Porous Li4Ti5O12-induced cathode electrolyte interphases enable sustainable single-crystalline NCM cathodes

Abstract

The cathode electrolyte interphase (CEI) layer derived on the cathode surface has a great influence on the coulombic efficiency, energy efficiency, rate performance, self-discharge, and cycling life of the lithium-ion batteries (LIBs). For layered nickel-rich cathodes, the electrochemically-inert residual lithium (e.g., Li2CO3, LiHCO3) combined with the continuous growth of CEI under high voltage (≥4.5 V vs Li/Li⁺) further increases the difficulty of regulating the activity and stability of LIBs. In this work, single-crystalline LiNi0.5Co0.2Mn0.3O2 (NCM523) is modified with porous Li4Ti5O12 particles through a simple solid synthesis process between residual lithium and metal–organic framework MIL-125(Ti). Here, metal–organic frameworks (MOFs) play an important role to thoroughly derive porous Li4Ti5O12 with a decentralized but not fully covered situation on the NCM523 surface. Moreover, Ti⁴⁺-rich Li4−xTi5O12−y would arise when the Li⁺ and O2– are removed from Li4Ti5O12 under a high voltage, which can promote the growth of cathode electrolyte interphase (CEI) along with enhanced capacity and stability of the NCM523 single crystals. Based on the MOFs materials, a new method of modifying cathodes with promoted CEI film was designed, which could be a reference for the development of high voltage and nickel-rich cathodes.

Full text

Review Article
Recent advances of SiO
x
-based anodes for sustainable lithium-ion
batteries
MengyuZhang1,NaiwenLiang1,DerekHao2,ZuxinChen3,4(
✉
),FanZhang1(
✉
),JiangYin1,YahuiYang1,andLi-
shanYang1,4(
✉
)
1Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education of China), National and Local Joint
Engineering Laboratory for New Petrochemical Materials and Fine Utilization of Resources, Key Laboratory of the Assembly and Application of
Organic Functional Molecules of Hunan Province, Hunan Normal University, Changsha 410081, China
2School of Science, STEM College, RMIT University, Melbourne 3000, Australia
3School of Semiconductor Science and Technology, South China Normal University, Foshan 528225, China
4International Iberian Nanotechnology Laboratory (INL), 4715–330 Braga, Portugal

Received: 22 March 2023 / Revised: 24 April 2023 / Accepted: 3 May 2023
ABSTRACT
The world is facing an ever-growing global energy crisis with unprecedented depth and complexity. The sustainable
developmentof highenergy densitylithium-ionbatteriesforelectric vehiclesand portableelectricdeviceshasbecome a
feasibleway todealwiththisproblem. Siliconsuboxides (SiOx)havebeendeemedas oneofthemostpromising anode
materialsbecauseof theirultrahightheoreticallithiumstoragecapacity,properworkingpotential,naturalabundance,and
environmental friendliness. However, the mass utilization of SiOx-based anodes is severely obstructed by their low
electricalconductivityandinevitablevolumeexpansion.Whilelithiumsilicateandlithiumoxideformedinthefirstlithiation
processactas bufferlayerstosomeextent,itisurgenttoaddresstheaccompanyinglowinitialCoulombicefficiencyand
unsatisfactorycyclingstability.Inthisreview,wesummarizedrecentadvancesinthesynthesis methods of SiOx-based
materials.Besides, thebenefits andshortcomingsofthevarious methodsare brieflyconcluded.Then,wediscussed the
effectivecombinationofSiOx with carbon materials anddesignsofporousstructure, which could considerably enhance
theelectrochemical performance indetail.Furthermore, progressesonthe modified strategies,advancedcharacteristics
andindustrialapplicationsforSiOx-basedanodesarealsomentioned.Finally,theremainingchallengesencounteredand
futureperspectivesonSiOx-basedanodesarehighlighted.
KEYWORDS
SiOxanodes,lithium-ionbatteries,synthesismethods,SiOx/Ccomposites,porous

1Introduction
Lithium-ion batteries (LIBs) as one of the most promising
electrochemical energy storage system has been playing an
essential role in the portable electronic market [1–8]. In recent
years, the increasing demands of portable electronic devices
(PEDs) and electric vehicles (EVs) require high-quality LIBs with a
long cycle life and excellent energy storage systems [9–12].
Therefore, the groundbreaking developments of novel anodes are
urgent priority for further practical applications of LIBs.
Conventional graphite anodes cannot satisfy the increasing energy
density demands of LIBs as a result of their inferior theoretical
capacity (372 mAh∙g−1) [13]. Silicon (Si) has been considered as an
ideal anode candidate for the next-generation LIBs because of its
plentiful reserves, a theoretical capacity of up to 4200 mAh∙g−1,
along with a suitable working voltage (< 0.5 V vs. Li/Li+) [14, 15].
However, the lithiation/delithiation process of Si are accompanied
by inevitable volume expansion (> 400%), resulting in
pulverization and crack of materials, formation of unstable solid
electrolyte interphase (SEI) and poor cycling stability [15–17].
Many strategies such as nanostructured materials [16, 18, 19],
surface coatings [20–22], and Si dispersion in active/inactive
matrices [15, 23–25] have been adopted to address the above
issues, while there are still many difficulties in the large-scale
application of Si anodes. Recently, Silicon suboxides (SiOx)-based
anodes (0 < x < 2) have attracted extensive attention due to their
ultrahigh theoretical lithium storage capacity, low cost, and natural
abundance [4]. The volume expansion leading to capacity fading is
mainly caused by the alloying reaction between Si and Li. With
unique microstructural combinations, SiOx possesses smaller
volume expansion than pure Si. Meanwhile, Li silicate and Li
oxide formed by the side reactions in the first lithiation process as

ISSN 2791-0091 (print); 2790-8119 (online)
https://doi.org/10.26599/NRE.2023.9120077


©The Author(s) 2023. Published by Tsinghua University Press. The articles published in this open access journal are distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction
in any medium, provided the original work is properly cited.
Address correspondence to ZuxinChen, chenzuxin@m.scnu.edu.cn; FanZhang, zhangfan@hunnu.edu.cn; Li-shanYang, lsyang@hunnu.edu.cn

buffer layers can maintain the structural stability. Compared to Si
anodes, SiOx-based anodes exhibit stabler cycling performance
and higher capacity retention due to the disconnection among
particles, conductive additives, and collectors [13, 26]. In the
development process of SiOx-based anodes, there are several
important milestones that play vital roles in their sustainable
development, which are depicted in Fig. 1(a). SiO, one of the most
typical oxides of Si, was firstly published by Charles F. Mabery in
1887 [27]. Silicon monoxide obtained by heating divided silicon
and silica in vacuum was reported in 1950 [28]. The value of x and
the size of Si nanodomains have significant influences on the
electrochemical performance of SiOx. Yang et al. firstly
systematically explored the electrochemical behaviors of
amorphous SiOx in 2002 [29]. To address the issue of low initial
coulomb efficiency (ICE), Kim et al. explored a scalable pre-
lithiation process in 2016 [30]. In 2016, Hirata et al. proposed a
heterogeneous structural model of non-proportional amorphous
SiO [31]. Stetson et al. successfully observed the SEI of SiOx
anodes in microscope [32]. Recent advancements especially focus
on the design of application-oriented composite structures and
various improvement strategies. Guo's team has designed special
composites such as graphite-like SiOx/C composites and flake-
graphite-like SiOx@G in 2018 [33, 34]. Li et al. explored the size
effect on the growth and comminution behavior of Si
nanodomains in SiO anodes [35]. Liu et al. designed conductive
polymers with hierarchically ordered structures in 2023 [36]. With
the diligent and persistent efforts of researchers, progress toward
the practical application of SiOx anodes is continually advancing.
According to our statistical results in Figs. 1(b) and 1(c), the
research and advancement on SiOx have been rapidly accelerated
during the past decades. Researches on the design of materials still
occupy the majority, while researches on other components and
mechanisms increase.
Nevertheless, it should also be noted that SiOx-based materials
are still limited in practical applications due to the inevitable
volume expansion and electrochemical irreversibility, leading to
an unsatisfied ICE of only 50%–60% [35]. To address these
drawbacks, compositing SiOx with carbon (C) and designing
porous structures are effective strategies (Fig. 2). Beyond SiOx-
based materials, several modified strategies including but not
limited to novel binders and electrolyte additives, prelithiation and
other LIBs components have been extensive explored recently.
Design of novel binders could not only stable the integrity of
electrode, but also promote the Li+/e– transport. Electrolyte
additives possess unique regulatory performance for the formation
of stable SEI, and prelithiation could significantly address the issue
of low ICE. The above-mentioned researches also occupy a crucial
role in the developments of SiOx-based anodes. In this review, we

