the re-

action kinetics.[, ] Therefore, it is necessary to carefully select

appropriate doping agents and adjust their concentrations to bal-

ance the trade-os between cycling life, capacity, and rate capabil-

ity. In addition to cation substitution, the combination of F−ions

with strong electronegativity to enhance the stability of lattice-O

has also proven eective in improving the electrochemical per-

formance of layered oxides.[ ] It was demonstrated that the in-

troduction of F ions reduces the thickness of the TMOlayer,

but broadens the ion-diusion channel to improve reaction kinet-

ics, alleviates abrupt phase transitions to suppress the formation

of microcracks, and enhances the resistance to electrolyte cor-

rosion to alleviate the dissolution of active materials.[ ] There-

fore, pouch-type full cells assembled with optimized cathodes

(Na.[Ni.Mn. ]O.F.) and HC anodes exhibited excellent

cycling stability, with a capacity retention of % after  cycles

at  mA g−. However, it should be noted that the introduction

of toxic F ions is unpopular in the industry, which limits the prac-

tical potential of these materials.

The frequent phase evolution accompanied by sluggish re-

action kinetics and multi-step redox characteristics is responsi-

ble for the poor electrochemical performance of NaNi.Mn.O.

Therefore, it is recommended to optimize the composition

to alleviate high-pressure phase transformation, thereby sup-

pressing the formation of microcracks and avoiding mechanical

failure.[ ] A systematic investigation of the eects of Sn doping

on the electrochemical performance and structural integrity of

NaNi. Mn.Oshowed that the introduction of Sn can alleviate

the abrupt contraction of crystal-cell parameters caused by the

P’ to O’ phase transition at high voltage (Figure 6a), thereby

inhibiting the formation of microcracks and extending the cy-

cling life; the capacity retention after  cycles at . C increased

from . to .% after Sn addition.[ ] In addition, Ti+, with

a similar ionic radius but a greatly dierent redox potential from

Mn+, was shown to improve the TM–O bonding energy, increase

the working potential, and disrupt Na/vacancy ordering.[] An

Mg/Ti co-substitution strategy was proposed to improve the

structural stability of NaNi.Mn.Ocathodes, in which Ti ex-

pands the interlayer spacing, and Mg acts as an HF scavenger.[]

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Unlike the drastic phase transition from P to O’ in non-

substituted samples under high voltage, co-doped samples with

extended TM–TM distances and suppressed Na/vacancy order-

ing provide a smooth phase transition, delay the O–O’ phase

transition, and form a stable PO intermediate buer phase in-

stead of the O’ phase (Figure b). Therefore, the optimized cath-

ode (Na[Ni/Mn/Mg/ Ti/ ]O) exhibits excellent structural

integrity as it eectively suppresses microcracks caused by the

signiﬁcant shrinkage of the interlayer spacing. A systematic in-

vestigation of the structure and electrochemical performance of

Ti- and Cu-doped NaNi.Mn.Oshowed that the crystal-cell pa-

rameters and oxidation peak potential of the samples increased

with increasing Ti/Cu substitution concentration (but the solu-

bility of Cu was limited to below . mol).[ ] The co-doped sam-

ple formed a mixed phase in the high-voltage region instead of a

new O phase, thereby reducing the unit-cell volume shrinkage.

Therefore, the optimized cathodes (NaNi.Cu.Mn. Ti. Oand

NaNi. Cu.Mn. Ti. O) exhibited a higher energy density

than the reference NaV(PO)Fmaterial. In addition, it was

demonstrated that co-substitution of Ti and Zn/Mg/Cu can also

suppress high-voltage phase transitions and reduce unit-cell vol-

ume changes, thus eectively improving the cycling stability of

NaNi. Mn.O.[] However, due to the presence of O domains

related to cation migration at the fully charged state, the cycling

stability of the modiﬁed cathodes is still unsatisfactory.

For the above reasons, by adjusting the electronic structure of

the TMOslab to expand the solid-solution zone, the structural

reversibility and Na-ion diusion ability of the obtained cathode

is fundamentally improved.[ ] For instance, Sn+(d) with

ﬁlled d orbitals cannot interact with O p orbitals, so orbital hy-

bridization occurs between the remaining TM and O atoms, thus

preventing charge delocalization in the TMO-layer, increasing

the output voltage, and improving the structural reversibility at

high voltages.[ ] The O–P phase transition can be delayed,

and the redox potential can be increased by the chemical substitu-

tion of Mn in NaNi.Mn.Oby Sn (Figure c).[] The improved

cycling stability of the resulting cathode (NaNi.Sn.O)comes

at the cost of sacriﬁcing some energy density due to the introduc-

tion of heavier Sn. In addition, Ti-doping can eectively alleviate

multi-step phase transformation because the slightly larger vol-

ume of TiOoctahedra than MnOoctahedra enables ﬂexibility

to absorb structural distortion and strain caused by Na-ion in-

sertion/extraction processes.[] The introduction of Ti+with a

large ionic radius broadens the TM–O bond length, inhibiting

the phase transition above . V to broaden the solid solution re-

action zone of the P phase (three charging/discharging plateaus

become sloped curves) (Figure d).[,] Therefore, the opti-

mized cathode NaNi.Mn. Ti. Oexhibited long cycling stabil-

ity (% capacity retention after  cycles at C) and fast ionic

diusion (. ×− cms−) over the voltage window of .–

. V.[ ] More importantly, the air stability of layered oxides

can be eectively improved by introducing dopants with large

ionic radii and dierent Fermi levels to weaken the hybridiza-

tion between O(p) and TM orbitals and enhance the Na–O bind-

ing energy (reducing the Na layer spacing).[,] Therefore,

NaNi. Mn.Ti.O[ ] and NaNi.Cu.Mn. Ti. O[ ] ex-

hibit excellent air stability and do not undergo structural changes,

even when soaked in water. It should be noted that due to the

structural similarity between the O- and P-type phases, some

studies mistakenly attribute the O-phase structure in the high-

voltage region to the P-type phase, thereby misidentifying the

solid-solution reaction zone.[, ] For O and P phases, the

intensity ratio between () and () diraction peaks can be

used as an indicator of the crystal structure, where the O and P

phases exhibit higher relative intensities of () and () peaks,

respectively.[]

3.2.3. NaTM1TM2O2

Owing to their abundant natural raw materials and low environ-

mental impact, Fe/Mn-based cathode materials are also attractive

systems. O-Na.Mn/Fe/Odelivered a discharge capacity of

 mA h g−over a voltage window of .–. V, but suers from

poor cycling stability due to the complex phase transitions dur-

ing the charge/discharge process.[] Although cation doping ef-

fectively improves the electrochemical behavior of Fe/Mn-based

cathodes, the abnormal ICE caused by Mn+/Mn+/Mn+redox

reactions at low voltage limits their commercial application.[]

Based on the typical O–P phase transition, NaNi.Fe. Ode-

livered a reversible capacity of  mA h g−and exhibited a

smooth charging/discharging proﬁle and stable cycling stability

in the voltage range of .–. V.[ ] However, the rapid capac-

ity degradation caused by Fe migration and irreversible changes

in the lattice parameters under high voltage still need to be ad-

dressed. Owing to the high price of Co, the large-scale application

of Co-containing cathode materials will weaken the competitive-

ness of SIBs. Furthermore, unlike LIB systems, the introduction

of Co may lead to cation mixing and weaken the layered structural

characteristics,[ ] making Co-containing layered oxides unsuit-

able as commercial cathode materials for SIBs. Moreover, cath-

ode materials with cationic-ordered superlattice structures (such

as honeycomb structures) can eectively inhibit the intercala-

tion reaction of HO due to enhanced interlayer interactions,

which enhances the air stability of layered oxides.[ ] A novel O’-

NaMn. Al.Ocathode with a superlattice layered structure was

obtained by Al substitution, in which AlOand MnOoctahe-

drons show a “queue-ordered” arrangement in TMOslabs.[]

The ordered arrangement of TM ions suppresses the lattice dis-

tortion caused by the JT eect, while the introduction of Al ions

induces electron transfer to enhance the TM–O bonding strength

and reduce the formation energy of layered structures. Therefore,

the capacity retention rate of the optimized cathode increased

from . to .% after  cycles at  mA g−. In addition,

some high-voltage binary metal-oxide cathode materials have

been presented. For example, a novel high-voltage cathode, O-

Na.Ni.Sn. O, with a . V operating voltage, demonstrated

a capacity of  mA h g−due to the contribution of the Ni+/Ni+

redox couple within a voltage window of .–. V.[ ] However,

subsequent studies showed that this material completely trans-

formed into its hydrated phase after being exposed to humid air

for  h, indicating its strong sensitivity to air.[] In recent years,

honeycomb layered oxides (NaNi/M/O, where M represents

As, Sb, and Bi) with high operating voltage have also attracted the

attention of researchers, as the small size of M enables more dis-

tortion of the NiOoctahedron and a decrease in the energy level

of the egorbitals.[ ] However, the presence of excessive inactive

ions leads to low reversible capacity (about  mA h g−)and

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Figure 7. a) Schematic diagram of the relationship between the discharge capacity, capacity retention, and thermal stability of O3-Na[NixCoyMnz]O2

increased production costs, which limits its potential for large-

scale commercialization.

3.2.4. NaM1M2M3O2

Inspired by the successful commercialization of NCM cathodes

in LIBs, researchers systematically studied the electrochemical

and thermodynamic behavior of O-Na[NixCoyMnz]O(/ ≤x

≤.) cathodes.[ ] Similar to lithium-based systems, increasing

the Ni content, especially above %, may increase the capacity,

but at the expense of the cycling life, rate properties, and ther-

mal stability (Figure 7a). This conﬁrms the importance of balanc-

ing the TM composition of layered oxides. NaNi/Mn/Co/O

with relatively high thermal stability and cyclability undergoe a

multi-step phase transition of O–O–P–P during the sodia-

tion/desodiation process, where the ab plane slightly contracts

and the interlayer spacing continues to expand.[ ] Ti-doped

NaNi. Co.Mn.Oshowed enhanced cycling stability (capac-

ity retention increased from  to % after  cycles at . C)

and thermal stability (exothermic peak increased by  °C) with-

out sacriﬁcing reversible capacity ( mA h g−).[ ] In addition,

this Ti-substitution strategy has been proven to improve air sta-

bility; the original crystal structure was maintained even when

immersed in water, with only a slight decrease in crystallinity.[ ]

It should be noted that unlike Li-ion systems, the development of

high-Ni layered oxides in SIB systems does not seem to be a pop-

ular research direction because Ni triggers the multiphase trans-

formation and lattice defects, and is quite expensive.[ ] There-

fore, surface engineering and optimizing the chemical composi-

tion and morphology are the key directions in the development

of Ni/Co/Mn-based cathodes.

The Ni/Fe/Mn-based cathode materials derived from Ni/Mn-

binary metal oxides have also received attention from re-

searchers. The introduction of Fe with a large ionic radius ex-

pands the TMOlayer and facilitates electronic delocalization.

Therefore, NaFe.Ni.Mn. Oshowed a reversible Ohex –

Phex phase transition over a voltage range of .–. V, without

forming other monoclinic structures, thus delivering higher aver-

age operating voltage (. V versus . V for NaNi.Mn.O),

reversible capacity (. mA h g−), and cycling stability (%

capacity retention after  cycles at C).[ ] However, it was

proposed that NaFe/Ni/Mn/ Otransforms into a monoclinic

O’ phase at . V and undergoes a multi-step phase transition

of O’–P’–O during discharge.[ ] In addition to the chemi-

cal composition, the proportion of each component in the TMO

slabs aects the electrochemical performance of the active mate-

rial. It was demonstrated that the Fe+/Fe+redox couple is quite

reversible at low Fe concentration (/ per unit), but signiﬁcantly

irreversible at high content (. per unit).[ ] Therefore, opti-

mizing the composition and proportion of active elements is the

key to the commercialization of SIBs. A systematic investigation

of the electrochemical performance of O-NaNixFeyMnzOwith

dierent compositional ratios showed that NaNi.Fe. Mn.O

exhibited the best cycling stability due to the suppressed JT ef-

fect and expanded interlayer spacing.[ ] However, the redox cou-

ple of Mn+/Mn+activated at low voltage decreases the energy

density. Na[Ni. Fe. Mn.]Owith high Ni content delivered

a high discharge capacity of  mA h g−and an average oper-

ating voltage of . V within a voltage window of .–. V.[]

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However, it undergoes a complex phase transition from O to

