Recent Progress in Capacity Enhancement of LiFePO4 Cathode For Li-ion Batteries

Abstract

LiFePO4 (lithium iron phosphate, abbreviated as LFP) is a promising cathode material due to its environmental friendliness, high cycling performance, and safety characteristics. On the basis of these advantages, many efforts have been devoted to increasing specific capacity and high rate capacity to satisfy the requirement for next-generation batteries with higher energy density. However, the improvement of LFP capacity is mainly affected by dynamic factors such as low Li-ion diffusion coefficient and poor electrical conductivity. The electrical conductivity and the diffusion of lithium ions can be enhanced by employing novel strategies such as surface modification, particle size-reduction, and lattice substitution (doping), all of which lead to improved electrochemical performance. In addition, cathode pre-lithiation additives have been proved to be quite effective in improving initial capacity for full cell application. The aim of this review paper is to summarize the strategies of capacity enhancement, to discuss the effect of the cathode pre-lithiation additives on specific capacity, and to analyze how the features of LFP (including its structure and phase transformation reaction) influence electrochemical properties. Based on this literature data analysis, we gain an insight into capacity-enhancement strategies and provide perspectives for the further capacity development of LFP cathode material.

Full text

Zishan Ahsan
1
Key Laboratory of Green Fabrication and Surface
Technology of Advanced Metal Materials,
Ministry of Education,
School of Materials Science and Engineering,
Anhui University of Technology,
Maanshan 243000, Anhui, China
e-mail: zishanahsan87@yahoo.com
Bo Ding
1
Key Laboratory of Green Fabrication and Surface
Technology of Advanced Metal Materials,
Ministry of Education,
School of Materials Science and Engineering,
Anhui University of Technology,
Maanshan 243000, Anhui, China
e-mail: www024151@163.com
Zhenfei Cai
Key Laboratory of Green Fabrication and Surface
Technology of Advanced Metal Materials,
Ministry of Education,
School of Materials Science and Engineering,
Anhui University of Technology,
Maanshan 243000, Anhui, China
e-mail: zhenfei_cai@163.com
Cuie Wen
School of Engineering,
RMIT University,
Bundoora,
Melbourne 3083, Victoria, Australia
e-mail: cuie.wen@rmit.edu.au
Weidong Yang
Future Manufacturing Flagship,
Commonwealth Scientific and Industry Research
Organization,
Melbourne 3168, Victoria, Australia
e-mail: weidong.yang@csiro.au
Yangzhou Ma
2
Key Laboratory of Green Fabrication and Surface
Technology of Advanced Metal Materials,
Ministry of Education,
School of Materials Science and Engineering,
Anhui University of Technology,
Maanshan 243000, Anhui, China
e-mail: yangzhou.ma@outlook.com
Shihong Zhang
Key Laboratory of Green Fabrication and Surface
Technology of Advanced Metal Materials,
Ministry of Education,
School of Materials Science and Engineering,
Anhui University of Technology,
Maanshan 243000, Anhui, China
e-mail: shzhang@ahut.edu.cn
Recent Progress in Capacity
Enhancement of LiFePO
4
Cathode
for Li-Ion Batteries
LiFePO
4
(lithium iron phosphate (LFP)) is a promising cathode material due to its environ-
mental friendliness, high cycling performance, and safety characteristics. On the basis of
these advantages, many efforts have been devoted to increasing specific capacity and
high-rate capacity to satisfy the requirement for next-generation batteries with higher
energy density. However, the improvement of LFP capacity is mainly affected by
dynamic factors such as low Li-ion diffusion coefficient and poor electrical conductivity.
The electrical conductivity and the diffusion of lithium ions can be enhanced by using
novel strategies such as surface modification, particle size reduction, and lattice substitu-
tion (doping), all of which lead to improved electrochemical performance. In addition,
cathode prelithiation additives have been proved to be quite effective in improving initial
capacity for full cell application. The aim of this review paper is to summarize the strategies
of capacity enhancement, to discuss the effect of the cathode prelithiation additives on spe-
cific capacity, and to analyze how the features of LFP (including its structure and phase
transformation reaction) influence electrochemical properties. Based on this literature
data analysis, we gain an insight into capacity-enhancement strategies and provide per-
spectives for the further capacity development of LFP cathode material.