Figure 1  The developments of SiOx-based anodes. (a) Major milestones in the history of developments of SiOx. Reproduced with permission from Ref. [30], ©
American Chemical Society 2016; Ref. [32], © American Chemical Society 2020; Ref. [34], © WILEY-VCH Verlag GmbH 2018. (b) Statistics of research results in
recent years: according to the JCR. (c) Classification of research results.
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discussed the structural model and electrode mechanism of SiOx-
based anodes to comprehend the inherent impediments. The
various synthetic routes of SiOx-based materials are also
demonstrated along with their respective characteristics. On this
basis, two confirmed effective optimization strategies of SiOx/C
composites and porous structure designs are summarized.
Furthermore, the cutting-edge modified strategies, advanced
characteristics and industrial applications of SiOx-based anodes
have also been mentioned. The analysis of SEI is highlighted.
Moreover, prospects and our personal opinions on SiOx-based
anodes are presented.
2Characters of SiO
x
2.1StructureofSiO
x
SiOx, including silicon monoxide and nonstoichiometric silicon
suboxides (0<x<1, 1<x<2), have been attracting much attention in
the past decades. SiO was firstly reported in 1990s and has been
extensively studied for decades. Nevertheless, the atomic structure
of SiO is of controversy in early literatures and mainly includes
two kinds of models, namely random bond model (RB) and
random mixture model (RM) [37]. Philipp firstly proposed the RB
model, pointing out Si–Si bonds and Si–O bonds were randomly
distributed throughout a continuous network and the entire
material possessed the single-phase structures [38, 39]. Conversely,
in the RM model, SiO is considered to be a multi-phase mixture of
nano-size amorphous Si and SiO2. The Si atom in SiOx can only
be surrounded by four Si atoms or four O atoms simultaneously
[40, 41]. For now, the interface cluster model (ICM) seems to be
more practical and acceptable [42]. Hohl et al. proposed the SiO
consists of amorphous Si and SiO2 clusters surrounded by the Si-
suboxide matrix [43]. This model was also verified by
Schulmeister through transmission electron microscope (TEM). It
was estimated that the proportion of interfacial domains between
amorphous Si and SiO2 was in the range of 20% ‒25% of the
total electrode materials [44]. Yu et al. proved that the surface of
SiO had different Si oxidation valence states from the interior of
SiO by X-ray photoelectron spectroscopy (XPS) analysis [45]. The
Si in SiO has various oxidation numbers from 0 to 4 and specific
atomic positions are also obtained from molecular dynamics and
reverse Monte Carlo simulations. The reconstructed
heterostructure model of amorphous SiOx constructed by Hirata is
shown in Fig. 3. SiOx exists in the interface region between Si and
SiO2. The model directly presented the structure of SiOx
experimentally at the atomic scale resolution, which effectively
overcomes the limitations of the conventional methods of X-ray
diffraction (XRD), XPS, and X-ray Raman scattering in terms of
spatial resolution [42].
2.2Electrodemechanism
A comprehensive understanding of the lithiation/delithiuation
mechanism of SiOx could help us to address the difficulties faced
and promote their development. Substantial efforts have been
devoted to analyze the electrochemical behaviors and mechanisms
of the SiOx anode for LIBs [46–50]. Based on these researches, the
electrochemical reactions with Li during the first discharge and
charge process can be expressed as follows [45, 51]:
SiOx+xLi++xe−→xLiO+Si 
SiOx+ (x/z)Li++ (x/z)e−→(x/z)LiSiyOz+ (−xy/z)Si 
Si +xLi++xe−⇌LixSi(x≤.)
It is universally acknowledged that LixSi, Li2O, and lithium
silicates (Li2Si2O5, Li6Si2O7, Li2SiO3, and mainly Li4SiO4) are
produced during the first cycle [52, 53]. Among these lithium
silicates, the lithiation of Li2Si2O5 is reversible. Li2O and Li4SiO4
could relieve the volume change of SiOx as buffer layers, which are
also likely to suppress the continuous SEI formation occurring in
the Si domains [2, 49, 54, 55]. Most of the irreversible capacity is
derived from the Li4SiO4 formed by the reaction of Li with
amorphous SiO2 or SiO [50, 53]. Besides, it was observed that
limited conditions like current density and molar rate had an
influence on reaction kinetics of reaction products in SiOx. When
the current density or molar rate increases, the reaction ratio of
irreversible products (Li2O and Li4SiO4) is significantly higher than
that of reversible products (Li-Si alloy) [56].
The value of x in SiOx also affects its performance for practical
applications [27, 57]. Specifically, the reversible capacity and the
ICE increase accompany poor cycling stability when the oxygen
content decreases [42]. Actually, O content simultaneously affects
the cycling life and low voltage hysteresis of SiOx anodes.
Prolonged cycle life requires high O content (e.g., more Li2O and
lithium silicates) to adapt to the volume changes, while high O
content usually leads to a high voltage hysteresis. The balance
between them is still extremely difficult to achieve. Wang et al.
demonstrated the initial size of Si nanodomains in SiOx was
related to these electrochemical behaviors and proposed that
the initial size of 4 ‒6 nm for Si nanodomains influenced
preventing the pulverization of Si domains [35]. For the

Figure 2  Research overview of SiOx anodes.
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advancement and practical applications of SiOx-based anodes,
more effective strategies for low-cost preparation and performance
improvement need to be explored.
3Synthesis methods of SiO
x
crystals
In industrial production, SiOx is mainly synthesized by heating Si
and SiO2 at extremely high temperatures to produce the gaseous
products, then condensing into steam [58]. However, the harsh
production conditions and uncontrollable process have affected
the practical application doubtlessly. Hence, it is crucial to explore
novel technologies for obtaining low-cost and high-quality SiOx-
based materials. There have been considerable studies on the
synthesis of SiOx-based materials, as shown in Fig. 4. We discuss
the recent progress of different synthesis methods in the following
sections, and propose feasible measures to the problems existing in
the synthesis of SiOx-based materials.
3.1Ballmilling
High-energy mechanical ball milling is a widely used method for
synthesizing SiOx, in which low-cost Si and SiO2 powders are
mechanochemically mixed. It is a simpler and cheaper approach
than the traditional high temperature heating method and energy
intensive vacuum process. SiOx with the different oxygen content
can be simply controlled through ball milling silicon in air
atmosphere [59]. During the milling process, adjusting the oxygen
content with different air exposure times is feasible and
controllable. SiOx materials synthesized by ball-milling are
compatible with high-temperature treatments such as chemical
vapor deposition (CVD) carbon coating, which would facilitate
the cycling performance effectively [60]. In addition, the ball-
milling time is closely relevant to the structure and electrochemical
performance of the obtained products. The chemical mixing of Si
and SiO2 could be realized by milling for at least 24 h, obtaining
products with structure like commercial SiO. XPS results also
confirmed that the structure of obtained products resembled
commercial SiO [61]. Nevertheless, the byproducts would increase
if the time exceeded 36 h owing to the formation of
contaminations such as Fe and Fe silicide. The SiOx electrode
produced by milling for 24 h exhibited excellent cycling
performance with a reversible capacity of 1060 mAh·g–1 after 100
cycles, which was significantly superior to commercial SiO.
Combining silicon oxide with metal oxide through a mechanical
milling process is effective to improve low ICE, which is scalable
and conducive to cost-effective mass production [62, 63].
Although the above method could reduce the product costs and
expand the production of SiOx with homogeneous distribution, its
lower efficiency and inevitable agglomeration phenomenon
cannot be ignored.
3.2Disproportion
It is universally known that disproportion is the most simple and
effective way to optimize the electrochemical performance of
anodes [64, 65]. SiO can spontaneously disproportionate at high
temperature to generate SiO2 and Si as follows:
SiO →Si +SiO
The active Si forms during the disproportion process
significantly improves the capacity of the anodes, and the
generated SiO2 acts as buffer layers to accommodate the volume
expansion of the Si, which is beneficial to the cycling stability of
SiOx. The main influencing factors of disproportion include
temperature and reaction time, which will influence the crystalline
grain size and the Si distribution in SiO2 matrix. The size and
distribution of Si domains can significantly affect the
electrochemical properties of SiOx-based materials. Recently,
Wang et al. investigated the evolution behavior of Si domains in
SiOx materials during the lithiation/delithiuation process by ex-situ
pair distribution function (PDF) and high resolution transmission
electron microscope (HRTEM). In this work, researchers
proposed a brand-new analysis method of PDF, which could
resolve the crystal structures of both short range and long range.
Researchers successfully synthesized disproportionated-SiO (d-
SiO) materials with different initial size of silicon domains by
regulating the temperature of disproportion to explore the effect of
the initial size of silicon domains on the electrochemical behaviors

Figure 3  Atomic models of amorphous Si (a), interfacial silicon suboxide (b), and amorphous SiO2 (c). Reconstructed heterostructure model of amorphous SiO (d).
Fractions of the five atomic coordinates existing in amorphous SiO (e). Reproduced with permission from Ref. [31], © The Author(s) 2016.
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of SiOx anode. After heat treatment at 450 and 900 °C, the size of
Si domains increased to 3.5 and 6 nm respectively, which were
significantly higher than in SiO (2 nm). It proved that the size of
the initial Si domains could be changed by disproportion of SiO
by heat treatment, thus improving the electrochemical
performance. The size of Si domains in SiOx is not as large as
possible, and there is a critical threshold. When the size
exceeds this value, the Si will gradually crack and
pulverization during the electrochemical cycling process, thus
fading the overall cycling stability. The experimental results
showed that the d-SiO heat treatment at 450 °C possess the
best cycling stability with the retention of 61.7%,
corresponding the size of the initial Si domains was 3.5 nm. In
addition, by combining with PDF analysis, researchers finally
concluded that silicon with the size of 4 ‒6 nm could
effectively avoid the pulverization of materials and is most
beneficial to the cycling stability of d-SiOx. This work showed
the electrochemical behavior and growth trend of Si domains
during the cycling and disproportion process, while the effect of
other components on the electrochemical performance is
unknown, and its electrochemical mechanism still needs to be
further explored.
Despite the SiOx obtained by disproportion have been
confirmed to have excellent electrochemical properties, the
insulating SiO2 matrix isolates the contact of active Si and Li+,
which severely affects the rate performance of materials. Recently,
Zhang et al. proposed a restricted-magnesium-vapor-reduction
(RMVR) method, where combined the disproportion and
magnesium-vapor-reduction in a simple step [66]. SiO@C was
utilized as raw material, and by controlling the amount of
magnesium to reduce the excessive insulating SiO2 to active Si.
The obtained polycrystalline Si/SiOx/C hybrid anodes exhibit
excellent cycling stability with a reversible capacity of 943 mAh·g–1
at 0.75 A·g–1 after 200 cycles.
Disproportion could significantly improve the electrochemical
performance and is compatible with large-scale production, while
the mechanisms and the factors affecting the disproportion still
need to be further explored.
3.3Metalreduction
Metal reduction seems to be a scalable method for SiOx
production. It utilizes suitable metals with low melting points and
suitable reduction potential to reduce silicon in higher valence
states in SiOx, including magnesium (Mg), zinc (Zn), aluminum
(Al), and so on [67, 68]. It can preserve the original structure of
the template. Different from traditional carbothermal
reduction, the reaction temperature is lower. For example, the
reaction temperature of magnesiothermic reduction is usually
500‒950 °C. Relatively mild reaction conditions are conducive
to the realization of controllable large-scale preparation.
Zhang et al. found that the porous surface structure of the
SiOx composites and Mg2SiO4 on the surface of the SiOx core were
formed spontaneously during the process of magnesiothermic
reduction. The double-layered SiOx/Mg2SiO4/SiOx structure
exhibited a high reversible capacity of 1381 mAh·g–1 with good
cycling stability. It is attributed to the porous outer of SiOx that
promoted deep penetration of the electrolyte on the surface, while
the Mg inner layer alleviated the volume expansion of the silicon
oxide core [58]. Wu et al. selected natural SiO2 as raw materials
and incorporated the ball-milling process to prepare SiOx. The
increases of micropores and mesopores from diatomite to SiOx
promoted the contact between anode materials and electrolyte,
thereby reducing the diffusion distance of lithium ions. In
addition, it effectively alleviated the volume expansion of silicon
domains after lithiation. Zn possesses lower melting point (419.5
°C) and boiling point (907 °C) [69]. After the process that mixed
Zn and SiO by high-energy mechanical milling, Kim et al.
evaporated the zinc-based materials by heat treatment and