O’–P–O’’ during the desodiation process, resulting in infe-

rior ion-diusion kinetics and the formation of microcracks. Al-

though limiting the charging cuto voltage to . V ensures that

the developed cathode maintains the P phase structure after

the extraction of . mol of Na, this sacriﬁces part of the re-

versible capacity. It was demonstrated that an appropriate in-

crease in the Fe content can alleviate unstable Ni+-induced O-

loss, thereby eectively improving the cycling and thermal sta-

bility of Ni/Fe/Mn-based cathode materials (Figure b).[ ] In

addition, cation substitution (such as Al, Ti, Zn, Mg, and Li) was

shown to further improve the high-voltage stability.[,] For ex-

ample, due to the introduction of Al enhancing the TM–O bonds,

widening the ion-diusion channels, alleviating the JT eect of

Mn+, and reducing the interfacial resistance, the resulting cath-

ode (NaNi/Mn. Fe/ Al. O) exhibited improved rate capa-

bility (. mA h g−at  C) and cycling stability (% capacity

retention after  cycles at C).[ ] An investigation of the struc-

ture and electrochemical performance of NaMn.Ni. Fe. O

and NaMn.Ni. Fe. Mg.Ocathode materials demonstrated

that Mg doping reduces the JT eect through a charge compen-

sation mechanism and improves the ionic diusion capability

by expanding the interlayer spacing.[ ] Therefore, the capacity

retention of Mg-doped samples after  cycles at . C signiﬁ-

cantly increased from  to %. The incorporation of Ti+with-

out d electrons strengthens the binding energy between TM ions

and O by increasing the O-electron density while increasing the

electrostatic repulsion of the O slab to expand the interlayer dis-

tance and reduce the Na-ion migration energy barrier.[  ] Hence,

Na[(Mn.Fe.Ni.). Ti. ]Oexhibited improved cycling stabil-

ity, with a capacity retention of % after  cycles at . C,

accompanied by some reduction in the initial discharge capac-

ity (from  to  mA h g−).[] Furthermore, it was demon-

strated that the introduced Ti ions are enriched on the surface

of bulk materials, which eectively suppresses monoclinic dis-

tortions and reduces the reactivity with water, thereby improving

the cycling and water stabilities.[] The morphology and crystal

structure of Na.Ni. Fe. Mn.Ti.Oremained unchanged

after  days of exposure to humid air and  h of immersion in

water, exhibiting high air stability (only .% reversible capacity

loss after  days of exposure) and excellent cycling stability (.%

capacity retention after  cycles at  C).[ ] It was shown that

doping with Li (. mol%) to support the TMOlayer can sup-

press the migration of Fe+ions, thereby improving the reversible

capacity (. mA h g−within .–. V), cycling stability

(.% capacity retention after  cycles at . C), rate capability

(. mA h g−at  C), and thermal stability ( °C exothermic

peak) of Na[Ni.Fe. Mn.]O.[ ] Furthermore, it was demon-

strated that the introduction of Li not only strengthens the TM–

O bonds and reduces JT distortions to improve the structural

reversibility under high potentials. Moreover, it also acts as a

sacriﬁcial agent to react with F−ions to form a stable cathode–

electrolyte interphase (CEI) layer containing LiF to alleviate the

dissolution of the active material.[ ] Therefore, the optimized

cathode (O-Na.Li.Ni.Mn. Fe. O) maintained % of

its initial capacity after  cycles over a wide voltage range of .

to . V at C.

Because Cu plays an active role in improving the structural

and air stabilities of layered oxides, Fe/Mn/Cu-based cathode ma-

terials composed of inexpensive metal ions are also considered

promising materials.[] The O-Na.[Cu.Fe. Mn.]Ocath-

ode with Cu/Fe active centers undergoes multiple phase transi-

tions from O to P and ﬁnally to O’ during the desodiation

process (.–. V), providing a reversible capacity of ≈ mA

−and an average storage voltage of . V.[ ] It was pro-

posed that the introduction of Cu reduces the average valence

state of Mn ions (forming electrochemical inactive Mn+)toavoid

the P–O phase transition and improve the cycling stability, while

the Cu+/Cu+redox couple compensates for the partial capac-

ity loss.[ ] In addition, a correlation between the solubility of

Cu and the valence state of Mn was demonstrated. An aver-

age oxidation state of Mn above +. induces the segregation

of CuO to satisfy a new electrostatic equilibrium that accommo-

dates low-valent Mn. Furthermore, the cation composition dia-

gram of the Nax(Cu–Fe–Mn)Osystem was investigated, and it

was shown that low-valent Mn provides high reversible capacity

(for example,  mA h g−for Na.(Cu.Fe. Mn. )O), Fe re-

duces the average operating voltage, and Cu improves the rate

performance, cycling stability, and air stability (Figures c,d).[]

In addition to the composition of the TMOlayer, the eect of

the Na content on electrochemical performance was studied.[]

It was demonstrated that reducing the Na content in the O-

NaxCu.Fe.Mn. Ocathode to a critical value (.) of the

O/P dual-phase structure can promote the phase evolution

of O–P, avoid changes in the unit-cell parameters of the O

structure and minimize voltage hysteresis. It should be noted

that similar to Fe/Mn-based cathode materials, the Mn+/+/+

redox reactions delivers high reversible capacity but low operat-

ing voltage, which reduces the energy density and leads to an ab-

normal ICE.[, ] Therefore, the Fe/Mn/Cu-based cathodes are

typically cycled over a narrow voltage window (for example, .–

. V), thereby exhibiting good cycling stability (% capacity re-

tention after  cycles at C) but low reversible capacity ( mA

−).[ ] A systematic investigation of the eect of Li-doping

on the electrochemical performance of NaCu.Fe. Mn. O

showed an inhibition of the JT eect and enhanced Fe+/+reduc-

tion, resulting in an optimized cathode with high reversible ca-

pacity, average discharge voltage, and cycling stability.[ ] It was

further demonstrated that the surface degradation caused by Mn

reduction and side reactions with electrolytes further degrades

the cycling stability of layered oxides, which implies that further

optimization of the surface structure is required.[]

3.2.5. High-Entropy Compounds

In recent years, high-entropy oxides with high fracture tough-

ness, tolerance to a wide operating temperature range, high

electronic delocalization, and high structural stability have re-

ceived extensive attention in the ﬁelds of electrochemical en-

ergy storage and electrocatalysis.[ ] Although a precise deﬁni-

tion of high-entropy oxides is still controversial, they are usually

deﬁned based on composition (single-phase complexes contain-

ing at least ﬁve metal atoms, with concentrations ranging from 

to % per atom) or conﬁgurational entropy (conﬁguration en-

tropy greater than .Rat room temperature, independent of

the phase number).[ ] Due to the synergistic eect of multiple

metal elements, high-entropy oxides with cocktail eects exhibit

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Figure 8. a) In-situ XRD patterns of high-entropy oxides in the ﬁrst two cycles within the voltage range of 2.0–3.9 V. Reproduced with permission.[ 201 ]

VCH GmbH. d) Charging/discharging proﬁle, cyclic voltammogram, and long-term cycling stability of high-entropy substituted cathodes. Reproduced

surprising electrochemical properties. As early as , a novel

quinary layered oxide cathode O-NaNi/Co/Fe/ Mn/Ti/O

was presented.[ ] The K-edge XANES results indicated that Ni,

Fe, and Co are responsible for charge compensation upon sodi-

ation/desodiation, the introduction of Ti enhances the ionic dif-

fusion ability, and the addition of Mn improves charge-transfer

properties. Similarly, the charge compensation reaction of the

ﬁve-component cathode Na.Ni/ Fe/ Co/ Mn/Ti/Oduring

charging/discharging is mainly attributed to the Ni+/Ni+re-

dox pair, with some contributions from Fe+/Fe+and Co+/Co+

pairs, while Ti+and Mn+act as structural pillars.[] Because

the multi-component structure suppresses unfavorable phase

transitions, the obtained cathode material only exhibited a voltage

plateau attributed to O and P phase transitions at ./. V,

along with a sloping voltage curve related to the solid solu-

tion reaction of the P structure between . and . V. This

extended solid-solution process and negligible unit-cell volume

change (≈.%) endowed the resulting ﬁve-component cath-

ode material with excellent structural reversibility and cycling

stability.[] The O-type high-entropy oxide cathode material

NaNi. Cu.Mg. Fe. Co. Mn.Ti.Sn. Sb.Owas ﬁrst

presented in , where Ni, Cu, Co, and Fe provide charge com-

pensation to achieve a high reversible capacity, Mg and Ti acts

as pillars to maintain the layered framework structure, Mn di-

rects the structure, and Sn and Sb increase the average operat-

ing voltage.[ ] The redox elements in small-component O-type

cathodes are uniformly distributed in the TMOslab, facilitating

the phase transition from O to P during charging/discharging.

However, the local changes in the redox elements in this multi-

component high-entropy cathode are easily accommodated by

the local characteristics of other stabilizers, thereby expanding

the region of the O phase structure (% of total capacity) and

increasing the energy density (average operating voltage of . V)

(Figure 8a). This delayed phase transition was also observed

in high-entropy O-type NaCu.Ni. Fe. Mn.Ti.Ocathodes

that maintained the O-type structure until . Na ions were ex-

tracted from the lattice unit.[ ] Although the high-entropy cath-

ode exhibited excellent cycling stability, with a high capacity re-

tention of % after  cycles at  C, a high proportion of inac-

tive elements resulted in a low reversible capacity of ≈ mA

−. To balance high entropy and high capacity, they further

optimized the concentration of each component in the TMO

slabs (NaNi. Mg.Cu.Fe.Mn. Ti. Sn.O).[ ] This high-

entropy conﬁguration enhances the surface energy of the ()

plane to increase the Na-ion transport channel, while the TMO

slab undergoes slight bending rather than producing a rock-salt

phase near the surface to accommodate the volume changes of

the unit cell during charging/discharging (Figure b). Therefore,

the obtained cathode exhibited high reversible capacity (. mA

−), excellent structural integrity (less TM dissolution and

cracks), long cycling life (% capacity retention after  cycles

at C), and thermal stability (exothermal peaks at  °C) in a nar-

row voltage window of . to . V. A further study investigated

the electrochemical properties of NaMn.Fe. Co. Ni. Ti. O

and Na/Li/Fe/Co/Ni/Mn/ Oand found that the quick

phase transformation from O to P metallic phase is the key to

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GmbH.

the high Na-ion diusion kinetics (Figure c).[,] It is neces-

sary to further clarify the optimal compositions and phase transi-

tion processes of high-entropy oxides. Recently, a novel layered

cathode material, NaNi.Mn.Co.Cu. Fe. Li.Ti.Sn. O,

was developed via high-entropy substitution, where Ni, Co, and

Mn provide charge compensation, Fe, Cu, and Ti serve as sta-

bilizers, Li alleviates lattice distortion, and Sn increases the

working potential.[ ] This multi-component high-entropy oxide

smoothed the successive phase evolution of O →O’ →P,

thereby delaying the phase transition from P to P’, and increas-

ing the Young’s modulus and indentation hardness resulting in

a signiﬁcant improvement in the cycling performance of the re-

sulting cathode (.% capacity retention after  cycles at

 mA g−) (Figure d). Although the introduction of inactive el-

ements slightly reduces the reversible capacity (. mA h g−),

the higher average discharge voltage of this high-entropy cathode

(. V versus . V for NaNi.Mn.O) provided a higher energy

density (. mW h g−). In addition, aging tests showed that

high-entropy structures can eectively alleviate phase changes,

TM dissolution, and the formation of NaCOimpurities. Based

on the discussion above reasons, high-entropy oxides are cur-

rently in the early research stage, but are promising candidates

for cathode materials in advanced SIBs. Future research needs

to focus on high capacity, high-voltage performance, air stability,

low cost, and morphology/interface engineering.

3.2.6. Multiphase Structure

The crystal structure of Na–TM–O compounds is aected by ther-

modynamics (such as the Na/TM ratio and synthesis tempera-

ture) and kinetics (including the preparation method, raw ma-

terials, and synthesis conditions), so they show rich structural

diversity.[] A systematic investigation of the phase evolution

process of Na.TMO(TM =Co, Mn) suggested that the for-

mation of the ﬁrst phase is controlled by thermodynamics, while

subsequent transitions are related to kinetics (Figure 9a).[]

Considering the structural diversity of layered oxides, it is

reasonable to consider the development of multiphase cath-

ode materials with dierent electrochemical properties, which

exhibit better performance than single-phase materials due

to synergistic eects.[ ] For example, the zNa/Ni/ Mn/O

(−z)NaNi.Mn. O(/ ≤x≤) system could have a two-

phase structure, consistent with the experimental results of

P-Na.Ni. Mn.Oand O-NaNi. Mn.O.[ ,] Aseriesof

cathodes (NaxNi.Mn. O;x=., ., and .) were pre-

pared with a mixed P/O structure by changing the sodium

content.[ ] However, due to the charge compensation mecha-

nism, the presence of Na vacancies requires an increase in the

content of JT-active Ni+and even the formation of NiO impu-

rities, which may have adverse eects on the electrochemical

performance. The synthesis phase diagram of NaxMnyNi−yO

materials showed that proper chemical compositional design is

the key to the synthesis of two-phase structures.[ ] A doping

method was used to control the ratio of the two-phase struc-

ture, demonstrating that the dopant-induced P phase forma-

tion trend is Mn >Ni >Cu >Fe >Co >Ti. In addition, by

controlling the sintering temperature, cooling rate, and calci-

nation time, even cathodes with the same chemical composi-

tion can form dierent crystal structures.[, ] Through theo-

retical simulations and XRD analysis, it was demonstrated that

the formation of the multiphase structure (P/O) originated

from ﬂuctuations in the local cation potential due to the un-

even distribution of metal ions, which is mainly dependent on

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temperature-dependent reaction kinetics and thermodynamics

(Figure b).[] Similarly, a NaxNi/Co/ Mn/Ocathode with a

three-phase structure (intergrowth P/O/O structure) was suc-

cessfully synthesized by controlling the calcination temperature

and cooling step (quenching).[ ] Unlike the speciﬁc synthesis

conditions of single-phase cathode materials, the optimal synthe-

sis conditions and reaction mechanism of multiphase materials

still need further clariﬁcation.