[DOI: 10.1115/1.4047222]
Keywords: carbon coating, cathode prelithiation additives, doping, Li-ion battery, lithium
iron phosphate, batteries
1
The authors contributed equally to the paper.
2
Corresponding authors.
Manuscript received January 6, 2020; final manuscript received March 8, 2020;
published online June 8, 2020. Assoc. Editor: Leela Mohana Reddy Arava.
Journal of Electrochemical Energy Conversion and Storage FEBRUARY 2021, Vol. 18 / 010801-1
Copyright © 2020 by ASME
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Guangsheng Song
2
Key Laboratory of Green Fabrication and Surface
Technology of Advanced Metal Materials,
Ministry of Education,
School of Materials Science and Engineering,
Anhui University of Technology,
Maanshan 243000, Anhui, China
e-mail: song_ahut@163.com
Muhammad Sufyan Javed
Siyuan Laboratory,
Department of Physics,
Jinan University,
Guangzhou 510632, China
e-mail: safyanjaved@yahoo.com
1 Introduction
Li-ion batteries (LIBs) have the potential to provide a break-
through to power electric vehicles, which requires both high-energy
density and high-power density simultaneously [1,2]. The cathode
materials with higher capacities, higher operating voltage, and
high current density have been studied extensively [3,4]. In this
regard, lithium-rich layered oxides Li [Li,Mn,Ni,Co]O
2
, which is
either solid solution or a fine-phase mixture of layered Li
2
MnO
3
and Li(TM)O
2
(TM =Ni, Co, Mn) (e.g., NMC 111, NCM 523,
NCM 622, and NCM 811) with excess lithium ions in the transi-
tion metal layer can deliver high capacities of ∼250 mAh/g when
charged more than 4.5 V [5,6]. However, Li-rich layered oxides
(e.g., (Li
1.2
Ni
0.13
Co
0.13
Mn
0.54
O
2
), (Li
1.2
Ni
0.13
Co
0.13
Mn
0.54
O
2
),
(Li
1.20
Mn
0.48
Ni
0.16
Co
0.16
O
2
), etc.) still have several challenges
which limit its commercial application, such as voltage decay,
capacity fade, poor rate capacity, the initial irreversible capacity
loss, and O
2
emission [7–10]. Also, lithium- and manganese-
rich (LMR) nickel-manganese-cobalt layered composites (e.g.,
(Li
1.2
Ni
0.15
Co
0.1
Mn
0.55
O
2
), Li[Li
0.2
Mn
0.54
Ni
0.13
Co
0.13
]O
2,
(Li
1.2-
Ni
0.16
Mn
0.56
Co
0.08
O
2
)) exhibit higher capacities (>240 mAh/g)
but experience continuous capacity/voltage decay during cycling
[7,11–17]. The spinel LiMn
2
O
4
(space group Fd3 m) belonging to
the Li-Mn-O system contains octahedral-coordinated Mn-cations
and Li-cations in tetrahedral positions of a cubic-closed-packed
O
2−
lattice has 3D channels for Li-ion diffusion, which is very
important for providing a high-power density [18]. However, high-
voltage doped LiMn
2
O
4
(e.g., LiNi
0.5
Mn
1.5
O
4
) has been investi-
gated extensively, but doped LiMn
2
O
4
suffers from continuous
capacity fade owing to Jahn–Teller distortion and Mn dissolution
into electrolyte [1,6]. Moreover, non-intercalation type materials
such as sulfur-based cathodes can deliver higher theoretical capac-
ities (>1000 mAh/g) but have stability limitations including large
volume expansion upon discharge, polysulfide dissolution, shuttle
effect, and insulation problems, which lead to inferior cycling per-
formance and poor capacity retention [19–23].