Figure 4  Overview of main methods for synthesizing SiOx. Reproduced with permission from Ref. [61], © Elsevier B.V. 2019; Ref. [35], © Elsevier Ltd. 2020; Ref. [68],
© Elsevier Ltd. and Techna Group S.r.l 2019; Ref. [70], © American Chemical Society 2014.
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simultaneously produced amounts of mesoporous silicon oxide.
This method possesses relatively low reaction temperature, and
the products with porous structures, which is worth to be further
investigated.
3.4Sol−gelreaction
In sol-gel method, silicane organic compounds are generally
dispersed in solvent as precursor, then hydrolyzed and
polymerized to generate gels with certain spatial structure. After
drying and heat treatment, nanoparticles of SiOx are produced.
Park et al. prepared Si/SiOx that crystal Si nanodots were
embedded into the amorphous SiOx matrix by sol-gel method.
Based on the sol-gel reaction of triethoxysilane in an aqueous
medium and the formation of carbon coating layer, the obtained
SiOx anodes showed excellent cycling performance and rate
capability [70]. Liu et al. utilized vinyltriethoxysilane and
resorcinol/formaldehyde as raw materials for the preparation of
homogeneous SiOx/C, maintaining 91.0% of the capacity after 400
cycles [71]. A dual-size Si nanocrystal-embedded SiOx
nanocomposite with a core shell structure also demonstrated good
electrochemical performance [37]. To attain the required
composition and structure of SiOx, the reaction conditions,
including the hydrolysis duration, drying time, and reagent
proportion are required strict regulation [72]. For example, the
higher oxygen content in the gel generates more SiO2, the lower
ICE will be caused. The SiOx products obtained by the sol-gel
method are homogeneously mixed at the atomic level and the size
can be effectively controlled. Nevertheless, the expensive silanes as
precursors and the complex multistep synthesis increase the cost
of preparation, which inhibits their practical application in
manufacture.
3.5Othersyntheticmethods
Besides the methods described above, other methods such as
CVD, physical vapor deposition (PVD), pyrolysis of biomass, and
microemulsion, have been also reported. The preparation of SiOx
by CVD has been focused and endeavored by many researchers
[73]. Kim et al. fabricated nano-sized SiOx (x ranging from 1.18 to
1.83) by evaporation and condensation processes, which employed
induction melting with the injection of various mixed gases [74].
Detailed experiments demonstrated that the number of particles
increased, cycling stability improved, and the ICE decreased as the
amount of oxygen in the incoming gas increased. As a matter of
fact, it is satisfactory that the ICE reached 99.8% at the 50th cycle
with a reversible specific capacity of 660 mAh·g–1 when x = 1.18.
CVD could realize the surface reaction and growth of the material
on the substrate. Takezawa et al. deposited amorphous SiOx films
by evaporating pure Si with the electron beam and introducing O2
gas near the surface of the Cu substrate [55]. Another approach to
obtain SiOx is by simultaneously evaporating silicon and SiO2 via a
thermal plasma process, followed by water granulation to form a
micron-sized powder [75]. It is worth mentioning that the thermal
plasma process requires a very high temperature, and the size of
amorphous Si particles is strongly influenced by the process
conditions. By rapidly condensing SiO vapors, undertaking
reduction and disproportionation reaction, Tashiro et al. prepared
core-shell nanosized particles consisted of amorphous SiOx and Si
nanocrystals embedded in SiOx. The addition of CH4 enhanced
the decrease of x content in SiOx, and the SiO0.46 demonstrated
higher ICE, better capacity than the raw material SiO [76, 77].
Although the above methods have certain guiding significance
for the development of SiOx-based anodes, their complex
preparation process and expensive cost make them challenging to
achieve large-scale industrial applications. SiOx has been utilized
into practical application. In industrial production, a small
amount of SiOx is usually mixed with commercial graphite anodes
to improve the capacity and energy density. High-capacity SiOx-
based materials will be put into production more widely in the
future, so large-scale preparation of SiOx-based anodes is urgent to
make further progress. Considering the research and actual
situation in recent years, ball milling and disproportion are still the
mainstream methods to improve the properties of SiOx-based
anodes because they are suitable for large-scale production. The
above two methods are usually combined in the general
production process. The relevant factors affecting ball milling and
disproportionation, including the time and proportion of ball
milling, reaction time and temperature of disproportion, and so
on, have been extensively studied by researchers in recent years,
and many achievements have been made to promote their large-
scale application. It seems that metal reduction could be put into
large-scale production. How to realize the controllable large-scale
preparation and avoid the occurrence of side reaction still yet to be
addressed.
4Fabrication of SiO
x
-based anodes
4.1SiO
x
/Ccomposites
A common phenomenon of SiOx anodes is that high capacity
always accompanied by large volume expansion which is not
expected. The combination of SiOx and carbon is one of the most
common and effective modification methods to optimize the
electrochemical properties of SiOx-based anodes [78–82]. Carbon
are ideal coating materials that are easy to synthesize with high
conductivity and controlled morphology [64, 83]. The complete
carbon coating effectively obstructs the migration of oxygen from
the core to the surface, which facilitates the alloy reaction between
lithium ions and the inner Si nanoclusters, acting as insulating
layer to inhibit the side reaction between the outer silicon dioxide
and the electrolyte [84]. Common carbon sources include
amorphous carbon, carbon produced by pyrolysis of organic
materials, graphite, carbon nanofiber, and carbon nanotubes [42,
64]. However, carbon is unstable at high temperature, which will
promote the formation of electrochemically inactive and
insulating silicon carbide. Besides, it is prone to contact poorly due
to the difference in morphology between carbon and SiOx [83, 85].
Amounts of studies would most likely facilitate the majorization of
SiOx/C composites anode, and we divide them into categories
including coated SiOx/C composites, embedded SiOx/C
composites, and assembled SiOx/C composites. Their
electrochemical properties are summarized in Table 1.
4.1.1CoatedSiO
x
/Ccomposites
Coated SiOx/C composites specifically refer to utilizing carbon
materials as the protective layer to cover outer layer of the SiOx. In
this way, the materials can effectively buffer the bulk effect of SiOx
and enhance the electronic conductivity, while generating a stable
SEI film to stabilize the interface between the composites and the
electrolyte. Depending on the various designs of SiOx mixed with
carbon, it has been extensively reported that core-shell structure,
yolk-shell structure, watermelon-like structure, and so on. We will
respectively discuss the characteristics of the above structures and
elaborate on promising coating methods as follows.
The core-shell structure of SiOx/C composites is a simple and
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effective modification strategy, which has been widely used in Si
anodes. On the one hand, it can isolate active materials and
electrolyte to avoid side reactions, and on the other hand, it can
alleviate the stress generated by volume expansion of SiOx, thus
promoting cycling stability. In the design process of core-shell
SiOx/C composites, heteroatoms with greater electronegativity and
electron affinity, such as B, N and P, are often introduced into the
carbon coating to further improve the e–/Li+ conductivity. Hu et al.
constructed SiOx@N-doped carbon (NC) with a core-shell
structure through a solvent-free and flammable gas-free method
which used m-phenylenediamine and formaldehyde decomposed
by hexamethylenetetramine to form NC coating on the
surface(Fig. 5i) [86]. The products obtained by the environmental-
friendly method exhibited high reversible capacity of 774 mAh·g–1
with capacity retention of 112% after 500 cycles. Based on research
on the core-shell structure, another yolk-shell structure was put
forward by introducing voids between core and shell. The yolk-
shell structure could accommodate large volume expansion and
provide additional active sites because of the void area between the
core and the shell [87]. Liu et al. reported a yolk-shell structured
SiOx/C anode with semi-graphitic carbon coatings on the exterior
and interior surfaces, which manifested a prominent cycling
performance of 972 mAh·g–1 after 500 cycles at 500 mA·g–1 [88].
In 2018, Guo et al. designed a novel watermelon-like structure
of Si/C anodes, which realized high tap density and long cycle life
simultaneously [89]. The unique watermelon-like structure
contributes to alleviate the volume expansion of Si crystals and has
been applied to SiOx-based anodes. Different from Si, the volume
expansion of SiOx is slight due to its low Si content, so more active
SiOx can be introduced into the watermelon structure. Li et al.
designed a unique watermelon-like structure of SiOx-TiO2@C
nanoparticles consisted of ultra-fine TiO2 nanocrystals (diameter ≈
3 nm) and an outer carbon shell (thickness ≈ 10 nm) [90]. At the
current density of 0.1 A·g–1, a reversible capacity of 910 mAh·g–1
was obtained after 200 cycles, while remaining 700 mAh·g–1 at 1
A·g–1 for over 600 cycles. In this work, in addition to the unique
watermelon structure, we also noticed that the unique role played
by TiO2 in composites. TiO2 has the potential to be one of the
ideal candidate components for electrode materials synthesis. It
can effectively improve the electronic conductivities of materials,
release the structure stress and maximize the capacity utilization
by altering the Si–O bonding properties and reducing the Si/O
ratio. Dou et al. introduced TiO2 to promote the generation of
ordered carbon layers on the SiO surface at low temperature [84,
91, 92]. The TiO2 layer and the carbon layer were interspersed and
composited with each other, so as to uniformly generate the
ordered carbon coating, resulting in better interfacial stability and
higher electrical conductivity. The Coulombic efficiency of the 3%
TC-SiO anode reached 99% at the first 10 cycles at 0.14 A·g–1.
Another effective coating method is conformal carbon coating
by in-situ polymerization [93–96]. By in-situ polymerization to
generate polymer precursors on the surface of SiOx and high-
temperature pyrolysis, a uniform carbon coating on the SiOx
surface could be successfully achieved. This method could not
only achieve controllable adjustment of coating amounts through
the regulation of monomer addition amounts, but also achieve
multi-component composites or doping through the introduction
of other additives during the polymerization reaction process.
Various polymers produced by in-situ polymerization have been
applied to the preparation of coated SiOx/C composites, with
polydopamine being a typical example. Dopamine could easily self-
polymerize at alkaline environment and spontaneously deposit a
uniform polydopamine coating on the surface of SiOx. After
carbonization, nitrogen heteroatoms are introduced into the
carbon coating in the form of pyridinic, pyrrolic, and quaternary
nitrogen, thereby improving the reaction activity and electronic
conductivity of nitrogen doped carbon [94]. Shi et al. designed
core-shell structured SiOx/nitrogen-doped carbon composites by
conformal polymerization of dopamine and carbonization [94].
This study realized the homogenic and thick carbon coating and
nitrogen doping by in-situ polymerization, and the obtained
anode delivered a reversible capacity of 1514 mAh·g−1 at 0.1 A·g–1
after 100 cycles. Yang et al. synthesized yolk-shell SiOx@NC
nanospheres to optimize the SEI film. The obtained two-
dimensional covalently bound Si–N interface effectively enable the
formation of a thin SEI film with accelerated diffusion kinetics of
ions [95].
Carbon coating is beneficial to improving the conductivity and
stability of SiOx anodes, but the thicker carbon coating layer is not
suitable. The addition of too much carbon will reduce the specific
capacity of the materials. Therefore, it is very important for the
design of carbon that introducing as little carbon as possible to get
the carbon layer that matches the material particle size and has
uniform and orderly morphology.