To combine the advantages of P-type and O-phase cathodes,

a series of layered cathode materials with two-phase or three-

phase structures have been developed. Previous studies have

demonstrated that the incorporation of heterometals (especially

Li) into TMOlayers can facilitate the formation of mixed-phase

structures, as these dopants can oer dramatically dierent elec-

tronic conﬁgurations, oxidation states, and sizes.[,, ] John-

son et al. ﬁrst incorporated Li as a dopant into NaNi.Mn.O

to induce the formation of a P/O biphasic cathode mate-

rial (Na.Li.Ni.Mn. O+d) with a nanoscale topotactic inter-

growth structure (Figure c).[] The volume change of the P

domain during the sodiation/desodiation process is less than

%, while the O phase transforms into the P phase and re-

mains unchanged throughout the discharge process, which en-

ables the presented cathode to exhibit improved structural sta-

bility and high-rate capability. However, this O-majority ma-

terial still suers from low cycling lifetimes, which needs to

be resolved. Later, a novel biphasic cathode material was de-

veloped, (Na.Li.Mn.Ni. Co. O+𝛿), which integrates a

small amount of O into a P-dominated layered material

to meet the requirements of high capacity and excellent cy-

cling stability.[] The developed cathode exhibits a high re-

versible capacity ( mA h g−) and good cycling stability (%

capacity retention after  cycles). Recently, a novel penta-

nary metal-oxide cathode with a P/O biphasic structure was

presented, namely Na/Ni/ Mn/Fe/Mg/ Li/O.[] Because

multi-element doping inhibits the dissolution of active materi-

als, the structural reversibility was improved. Furthermore, the

topologically intergrown structure suppressed the high-voltage

phase transition and lattice mismatch, resulting in a cathode with

excellent rate properties (. mA h g−at  C) and cycling

stability; the capacity retention increased from  to % after

 cycles at C. It should be noted that Li-substituted cathode

materials do not all exhibit a layered structure, and the intro-

duction of excess Li can result in the formation of anomalous

spinel phases. The Na.Li.Ni.Fe.Mn. O+𝛿cathode mate-

rial with a Li-substituted layered (O) and spinel intergrowth

structure was prepared with an alkali metal content greater than

.[ ] Compared with the pristine samples, the layered/spinel in-

tergrown cathode exhibited high rate capability ( mA h g−at

 mA g−) and Na-ion diusion coecient (. ×− cms−)

because the additional spinel phase provides D ion-diusion

channels to facilitate ion transport.

Although the P/O heterostructure induced by Li doping re-

sults in excellent Na-ion storage performance, the introduction of

Li leads to an increase in the production cost of the cathode ma-

terial, which reduces the scalability of SIBs. Therefore, cations

(such as Ti, Sn, Sb, Mg, Cu, and Fe) with dierent ionic radii and

valence states are introduced to induce the formation of multi-

phase structures.[– ] Hu et al. proposed that the crystal type of

the Na layered oxide can be determined from the cationic poten-

tial, which is beneﬁcial for guiding the design of O- and P-type

cathodes.[ ] However, this empirical formula has limited impact

on the development of non-equilibrium and distorted phases.

The inﬂuence of chemical composition changes on the propor-

tion of the bi-phasic structure was evaluated using a radial poten-

tial predictor (combining the cation potential and ionic radius)

(Figure d).[] The proportion of P phase in the cathode ma-

terials with the same Na content decreased linearly with increas-

ing radial potential. Structural analysis and theoretical calcula-

tions showed that the intergrowth structure with an interlocking

eect can eectively inhibit unfavorable cation migration and O-

loss, thus improving the structural stability and reversibility of

the anion redox reaction. In addition, the local chemistry modu-

lation of dopants can induce the formation of P/O intergrowth

structures by regulating the structural evolution and formation

energy, thereby suppressing the sliding of TMOslabs and en-

hancing the reversibility of crystal structures.[ ] Hence, Ti with

low cationic potential was introduced (Na.Ni. Mn.Ti.O)to

trigger the formation of an O structure and adjust its pro-

portion in the biphasic cathode.[ ] Beneﬁting from the sup-

pressed high-voltage phase transition and reduced lattice param-

eter changes, the developed heterostructured cathode exhibited

improved structural integrity and excellent cycling stability (with

a capacity retention of % after  cycles at  C and a reversible

capacity of  mA h g−at . C within .–. V). Moreover,

an interfacial interlocking reaction mechanism was proposed to

explain the excellent Na-ion storage performance of heterostruc-

tured cathodes, whereby the mutual anchoring of dierent crys-

tal domains reduces the strain energy and mechanical damage at

the phase boundary (Figure e).[] Accordingly, the optimized

P/O-Na.Ni. Mn.Ti.Ocathode exhibited high cycling

stability with a retention rate of .% after  cycles at  C.

Because the introduction of dopants can reduce the capac-

ity, the strategy of synthesizing multiphase structures by ad-

justing the sintering temperature and/or Na content is receiv-

ing increasing attention. In addition, introducing Na vacancies

to reduce the amount of surface residual Na and improve the

reaction kinetics improves the Na-ion storage performance of

O-type cathodes, making it possible to achieve long cycling

life under high reversible capacity.[,] A series of  com-

pounds were synthesized by adjusting the Na content and cal-

cination temperature to evaluate the temperature dependence

of the cathode structure.[ ] Although the in-situ XRD results

suggested that the optimized biphasic cathode undergoes com-

plex structural evolution during the sodiation/desodiation pro-

cess, it was proposed that the interlocked intergrowth struc-

ture can reduce internal stresses and alleviate lattice parame-

ter changes (biphasic clamping reaction mechanism), leading

to improved electrochemical performance (Figure f). A three-

phase cathode (P/P/O-Na.Ni. Mn.O) was synthe-

sized by only adjusting the calcination conditions (temperature

and atmosphere).[ ] Beneﬁting from the structural constraint

eect of this multiphase structure that suppresses detrimen-

tal phase transitions, the developed cathode material showed

high rate capability ( mA h g−at  mA g−) and cy-

cling stability (the residual capacity increased by  mA h g−

after  cycles). A series of P/O mixed-phase intergrowth

layered cathode materials were synthesized by optimizing

the Na content (combining P-Na.[Ni.Fe. Mn. ]Oand

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O-Na[Ni.Fe. Mn.]O).[ ] In-situ XRD results showed that

the optimized biphasic cathode undergoes a reversible phase

transition and negligible volume change during cycling, thereby

providing high reversible capacity (. mA h g−) and improved

cycling stability (% capacity retention after  cycles at .C)

over a wide voltage window of .–. V.

3.3. Potential Coating Medium for O3-Type NaTMO2

To suppress interfacial parasitic reactions and alleviate the disso-

lution of active materials, researchers proposed a surface coating

strategy to improve the interface stability, including coatings of

C,[, ] oxides,[, ] ﬂuorides,[ ] and phosphates.[ ,] The

introduced surface-modiﬁcation layers have the following func-

tions: ) Improve ionic/electronic conductivity. A surface modiﬁ-

cation layer with high ionic/electronic conductivity can eectively

improve the surface properties of cathode materials and reduce

the internal reaction resistance.[] ) Physical protection. The

coating isolates the active material from the corrosive electrolyte

to prevent the corrosion by harmful by-products such as HF. In

addition, the coating also acts as a buer to alleviate the volume

change of the crystal during the charging/discharging process,

thus improving the integrity of the composite. ) Reduce resid-

ual Na on the surface. As mentioned in section , residual Na on

the surface may cause gel of the slurry and loss of capacity.[]

Therefore, in-situ construction of the Na-containing coating can

eectively improve the electrochemical performance and inter-

facial stability by consuming the insulating surface residue.[ ]

It should be mentioned that owing to the sensitivity of Na-rich

compounds to humid air, the post-treatment of sodiated TM ox-

ides may lead to surface degradation.[] Therefore, it is necessary

to carefully explore the synthesis process and preparation envi-

ronment.

3.3.1. Carbon Coating

Carbon-based materials with high electronic conductivity and an

elastic network are eective as a coating layer, so they are widely

used to modify electrode materials for LIBs and SIBs. However,

most Na–TM–O composites are calcined in air or O,sothisC-

coating strategy is usually applied to speciﬁc cathode materials,

such as Cr- and V-based oxides.[] Carbon-coated Fe/Mn-based

cathode materials can be achieved using solid-state-assisted pro-

cesses or secondary high-temperature carbonization steps (Ar

atmosphere).[, ] However, due to unstable coatings or intro-

duced O-vacancies, the resulting cathode materials exhibit unsat-

isfactory electrochemical performance. Therefore, from the per-

spective of production costs and Na-ion storage performance, the

C-coating strategy does not seem to be fully suitable for modify-

ing layered oxide materials.

3.3.2. Oxide Coatings

In addition to C-coatings, metal oxides are widely used for surface

modiﬁcation of layered oxide cathodes owing to their good con-

ductivity, structural stability, and interfacial compatibility.[,]

For instance, atomic layer deposition technology was used to

precisely encapsulate NaNi.Mn.Oin an AlOcoating with

controllable thickness ( nm) to reduce side reactions and sup-

press electrolyte corrosion (Figure 10a).[] Furthermore, the sur-

face of O-Na[Ni.Co.Mn.]Ocathodes were uniformly coated

with nano-AlOvia dry ball-milling in a dry room to prevent the

formation of surface by-products.[ ] The introduced protective

coating can scavenge the HF produced by the adverse parasitic re-

action with the electrolyte, accompanied by the formation an AlF

layer on the outer surface, thus inhibiting the dissolution of TM

ions and the generation of microcracks (Figure b). Therefore,

the pouch-type full-cell assembled with this surface-optimized

cathode and HC anode exhibited excellent cycling stability (%

capacity retention after  cycles). Further, a multi-level strat-

egy was proposed, which combines a core–shell structure, sur-

face AlOcoating, and bulk Ti doping to alleviate the structural

degradation and sluggish ion-diusion kinetics of a Ni-rich ox-

ide cathode (NaNi.Co.Mn.O).[ ] The introduction of Ti im-

proved the ion-diusion dynamics without reducing crystallinity.

In addition, the electrochemically active Ni ions were enriched in

the inner core to provide high capacity, while the Mn-rich shell

and thermally stable AlOcoating inhibit surface parasitic re-

actions and prevent the growth of microcracks to maintain the

structural integrity. Therefore, the modiﬁed cathode exhibited a

wide operating-temperature range (− to  °C) and excellent

sodium storage performance (. mA h g−at  C; .%

capacity retention after  cycles at .C) (Figure c). There-

fore, the introduction of these oxide coatings physically protects

the active material from electrolyte attack, thereby maintaining

the structural integrity and electrochemical activity of the cath-

ode material.

It was reported that heat treatment can enhance the contact

between the coating and bulk material and also promote the

doping of some of the metal ions into the crystal lattice of the

cathode.[ ] Sucient evidence indicates that doping or substi-

tution of cations (Mg, Cu, Ti, Sn, and others) can eectively

maintain the stability of the layered framework, reduce the dif-

fusion barrier, and alleviate/suppress phase transitions.[,,]

Therefore, the integrated doping–coating strategy eectively en-

hances the electrochemical performance of the modiﬁed cath-

ode. It was reported that the increase in entropy mixing is a

driving force for the random substitution of Ni and dopants

(for example, Mg) in the lattice, enabling the synthesize of cath-

odes with a surface coating and bulk substitution via a one-

pot method.[ ] Accordingly, an O-Na[Ni.Mn.]Ocathode

coated with MgO and doped with Mg was demonstrated, in

which the coating layer minimizes parasitic reactions between

the active material and the electrolyte to prevent HF attacks,

while Mg+doping eectively reduces irreversible phase transi-

tions to improve structural stability (Figure d). A TiO-coated

O-NaMn.Fe. Ni. Ocathode was synthesized by dry mix-

ing followed by high-temperature calcination, and it was con-

ﬁrmed that some of the Ti+was doped into the lattice us-

ing cross-sectional EDS mapping and X-ray photoelectron spec-

troscopy depth proﬁling.[ ] The coating layer prevented surface-

side reactions to improve structural integrity, while Ti+doping

shortens the TM–O bond length and expands the interlayer dis-

tance to enhance the rate capability. A strategy combining bulk

Sn+substitution and surface nanolayer Sn/Na/O composite

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d) Schematic diagram of the synthesis process of MgO-coated Na[Ni0.5Mn0.5 ]O2(top). TEM image and corresponding energy-dispersive spectroscopy

coating was shown to enhance the Na-ion storage performance

of O-NaNi/Fe/ Mn/Ocathodes.[ ] The lifetime of the Sn-

modiﬁed cathode was doubled (Figure e). This was attributed

to the surface coating reducing undesirable parasitic reactions,

Sn doping inhibiting adverse high-voltage phase transforma-

tion to improve structural reversibility, expanding the interplanar

spacing to enhance the ion diusion coecient, and increasing

the ionicity of the TM–O bond to increase the average operat-

ing voltage. In addition, the combined coating–doping strategy

not only extends the service life by improving structural integrity

and reducing internal reaction resistance, but also eectively im-

proves air stability and anion redox reversibility.[,] However,

it should be noted that this dual modiﬁcation strategy requires

further optimization as it is still dicult to accurately control the

amount and location of the dopant.

In addition to improving chemical stability, the introduced pro-

tective coating eectively improves the environmental stability of

layered oxides by physically isolating the sensitive cathode from

moist air.[] The air stability of NaNi.Mn. Owas enhanced

using surface coatings of CuO[ ] or AlO.[  ] The chemical ag-

ing results indicated that this surface-modiﬁcation strategy can

eectively alleviate the structural degradation (reduce the con-

tent of hydrated compounds), inhibit the formation of impurities

(NaHCOand NaCOby-products), and suppress the dissolu-

tion of TM ions. Manthiram et al. used inert ZrOto modify the

surface of NaNi.Mn.Co.Oto suppress its intercalation re-

action with HO/CO.[] Electrochemical tests showed that the

modiﬁed cathode delivered % of the initial capacity after  days

of exposure to air, while the unmodiﬁed anode completely failed.