As a promising cathode material, lithium iron phosphate (LFP)
has been widely studied for powering Li-ion batteries due to its
good cycling and thermal stability, high-energy density, and envi-
ronmental friendliness, as well as low cost. However, its intrinsic
poor electronic conductivity (10
−9
–10
−10
S/cm) and low Li-ion dif-
fusion coefficient (10
−14
cm
2
/s) have restricted its extensive appli-
cation [24–26]. LFP cathode materials exhibit sluggish kinetics in
their Li-ions and electrons in comparison with most conventional
cathode materials because of the structural limitations. The theoret-
ical capacity (170 mAh/g) and the tap density of LFP are lower than
those of some other common cathode materials [27,28]. Unfortu-
nately, it is very challenging to achieve the theoretical value of
capacity (170 mAh/g) of LFP because of its intrinsic slow Li-ion
diffusion and very low electronic conductivity. The specific
capacity of commercial carbon-coated nanosized LFP (10–
100 nm) cathode is typically 120–160 mAh/g [29]. So, a lot of
research is focused on LFP to increase its capacity and energy
density as well, without compromising its structural stability. The
structure of LFP makes it a potential candidate as a cathode for
Li-ion batteries. The structure of LFP is shown in Fig. 1; the PO
4
tetrahedron is located between the LiO
6
octahedron and the FeO
6
octahedron, with only narrow one-dimensional “holes”formed for
Li-ion diffusion, which restricts the Li
+
intercalation and
de-intercalation during the processes of charging and discharging
[30].
Lithium intercalation and de-intercalation occur through a two-
phase reaction with LiFePO
4
and FePO
4
as end members. Both
LiFePO
4
and FePO
4
have the same structure, and the Li-ions in
the lattice of LiFePO
4
can only travel along the [010] direction
(b-axis) of the structure because no continuous LiO
6
octahedra
exist along with the directions of aand c-axis [26]. Also, Islam
et al. [31] calculated through first-principles that one-dimensional
diffusion channel with the lowest activation energy path was
along [010] direction for Li-ion diffusion. Hence higher mobility
along [010] direction due to low energy barrier compared with
the other two transport channels which have higher energy barriers.
Compared with other cathode materials such as layered cathode
materials which have two-dimensional Li-ion diffusion and spinel
structured cathode materials LiMn
2
O
4
(e.g., LiNi
0.5
Mn
1.5
O
4
)
which have three-dimensional Li-ion diffusion channel, olivine
LFP has one-dimensional diffusion [32–36]. So, the diffusion of
lithium ions in LFP is not only insufficient but also easily influenced
by defects [37,38].
During the electrode reactions, lithium ions and electrons both
migrate in certain directions. For LiFePO
4
, during the charging
process, lithium ions de-intercalate from the crystal of LiFePO
4
,
move toward the anode through the electrolyte. Simultaneously,
electrons also depart from LiFePO
4
to maintain the electric neutral-
ity along with the exportation of Li-ions (the discharge step is
reverse). The time taken by all the movable lithium ions to
de-intercalate from a certain LiFePO
4
crystal is mostly affected
by the diffusion dimensionalities, path patency, and length of
lithium ions in the LiFePO
4
(or FePO
4
) crystal. If time is long
either due to limited diffusion dimensionality or the path is long
and not unobstructed, lithium ions will not migrate efficiently to
maintain the electrode reactions which cause concentration polari-
zation. If the conductivity is not high enough, which means the elec-
trons cannot transfer efficiently enough to sustain the electrode
reactions, electrochemical polarization occurs [39]. In relation to
Li-ion insertion/extraction in the host LFP, to find out the LFP
electrode intrinsic kinetics during charge/discharge processes, the
diffusion of Li-ions needs to be determined. It is known that the dif-
fusion of Li-ions in LFP is a one-dimensional phenomenon along
the b-axis [40] and Li-ion intercalation/de-intercalation in LFP is
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a two-phase transition phenomenon. This two-phase transition reac-
tion is described by several models, such as the shrinking-core [41],
mosaic [42], core-shell [43], and domino-cascade [44] models. In
the shrinking-core model proposed by Srinivasan and Newman,
the lithium de-intercalation would occur from the surface of the par-
ticle to the center. On contrary, Laffont et al. [43] investigated by
using high-resolution transmission electron microscopy coupled
with electron energy loss spectroscopy that FePO
4
remains in the
core of the particle and LFP on the shell of chemically delithiated
nanosized particles. This experimental study established the core-
shell model. Delmas et al. [44] developed a different approach
called the domino-cascade model, which describes the existence
of single-phase at the particle level, while a two-phase repartition
at the scale of the electrode and shows the influence of these collec-
tive effects in the case of electrochemical delithiation. So, the par-
ticle size is associated with the existence of solid-solution and
two-phase transition mechanism, which can severely influence the
electrochemical properties of the cathode material [45].