Table 1  Summary of electrochemical properties of various SiOx/C anodes in LIBs
Anode active materials Carbon
source Composite form Mass loading
(mg·cm–2)
carbon contents
(wt.%)
Initial discharge
capacity/
(mAh·g–1)
ICE (%)
Capacity
retention
(%/cycles)
Ref.
SiOx in N-doped Carbon M-phenylenediamine Coated 1.0−1.5 65.2 1309 59.1 112/over 500 [81]
SiOx-TiO2@C Sucrose Coated 1.3 11.8 1618 62.5 — [84]
TC-SiO Carbon nanotube Coated 1−1.5 12.49 1251 71.9 — [86]
SiOx@C Epoxy resin Coated 1.2 15.7 2875.2 44 88.6/1000 [87]
SiOx/MWCNT/N-doped C CNT Embedded 1.3−1.4 41.4 1093 66 — [90]
Si/CNT/SiOxCNT Embedded ~1.5 58.67 1588 70.5 ~88/100 [93]
SiOx/NCS Graphite Embedded 3.3 ~30.5 1898 66.1 80/ over 200 [85]
SiOx/C Graphite, GO Embedded ~3.5 2.5 785 82.2 ~90/500 [28]
SiOx/NC Chitosan Micro-nanosized 0.9−1.2 ~34−48 1270 74.9 89/350 [97]
SiOx/C Resorcinol formaldehyde Micro-nanosized 1.5−2.0 39.75 1460 66.1 85.5/150 [66]
VG@SiOx/NC Graphene BTMS Micro-nanosized ~1.5 38.4 1796.7 73.7 84.2/500 [98]
SiOx/C Resorcinol Micro-nanosized ~1.2 ~28.9 ~1500 68 67/40 [99]
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4.1.2EmbeddedSiO
x
/Ccomposites
Embedded SiOx/C materials generally embed SiOx into the carbon
matrix to form secondary particles relying on conductive carbon
medium to improve the structural stability and the electroactivity
of the electrode. Compared with the coated SiOx/C composites,
the embedded SiOx/C composites usually possess better stability
with the high carbon content and lower reversible capacity owing
to the lower SiOx content. Among numerous studies on the
synthesis of carbon modified SiOx-based materials, graphene, and
carbon nanotubes (CNTs) are considered as promising and ideal
carbon sources. The improved electrochemical performance of
SiOx@CNTs can be attributed to the benefits of CNTs: (i)
providing an effective conductive network for SiOx; (ii) promoting
ion/electron transport and improving the conductivity and
electrochemical properties; (iii) reducing the resistance of the
electrode and buffering the volume change of SiOx. Preparing
SiOx/graphene composites is also one of the most effective
strategies to construct conformal conductive materials. Graphene,
a sp2 bonded carbon nanosheet with two-dimensional (2D)
honeycomb lattices, has attracted much attention with high
flexibility to withstand large stresses generated by volume
expansion of SiOx [97, 98].
CNTs with prominent flexibility, unique structure and excellent
chemical stability can perfectly form an interconnected network
with SiOx for rapid electron transport. The SiO2 surface layer
obtained by disproportionation of SiO provided many O sites for
CNT nucleation and was also acted as a barrier layer to prevent
the formation of electrochemically inert SiC. Ren et al. synthesized
network-structure of SiOx/multiwall carbon nanotube/N-doped
carbon composites, with amorphous silicon oxide distributed on
the outer surface and in the mid-space of multi-walled carbon
nanotubes. It provided a porous scaffold that facilitated the
transport of electrons and Li+, while highly conductive N doping
mitigating interfacial contact problems caused by the
agglomeration of SiOx [99]. After 450 cycles, a reversible capacity
of 621 mAh·g–1 was maintained. Xue et al. grew CNT directly on
SiO particles by a simple CVD process with acetylene as the
carbon source [100]. The SiOx-C anodes prepared by the situ self-
catalytic method are more firmly contacted than those prepared
by the “hard bonding” method such as ball milling [100, 101]. It
avoided the disadvantage that the added carbon tended to phase
separate after mixing with the slurry. The SiOx@CNTs anode
showed a reversible capacity of 1012 mAh·g–1 after 500 cycles at 2
A·g–1. Designing ternary SiOx-C composites system is also an
effective way to improve the electrochemical properties of SiOx-
based materials. Triethoxysilane-derived SiOx was used to form
solid bonds between the Si nanoparticles, CNTs and among