Although the aged cathode can be restored by high-temperature

annealing, this increases production costs.[ ] In summary, sur-

face engineering and element substitution strategies remain key

to improving environmental stability and achieving commercial

applications of layered-oxide cathode materials.

3.3.3. Polyanionic Composite Coatings

Polyanionic composites are considered suitable coating mate-

rials because of their potentially high ionic conductivity and

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Figure 11. a) Schematic diagram of the synthesis process of an AlPO4-coated Na[Li0.05Mn0.50 Ni0.30Cu0.10 Mg0.05 ]O2cathode and the correspond-

Na3−3xAlxPO4coating on active materials during repeated cycles and HRTEM images of the coated sample. Reproduced with permission.[ 240 ] Copyright

2023, the American Chemical Society. c) Structural model and corresponding TEM images of a Ti-rich spinel coating on a Mn-rich layered cathode. Repro-

synthesis process of P2-coated O3-structured cathode and a corresponding TEM image of the coating. f) Cycling stability of the pristine and coated

stable framework structure.[ ] In addition, compared to com-

mon mechanical mixing methods for synthesizing oxide coat-

ings, polyanionic coatings are typically prepared by wet-chemical

methods, which facilitate the formation of uniform and well-

adhered layers.[ ] A thin AlPOion conductor was coated on

the surface of an O-Na[Li.Mn.Ni.Cu. Mg.]Ocathode

to enhance the ionic conductivity and alleviate the dissolution of

Mn (Figure 11a), thereby increasing the capacity retention from

% to % after  cycles at C.[] In addition, NaSiOand

NaTi(PO)ion conductors with D Na-ion diusion channels

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were developed to protect the host material and reduce the migra-

tion energy barrier, thereby improving the rate capability.[, ]

A new type of ionic conductor NaZrSiPO coating was de-

posited on the NaNi/Mn/Fe/Omodel cathode by a sol-gel

process, followed by high-temperature calcination.[ ] The pro-

tective coating provided fast Na-ion diusion channels and re-

duced cracking and particle peeling. In addition, density func-

tional theory and neutron powder diraction analyses showed

that some of the Zr atoms were successfully doped into the TMO

layer, which expanded the Na-layer spacing, increased the self-

diusion coecient, and delayed the phase transition. Therefore,

the interface-optimized cathode exhibited improved rate capabil-

ity ( mA h g−at  C) and reduced electrochemical polariza-

tion (. V). Considering that one of the main factors lead-

ing to rapid capacity degradation is the residual Na-based im-

purities, such as NaOH and NaCOwith poor conductivity, re-

ducing or removing such surface compounds to improve struc-

tural stability is an eective modiﬁcation strategy for layered ox-

ide cathodes.[, ] Therefore, simultaneously consuming sur-

face residues and applying a protective coating in situ is an ef-

fective method for overcoming the shortcomings related to Na

residues and interfacial instability.[] The in-situ formation of

a plastic-crystal Na−xAlxPOcoating on the outer surface of an

O-NaNi.Fe. Mn.Ocathode was demonstrated using a wet-

chemical method, while a thin Na-deﬁcient phase was formed

on the inner surface due to the extraction of Na from the crys-

tal lattice.[ ] This protective layer formed by consuming resid-

ual alkali compounds on the surface inhibits electrolyte corro-

sion and the accompanying parasitic reactions, isolates the sen-

sitive active materials from humid air, and provides a rapid dif-

fusion channel for Na-ions based on the paddle-wheel mech-

anism associated with coupled anion rotation and cation mi-

gration (Figure b).[] This strategy of coupling surface en-

gineering and Al doping can eectively alleviate the generation

of microcracks and particle fragmentation, resulting in an opti-

mized cathode with excellent rate performance (. mA h g−at

 C) and cycling stability (capacity retention increased from %

to % after  cycles at  C). Recently, the surface residual

Na of O-NaNi.Cu.Mn. Ti.Owas transformed into a solid-

electrolyte NaMgPOcoating to improve the rate capability of

the modiﬁed cathode.[ ] The introduction of protective coat-

ings with fast ion-transport channels inhibits the dissolution of

the active material and improves the reversibility of the P–OP

phase transition within .–. V. Therefore, the resulting full-

cell showed a reversible capacity of  mA h g−and a high

capacity retention of % after  cycles at . C. To date, this

strategy of removing surface residues while constructing a pro-

tective surface layer has not yet been fully developed, and further

optimization is required.

3.3.4. Heterostructure Coating

Although heterostructured cathode materials have been proven

to have excellent electrochemical performance, Mn segregation

to the particle surface is the main cause of capacity loss dur-

ing cycling.[ ] Contrastingly, coupling additional phases on the

surface of secondary particles enhances the direct interaction

between heterostructures.[ ] It has been shown that the intro-

duction of inactive protective coatings can eectively improve

the structural stability and environmental stability of layered

oxides, but at the expense of some capacity.[,] Therefore,

researchers have developed advanced core–shell cathode mate-

rials by coating fast ion/electron-conducting structural materi-

als (such as the P phase) on high-capacity structural materi-

als (for example, the O phase). The coating protects the sensi-

tive core material from the corrosive electrolyte and provides a

buer layer to reduce the eect of lattice changes in the cathode

during cycling. For example, a multiphase NaMn.Ti.Ni.O

cathode containing P and O’ layered core and a spinel-like

titanium(III)-oxide interface was fabricated using Ti-enrichment-

induced surface reconstruction (Figure c).[] The introduc-

tion of an atomically thick Ti-rich spinel-like interface pre-

vents the direct contact of the layered oxide with the moist

air/electrolyte and improves the electronic/ionic conductivity, so

the resulting cathode material exhibits excellent cycling stabil-

ity (% capacity retention after  cycles at  C). To elimi-

nate surface NaCO, a thermal activation process was demon-

strated, which converts surface residual Na into an electrically

active P/O composite structure, thereby increasing the initial

charging capacity of NaxMnO(from  to  mA h g−)de-

spite the dominant P phase leading to an abnormal ICE.[ ]

Furthermore, an O/O’-P core–shell structured cathode ma-

terial (Na.[(Ni.Co.Mn. ).(Ni. Mn.). ]O) was prepared

by a co-precipitation method, which beneﬁted from the high-

capacity O-type core and stable P-type shell (Figure d).[]

The precursor with an Mn-rich surface is favorable for the for-

mation of the P phase, so the P-phase shell on the spher-

ical particles can alleviate the mechanical damage caused by

volume changes and provide fast Na-ion diusion paths. Al-

though introducing P coatings with Na vacancies can slightly

increase the discharge capacity, it can also lead to an abnor-

mal ICE. Hence, a -nm P-Na/MnOlayer was successfully

coated on an O-NaNi.Mn.Ocathode using a wet-chemical

process (Figure e).[] By carefully adjusting the thickness of

the P coating, the optimized cathode ( mol% coating) exhib-

ited a slightly higher ICE (%) and excellent cycle stability (%

capacity retention after  cycles at  C) without sacriﬁcing the

reversible capacity ( mA h g−) (Figure f).

3.3.5. Other Coatings

In addition to the modiﬁed layers discussed above, researchers

have developed various coating materials to improve the struc-

tural stability of layered cathode materials. For example, organic

polymer coatings with mechanical ﬂexibility and appropriate

electrical conductivity have been developed.[ ] For example, a

composite organic solvent “cocktail” was used to form an artiﬁcial

CEI layer composed of metal–organic compounds and reduced

TM cations on the surface of NaNi/Fe/ Mn/Ocathodes to en-

hance the electrochemical stability of active materials in a propy-

lene carbonate (PC) based electrolyte, especially at high current

rates.[ ] A surface-modiﬁcation strategy for a methacrylic-acid–

acrylonitrile copolymer was shown to improve the interfacial sta-

bility of P/O-Na.Li.Ni.Mn. O+𝛿cathodes under high

potentials.[ ] The introduced polymer with a strong electron-

donating group shares electrons with oxidized TM ions to

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anchor them and inhibit the dissolution of the active material.

In addition, the slightly higher valence of O caused by a charge-

balancing mechanism leads to an increase in the lattice-O activity

and Na-ion diusion rate. Furthermore, an AlF-coated nanorod

O-Na[Ni.Co.Mn.]Omaterial was developed to limit the

structural degradation associated with the formation of a thick

rock-salt layer and inhibit the mechanical failure caused by the

growth of intra-particle cracks.[ ] Therefore, the pouch-type full

cell assembled with this optimized cathode and HC anode exhib-

ited enhanced cycling stability, with a high capacity retention of

% after  cycles at . C.

3.4. Optimal Synthesis Process: Co-Precipitation versus Other

Methods

Coprecipitation is a common method for preparing cathode ma-

terials with controllable morphology and high tap density for

both LIBs and SIBs.[, ] The method usually includes the syn-

thesis of precursor particles (hydroxide, oxalate, or carbonate)

by a coprecipitation reaction. The precursors are then mixed

with a Na source and processed by high-temperature calcina-

tion. Spherical precursors with a diameter of several microns are

usually obtained by the coprecipitation method and their mor-

phology is inherited by the ﬁnal product after calcination.[, ]

In addition, because the calcination temperature of the sodi-

ated oxide cathode is usually higher than that of the Li analogs,

grain-boundary fusion can occur. Therefore, polyhedral spheri-

cal cathode materials with dense surfaces have been reported

(Figure 12a).[] Conversely, the introduction of dopants can af-

fect the rate of crystal growth and ultimately alter the morphol-

ogy of the product.[ ] For instance, it was demonstrated that

the introduction of Ti as a dopant can induce the growth of pri-

mary particles during heat treatment, which resulted in the syn-

thesis of densely packed secondary particles with a lower void

volume and enhanced mechanical strength (Figure b).[] Be-

cause Mg/Ti co-doping accelerates the growth of the O struc-

ture, the ﬁnal product exhibited spherical and plate-like mor-

phologies (Figure c).[] To further improve the Na-ion storage

performance, researchers have prepared cathode materials with

unique morphologies by adjusting the synthesis conditions. In

, a new concept of assembling spherical particles using a

radially aligned hierarchical columnar structure was presented,

and the product was characterized by a concentration gradient

of TM ions (Figure d).[] The closely arranged secondary par-

ticles minimize the contact between sensitive Ni and corrosive

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Springer Nature. d) SEM and TEM images of Na[Li0.05Ni0.3 Mn0.5Cu0.1 Mg0.05 ]O2with exposed (010) active facets. Reproduced with permission.[278 ]

electrolyte, thus achieving extended service life (% capacity

retention after  cycles) and improved thermal stability (in-

creasing the exothermic peak by  °C). To highlight the ad-

vantages of this structure, the mechanical and electrochemical

characteristics of the cathode composed of spoke-like nanorods

were compared with those a constant-concentration sample.[ ]

The results of micro-compression tests and high-resolution TEM

reveal that the compact structure can enhance the mechanical

strength by minimizing the porosity, and improve the ionic dif-

fusion dynamics through the directional transmission of elec-

trons/ions. In addition, radially distributed primary particles can

be synthesized in other constant-concentration cathode materi-

als, which indicates that the coprecipitation conditions have a

great impact on the particle morphology.[] By optimizing the

synthesis process (pH, temperature, and feeding rate), a sub-

micron spherical precursor was formed by the aggregation of

nanoplate primary particles, which exhibited high industrial fea-

sibility (Figure e).[] Due to the short Na-ion diusion dis-

tance, the obtained cathode material showed excellent rate prop-

erties. In addition, O-type NaNi.Mn.Ocathodes with a core–

shell structure were developed by adjusting the chemical compo-

sition, in which the Ni-rich core provides high capacity, and the

Mn-rich shell improves the structural stability (Figure f).[]

However, the core–shell structured cathode exhibited unsatisfac-

tory cycling stability, which may be related to the formation of

irregular ﬂaky particles. In addition to conventional spherical

precursors, coprecipitation can be used to synthesize cathodes

with unique morphologies by adjusting the solvents and chelat-

ing agents. For instance, hollow O-NaNi.Mn.Omicrobars

prepared using ethanol-mediated coprecipitation were composed

of oriented stacks of nanoplates with exposed () crystal facets

(Figure g).[] The resulting cathode material exhibited excel-

lent rate capability and an extended service life owing to the en-

hanced exposure of electrochemically active planes, which short-

ens the diusion distance and reduces volume changes during

the sodiation/desodiation process. A dispersed microscale O-

NaMn. Fe. Ocathode material was synthesized using a cubic

Mn[Fe(CN)]·HO precursor (Figure h).[] Although us-

ing PBAs as templates can eectively control the morphology of

the ﬁnal product, this method can only be used to synthesize cath-

ode materials with speciﬁc chemical components.

Solid-state fabrication is one of the traditional methods for

synthesizing cathode materials, which has the advantages of

simple processing steps and accurate product stoichiometry.