In order to improve the capacity and rate performance of LFP,
several effective approaches have been used such as carbon
coating, cation doping, size, and crystal morphology control and
the addition of prelithiation additives. Since the diffusion of
Li-ions takes place along the [010] direction, the first approach of
particle size reduction and morphology control has been confirmed
very effective to improve the rate performance of LFP. Therefore,
the controllable synthesis of nanoscale particles is key to improve
electron conduction and Li-ion diffusion. The second approach
includes surface modification (both coating and making compos-
ites). Surface modification is one of the most efficient methods
which can completely alter the cathode properties. The require-
ments for surface modification include high stability and high elec-
tronic conductivity. For this purpose, electronic conductor materials
are preferred due to the low conductivity of LFP. Third, ion doping
can expand the Li
+
diffusion pathways and is expected to enhance
the voltage, electronic conductivity, and the charge/discharge prop-
erties at high current densities. Fourth, to compensate for the loss of
lithium due to the SEI layer formation at the anode surface, the
effect of the addition of cathode prelithiation additives on specific
capacity is discussed in detail. Fig. 2illustrates the typical strategies
for capacity enhancement of LFP.
It is a fact that the performance of active materials is strongly
influenced by their synthesis methods. The traditional method for
LFP synthesis is the solid-state reaction, which consists of two
heating stages, each of which can take between 5 and 24 h for the
generation of desirable solid phases under an inert atmosphere to
avoid the iron oxidation [46]. For the solid-state reaction method,
the common precursors for iron and lithium are carbon-based mate-
rials such as Fe (II)-acetate or Fe (II)-oxalate (as iron and carbon
sources), Li
2
CO
3
(as both lithium and carbon precursors), and
ammonium phosphate as a phosphate source. The solid-state synth-
esis has limitations, including the need for a long sintering process
at a high temperature, controlling particle size, agglomeration, and
the final product with large particle sizes and irregular morpholo-
gies. As compared with the traditional solid-state method, solution
chemistry methods such as hydrothermal, sol-gel, spray pyrolysis,
and co-precipitation have recently attracted considerable attention
for their advantages such as intimate mixing of starting materials
at the atomic level, homogeneous nucleation of fine particles with
high purity, introducing carbon source into the synthesis route,
and tailoring the size and morphology of nanostructure. In addition,
starting materials, which are not suitable for solid-state reaction
(e.g., iron chloride and iron sulfate), can be successfully used in
solution methods because Cl
−
and SO
4
2˗
can be dissolved and
removed with the solution. Also, some impurities can also be
removed with the solution, which lowers the fabrication costs and
the requirement of purity for the precursors [47].
We know that LFP is a commercially used material and the extent
of its application is limited by its low capacity (compared with other
cathode materials). In order to understand how to achieve and
exceed the theoretical value of 170 mAh/g, the strategies for perfor-
mance improvement of the cathode material LFP are discussed in
this review paper with the aim of enlarging industrial application.
2 Morphology and Particle Size Control
Through morphology and particle size control, the crystal struc-
ture of LFP is modified by shortening the pathways for Li-ion diffu-
sion and optimizing well-oriented {010} facets, and this can be very
effective to improve the rate capacity of LFP. The reduction of LFP
particles to the nanosize allows high-power density, which is mainly
ascribed to shortened pathways for Li
+
diffusion inside the 1D
channel of LFP. The fabrication of well-oriented nanometric phos-
pholivine LFP is usually carried out by low-temperature processes
occurring in solvothermal, hydrothermal, and co-precipitation
methods, compared with conventional dry synthesis (solid-state)
Fig. 1 The crystal structure of olivine LFP along [001] projection [30]
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methods. Due to the strong influence on morphology, defect chem-
istry, and the properties of LFP, precise control of the synthesis
parameters (reactant concentration, reactant sources, reaction
time, reaction temperature, reaction pH, and reaction procedure)
is essential [48]. However, particle size reduction adversely
affects the volumetric energy density, as the increased surface
area requires a higher amount of binders and causes unwanted reac-
tions, all of which lead to poor cycling life. Considering these lim-
itations in reducing the particle size to nanoscale, the fabrication of
micro-nanostructure materials is currently a feasible approach for
improving LFP performance as it can take full advantage of both
nanoparticles as well as micro-sized structures. LFP nanoparticles
can lead to a high-rate performance by shortening pathways for
Li
+
diffusion, and micro-sized structures promise the benefits of
both high volumetric energy density and cycling performance
[49]. Similarly, Chen et al. [50] synthesized the porous micro-nano
structured, starfish-like LFP/C via the solvothermal route and
reported high-rate capabilities (157.5, 113.7, 86.7 mAh/g at 1 C,
10 C, and 20 C, respectively) and excellent cycling stability
(98.4% after 100 cycles at 1 C) as shown in Figs. 3(a)and 3(b).