Figure 5  Different categories of SiOx/C composites: (i) Coated SiOx/C composites. Reproduced with permission from Ref. [86], © The Royal Society of Chemistry
2020. (ii) Embedded SiOx/C composites. Reproduced with permission from Ref. [102], © The Royal Society of Chemistry 2020. (iii) Micro-nanosized SiOx/C
composites. Reproduced with permission from Ref. [106], © The Royal Society of Chemistry 2020.
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neighboring CNTs, which improved the maintenance of
conductive paths and structural integrity of SiOx [102]. The stable
cycling performance of the composites can be maintained with the
retention of 88% of its initial charge capacity after 100 cycles (Fig.
5ii). Zhang et al. prepared three-dimensional (3D) graphene and
CNT dual decorated SiOx composites by a simple one-step high-
energy ball milling method. The SiOx-Gr-CNT anode materials
demonstrated more than 800 mAh·g–1 reversible specific capacity
after 200 cycles under a large current density of 1 A·g–1 [103]. Tian
et al. synthesized hollow-structured SiOx@CNTs/C composites
with graphitic carbon coatings and in-situ growth of CNTs [101].
Since the metal-organic-framework (MOF) structure includes
inorganic metals and organic ligands, the transition metals
generated in-situ during pyrolysis catalyze CNT growth, and the
presence of Co enhanced the electrochemical performance. The
SiOx@CNTs/C anodes with unique composite nanostructure
showed high specific capacity of 1200 mAh·g–1 and excellent ICE
of 88%.
Graphene and graphene oxide (GO) are also considered as ideal
carbon sources because of their high conductivity, facile
preparation, and excellent physical and chemical properties [91].
Common effective methods of preparing graphene include
mechanical exfoliation of graphite by ball milling,
micromechanically cracking, ultrasonic treatment and so on. Xu et
al. obtained SiOx/C anode by stripping and recovering the artificial
graphite and loading SiOx particles between the inside and the
surface of the graphite [34]. Benefitted from the rational design of
multi-component carbon materials (chitosan, GO and emulsified
asphalt) and the flaky structure to avoid agglomeration of Si
nanomaterials, as a result, the materials exhibited high reversible
capacity and excellent cycling stability. The SiOx/C with graphite-
like structure exhibited reversible capacities of 645 mAh·g–1,
excellent cycling stability (≈90% capacity retention after 500
cycles), and superior rate performance (565 mAh·g–1 at 5 A·g–1).
Chen et al. prepared multicomponent nanosheets including
reduced graphene oxide (rGO) nanosheets substrate, porous SiOx
as intermediate layer, and N-doped porous carbon nanoparticles
(NNC) as shell [104]. The obtained anodes exhibited remarkable
electrochemical properties due to multiple carbon components
and controllable pore structures. Zhang et al dedicatedly
synthesized SiOx embedded in N-doped carbon nanoslices by
using melamine to embed in graphite on account of their strong
binding properties and then stripping in the process of ball milling
to form carbon nanoslices [91]. Melamine not only played a role
in exfoliating graphite, but also acted as a nitrogen source to
achieve nitrogen doping. The resultant SiOx/C anode
demonstrated excellent cycling stability (~900 mAh·g–1 over 600
cycles at 1 A·g–1) and impressive rate performance (565 mAh·g–1 at
5 A·g–1). During cycling, graphene provides the security of
mechanical strength and flexible cushioning, but the disadvantages
of potential disengagement problems and the low specific surface
area of it still undetermined, which affected the loading of SiOx on
the graphite surface. Therefore, exploring the strong and durable
bond between SiOx-based electrodes and graphene is essential for
outstanding cycling performance.
4.1.3Micro-nanosizedSiO
x
/Ccomposites
Although nano-sized SiOx-based materials could effectively
accommodate the volume stress due to their unique strength of
size, they would create more interparticle space and surfaces,
resulting in low tap density and low volumetric capacity which
could not satisfy the demands of practical applications [105, 106].
To address the above issues, micro-sized SiOx-based materials
with nanostructure characteristics are of tremendous potential for
high energy density LIBs. Micro/nano-sized SiOx-based materials
could be obtained by integrating SiOx nanoparticles with micro-
sized carbon metrics. Although the low content of SiOx causes
their specific capacity to be far lower than the theoretical specific
capacity of SiOx, their higher theoretical specific capacity is still
significantly better than commercial graphite anodes. Besides, the
high tense density could address the application defects of
nanosized materials. The stable micron-sized carbon metrics could
promote the transportation of Li+/e– while maintaining the stability
of the structure and ensuring the cycle life, thus improving the rate
performance of the anodes, namely the fast charging/discharging
performance. Liu et al constructed monodisperse SiOx/C
microspheres by the gel-solution process with uniform
distribution of ultrafine SiOx nanodomains in an amorphous
carbon matrix [71]. The microspheres showed high capacity and
cycling stability, achieving a reversible capacity of 689 mAh·g–1 and
maintains 91.0% capacity after 400 cycles even at 5 A·g–1. In
another team, micro-/nano-structured SiOx/C composites were
designed via sol-gel and heat-treatment process based on
carbothermal reduction of silica-carbon binary xerogel [107]. It
exhibited high reversible capacity of 830 mAh·g–1 for 100 cycles,
and excellent cycling stability with 67% capacity retention after
400 cycles. With vinyltriethoxysilane and chitosan as precursors,
Liu et al. synthesized SiOx/N-doped carbon microspheres by a
scalable microemulsion method (Fig. 5iii). It effectively mitigated
the inherently low electron/Li+ conductivity and enhanced the
structural integrity of SiOx. The assembled SiOx/NC-2/LiFePO4
full cell showed excellent capacity retention of 89% after 350 cycles
at 2C [106]. Han et al. reported vertical graphene (VG)@SiOx/NC
microspheres using a thermal chemical vapor deposition method
[108]. The process promoted the homogeneous dispersion of SiOx
and NC at sub-nanoscale, followed by VG coating which could
effectively construct a 3D conductive network and provide high
electric conductivity. Uniform dispersion of SiOx and C
components on the atomic scale is the key to preparing SiOx@C
microspheres with nanostructure. The uniform dispersion
structure and high electrochemical activity of SiOx component
combinedly contributed the satisfactory electrochemical
performance. Micro-nanosized SiOx/C materials are of certain
research value owing to its suitable particle size and superior
interconnection, while how to rationally realize them in large-scale
production still need to be explored.
4.2Porousstructures
4.2.1Anoverviewofporosity
According to the International Union of Pure and Applied
Chemistry (IUPAC), porous materials can be divided into
three types according to their pore size: microporous
materials (pore size < 2 nm), mesoporous materials (pore size
between 2‒50 nm) and microporous materials (pore size > 50
nm). For porous materials, their multiple characteristics, such
as pore structure, arrangement, specific surface area, surface
structure, symbiosis, and defects, have seriously affected their
properties. The porous SiOx-based materials with large specific
surface and abundant pores not only facilitate the transportation
of Li+, resulting in improvement of the rate performance; but also
improve the cycling stability due to their porous structure, which
could provide enough voids to accommodate the volume
expansion. As follows, we summarized the different methods for
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synthesizing porous SiOx-based materials, and put forward for
future developments.
4.2.2SynthesisofporousSiO
x
-basedmaterials
Porous SiOx-based materials can be divided into porous SiOx and
the combination of SiOx and porous framework, mostly porous
carbon. On the synthesis of porous SiOx, the preparation methods
can be divided into the top-down type of porous SiOx prepared by
etching technology and the bottom-up type of porous SiOx
formed by weak interaction self-assembly according to the
different construction strategies [109]. Therefore, there are
different preparation methods, such as etching, template-assisted
method and so on. Attributed to their large pores, the porous SiOx
as lithium storage materials possess adequate spaces to
accommodate the volume change, so that their structures could
remain integrity of structure during cycling and avoid the
pulverization of electrode materials. Besides, the large specific
surface area is conducive to the transportation of Li+, improving
the rate performance of SiOx.
Metal-assisted chemical etching (MACE) is a typical top-down
synthesis method. In 2013, Jung-In Lee et al. prepared three-
dimensional porous SiOx as anodes by a combination of current
displacement reaction and MACE [110]. Silver nanoparticles as
catalysts were electrodeposited on the surface of SiOx by
electroplating, and SiOx particles deposited with silver were
chemically etched to synthesize porous SiOx particles. The anode
exhibited a high capacity of 730 mAh·g–1 and outstanding cycling
performance after 100 cycles without significant dimensional
changes. Chemical etching also achieved results in the same year
by Yu et al. used Si as pore-forming agent, silicon oxide as
template and NaOH solution [51]. The original SiO was heat-
treated at 900 °C, and nanocrystalline Si was provided in the SiOx
matrix. Nanocrystalline Si was dissolved to provide pore size
of 200 ‒500 nm, and the surface area of etched SiO was
increased by more than 5 times. The porous SiOx anode
exhibited a stable reversible capacity of about 1240 mAh·g–1 after
100 cycles at 0.2 C. Template-assisted method is also one of the
important methods for preparing porous SiOx. Park and Eunjun
et al. proposed the sol-gel reaction of triethoxysilane and the
introduction of oil as additives for large-scale production, which
was not only used to generate mesopores in SiOx matrix, but also
as the precursor of carbon layer on the surface of porous SiOx
particles [111]. It exhibited excellent cycling performance and had
no obvious size change after more than 100 cycles. The porous
SiOx of carbon coating also showed a highly stable thermal
reliability like that of graphite. The most mainstream method for
preparing porous SiOx is magnesiothermic reduction, which has
been mentioned in Section 3. Porous structures could
accommodate volume expansion and achieve long cycle life, but
there are still many unresolved issues in the large-scale production
of porous SiOx. Magnesiothermic reduction has achieved small-
scale application in the production of porous Si, and how to
expand to SiOx anodes and industrial production still needs to be
considered.
Another porous SiOx-based anode is the combination of SiOx
and porous framework, such as porous carbon. Biomass carbon is
a potential source for large-scale production [112–116].
Compared with graphene and carbon nanotubes, biomass carbon
can significantly reduce the cost of raw materials and possess
inherently special structure. Special natural products enriched with
Si and C include rice husk, bamboo and reeds have aroused the
attention of researchers due to their abundant reserves, low cost
and unique structural characteristics inheriting from the natural
resources. Bamboo charcoals possess a special well-connected 3D
microstructure as a natural material that is rich in both Si and C
[115]. Yu et al. synthesized hierarchical porous SiOx dispersed in
3D carbon network by following a top-down route [115]. In-situ
construction of the hierarchical composites used bamboo
charcoals as raw materials without adding any additional silicon
and carbon sources. The 3D porous SiOx/C composites
demonstrated a high specific capacity of (1100 mAh·g–1) at 200
mA·g–1 after 300 cycles with excellent prominent cyclic stability.
According to the existing research, the development prospect of
porous structure in SiOx-based anode can be proved. However,
the above preparation methods have not been widely used in
production due to their respective defects. The etching method
inevitably involves the use of HF, which is not environmentally
friendly and toxic. The template-assisted method has its unique
strengths in constructing various porous structures, while
expensive templates and complex processes are doomed to be
unable to be applied on a large scale. The preparation of porous
structure in large-scale production is still a difficulty to be
addressed.
5Beyond SiO
x
-based materials
5.1Novelbinders
Besides active materials, there are many crucial components in
LIBs which will severely influence the cycling life. Among them,
binders have attracted extensive concentrations in recent years,
which could not only adhere the active materials and conductive
additives together, but also combine electrode materials with
copper collectors together. For SiOx-based anodes, appropriate
amounts of binders could effectively alleviate the volume
expansion of SiOx and thus prolonging the cycling life, while too
much binders will reduce the proportion of active materials and
thus reducing the specific capacity. It is noticed that the binder
network is an ion-conducting network that does not effectively
conduct electrons. How to simultaneously realize the Li+/e–
conduction possess outstanding scientific significance and
application value. As widely used binders for commercial LIBs,
polyvinylidene difluoride (PVDF) provide weak van der Waals
force for Cu current collectors and electrode materials, result in
unsatisfactory cycling performance even though the volume
expansion of SiOx is slight compared with Si. It is urgent to
develop novel binders with high mechanical properties and strong
interaction with electrode materials to promote the developments
of SiOx-based anodes. In recent years, water-soluble polymer
binders with plentiful hydroxyl and carboxyl groups have been
widely studied because of their strong interaction with SiOx and
excellent adhesion. There are considerable reports about the
application of carboxylmethyl cellulose (CMC), sodium alginate
(SA), polyacrylic acid (PAA), styrene butadiene rubber (SBR), etc.
in SiOx-based anodes [26, 87, 117–119]. The CMC-SBR binders
have been partially utilized for commercial Si-based anodes.
We hope that the ideal binders can effectively promote the
conduction of Li+/e– while possessing sufficient mechanical
elasticity and adhesion ability with materials. The abundant
natural reserves and low cost, as well as the low proportion in the
electrode composition, are proposed to meet the requirements of
industrial production. Based on the above-mentioned summary,
cross-linked polymer binders based on water-soluble binders,
binders with supramolecular and conductive polymer binders are
discussed here (Fig. 6).
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Due to the abundant groups, water-soluble polymers not only
can be tightly combined with the surface of SiOx, but also can
form cross-linking networks with chemical bonds and hydrogen
bonds through dehydration-condensation and other
polymerization reactions. Jing et al. evaluated the effects of PVDF,
SA and PAA on the electrochemical performance of SiOx/G@C
anodes [120]. Water-soluble binders of PAA and SA exhibited
significantly better cycling stability than PVDF, with capacity
retention of 77.2%, 37.8%, and 10.6%, respectively. In 2018, Lee et
al. designed a novel binder, by combining dopamine-grafted
heparin with CMC/SBR [121]. The obtained graphite/SiOx anodes
remained a high retention of 73.5% after 150 cycles, and showed
excellent rate performance with 200 mAh·g–1 at 20 C. Tang et al.
introduced the small molecule tannic acid (TA) into PAA for SiOx
anodes [122]. The designed crosslinked PAA-TA binders
delivered a reversible capacity of 1025 mAh·g–1 after 250 cycles.
Liao et al. fabricated three-dimension crosslinked binders by in-
situ thermal crosslinking of CMC and iminodiacetic acid (IDA)
[123]. The SiOx anode showed a reversible capacity of 899 mAh·g–1
at a high current density of 1 A·g–1 after 200 cycles.
Supramolecular chemistry has been utilized to the design of
binders in recent years. In 2017, Choi et al. reported research on
highly elastic binders integrating polyrotaxanes (PR) used in
microparticle Si [124]. The conjunction of sliding-ring PR could
alter the mechanical properties of PAA and construct a highly
elastic binder network. This binder attained stable mechanical
properties and cycling life of microparticle Si at commercial area
capacities (initial capacity of 2.67 mAh·cm–2), with excellent
capacity retention of 91% after 150 cycles. Based on this study, The
PR-PAA binders were applied to SiOx anodes by the same team in
2019 [125]. The obtained SiOx/NCA full cell delivered initial
capacity of 2.79 mAh·cm–2, with a high retention of 82.5% after
150 cycles at 0.5C. Supramolecular chemistry plays an crucial role
in the design of binders for high capacity batteries in the future,
and has the potential for expanding applications.
Conductive polymer binders with certain mechanical elasticity
are also considered as promising binders, which can achieve good
e– conductivity that cannot be achieved by traditional binders
[126, 127]. Song et al. synthesized polyfluorene-type cross-linked
conductive binders for micro-sized SiOx by connecting linear
conductive binder onto conjugated anchor points [127]. The
conductive polymer binders aided the SiOx anodes remain a
cycling retention of 88.0%. The introduction of conductive
polymers into binders is an effective way to improve the
conductivity of electrodes, while the complexity of synthesis
process remains to be resolved.
5.2Electrolyteandelectrolyteadditives
As the medium of Li+ conduction, electrolytes have profound
significance for the electrochemical behaviors of LIBs. The Li+
concentration and solvent composition will affect the performance
of Li+ with active materials in the electrochemical process. More
importantly, the composition of electrolytes and the introduction
of electrolyte additives directly affect the morphology and
composition of SEI. Mostly, the traditional electrolytes consist of
solutes of lithium hexafluorophosphate (LiPF6) and organic
carbonate solvents of ethylene carbonate (EC) and diethyl
carbonate (DEC), and electrolyte additives, which are shown in
Fig. 7. The latest progresses in SiOx-based anodes have made
breakthroughs in the study of each component of electrolytes,
especially electrolyte additives [128–130].
We hope that the SEI obtained possess excellent mechanical
properties, with the outer layer rich in inorganic components (LiF)
and the inner layer mainly composed of organic components. The
ultimate purpose of using electrolyte additives is to form SEI with
good ion conductivity and mechanical elasticity, thereby
maintaining interface stability and high energy density for LIBs.
Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are
currently the most widely used electrolyte additives. Besides, there
are many electrolyte additives widely used by researchers,