Nanoscale O-NaNi.Mn.Owith controllable grain orienta-

tion was achieved by Sb substitution using a solid-state method

(Figure 13a).[] The introduced dopant can reﬁne the grains by

modifying the surface energy of layered cathodes,[ ] broaden

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the Na-ion transport channels, and reduce the ion-diusion en-

ergy barrier. Therefore, the optimized cathode delivered an out-

standing rate capability of . mA h g−at  C. The solid-state

synthesis process is relatively simple, but the subsequent long

high-temperature sintering process ( to  °C) consumes

a lot of energy, resulting in low processing eciency.[] Al-

though the sol-gel method can reduce the sintering temperature

( °C), the ﬁnal product is usually composed of agglomerated

nano/microplates, which limits the transfer of ions/electrons

during the charge/discharge process.[ ] Similarly, a solution

combustion synthesis technique was proposed to prepare chem-

ically uniform cathode materials.[ ] Speciﬁcally, nitrate and

glycine were used as the oxidant and fuel, respectively, to ob-

tain micron-sized secondary particles composed of hexagonal

plate-like primary nanoscale particles by auto-ignition at  °C

(Figure b).[] However, the electrochemical performance of

the resulting cathode was not satisfactory, which may be due to

its loose stacking. In addition, the consistency of the morphology

of products prepared by sol-gel and solution-combustion meth-

ods is worse than that of products prepared by coprecipitation,

which is not conducive to large-scale production.

Hydrothermal methods are also eective for synthesizing ac-

tive materials with controllable morphology. The synthesis of a

nano-ﬂower quinary cathode material was achieved using a urea-

assisted hydrothermal method, combined with subsequent high-

temperature calcination (Figure c).[] The resulting cath-

ode material provided surprisingly long cycling stability, with

a high capacity retention of .% after  cycles at  C.

By controlling the synthesis conditions, a series of thin sheet-

like structures and nano/micro-spherical cathode materials have

been developed, which demonstrated enhanced electrochemical

performance.[– ] In addition, appropriate crystal facet design

to expose more active sites is an eective strategy to improve

the rate capability of layered cathodes. A ﬁve-component cathode

material (O-Na[Li.Ni. Mn.Cu. Mg.]O)withanexposed

() crystal facet was synthesized by thermal polymerization,

which comprised multilayers of oriented stacked nanosheets

(Figure d).[] By increasing the exposed electrochemically ac-

tive plane to shorten the diusion distance and maintain high

electrochemical reversibility through multi-element substitution,

the resulting cathode material exhibited outstanding rate perfor-

mance (. mA h g−at  C) and long-term cycling stability

(.% of the initial capacity after  cycles). Although the prod-

uct synthesized by a hydrothermal method showed unique mor-

phology and enabled subsequent calcination at moderate temper-

atures, the feasibility of upscaling this method needs to be con-

ﬁrmed.

Micro-spherical cathode particles assembled by primary par-

ticles can also be obtained by spray drying. For instance, TM-

oxides were used as raw materials to prepare spherical Ni/Mn/Fe-

based cathode precursors by a combined ball milling–spray dry-

ing process (Figure e).[,] The obtained porous hollow prod-

ucts are beneﬁcial for electrolyte permeation and exhibit im-

proved ion-diusion kinetics. Although rapid spray-drying meth-

ods have commercial potential, they still require further op-

timization to obtain uniform cathode materials. In addition,

due to the loose characteristics of the precursors, it is neces-

sary to avoid particle cracking during high-temperature calcina-

tion to further increase tap density and structural integrity.[]

Electrospinning to synthesize one-dimensional nanoﬁbers with

uniformly adjustable diameters and interconnected framework

structures is an eective strategy for achieving a high rate perfor-

mance and low internal resistance of cathode materials.[, ]

Hollow-ﬁber NaNiSbOcathode nanoparticles were fabricated

using such a method (Figure f).[] Compared with equivalent

materials with an irregular morphology, the ﬁber-based cathode

showed improved rate performance due to exposed active sites

and reduced volume changes. However, the subsequent high-

temperature calcination process in air may lead to the growth

of primary nanoparticles and interrupt the ion/electron trans-

port paths. Thus, the practicality of electrospinning technology

for producing oxide cathode materials needs further analysis.

3.5. Single-Crystal Particles

It should be noted that many grain boundaries exist in the sec-

ondary particles composed of primary particles, which induce

the formation of microcracks that result in capacity decay and

thermal instability during the charging/discharging process.[]

In LIBs, the single-crystal cathode without inter-particle bound-

aries has attracted the attention of academia and industry, be-

cause it can avoid the formation of cracks to obtain extended

service life.[ ] Based on this, a solvothermal method was used

to prepare a single-crystal O-NaFeOcathode material with

a hexagonal morphology.[] Furthermore, micron-sized trun-

cated octahedra or platelet-like single-crystal particles were ob-

tained by molten salt-assisted synthesis.[, ] First, the synthe-

sis conditions were optimized to produce Na(Ni.Fe. Mn.)O

single crystals with enhanced cycling stability. These modiﬁca-

tions included using metal oxides instead of coprecipitation par-

ticles as the starting materials, % molten-salt (NaOH) con-

tent, a temperature of  °C, and rinsing the particles in wa-

ter (Figure 14a).[] To avoid parasitic reactions between the

alumina crucible and NaOH ﬂux, a two-step synthesis method

was further proposed, which uses NaCl as the molten salt and

metal oxides as raw materials to prepare single-crystal trun-

cated octahedral NaNi.Mn.Oparticles with high tap density

(. g cm−).[ ] The resulting single-crystal cathode with high

crystallinity and consistent lattice orientation eectively mini-

mized surface parasitic reactions, reduced the generation of in-

tergranular cracks, and exhibited a longer lifetime than polycrys-

talline particles (Figure b). Although single-crystal particles ex-

hibit improved cycling stability, there are still some issues that

need to be addressed. For example, synthesizing single-crystal

cathodes by molten-salt methods requires an additional water-

washing step, which adds complexity to the production process

and may induce structural transformation of the active mate-

rial. Therefore, it is necessary to develop alternative methods

to simplify the synthesis process.[ ] Due to the lengthening of

Na-ion diusion pathways and reduced electrolyte accessibility,

large single-crystal particles typically exhibit slightly reduced re-

versible capacity and sluggish reaction kinetics (rate capability),

which require further optimization. In addition, since selected

area electron diraction analysis is conﬁned to a narrow area, it is

not suitable to prove that micron/submicron particles are single

crystals.[ ] Therefore, it is recommended that electron backscat-

ter diraction analysis is conducted as this method can detect

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Figure 14. a) Cycling performance of Na(Ni0.3Fe0.4 Mn0.3 )O2single crystals synthesized by a molten-salt method with diﬀerent raw materials and rinsing

grain boundaries and provide direct evidence for the synthesis of

single crystals. Although the research on single-crystal cathode

materials for SIBs is still in its early stages and the cycling stabil-

ity of these materials is slightly inferior to some modiﬁed poly-

crystalline cathodes,[,, ] it is proposed that with further re-

search eorts, the electrochemical performance of single-crystal

cathodes will be signiﬁcantly improved soon.

4. Anodes and Electrolytes for Use with Practical

Cathodes

4.1. Hard-Carbon Anodes

The anode materials for SIBs mainly include C-based materials,

alloying materials (P, Sn, Sb, and others), and metal oxide/sulﬁde

materials.[] Although anodes that undergo alloying and conver-

sion reactions can deliver high reversible capacity, their large vol-

ume expansion during sodiation leads to the crushing and struc-

tural collapse of the active material, which greatly limits the cy-

cling life of SIBs.[ ] Furthermore, the high reversible capacity

of an anode material does not directly correspond to high-energy

batteries, which is also closely related to their average charg-

ing voltage.[ ] Titanium-based oxide/phosphate anode materi-

als based on intercalation reactions have attracted widespread

attention because of their excellent cycling stability and rate ca-

pacity, and small volume changes. However, their high operat-

ing voltage and low speciﬁc capacity limit the energy density of

the full-cell system. Therefore, insertion-based carbonaceous ma-

terials are considered the most promising anode materials for

commercial SIBs because of their diverse structures, high nat-

ural abundance, and low cost. Carbonaceous materials include

graphite, soft carbon (SC), HC, CNTs, and graphene. Graphite is

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VCH GmbH. d) ICE as a function of various C characteristics (CO2speciﬁc surface area, COxgroups, and active surface area). Reproduced with

Society.

currently used as the anode material for commercial LIBs. Al-

though graphite with solvent co-intercalation mechanism pro-

vides excellent cycling stability and rate capability in SIBs sys-

tems, its low speciﬁc capacity (≈ mA h g−) and high operat-

ing voltage (≈. V versus Na+/Na) are not conducive to achieving

a high energy density in the full cell, which greatly limits its appli-

cation potential.[ ] Compared to the high price and low packing

density of CNTs and graphene, as well as the low ICE and high

voltage plateau of SC, HC with high reversible capacity (–

 mA h g−) and low discharge voltage (≈. V versus Na+/Na)

is considered the most promising anode material for commer-

cial SIBs, which can be demonstrated through the research path

of SIB startups.[ ]

4.1.1. Na Storage Performance

The electrochemical behavior of Na ions in HC anodes involves

adsorption on surfaces and defects, intercalation reactions, and

ﬁlling of micropores.[ ] At present, the main controversy re-

garding the Na storage mechanism of HC anodes is the con-

tribution of various reactions to the slope regions (above . V)

and plateau region (below . V). To achieve commercial appli-

cations of SIBs, various challenges related to HC anodes still

need to be addressed, including their poor rate capability and

unsatisfactory cycling stability caused by slow ionic diusion

in the low voltage plateau region, as well as diculties in con-

structing practical full cells due to the low ICE. Shortening

the Na-ion transfer distance by morphological engineering can

eectively improve the poor rate capability of HC materials. For

example, porous HC ﬁbers synthesized by electrospinning and

subsequent high-temperature carbonization processes can eec-

tively enhance the electrolyte accessibility and ionic diusion

eciency.[,] In addition, C-microspheres synthesized by hy-

drothermal methods or hollow C-spheres prepared by templat-

ing methods (with templates of SiO, TiO, and others) not only

shorten the ionic diusion paths, but also avoid the formation of

uneven and thick solid electrolyte interphase (SEI) layers, thereby

achieving high reversible speciﬁc capacity and improved Na stor-

age performance of the obtained HC anode.[, ] HC mate-

rials with nanoporous networks and wide interlayer spacings

were synthesized using a MgO-templating method, which ex-

hibited low operating voltage (. V), high reversible capacity

( mA h g−), and appropriate ICE (%).[ ] HC materials

synthesized by carbonization of inexpensive and easily available

biomass-derived materials, such as discarded fruit shells, cotton,

poplar, and straw have attracted the attention of researchers ow-

ing to their low cost and unique inheritable structure. HC anodes

with a hierarchical porous structure were fabricated from waste

cork (Figure 15a) and exhibited a reversible capacity of  mA

−and an ultra-long lifespan of over  cycles (% ca-

pacity retention) in a Na half-cell.[] Although morphological

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engineering can eectively improve the reversible capacity and

rate capability of HC materials, an increase in speciﬁc sur-

face area may lead to poor cycling stability and high initial

irreversibility.[] In addition, the nanoscale design of anode

materials may lead to processing diculties (including mate-

rial synthesis and electrode manufacturing), which is not favor-

able for industrial production. Moreover, the C-conversion ef-

ﬁciency of HC materials formed by the carbonization of poly-

mer/biomass materials should also be considered to evaluate

their feasibility.[,] Therefore, more eorts are required to op-

timize the synthesis process to balance the manufacturability,

morphology, and electrochemical performance of HC materials,

which is the key to promoting the commercialization of SIBs.

Dopingwithheteroatoms(suchasN,S,O,P,B,andF)is

also an eective strategy for improving the Na-ion storage per-

formance of HC materials by changing the electronic state of C

to increase the electronic conductivity while also increasing the

number of defect/surface functional groups to increase the re-

versible capacity.[] For example, it was demonstrated that phos-

phorus functionalization of HC can broaden the interlayer spac-

ing and increase the Na-adsorption energy, resulting in a high

initial reversible capacity (. mA h g−) and excellent rate per-

formance (. mA h g−at  A g−) of the resulting anode

(Figure b).[] Furthermore, a multistage porous HC anode

with a high concentration of pyridine N was formed by polymer-

ization and pyrolysis using tri-sodium citrate (C-source and tem-

plate) and hexamethylenetetramine (N source and pore-forming

agent) as raw materials.[ ] The introduction of C–N•and

C–C•radicals provide additional active sites for the adsorption of

Na ions under low voltage, so the obtained anode shows high re-

versible capacity ( mA h g−) and rate capability ( mA h g−

at  A g−). A novel pre-oxidation treatment was proposed to form

a more disordered structure and introduce more O-functional

groups, thereby improving the reversible capacity (from  to

. mA h g−), C-yield (from % to %), and ICE (from

.% to .%) of carbonized and pre-oxidized pitch.[ ] Simi-

larly, carboxyl groups were introduced into carbonized anthracite

by a mechanochemical post-treatment process to expand the in-

terlayer distance and enhance the porosity and disorder/defect

structure.[ ] As shown in Figure c, this modiﬁed HC anode

with a precise content of  at% carboxyl groups (C-M) ex-

hibited high reversible capacity ( mA h g−) and excellent rate

performance ( mA h g−at  A g−). It should be noted that

compared to traditional HC, modiﬁed HC materials with more

defects induced by heteroatom doping exhibit increased capac-

ity in the sloped region of the current–voltage curve, which may

lead to an increase in the average charging voltage and oset the

beneﬁts of increased capacity.[,] In addition, introducing de-

fects and porous structures into HC materials can increase the

reversible capacity, but it may also lead to an increase in initial ir-

reversible capacity, making it dicult to fabricate practical Na-ion

full cells.[, ]

4.1.2. Increasing the ICE

The practical application potential of SIBs is limited by the low

ICE of the anode material caused by electrolyte decomposition

and side reactions, as all Na-ions are originally stored only in

the cathode, not in the HC anode. After years of development,

signiﬁcant progress has been made in the electrochemical per-

formance of HC anodes, but the ICE of most reported HC ma-

terials is only about %, which does not yet meet the require-

ments for practical applications.[, ] Although pre-sodiation

treatment (by electrochemical methods, direct contact, or sacri-

ﬁcial agents) can improve the performance of Na full cells, it in-

creases the number of processing steps and production costs.[]

Therefore, it is necessary to further optimize the HC anode to

meet the technical requirements for the commercial application

of SIBs. It should be noted that the service life and ICE of HC

materials are also aected by the electrolytes, binders, and con-

ductive agents, which require evaluation of the samples under

the same conditions to enable objective comparisons. For exam-

ple, selecting water-soluble binders (sodium carboxymethyl cel-

lulose, sodium polyacrylate, sodium alginate, and others) or re-

ducing the amount of super C with a large speciﬁc surface area

can improve the ICE of HC electrodes.[, ]

A systematic investigation of the relationship between the

defect concentration and ICE in HC materials demonstrated

that Na ions trapped by defect/surface functional groups gen-

erate electrostatic repulsion against other ions, thereby reduc-

ing the low-voltage intercalation capacity and increasing initial

irreversibility.[] Therefore, they proposed reducing the pyrol-

ysis rate to provide sucient time for gas removal and C-atom

reorganization, thereby increasing the ICE from .% to .%.

In addition, the pyrolysis temperature aects the defects, poros-

ity, and graphitization degree of HC materials.[] It is gener-

ally believed that a high carbonization temperature will lead to

the closure/collapse of pores and the reduction in the number of

defects, thereby increasing the ICE and low-potential plateau ca-

pacity of the obtained HC anode.[ ] However, HC synthesized

at high pyrolysis temperatures exhibits a high degree of graphi-

tization and narrow interlayer spacing, which degrades the Na

storage performance. By investigating the electrochemical perfor-

mance of HC anodes fabricated by direct pyrolysis of pitch and

phenolic resin at temperatures ranging from  to  °C,

the optimal carbonization temperature was determined.[ ] The

sample synthesized at  °C exhibited the highest ICE (%)

and improved cycling stability (% capacity retention after 

cycles at .C), owing to a balance between electronic conduc-

tivity (graphitization degree) and ionic conductivity (interlayer

spacing). In addition, research on the carbonization synthesis

of HC microspheres using phenolic resin precursors showed

that increasing the pyrolysis temperature can eectively enhance

the closed porosity and tap density.[] Therefore, a rolled HC

anode with low electrode porosity (%) and high tap density

(. g cm−at  °C) exhibited an impressively high ICE of

% (Figure d). Furthermore, it was shown that graphite crys-

tal templates can induce the epitaxial growth of cotton precur-

sors under intimate contact, thereby forming an intergrowth HC

anode composed of graphite-like crystals, graphite phases, and

pseudo-graphite domains.[ ] The HC with graphite-like crystals

and wide interlayer spacing (. Å) exhibited an ultra-high ICE

(%), suitable reversible capacity ( mA h g−), and a long

low-voltage plateau.

In addition to the carbonization temperature, the graphiti-

zation and defect degree of HC materials are inﬂuenced by

other conditions, such as the addition of transition metals with

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catalytic graphitization ability. The graphitization degree of HC

anodes was carefully controlled by introducing appropriate con-

centrations of Mn ions into the precursor of paper towels.[ ]

Mn ion-assisted catalytic graphitization eliminates O-defects and

maintains eective ionic transport channels, thereby increasing

the ICE of HC samples from . to .% (Figure e). Simi-

larly, atomically dispersed Zn was added into HC microspheres

to catalyze the decomposition of the electrolyte and form a stable

inorganic-rich SEI layer.[] The optimized HC anode exhibited

a sucient ICE (%) and enhanced low-temperature capacity

( mA h g−at − °C).

Considering the inﬂuence of the electrode interface behav-

ior on the formation and properties of SEI layer, surface-coating

technology is considered an eective strategy to enhance the ICE.

For example, an AlOlayer (artiﬁcial SEI) was evenly coated on

HC spheres by a simple liquid-phase method, which reduces the

surface active sites and inhibits parasitic reactions to increase the

ICE (from . to .%).[] Similarly, a vapor C-coating strat-

egy was demonstrated via the pyrolysis of polypropylene to en-

hance the ICE (%) of C-spheres, as the coating reduces surface

defects and ﬁlls the pores.[ ] HC anodes coated with SC were

fabricated by carbonizing paper towels soaked in coal-tar pitch

at  °C.[ ] The high H/C ratio of pitch reduces O-defects in

HC during carbonization and the long-range-ordered SC coating

avoids the formation of a thick SEI layer and subsequent con-

sumption of active Na-ions, resulting in the modiﬁed HC elec-

trode exhibiting a high ICE of .% (Figure f). However, most

surface-engineering technologies are still in the laboratory stage

and not yet suitable for large-scale application. Furthermore, they

have limitations related to the complex production steps and high

costs, requiring careful evaluation of their applicability.

4.2. Electrolyte Engineering

Electrolytes are an important component of electrochemical

energy-storage systems and largely determine the electrochem-

ical performance and safety of the batteries. An ideal electrolyte

should include the following characteristics: wide operating volt-

age window (large energy gap between the highest occupied

molecular orbitals (HOMO) and lowest unoccupied molecular or-

bitals (LUMO)), good compatibility with cathode and anode mate-

rials, high ionic conductivity, low viscosity, chemical/thermal sta-

bility, and low cost.[] Traditional non-aqueous electrolytes in-

clude sodium salts, organic solvents, and additives, as discussed

below.

Sodium salts should have high solubility and excel-

lent ionic conductivity, among which sodium perchlorate

(NaClO), sodium hexaﬂuorophosphate (NaPF), and sodium

bis(triﬂuoromethane)sulfonimide (NaTFSI) have been widely

investigated. In PC-based electrolytes, NaPFexhibits the highest

conductivity (. mS cm−), followed by NaClO(. mS cm−)

and NaTFSI (. mS cm−)(Figure 16a).[ ] It was demonstrated

that the thermal stability of electrolyte salts follows the order:

NaClO>NaTFSI >NaPF>Na bis(ﬂuorosulfonyl)imide

(NaFSI).[] Due to its fast ion-transport rate, high thermal

stability, good compatibility, low water sensitivity, and low cost,

NaClOis widely used in prototype SIBs.[, ] However, its

toxicity and explosiveness in a dry state limit its large-scale

production, making it more popular in research than industry.

NaPFwith high ionic conductivity is a suitable sodium salt,

but it still faces challenges such as high toxicity, high price,

and low decomposition temperature. In addition, the solubility

of NaPFin a single solvent is limited, requiring the use of

mixed solvents.[ ] NaTFSI and NaFSI are also considered

promising sodium salts owing to their high ionic conductivity,

and non-toxicity, but their application is hindered by their severe

corrosion reaction with the Al current collector.[] Therefore,

NaPFprovides a suitable compromise in terms of electrochem-

ical stability, ionic conductivity, and safety, so it is considered the

best commercial sodium-salt candidate.[ ]

As a medium for Na-ion transport, organic solvents need to

meet the requirements of high dielectric constant (ɛ; conducive

to the dissociation of sodium salt), low viscosity (rapid ionic diu-

sion), high electrochemical stability (wide operating voltage win-

dow), high security (high boiling point and ﬂash point), low tox-

icity, and low cost.[] Common organic solvents include car-

bonate (for example, PC, ethylene carbonate (EC), dimethyl car-

bonate (DMC), methyl ethyl carbonated (EMC), diethyl carbon-

ate (DEC)) and ether-based solvents (such as dimethoxyethane

(DME), diglyme, triglyme). Ether-based electrolytes can form

functional SEI layers on the surface of active materials during the

electrochemical process, so they show satisfactory electrochem-

ical performance in HC half-cells and Na metal batteries.[]

However, their low thermal stability (DME <DMC <DEC <PC <

EC) and incompatibility with high-voltage cathode materials limit

their application in Na full-cells.[] Organic electrolytes in SIBs

typically contain EC (ɛ=. at  °C) and PC (ɛ=. at  °C)

with high dielectric constants, although their viscosity is rela-

tively high (Figure b).[] However, EC exists in solid form at

room temperature (melting point . °C), so it is usually neces-

sary to mix one or more co-solvents with low viscosity (including

DMC, EMC, DEC) to optimize the electrochemical performance

of the resulting SIB. An EC:PC mixture with high thermal sta-

bility and a wide electrochemical stability window was proposed

as an optimal electrolyte formulation, but electrolytes based on

EC:DMC and EC:EMC have kinetic advantages.[ ] For example,

HC anodes exhibited high reversible capacity, low initial capacity

loss, and excellent cycling stability in EC:PC-based electrolytes

due to the formation of a thin and uniform SEI layer.[,]

In addition, high-voltage SIBs assembled with ternary solvent-

based electrolytes with high ionic conductivity and low viscos-

ity, such as EC/PC/DMC (././.), EC/PC/DEC (//),

and EC/DMC/DEC (//), exhibited excellent electrochemical

performance.[, ] However, there is currently no consensus on

the optimal solvent composition. Considering the inﬂuence of

the solvation structures of dierent electrolyte components on

battery performance, it is necessary to further optimize the sol-

vent design.[ ]

Because organic electrolytes decompose and corrode the elec-

trode during the electrochemical process, ﬁlm-forming additives

such as ﬂuoroethylene carbonate (FEC), vinylene carbonate (VC),

ethylene sulﬁte (ES), and diﬂuoroethylene carbonate (DFEC) are

commonly added to extend the service life of the entire battery

to thousands of cycles.[ ] To date, FEC remains the most ef-

fective additive for SIBs. Adding a small amount of FEC (.%)

to NaPF-based electrolytes can signiﬁcantly improve the re-

versible capacity, ICE, and cycling stability of HC anode materials

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GmbH. d) Schematic diagram of the formation of CEI on the cathode after cycling in electrolytes with and without FEC additives. Reproduced with

based electrolytes. f) Flammability test in a blank electrolyte (left) and electrolyte with 5% EFPN additive (right). Reproduced with permission.[ 324]

(Figure c), as it is preferentially reduced to form a functional

SEI layer containing NaF and Na polycarbonate.[] In addition,

electrolytes containing FEC additives induce the formation of a

F-rich CEI layer on the surface of the cathode material at high

voltage (≈. V) to promote Na-ion diusion (Figure d).[]

Although this uniform passivation layer slightly increases the

charge transfer resistance of the cathode, it eectively avoids

electrolyte consumption caused by the continuous decompo-

sition of oxidation products under high voltage, making elec-

trolytes containing FEC additives popular in SIBs systems.[, ]

A dual-additive electrolyte containing FEC and succinic anhy-

dride (SA) was used to improve the electrochemical performance

of Na.Li.Ni.Mn. Cu.Oand reduce the generation of

COduring electrolyte decomposition.[ ] The introduction of

SA induced the formation of a uniform and stable CEI layer,

which contains more O-rich organic compounds and less high-

resistance NaF compared to electrolytes containing only FEC ad-

ditives. In addition, compared with LIB systems, the Na-based

SEI layer composed of non-cross-linked or non-polymerized or-

ganic species is more easily dissolved, leading to an increase in

the self-discharge rate; thus, only a few LIB electrolyte additives

can be applied to SIBs.[ ] For example, the popular LIB elec-

trolyte additive VC is not highly eective in SIB systems.[]

To achieve high energy-density SIBs, it is necessary to fur-

ther develop stable high-voltage electrolytes, which can be

achieved by introducing new ﬁlm-forming additives,[ ] ﬂu-

orinated electrolytes,[ ] or high-concentration electrolytes.[]

More importantly, because of the inherent ﬂammability of or-

ganic solvents, improving the thermal stability of electrolytes is

particularly important for improving the safety of SIBs, which

can be optimized by applying the following strategies: ) employ-

ing nonﬂammable solvents or adding ﬂame-retardant additives;

) adopting high concentration/localized high-concentration

electrolytes; and ) developing suitable ionic liquids or solid

electrolytes.[ ] Commonly used phosphorus/ﬂuoride-retardant

additives terminate combustion reactions by physically iso-

lating O via a ﬂame-retardant gas vapor or chemically cap-

turing free hydrogen active radicals. For instance, it was

shown that NaPF/EC–DEC-based electrolytes containing  wt%

ethoxy(pentaﬂuoro)cyclotriphosphazene (EFPN) can eectively

reduce the self-extinguishing time from  to  s, demon-

strating the successful synthesis of nonﬂammable electrolytes

(Figures e,f).[] However, it should be noted that introducing

EFPN with a low dielectric constant reduces the ionic conductiv-

ity of the resulting electrolyte. Moreover, other ﬂame-retardant

additives, such as perﬂuoro--methyl--pentanone, trimethyl

phosphate (TMP), triethyl phosphate, triphenyl phosphate, di-

ethyl ethylphosphonate, and cresyl diphenyl phosphate, have

been developed, but research on ﬂame-retardant additives in

SIBs systems is still limited.[, ] Recently, high-concentration

electrolytes have attracted attention in both LIB and SIB

systems owing to their unique electrochemical properties,

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including wide voltage windows, enhanced thermal stability, and

inhibition of dendrite growth.[ ] However, the high ﬂame re-

tardancy of high-concentration electrolytes is related to the low

content of ﬂammable organic solvents, so high-concentration

electrolytes without ﬂame-retardant additives may still ignite.

In addition, the intrinsic drawbacks of high-concentration elec-

trolytes, such as poor wettability toward the electrode/separator,

high viscosity, and high cost, cannot be ignored.[] To balance

the ionic conductivity, viscosity, and electrochemical/thermal

stability, researchers introduced inert and poorly solvating sol-

vents (ﬂuorinated ether (BTFE)) into the high-concentration elec-

trolyte to obtain a localized high-concentration electrolyte (. 