Figure 6  The designs of different binders for SiOx anodes: (a) Cross-linked polymer binders of CMC-IDA. Reproduced with permission from Ref. [123], © American
Chemical Society 2021. (b) Supramolecular binders of PR-PAA. Reproduced with permission from Ref. [125], © Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2019. (c) Cross-linked conductive polymer binders. Reproduced with permission from Ref. [127], © Wiley-VCH GmbH 2021.
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including but not limited to lithium bis(fluorosulfonyl)imide
(LiFSI), lithium difluorophosphate (LiPO2F2), octadecylamine
(ODA), 1,3,2-dioxathiolane 2,2-dioxide (DTD), 1,3-propane
sultone (PS), LINO3, trans-difluoroethylene carbonate (DFEC),
etc. [131–142] The current researches are mostly based on FEC
and VC, and then promotes the generation of stable SEI through
the combined use of multiple additives. The positive role of FEC
and VC in producing mechanically stable LIF-rich SEI has been
confirmed. They will be reduced at a higher potential than organic
solvents such as EC, producing stable and uniform SEI with good
ionic conductivity. Kang et al. used FEC as electrolyte additives to
improve the high temperature performance of
LiNi0.8Mn0.1Co0.1O2||SiOx@graphite pouch cells [131]. By adding
5% FEC, the capacity retention increased from 54.5 % to 67.0%
after 100 cycles at 45 °C. Besides, the use of FEC as a cosolvent
further increased the capacity retention to 83.3%, which attributed
to the multifunctional effect of FEC, preventing electrolyte
decomposition, mitigating the resistance increment and Mn ions
dissolution. Nonetheless, FEC is not flawless. Zhou et al. found
that FEC will accelerate the generation of gas and HF after 60 °C
storage, causing cathodic corrosion [132]. Based on this study,
Zhou et al. reported two kinds of nitriles additives to optimize the
high-temperature performance of SiOx/Gr||NCM811 pouch cell
[133]. The introduced additives could not only inhibit the
decomposition of FEC additive to release HF, but also reduce the
dissolution of metal ion coming from NCM811 cathode. Ouyang
et al. designed two kinds of two-component electrolyte additives,
i.e., 2% vinylene carbonate+1% ethylene sulfate (2VC1DTD) and
2% fluoroethylene carbonate+1% lithium difluorophosphate
(2FEC1LFO) to explored the safety characteristics of cells [134].
They confirmed that these electrolytes enabled to improve the
thermal stability between cathode materials and electrolytes, while
brought about little effect on suppressing the reactivity between
anode materials and electrolytes. ODA as electrolyte additives
could promote the formation of Li-N rich SEI and are often used
in combination with FEC or VC [135, 136]. Yang et al. reported
the use of ODA in a conventional electrolyte with VC. The VC-
ODA additives showed excellent capacity retention of 77.0% at 1C
after 1000 cycles [135]. Researches on other electrolyte additives
have also been mentioned and with potential guiding significance
for industrial applications. Kim et al. designed anti-corrosive and
surface-stabilizing functional electrolytes for SiOx/NCM811
batteries, which contains LiFSI and LiPO2F2 [137]. LiFSI enabled
for the formation of LiF-based SEI, and LiPO2F2 effectively
suppressed the undesirable LiFSI-induced corrosion reactions at
the cathode by forming stable cathode–electrolyte interphases on
the surface of the Al current collector. This novel system exhibited
improved cycling retention (94.6%). Qi et al. introduced LiNO3
additive during the preparation of electrolytes. The results of
relative characteristics showed that LiNO3 decomposed into
LiNxOy and Li3N with high ion conductivity, which deposited on
the anode surface during the first discharge process [138]. An et al.
explored the S-containing and Si-containing compounds as highly
effective electrolyte additives for SiOx-based coin-type half cells
and pouch cells [139]. They evaluated multiple electrolyte systems:
DTD, DTD+prop-1-ene-1,3-sultone (PES), tris(trimethylsilyl)
phosphate (TTSP), tris(trimethylsilyl) borate (TMSB), and
methylene methanedisulfonate (MMDS) were added to the
baseline electrolyte (BL=1.0 M LiPF6+3:5:2 w:w:w EC: EMC:
DEC+0.5 wt.% LiDFOB+2 wt.% LiFSI+2 wt.% FEC+1 wt.% PS).
Si-containing additives exhibited a lower impedance increase in
the full cell, better beginning-of-life performance, less reversible
capacity loss and better storage at elevated temperatures than S-
containing additives. S-containing additives exhibited the
advantages of low SEI impedance, while yield a worse
performance in the full cell than Si-containing additives.
More functional electrolyte additives include conductive
additives, flame retardant additives, additives to improve low
temperature performance, et al. are urgent to be developed and
used simultaneously with the current mainstream additives. In
addition, we should pay more attention to the research of new
solvents to replace carbonate solvents in order to address the safety
hazards of traditional solvents.
5.3OtherLIBscomponents
There are also views on the research of other battery components,
such as conductive additives, current collectors, and separators.
The low intrinsic conductivity of SiOx severely limits the e−
conduction and high-rate performance. Conductive additives
provide paths for e– conduction between active materials. Carbon
black is widely used in LIBs due to their excellent Li+ and e–
conductivity. In addition, the carbon black particles with size of 50
nm certainly agglomerate to form a branched chain structure,
which is helpful to improve the electronic conductivity of active
materials. Undoubtedly, practical application tends to favor less
contents of conductive additives. Because of their excellent
conductivity and unique structures, which could effectively realize
e− conduction between active materials, CNTs and graphite
conductive additives are also considered as promising conductive
additives. Despite their applications are confined by their high-cost
and synthesis-complexity, a trace amount of addition in carbon
black could significantly improve the e– conductivity of the
electrode, which is widely recognized and applied. It is noticed
that the current collectors and separators will also affect the
electrochemical performance. There have been many related
studies on these two components in recent years, while the current
commercial technologies are quite mature, and the future
developments will be based on Cu foil with good conductivity,
stability, and low-cost and polyethylene (PE)/polypropylene (PP)
based microporous separators.

Figure 7  Overview of electrolytes and electrolyte additives for LIBs with SiOx
anodes.
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5.4Prelithiation
Low ICE cannot be ignored for SiOx-based anodes. The
irreversible side reaction of Li+ intercalation in the first cycle and
the side reaction of the formation of SEI between the active
materials and the electrolytes collectively lead to the low ICE. Most
of researches are based on the half-battery using Li metal as the
counter electrode, which can provide excess Li+ so that we can
ignore the impact of irreversible capacity loss in the first cycle.
However, in practical application of full cell, Li ions are provided
by the cathode materials, which means that the loss of Li+ cannot
be compensated, thus reducing the energy density of the battery.
The strategies of prelithiation can compensate for the irreversible
lithium loss during the first cycle of charging/discharging by
providing additional lithium sources, which has remarkable
research significance and value, and is in urgent need of
breakthroughs. Nowadays, the strategies reported for acting on the
anodes mainly include the following four types, as shown in Fig. 8:
lithium foil direct contact, lithium powder direct contact,
electrochemical prelithiation and chemical prelithiation [4, 30,
143–145].
Li foil direct contact with anodes was convenient for large-scale
application, while this method was uncontrollable of the rate and
extent of prelithiation. Guo et al. modified this method by
inserting a resistance buffer layer between anode and Li foil. This
work realized controllable prelithiation with a tunable ICE ranging
from 78% to 137% based on half-cell, and a high ICE of 87% was
obtained based on full-cell [143]. Lithium powders as lithium
sources of prelithiation could achieve controllable and uniform
prelithiation instead of Li foil. Stabilized lithium metal powder
(SLMP) was a promising Li resource. Zhao et al. improved the
ICE of SiO/NMC full cell from ~ 48% to ~ 90% by using SLMP
[144]. Although SLMP is more stable for the prelithiation process
than Li foil, the prelithiation processes involved in SLMP include
adding to the anode slurry and adding to the electrode. Due to the
instability of SLMP in solvents, the lithium powder contact
method is incompatible with current anode fabrication processes
[4]. Besides, the preparation of SLMP involves molten Li metal,
which is high-cost and with safety hazards. How to achieve its
large-scale application remains to be investigated. A novel lithium
thermal evaporation technology was applied to the prelithiation of
Si-based anodes to address the issue of inhomogeneous Li
distribution caused by direct contact with Li foil/SLMP. Adhitama
et al. applied this novel prelithiation method to micrometer-sized
Si anodes and Si thin films, successfully achieving controllable
extent of prelithiation and highly homogeneous Li deposition
[146, 147]. They evaluated the Li nucleation, mechanical cracking,
and the ongoing phase changes. At present, there are few reports
on this new prelithiation method in research on SiOx-based
anodes. In 2017, Takezawa et al. achieved prelithiation of SiOx film
anodes by Li thermal evaporation [148]. The obtained anodes
showed excellent ICE of over 100%, and exhibited superior
cyclabilities for 30 cycles. Although there is currently limited
research and more investment and exploration are still needed,
this method is simple and could achieve controllable Li deposition,
with the potential to expand to industrial scale applications.
Electrochemical prelithiation method improves the ICE by
making the contact between the anodes and the lithium metal
under constant current charge or external short circuit conditions
to achieve Li+ migration [30, 149]. Kim et al. designed a scalable
prelithiation scheme based on electrical shorting with Li foil [30].
The Coulombic efficiency in the first three cycles reached 94.9%,
95.7%, and 97.2%. Perhaps this method is precise and controllable,
the inevitable complex processes including disassembling battery,
extracting materials, and reassembling battery severely limit its
practical application.
Chemical prelithiation is implemented in solution, which could
achieve prelithiation through the redox reaction of Li containing
compounds with strong redox properties and anode materials in
solution, thus improving the ICE [145, 150–152]. Compared to

Figure 8  Different strategies of prelithiation: (i) Lithium foil direct contact. Reproduced with permission from Ref. [143], © American Chemical Society 2019. (ii)
Lithium powder direct contact. Reproduced with permission from Ref. [144], © American Chemical Society 2014. (iii) Chemical prelithiation. Reproduced with
permission from Ref. [145], © American Chemical Society 2020. (iv) Electrochemical prelithiation. Reproduced with permission from Ref. [30], © American Chemical
Society 2016. (v) Thermal evaporation prelithiation. Reproduced with permission from Ref. [147], © The Author(s). Published by Wiley-VCH GmbH 2022.
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direct contact Li methods, solution-based chemical prelithiation
has been widely studied in recent years due to its easy to scaling up
production and uniform solution process. The extend of
prelithiation can also be effectively adjusted by controlling the
composition of the solution. Yan et al. respectively evaluated the
effects of lithium powder direct contact and chemical prelithiation
on the electrochemical performance of SiOx/C anodes [145]. The
NCM811/SiOx/C pouch cells possessed high energy density of 301
Wh·kg–1 and satisfactory cycling stability of 93.3% capacity
retention after 100 cycles. Although different anodes require
different solutions for chemical prelithiation, this stable and safe
prelithiation method, which is compatible with anode production
processes, is effective and promising for future developments.
5.5AdvancedcharacteristicsandSEIanalysis
After more than ten years of developments, the researches on SiOx-
based anodes have expanded from the materials designs to
mechanism researches. The revelation of the lithiation mechanism
of SiOx-based anodes and other theoretical studies will help
promote their practical applications. Many advanced
characterization techniques and simulation methods will be used
in this process, some of which are shown in Fig. 9. Various in-situ
characterization techniques have been applied to the study of the
electrochemical behaviors of SiOx. Park et al. confirmed the
differences of electrochemical behaviors between graphite anodes
and graphite-SiOx anodes by in-situ XRD [153]. They found that
the Li+ insertion into graphite was delayed because of the presence
of SiOx, and the Li+ insertion into SiOx taken place prior to the
graphite. Chou et al. performed density functional theory (DFT)
calculations to to examine lithiation behaviors and the structural
evolution of lithiated a-SiO1/3 [154]. This study showed that both
Si and O are active toward Li, and it is worth noting the formation
of highly symmetric Li6O complexes with unique Oh symmetry.
The importance of the O concentration and spatial distribution
were revealed, which could be utilized in designs of SiOx anodes.
Adkins et al. revealed the lithiation/delithiation behaviors of Si
nanowires with a SiOx shell by in-situ TEM [155]. They
demonstrated that SiOx coatings would significantly limit the
lithiation of Si in some extent, and induced the formation of pores
in Si nanowires. Jung et al. illustrated the lithiation mechanism of
SiOx nanowires based on molecular dynamics simulations
employing the ReaxFF reactive force [156]. During the lithiation
process, Li+ interact with the a-SiO2 layer to generate Li2O and
Li4SiO4. Subsequently, other unreacted Li+ diffuse into the c-Si
core, where they preferentially penetrate the c-Si along the <110>
or <112> direction. This study further proved that an a-SiO2 layer
with a thickness of 1 nm could suppress the volume expansion of
SiOx nanowires during lithiation. Currently, there are still many
perplexities regarding the lithiation mechanism of SiOx, and more
advanced characteristics such as in-situ nuclear magnetic
resonance (NMR), Raman spectra, theoretical simulation, and so
on are yet to be utilized.
The studies of SEI are crucial for the developments of SiOx-
based anodes. Stable SEI is essential for high energy LIBs. In recent
years, the analysis of SEI has become a research hotspot, with
more and more reports discussed how to form dense and elastic

Figure 9  Advanced characterization techniques and simulation methods for researching the electrochemical behaviors of SiOx, including in-situ XRD, in-situ TEM,
reactive molecular dynamics simulations and density functional theory. Reproduced with permission from Ref. [153], © Elsevier Ltd. 2013; Ref. [154], © American
Chemical Society 2013; Ref. [155], © American Chemical Society 2018; Ref. [156], © The Royal Society of Chemistry 2016.
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SEI. Various methods have been used to characterize the physical
and chemical properties of SEI (Fig. 10). Atomic force microscopy
(AFM) could analyze the morphological features on the electrode
surface. Stetson et al. analyzed the electrode with AFM in their
pristine state and after the first process of lithiation/delithiation
[32]. The changes of roughness in electrode revealed the
formation of SEI during the first discharge cycle and the remnants
of SEI after the first charge/discharge cycle. The SEI on the surface
of SiOx-based electrodes with excellent cycling performance is
usually thin and uniform. The thickness and morphologies of SEI
usually observed by HRTEM. Xiao et al. tested the coating
integrity of SiOx@C composite by a developedselective alkali
dissolution. In this work, the samples with the best cycling life
possess the most thin-flat SEI, which was observed by HRTEM
[157]. The analysis of the surface composition of SEI usually
involves sensitive characterization techniques, such as Fourier
transform infrared spectrometer (FT-IR), high-resolution XPS,
scanning transmission electron microscopy (STEM), electron
energy loss spectroscopy (EELS) and energy dispersive X-ray
spectroscopy (EDS), and time-of-flight secondary ion mass
spectrometry (ToF-SIMS). Many studies have confirmed that SEI
mainly consists of inorganic components of LiF, Li2CO3, etc. and
organic compoments through XPS analysis of the electrode
surface after cycling [158, 159]. Yang et al. analyzed the differences
composition of SEI on the surface of SiOx electrode and SiOx@NC
electrode after cycling through FT-IR and high-resolution XPS
[95]. The results proved the effect of two-dimensional covalently
bound Si−N interface, which was caused by introduction of N-
doped carbon coating. Stetson et al. compared the differences in
SEI composition on the surface of SiOx anodes before and after
HF etching through STEM analysis, confirming that HF etching
was beneficial for the formation of elastic SEI with inorganic
components as the outer layer, which may be partially responsible
for improved cycle life observed in SiOx-based anode materials
[32]. ToF-SIMS has been reported for SEI research in Si-based
anodes, while it has not been reported in SiOx-based anodes, and
yet to be expanded.
5.6IndustriaapplicationsofSiO
x
Due to the limited theoretical specific capacity of graphite anods,
Si-based anodes have been highly attached in industrial
applications in recent years. Despite SiOx is unstable existence in
nature and require artificial synthesis, many efforts have been
made for SiOx due to its excellent cycling stability. We have
already introduced the synthesis of SiOx in the third section. The
research in the laboratory mainly focuses on button batteries,
pursuing high mass specific capacity and cycling stability, without
considering large-scale applications. LIBs are mainly used in
electric vehicles and portable electric vehicles, requiring high
loadings and energy density. In industrial applications, how to