NaTFSI in TMP/BTFE/VC).[] The potential of this non-

ﬂammable electrolyte was proven by the electrochemical and

safety performance of practical pouch cells. In fact, high pro-

duction costs are the main bottleneck for the practical appli-

cation of high-concentration/localized high-concentration elec-

trolytes. Ionic liquids with high electrochemical/thermal stabil-

ity and non-ﬂammability are expected to enable the development

of highly safe SIBs, but they need to overcome the current lim-

itations of high cost, high viscosity, and low conductivity.[ ] To

date, the optimization strategies for ionic liquids have focused

on the development of hybrid electrolytes, such as those con-

taining gel and organic electrolytes. As the “Holy Grail” of high-

energy-density battery systems, the development of all-solid-state

SIBs has also received signiﬁcant attention.[ ] Because solid-

state electrolytes used in SIBs are still in the research phase, some

key challenges need a major breakthrough, such as the low ionic

conductivity, high solid–solid interfacial resistance, and diculty

in manufacturing solid-state batteries. However, these issues are

beyond the scope of this review and will not be discussed further.

The conventional electrolytes for SIBs that can match with

high-performance cathode materials are mainly binary (PC +

FEC) or ternary (EC +DEC +FEC, EC +PC +FEC, EC +

DMC +FEC) solvents with  мNaPFor NaClO. Among them,

the PB cathode shows similar electrochemical performance in

NaPF-andNaClO

-based electrolytes, which may be related to

the low water sensitivity of NaClO.[] On the contrary, the CEI

layer formed by the decomposition of NaPFexhibits higher Na-

conductivity than NaClO, resulting in lower interfacial resis-

tance and higher cyclic stability.[] In addition, the dissolved

SEI species produced by linear carbonates (DMC, EMC, DEC)

reduction shuttle from the anode to the cathode side for oxida-

tion, resulting in Na loss and capacity decay.[  ] Therefore, the

electrolyte composed of NaPFand single cyclic carbonates (PC

and EC) is more popular in layered oxides and polyanionic com-

pounds. As the most eective electrolyte additive, introducing

FEC into the electrolyte can induce the formation of a robust F-

rich CEI layer on the surface of cathode material, thereby improv-

ing cycle stability. However, it should be noted that FEC is not a

perfect electrolyte additive as it does not match with some an-

odes and is prone to decomposition at elevated temperatures.[ ]

On the other hand, although ﬂuorinated electrolyte components

exhibit high electrochemical stability, the production of toxic

byproducts (HF, NaF) can pose safety and environmental haz-

ards. Therefore, it is suggested to develop novel functional elec-

trolytes that are inexpensive, non-toxic, and can form thin and

uniform interface layers on the cathode and anode, so as to fur-

ther promote the development of SIBs. In summary, optimiz-

ing cathode materials and matching appropriate anodes and elec-

trolytes are the key to realize commercial application of SIBs.

5. Conclusions and Perspectives

SIBs with cost and safety advantages have shown great potential

for replacing LIBs in next-generation large-scale energy-storage

systems and moderate-range electric vehicles. In full-cell appli-

cations, O-type layered oxides with sucient Na are considered

the most promising cathode candidates owing to their high oper-

ating voltage, high reversible capacity, long service life, low pro-

duction cost, and practicality. However, their sluggish Na dynam-

ics, complex phase-evolution processes, tendency to form micro-

cracks and undergo surface reconstruction during cycling, and

air sensitivity are the main factors limiting their large-scale ap-

plication. Recent literature shows that customizing layered ox-

ides by rational compositional/structural design can eectively

overcome some of the inherent shortcomings, which is of great

signiﬁcance for promoting the commercialization of SIBs. There-

fore, this review summarizes the current development status of

O-type cathodes in SIBs and provides constructive prospects for

their large-scale applications.

) Appropriate compositional design not only improves the

structural stability/reversibility of layered oxides through the

synergistic eect of multiple TM ions, but also fundamentally

overcomes the capacity loss caused by structural degradation

by smoothing the phase evolution process or extending the

solid-solution reaction zone. In addition, coupling metal ions

with high redox reaction pairs (such as Cu+/+) or with dif-

ferent cationic potential (such as Ti and Mg) in the TMO

slab can increase the energy density by increasing the aver-

age working potential, or improve the air stability by adjust-

ing the interlayer distance. In recent years, high-entropy ox-

ides and multiphase cathodes synthesized based on compo-

sitional design have also received research attention owing to

their unique electrochemical performance, but their practical

application potential needs further veriﬁcation.

) Surface-passivation layers provide a physical barrier to pre-

vent direct contact between the active material and corrosive

electrolytes and/or humid air, thereby suppressing parasitic

reactions and improving interfacial and air stability. These

surface-modiﬁed layers act as buer matrices to alleviate the

volume expansion and dissolution of active materials dur-

ing the electrochemical process. In addition, the combina-

tion of surface coating and bulk doping can enhance the sur-

face/structural stability and reduce ionic-diusion barriers,

thus further improving the cycling stability and rate proper-

ties. Moreover, the consumption of surface residual Na and

the in-situ formation of Na-containing layers (for example,

phosphate and P phase layers) can further improve the Na

storage performance and air/thermal stability of layered ox-

ides.

) Optimizing the synthesis methods to design cathode parti-

cles with unique morphology, including microspheres, ﬁbers,

ﬂakes, and single-crystal structures, is beneﬁcial for shorten-

ing the Na-ion diusion paths, exposing active planes, and ac-

commodating internal stress. Among them, spherical materi-

als have high tap density and low speciﬁc surface area to meet

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the requirements of high volume energy density and minimal

surface side reactions, providing insights to support the large-

scale production of SIBs.

Although the Na-ion storage performance of O-type cathode

materials has been greatly improved after years of eort, the fol-

lowing topics require further attention.

) Balancing production costs and electrochemical performance.For

various layered oxide candidates, it is recommended to fur-

ther develop binary or ternary cathode materials contain-

ing Ni (low/medium content), Mn (medium content), Fe

(low/medium content), and Cu (low content). In addition, it

has been proven that metal-ion substitution can further im-

prove the electrochemical performance of oxide materials, but

the doping content should not be high enough to risk sacriﬁc-

ing the reversible capacity (especially for high-entropy oxides).

Furthermore, it is necessary to avoid introducing toxic and/or

expensive metals (such as Li, Sn, and Sb) that will weaken

the competitiveness of SIBs. Surface modiﬁcation can protect

air-sensitive layered oxides by inhibiting structural degrada-

tion, but it also increases the number of production steps and

costs. Therefore, a one-step integrated doping–coating syn-

thesis method for cathode materials could provide a promis-

ing modiﬁcation strategy.

) Expand production capacity and batch stability.Themain

method used for the large-scale production of cathode ma-

terials is the fabrication of uniform spherical secondary par-

ticles by a coprecipitation method, which has been demon-

strated for layered LIBs. Therefore, subsequent studies are

required to optimize the synthesis process to develop parti-

cles with unique structures, such as core–shell and radiation

structures, and exposed active planes.

) Evaluate practicality using pouch-type Na-ion full cells. A prac-

tical Na full-cell system requires combining a cathode mate-

rial (with excellent electrochemical performance) with a suit-

able HC anode material (to provide a high ICE) and a non-

ﬂammable electrolyte (to enable a wide voltage window). In

addition, it should be noted that the unit energy cost of SIBs

is closely related to the production cost of each component

and the electrolyte usage. More importantly, the practicality

of SIBs needs to be evaluated using pouch-type cells to clar-

ify the potential operating temperature range, gas production

issues, and overall safety.

) Investigate the mechanism of capacity fading. It is necessary

to further investigate the capacity fading caused by struc-

tural degradation and side reactions to guide the develop-

ment of SIBs. Therefore, it is necessary to combine ad-

vanced characterization techniques such as neutron dirac-

tion, synchrotron X-ray diraction, spherical aberration-

corrected scanning transmission electron microscopy, nu-

clear magnetic resonance, X-ray absorption spectroscopy,

and theoretical calculations to provide a deep understand-

ing of the relationships between the crystal structure, chem-

istry/electrochemistry properties, and Na-ion storage perfor-

mance of O-type cathode materials.

) Develop next-generation advanced cathode materials. SIBs that

can be used in electric vehicles require fast charging, so it is

necessary to further optimize the cathode materials to achieve

high power density and alleviate the long charging times of

current electric vehicles. In addition, the reversible capacity

of cathode materials can be further improved by inducing

anionic redox reactions through doping and defect forma-

tion. However, because anionic redox reactions usually oc-

cur under high voltage (. V), which may lead to O-loss and

structural degradation, it is necessary to conduct in-depth re-

search on the reaction mechanism and investigate possible

solutions.

By combining multiple optimization strategies,[] O-type

cathodes have achieved high capacity (> mA h g−), high op-

erating voltage (>. V), and high thermal stability (exothermal

peak > °C) in SIB systems, which are now considered a vi-

able competitor to commercial LIBs.[ ] In the coming years, we

hope that the electrochemical performance of SIBs will be further

improved, creating opportunities in both academia and industry.

Acknowledgements

X.L. and J.-Y.H. contributed equally to this work. This work was supported

by the Human Resources Development Program (No. 20214000000320)

of the Korea Institute of Energy Technology Evaluation and Planning

(KETEP), funded by the Ministry of Trade, Industry, and Energy of the Ko-

rean government. The authors acknowledge Hoon-Hee Ryu and Gwan-

geon Oh for useful discussions.

Conﬂict of Interest

The authors declare no conﬂict of interest.

Keywords

cathode materials, commercialization, O3-type structures, sodium ion

batteries, strategies

Received: June 23, 2023

Revised: August 1, 2023

Published online:

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Xinghui Liang received his Ph.D. degree (2023) in energy engineering from Hanyang University.He is

currently a postdoctoral researcher in the group of Professor Yang-Kook Sun at Hanyang University.

His research interests include the preparation and characterization of advanced cathode materials for

sodium-ion batteries.

Jang-Yeon Hwang received his Ph.D. degree (2018) at Hanyang University in Seoul. He is an associate

professor in the Department of Energy Engineering & Battery Engineering at Hanyang University in

Seoul. His research is mainly focused on the synthesis and characterization of advanced electrode

materials for the high-energy alkali-ion and alkali-metal batteries.

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Yang-Kook Sun is currently a distinguished professor at Hanyang University in Seoul. After receiving

his Ph.D. at Seoul National University in 1992, he has pursued his passion for energy storage and

conversion materials for innovative and high-level scientiﬁc work. One of his major achievements is

the discovery of a new concept of layered concentration gradient NCM cathode materials for lithium-

ion batteries. His major research interests are design, synthesis of novel materials for lithium-ion

batteries, sodium-ion batteries, lithium-sulfur batteries, and all-solid-state batteries.

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Sodium-ion batteries (SIBs) have attracted enormous attention as candidates in stationary energy storage systems, because of the decent electrochemical performance based on cheap and abundant Na-ion intercalation chemistry. Layered oxides, the workhorses of modern lithium-ion batteries, have regained interest for replicating their success in enabling SIBs. A unique feature of sodium layered oxides is their ability to crystallize into a thermodynamically stable P2-type layered structure with under-stoichiometric Na content. This structure provides highly open trigonal prismatic environments for Na ions, permitting high Na+ mobility and excellent structural stability. This review delves into the intrinsic characteristics and key challenges faced by P2-type cathodes and then comprehensively summarizes the up-to-date advances in modification strategies from compositional design, elemental doping, phase mixing, morphological control, and surface modification to sodium compensation. The updated understanding presented in this review is anticipated to guide and expedite the development of P2-type layered oxide cathodes for practical SIB applications.

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Article

Jun 2024

Sodium-ion batteries (SIBs), which serve as alternatives or supplements to lithium-ion batteries, have been developed rapidly in recent years. Designing advanced high-performance layered NaxTMO2 cathode materials is beneficial for accelerating the commercialization of SIBs. Herein, the recent research progress on scalable synthesis methods, challenges on the path to commercialization and practical material design strategies for layered NaxTMO2 cathode materials is summarized. Co-precipitation method and solid-phase method are commonly used to synthesize NaxTMO2 on mass production and show their own advantages and disadvantages in terms of manufacturing cost, operative difficulty, sample quality and so on. To overcome drawbacks of layered NaxTMO2 cathode materials and meet the requirements for practical application, a detailed and deep understanding of development trends of layered NaxTMO2 cathode materials is also provided, including high specific energy materials, high-entropy oxides, single crystal materials, wide operation temperature materials and high air stability materials. This work can provide useful guidance in developing practical layered NaxTMO2 cathode materials for commercial SIBs.

Expediting Layered Oxide Cathodes Based on Electronic Structure Engineering for Sodium-ion Batteries: Reversible Phase Transformation, Abnormal Structural Regulation, and Stable Anionic Redox

Article

Jun 2024

Realizing Long-Term Cycling Stability of O3-Type Layered Oxide Cathode for Sodium-Ion Batteries

Article

Jun 2024

O3-type layered oxide cathodes are promising in practical sodium-ion batteries (SIBs) owing to their high theoretical capacity, facile synthesis, and sufficient Na+ storage. However, they face challenges such as rapid...

Insights Into Scalable Technologies and Process Chains for Sulfide‐Based Solid‐State Battery Production

Article

Full-text available

May 2024

The successful utilization of innovative sulfide‐based solid‐state batteries in energy storage hinges on developing scalable technologies and machinery for upscaling their production. While multiple Gigafactories for lithium‐ion batteries are already operational worldwide, the upscaling of solid‐state batteries exhibiting their full potential remains to be seen in the near future. In this study, the conventional production of lithium‐ion batteries is reconsidered, and the feasibility of seamlessly integrating sulfide‐based solid‐state batteries into the existing process chains is discussed. Scalable technologies and key challenges along the process chain of sulfide‐based solid‐state batteries are accordingly addressed. Experimental investigations yield crucial insights into enabling large‐scale production of sulfide‐based battery components while highlighting remaining challenges from a production perspective. An overview of the roll‐to‐roll machinery housed in microenvironments under an inert atmosphere in the “Sulfidic Cell Production Advancement Center” at the Institute for Machine Tools and Industrial Management at the Technical University of Munich is given.

Mo6+ bifunctional substitution of P2-type manganese oxide for high performance sodium-ion batteries

Article

May 2024

High-Entropy Configuration Strategy to Build High Performance Na-Ion Layered Oxide Cathodes Derived from Simple Techniques

Article

May 2024

Facilitating an Ultrastable O3-Type Cathode for 4.5 V Sodium-Ion Batteries via a Dual-Reductive Coupling Mechanism

Article

May 2024

Air Corrosion of Layered Cathode Materials for Sodium-Ion Batteries: Cation Mixing and a Practical Suppression Strategy

Article

May 2024
ACS NANO

Long‐Cycle‐Life Cathode Materials for Sodium‐Ion Batteries toward Large‐Scale Energy Storage Systems

Article

Full-text available

May 2023
Adv. Energy Mater.

The development of large‐scale energy storage systems (ESSs) aimed at application in renewable electricity sources and in smart grids is expected to address energy shortage and environmental issues. Sodium‐ion batteries (SIBs) exhibit remarkable potential for large‐scale ESSs because of the high richness and accessibility of sodium reserves. Using low‐cost and abundant elements in cathodes with long cycling stability is preferable for lowering expenses on cathodes. Many investigated cathodes for SIBs are dogged by structural and morphology changes, unstable interphases between the cathode and the electrolyte, and air sensitivity, causing unsatisfactory cycling performance. Therefore, understanding the mechanism of capacity degeneration in depth and developing precise solutions are critical for designing low‐cost cathodes that are highly stable under cycling. Herein, recent progress in long‐cycle‐life and low‐cost cathodes for SIBs is focused on, and a comprehensive discussion of the key points in SIBs toward large‐scale applications is provided. The roots of the unstable cycling performance of low‐cost cathodes are discussed. Also, effective strategies are summarized from the recent progress on long‐cycle‐life and low‐cost cathodes. This review is expected to encourage deeper investigation of long‐lifespan cathodes for SIBs, particularly for potential large‐scale industrialization.

Four‐In‐One Strategy to Boost the Performance of Nax[Ni,Mn]O2

Article

Full-text available

May 2023
ADV FUNCT MATER

High‐performance layered oxide cathode materials are vital to the industrialization of Na‐ion batteries (NIBs) due to their high theoretical capacity and facile production. These materials, however, suffer from complicated phase transitions and poor capacity retention cycling. The modifying impacts of classic strategies, such as doping and coating, are generally restricted. Herein, a rational four‐in‐one strategy to boost the overall performance of Nax[Ni,Mn]O2 materials by innovative composition design and elegant synthesis management is provided. After modification, the half‐cell capacity retention rate increases from 45% to 77% after 250 cycles at 1 C rate. The capacity of the full cells is maintained at 83% after 300 cycles at 0.5 C rate. This four‐in‐one strategy, which encompasses optimization of the energy band structure, ion diffusion, crystal structure, and particle morphology, sheds new insight into the overall optimization of cathode materials for NIBs.

Binder‐Induced Ultrathin SEI for Defect‐Passivated Hard Carbon Enables Highly Reversible Sodium‐Ion Storage

Article

Full-text available

Apr 2023
Adv. Energy Mater.

Hard carbon is one of the most promosing anodes for resource‐rich sodium‐ion batteries. However, an unsatisfactory solid–electrolyte‐interphase formed by irreversible electrolyte consumption caused by defects or oxygen‐containing functional groups of hard carbon impedes its further application. Herein, a novel composite binder that is composed of polar polymer chondroitin sulfate A (sodium salt) and polyethylene oxide by hydrogen bonding demonstrates defect passivation capability. This composite binder can reduce the exposure of defects by forming hydrogen bonds with oxygen functional groups on the hard carbon surface and inhibit the decomposition of electrolyte confirmed by in situ differential electrochemical mass spectrometry. In situ Raman and theoretical calculations reveal that multiple polar functional groups in chondroitin sulfate A (sodium salt) can accelerate the transport of Na⁺ by adsorbing and facilitate the decomposition of PF6⁻ to form NaF. Additionally, polyethylene oxide in the composite binder can increase viscosity and accelerate the transport of Na⁺. As a result, an ultra‐thin (9 nm, cyro‐TEM) and NaF‐rich solid–electrolyte interphase is obtained, thereby the hard carbon anode achieves improved initial Coulombic efficiency (84%) and high‐capacity retention of 94% after 150 cycles in a NaPF6/ethylene carbonate/dimethyl carbonate electrolyte.

Conversion of Surface Residual Alkali to Solid Electrolyte to Enable Na‐Ion Full Cells with Robust Interfaces

Article

Full-text available

Sep 2023
ADV MATER

The deposition of volatilized Na ⁺ on the surface of the cathode during sintering results in the formation of surface residual alkali (NaOH/Na 2 CO 3 /NaHCO 3 ) in layered cathode materials, leading to serious interfacial reactions and performance degradation. This phenomenon is particularly evident in O3‐NaNi 0.4 Cu 0.1 Mn 0.4 Ti 0.1 O 2 (NCMT). In this study, we propose a strategy to transform waste into treasure by converting residual alkali into a solid electrolyte. Mg(CH 3 COO) 2 and H 3 PO 4 are reacted with surface residual alkali to generate the solid electrolyte NaMgPO 4 on the surface of NCMT, which can be labeled as NaMgPO 4 @NaNi 0.4 Cu 0.1 Mn 0.4 Ti 0.1 O 2 ‐X (NMP@NCMT‐X, where X indicates the different amounts of Mg ²⁺ and PO 4 ³⁻ ). NaMgPO 4 acts as a special ionic conductivity channel on the surface to improve the kinetics of the electrode reactions, remarkably improving the rate capability of the modified cathode at a high current density in the half‐cell. Additionally, NMP@NCMT‐2 enables a reversible phase transition from the P3 to OP2 phase in the charge–discharge process above 4.2 V and achieves a high specific capacity of 157.3 mAh g ⁻¹ and outstanding capacity retention in the full cell. The strategy can effectively and reliably stabilize the interface and improve the performance of layered cathodes for Na‐ion batteries (NIBs). This article is protected by copyright. All rights reserved

Catalytic Defect‐Repairing Using Manganese Ions for Hard Carbon Anode with High‐Capacity and High‐Initial‐Coulombic‐Efficiency in Sodium‐Ion Batteries

Article

Full-text available

Mar 2023
Adv. Energy Mater.

Hard carbon (HC) anodes have shown extraordinary promise for sodium‐ion batteries, but are limited to their poor initial coulombic efficiency (ICE) and low practical specific capacity due to the large amount of defects. These defects with oxygen containing groups cause irreversible sites for Na⁺ ions. Highly graphited carbon decreases defects, while potentially blocking diffusion paths of Na⁺ ions. Therefore, molecular‐level control of graphitization of hard carbon with open accessible channels for Na⁺ ions is key to achieve high‐performance hard carbon. Moreover, it is challenging to design a conventional method to obtain HCs with both high ICE and capacity. Herein, a universal strategy is developed as manganese ions‐assisted catalytic carbonization to precisely tune graphitization degree, eliminate defects, and maintain effective Na⁺ ions paths. The as‐prepared hard carbon has a high ICE of 92.05% and excellent cycling performance. Simultaneously, a sodium storage mechanism of “adsorption‐intercalation‐pore filling‐sodium cluster formation” is proposed, and a clear description given of the boundaries of the pore structure and the specific dynamic process of pore filling.

Cation–Oxygen Bond Covalency: A Common Thread and a Major Influence toward Air/Water‐Stability and Electrochemical Behavior of “Layered” Na–Transition‐Metal‐Oxide‐Based Cathode Materials

Article

Full-text available

Mar 2023
Adv. Energy Mater.

This work evolves a universal strategy, toward simultaneously addressing the air/water‐instability and structural‐cum‐electrochemical instability of “layered” Na–transition‐metal (TM)–oxide‐based cathode materials for Na‐ion batteries, by way of varying the “interslab” spacing via tuning the TMO bond covalency. In this regard, model O3‐structured NaTMoxides, with varied “charge‐to‐size” ratio of the cation‐combination (viz., TM‐ + non‐TM‐ions) in the TM‐layer [i.e., (C:S)TM], are designed and subjected to structural characterizations, density‐functional‐theory‐based studies, air/water‐exposure studies, electrochemical cycling, and operando investigations. Such studies have yielded a clear correlation‐cum‐trend concerning lower (C:S)TM => lower TMO covalency => higher effective negative charge on O‐ion (which gets shared by both TM‐ and Na‐ions) => stronger‐cum‐shorter NaO bond => reduced “interslab” spacing => lower Na‐transport kinetics => suppressed spontaneous Na‐extraction upon air/water‐exposure => concomitant vastly improved air/water‐stability => suppressed/delayed O3 → P3 transformation during electrochemical Na‐extraction (i.e., charging) => concomitant vastly improved electrochemical cyclic stability. Furthermore, a critical d(ONaO)/d(OTMO) of ≈1.38 for the O3 structure, corresponding to the initiation of O3 → P3 transformation during desodiation/charging is revealed. NaTMO2s having higher initial (C:S)TMs reach this critical d(ONaO)/d(OTMO) earlier (i.e., upon minimized Na‐removal) and, thus, suffer from more transformations during continued desodiation/charge, resulting in structural‐cum‐electrochemical instability.

Li-Doped Layered Na1.0Cu0.22Fe0.30Mn0.48O2 Cathode with Enhanced Electrochemical Performance for Sodium-Ion Batteries

Article

Full-text available

Mar 2023

The introduction of copper (Cu) element to iron-manganese-based layered cathode materials can effectively enhance their cycling stability and air tolerance. However, the low redox reactivity of Cu2+ decreases the capacity of the copper-iron-manganese layered oxide cathode material. Recently, lithium (Li) doping has been regarded as an efficient strategy to exploit high-capacity cathode materials by enabling high-covalency transition metals. Here, we report a Na1.0LixCu0.22Fe0.30Mn0.48O2 (x = 0.025, 0.05, 0.075) cathode material with increased capacity by adding Li into a Na1.0Cu0.22Fe0.30Mn0.48O2 cathode via a simple solid-phase sintering method. The doped Li element can regulate the redox reactivities of the adjacent Fe and Mn elements, leading to the promotion of the Fe redox reactivity and the suppression of Mn redox reactivity, which prevents both the Jahn–Teller effect and the structure collapse during the charge/discharge process. In conclusion, Li doping can not only improve the capacity of the cathode material but also improve its stability. When x = 0.075, the capacity of Na1Li0.075Cu0.22Fe0.30Mn0.48O2 cathode can reach 114.2 mAh g−1 with a high capacity retention of 90.2% after 300 cycles at 1 C. These results shed light on the role play of Li in the transition metal layer, and can guide the design and modification for high-performance SIBs of layered materials.

Benchmarking the Performance of Moisture-Sensitive Battery Materials: the Importance of the Electrode Preparation Method

Article

Jun 2023

O3-Type Na0.95Ni0.40Fe0.15Mn0.3Ti0.15O2 Cathode Materials with Enhanced Storage Stability for High-Energy Na-Ion Batteries

Article

May 2023
ACS APPL MATER INTER

O3-type layered oxides with high initial sodium content are promising cathode candidates for Na-ion batteries. However, affected by the undesired transition metal slab sliding and reaction with H2O/CO2, their further application is typically hindered by unsatisfactory cycling stability upon charging to high voltage and poor storage stability under humid air. Herein, we demonstrate a Fe/Ti cosubstitution strategy to simultaneously enhance the electrochemical performance and storage stability of pristine O3-NaNi0.5Mn0.5O2 cathode material, via employing high redox potential and inactive stabilized dopants. The resultant Fe/Ti cosubstituted Na0.95Ni0.40Fe0.15Mn0.3Ti0.15O2 undergoes highly reversible O3-P3-OP2 phase transitions with a small cell volume change of 2.8%, instead of complex O3-O'3-P3-P'3-P3'-O1 phase transitions in NaNi0.5Mn0.5O2. Consequently, the cathode displays a high specific capacity of 161.6 mAh g-1 with an average working voltage of 3.28 V and 81.8% capacity retention after 200 cycles at 5C. Furthermore, the cathode material remains very stable after exposure to air for 7 days and even after soaking in water for 1 h, owing to the prohibition of sodium losing by elevating redox potential and contracting sodium layer spacing. This work proposes an effective method to enhance the electrochemical performance and storage stability of O3-type layered oxide cathodes and promises advancing Na-ion batteries toward large-scale industrialization.

Dual-strategy of Cu-doping and O3 biphasic structure enables Fe/Mn-based layered oxide for high-performance sodium-ion batteries cathode

Article

May 2023
J POWER SOURCES

Practical Cathodes for Sodium‐Ion Batteries: Who Will Take The Crown?

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