Figure 10  Various methods for studying the properties of SEI. Reproduced with permission from Ref. [32], © American Chemical Society 2018; Ref. [95], ©
American Chemical Society 2021; Ref. [157], © Wiley-VCH GmbH 2023.
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achieve long cycling life with high loadings and active materials
contents is a key issue. As shown in Fig. 11, the current method
for industrial production of SiOx is to uniformly mix Si and SiO2,
generate gas-phase products through heat treatment, and then
condense to obtain SiOx materials, which are then subjected to
subsequent treatment.
The specific capacity of SiOx/C anodes for current
commercial application is mainly between 450‒500 mAh·g–1,
which is significantly higher than graphite of 372 mAh·g–1.
Actually, SiOx are limited in commercial LIBs due to low ICE and
unsatisfactory cycling performance. In order to meet the ever-
increasing market demands, industrial applications mainly adopt
carbon coating through CVD, pre-magnesium (Mg) and
prelithiation to improve the cycling stability and ICE, respectively
[30, 160]. In 2014, BTR began researching carbon coating
technologies. In fact, the technical difficulty of carbon coating
is not significant, while the requirements for equipments are
relatively high. Around 2018‒2019, BTR also achieved large-
scale production and improved product cost-effectiveness,
gradually being used in TESLA and other industries. Tesla’s
North American market currently mainly focuses on SiOx/C
produced by the first generation CVD carbon coating technology.
Both pre-Mg and prelithiation could effectively improve the ICE
of SiOx-based anodes, thereby achieving high energy density and
satisfy cycling performance. Pre-Mg, also known as Mg doped, has
been a concern for enterprises since 2000. Its difficulty of process
is relatively low, and it can increase the ICE of SiOx-based anodes
to 82%. Prelithiation applied in industrial production could
increase the ICE to over 90%, while it is limited by high cost and
technical difficulty. Herein, we introduced dry-cells+prelithiation,
which is expected to break through the bottleneck of large-scale
application of SiOx-based anodes. The traditional wet-electrode
technology involves high-speed stirring and mixing of active
materials, conductive agents, and binders with solvent 1-methyl-2-
pyrrolidinone (NMP), followed by coating the slurry on the
collector and drying. During the coating process, the solvent is
evaporated and removed. NMP is toxic and requires recovery,
purification, and reuse. The dry-electrode process is simpler,
without the use of solvents. The active material, conductive agent,
and solid binder (PTFE) are extruded into a thin film, and then
compounded with copper foil through hot rolling mode. Due to
NMP being a polar solvent that reacts with Li metal, preithiation
supplementation cannot be achieved in wet-electrode. The use of
dry-electrode technology does not require the addition of solvents,
and prelithiation technology could be implemented smoothly. The
additional Li could compensate for the Li consumed by the
formation of SEI film during initial charging process, thereby
improving the ICE and increasing the energy density of the
battery. Meanwhile, due to the fact that during the
charging/discharging process, the SEI film will continuely grow at
a small rate, result in consuming Li+. According to the Maxwell
dry-cells technology solution acquired by Tesla, battery
performance is expected to significantly improve. The use of dry
electrode technology is expected to increase the energy density of
the battery to 500 Wh·kg–1, extend the cycle life by one time,
increase production efficiency by 16 times, and reduce the
cost by 10% ‒20% compared with the most advanced wet
electrode technology.
In 2006, BTR began researching Si/C anodes and obtained its
first invention patent. At present, the company's annual
production capacity is 3000 tons, and the specific capacity of
SiOx/C has been increased to 1500 mAh·g–1. In 2013, BTR passed
the validation of Samsung SDI. Afterwards, the products
successfully entered the Panasonic-Tesla system. Pu-Tai-Lai has
been laying out SiOx products since 2014, and was the first to
cooperate with the Chinese Academy of Sciences to establish a
pilot plant in the Zichen factory in Jiangxi. The company has also
established a SiOx pilot line in Liyang. The planned production
capacity of SiOx anodes is 1000 tons, and it is expected to be mass-
produced by the end of 2022. Shanshan’s Si-based anode products
are mainly made of SiOx, which have been widely used in
consumer and small power markets. In the field of power battery
applications, it has passed multiple rounds of evaluation. At
present, the annual shipment volume is in the 100 ton level, with
the main customer being Contemporary Amperex Technology
Co., Limited (CATL). The large-scale commercial applications of

Figure 11  Schematic diagram of the industrial applications of SiOx, including characteristics of fundamental researches in labs and industrial applications, production
process in industry, the mainstream modified strategies, and key issues. Reproduced with permission from Ref. [30], © American Chemical Society 2016; Ref. [160], ©
American Chemical Society 2021.
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SiOx-based anods is steadily advancing. However, there are still
many inevitable problems, including the high cost and the
difficulties of high load and high content of active materials.
Compared to the cost of 8000 dollars per ton for graphite anodes,
the cost of tens of thousands of dollars per ton for SiOx anodes
limits their large-scale application. The high cost of Li has a
constraining effect on improving the ICE and energy density of
SiOx anodes. The power battery market has put forward
requirements for high energy density LIBs. This means higher
loadings and capacity, which often leads to unsatisfactory cycling
performance accompanied by pulverization of materials and
electrodes. Although there are still many difficulties urgent to be
solved before large-scale commercial applications, the Si-based
anodes are considered as the next industry trend, and requires the
joint efforts of researchers and enterprises.
6Conclusions
In summary, SiOx is an attractive material for LIBs due to its high
theoretical capacity of 2400 mAh·g–1, which is significantly greater
than graphite anodes. Compared with the silicon anode, SiOx-
based anodes have smaller volume expansion of Si and maintain a
stable structure during cycling. The lithium silicates and Li2O
formed during the first cycle not only act as buffer layers to
alleviate the volume expansion of active Si, but also effectively
isolate the materials from the electrolytes to prevent the
occurrence of side reactions. In recent years, SiOx-based materials
have been partially put into production, and will occupy a larger
market in the future. It is urgent to develop a suitable process for
large-scale production of SiOx-based materials. In this review, we
briefly summarized the different methods of synthesizing SiOx-
based materials. Ball milling, disproportion, and metal reduction
have unique strengths in large-scale preparation, and their
respective influencing factors have also been widely studied.
Although SiOx-based materials possess excellent capacity and
cycling stability, the non-negligible volume expansion and low
ICE caused by side reactions have seriously obstructed their
practical application. Herein, we proposed two feasible strategies
for the modification of SiOx-based materials: carbon composite
and porous structure design. We present three types of SiOx/C
composites. Among them, micro/nano-sized SiOx/C composites
can not only reduce the surface area without sacrificing the
superiority of nanostructure, but also form a conducting
framework among nanoparticles to improve electrical
conductivity. It provides unique eyesight into the preparation of
SiOx-based composites. We have tried to give some consideration
to techniques of SiOx/C composites and porous structure design.
We also summarized the studies other components of anodes and
LIBs. In addition, we reflected on the latest progress in advanced
prelithiation technologies, characterization technologies, and
analytical methods.
Despite the development of SiOx-based materials has obtained
remarkable achievements in a common effort, there is still a huge
gap for to the large-scale commercialization. The unsatisfactory
cycling stability and low ICE have not been completely resolved.
LIBs with high capacity and energy density are the demands of
future developments. According to data forecasts, the demands for
anode materials in the global LIBs market will reach 1.8 million
tons in 2023, and the output is expected to exceed 3 million tons
in 2025. The developments of SiOx-based anodes are of great
significance for LIBs with high density energy. Nowadays, most
companies grind and mix Si and SiO2 to obtain SiOx and carry out
subsequent processing. A small amount of SiOx have been mixed
with graphite anode to improve the capacity. It is worth noting
that when the amount of SiOx exceeds a critical value, the cycling
life of anodes will decline sharply and cannot meet the commercial
requirements. In order to promote the rapid development of high
energy density LIBs in the future, we believe that we should begin
with the following aspects:
(i) The low-cost and large-scale preparation of SiOx-based
materials with excellent electrochemical performance needs to be
further developed. Besides ball milling and disproportionation,
how to prepare SiOx-based materials on large-scale by metal
reduction still needs to be explored. Machine learning could
effectively accelerate the research and development of functional
materials. Machine learning be wide applied in the field of
chemistry and materials in recent-years and have the potential to
serve in the research of large-scale preparation of silicon-based
materials.
(ii) The modification strategies of SiOx-based anodes to solve
the issues of unsatisfactory cycling performance and low ICE
needs to be developed urgently. The ICE of SiOx is as low as
50%−60%, and the cycle performance still needs to be improved
due to the volume expansion that cannot be ignored. Therefore,
feasible, and scalable strategies such as composite with carbon and
prelithiation should be more considered. Besides, novel binders
and electrolyte additives should also receive the attention of
researchers.
(iii) The lithiation and delithiation mechanisms of SiOx
required to be comprehensively researched and serve to address
the issues, including but not limited to poor cycling life,
insufficient initial conductivity, and low ICE. In particular, the
formation of SEI on SiOx composites surface determines the
performance of the batteries. Scanning electron microscope (SEM)-
EDS, AFM, FT-IR, XPS etc. are common tools to obtain surface
information of batteries. ToF-SIMS techniques with high
sensitivity, can be successfully applied to detect the SEI layer and
determine the lithium depth profile during the first lithiation of
electrodes [161–164]. It gives a thorough insight on lithium
distribution into the electrode material and a deeply
understanding of electrode dynamics. Research on SEI
mechanism usually involved in-situ characterizations, including
TEM, NMR, etc. Cryogenic electron microscopy has been applied
in the study of SEI of Si, and has the potential to expand to the
study of SiOx system. Recently, the COMSOL Multiphysics
simulations are have been used to systemically investigate the Li
ions diffusion behaviors and variation tendency of stress [165,
166]. Therefore, the use of more advanced characterization
methods can be a powerful tool in advancing SiOx developments.
Although there still have a long way to go before performance
optimization and widespread commercialization of SiOx, we are
sure SiOx-based composites will play a crucial role for the anodes
of LIBs and make significant breakthroughs soon.
Acknowledgements
This work was supported partially by the National Key Research
and Development Program (No. 2022YFC3900905), the National
Natural Science Foundation of China (Nos. 52234001, 62104703,
and 52074119), the Science and Technology Planning Project of
Hunan Province (No. 2018TP1017), the Scientific Research Fund
of Hunan Provincial Education Department (No. 22A0045), the
Science and Technology Innovation Program of Hunan Province
(No. 2021RC1003), the Changsha Science and Technology
Foundation (No. kq2208162) and Joint Funds of Hunan
Nano Research Energy2023,2:e9120077 17


https://www.sciopen.com|https://mc03.manuscriptcentral.com/nre|Nano Research Energy

Provincial Innovation Foundation for Post-graduate (No.
CX20220512).
Declaration of conflicting interests
The authors declare no conflicting interests regarding the content
of this article.
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
Mengyu Zhang is currently studying for her Bachelor degree under the supervision of Dr. Lishan Yang at Hunan Normal
University. Her research interests focus on the design and fabrication of Si-based anode materials for Li-ion batteries.
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22 Nano Research Energy2023,2:e9120077
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Zuxin Chen earned his Ph.D. from Sun Yat-sen University in 2018. From 2018 to 2020, he conducted postdoctoral re-
search at the International Institute for Nanotechnology in Iberia, Portugal, before accepting an associate professor posi-
tion at South China Normal University in 2020. He has published over 30 SCI papers and is currently focused on research-
ing two-dimensional ferroelectric and ferromagnetic materials and devices.

Li-shan Yang has received his Ph.D. degree in inorganic chemistry from Shandong University (China) in 2011, and moved
to the Shandong University and then the Institute of Chemistry - Chinese Academy of Sciences as postdoctoral scholars.
He is currently a professor at National and Local Joint Engineering Laboratory for New Petrochemical Materials and Fine
Utilization of Resources of Hunan Normal University. His research interests are in commercial anode/cathode materials for
lithium-ion batteries and sodium-ion batteries.
Nano Research Energy2023,2:e9120077 23
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https://www.sciopen.com|https://mc03.manuscriptcentral.com/nre|Nano Research Energy