Since Li−Fe antisite defects can limit the electrochemical perfor-
mance of LFP via blocking the Li-ion diffusion pathways (010),
Huang et al. [52] showed the effects of temperature and time of
heating on the structure and electrochemical performance of the
LFP. The LFP formation reaction occurred at a very low tempera-
ture of 89 °C (using ethylene glycol (EG) as solvent) and carbon-
coated nano LFP materials synthesized after 4 h of solvothermal
treatment (at 180 °C) showed discharge capacities of 160.6 mAh/
g at current rate of 0.1 C and of 129.6 mAh/g at 10 C. Defect anal-
ysis results showed that the concentrations of lithium vacancies and
Li−Fe antisite defects were too low to be detected after 4 h of sol-
vothermal treatment.
In order to show how solvent composition may effect the mor-
phology and electrochemical performance of LFP particles, Ma
et al. [53] investigated that the morphology of LFP particles gradu-
ally transformed from nanoplates to hexagonal prism nanorods as
the water content in the solvent increased.. The LFP@C rectangular
prism nanorods exhibited superior electrochemical performance
(163.8 mAh/g) to those of LFP@C nanoplates (153.3 mAh/g) and
hexagonal prism nanorods (144.4 mAh/g) due to the reasonable
size and short Li-ion diffusion distance along [010] direction. In a
similar work, Yao and coworkers analyzed the effect of concentra-
tion of starting materials on LFP morphology via a facile solvother-
mal method. It was found that the shape of the LFP particles
changed with the precursor concentration from 0.15M to 0.90M,
but their shape was unchanged as nanoplates when prepared at dif-
ferent reaction temperatures and residence times with 0.30M pre-
cursor. The comparison of LFP/Cmaterials synthesized at various
precursor concentrations showed that the LFP/C nanoplates synthe-
sized in the optimum condition of the 0.30M precursor solution
showed a high-rate performance of 140.8 mAh/g at 10 C, excellent
rate capability, and cycling stability [54].
Since Li-ions preferentially migrate along the [010] facet in LFP,
Zhao et al. [55] reported on the synthesis of single-crystalline
LFP nanosheets with well-oriented [010] facets. The [010] ori-
ented LFP nanosheets showed high-specific capacities of 151 and
140 mAh/g at low charge/discharge rates of 0.5 C and 1 C, respec-
tively. These higher capacities were ascribed to the easy Li
+
diffu-
sion along the [010] direction. However, the [100]-oriented LFP
exhibited poor electrochemical performance along with low
Fig. 2 Illustration of typical strategies in LFP cathode
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discharge capacity as well, due to the excessively high Li
+
migra-
tion energy barrier along the [100] direction [56]. In contrast,
Wang et al. reported on the production of 12 nm thick
[100]-oriented LFP nanoflakes by the solvothermal method. The
LFP/C composite showed unexpectedly excellent performance,
delivering reversible capacities of 164 and 122 mAh/g at discharge
rates of 0.1 C and 20 C, respectively, and cycling stability of 90%
after 1000 cycles at 10 C. This surprisingly high performancea of
[100]-oriented LFP defies the belief that the migration of Li
+
along the [100] axis is synonymous to poor electrochemical perfor-
mance and encourages the new idea of Li
+
migration along [100]
direction [57].
The hollow structure of LFP not only offers short Li-ion diffusion
pathways but also offers the void space to accommodate the stress
induced by volume changes during charge/discharge process and
inreases the fatigue resistance, improving the stability of LFP elec-
trode. Zheng et al. [58] fabricated hollow LFP nanoparticles via the
solvothermal method, using ammonium tartrate as the additive and
EG/water as the solvent. The hollow structure not only supports
lithium diffusion by offering pathways but also bears the stress of
volume changes during charging/discharging, so offering high
capacity retention (stability). The hollow-structured LFP cathode
exhibited the high performance of 165.2 mAh/g at a rate of
0.1 C. The better rate capacity and cycling stability was ascribed
to hollow structure, small particle, and grains sizes, and relatively
shorter parameter bshortens Li-ion diffusion channels and hence
leading to larger Li-ion diffusivity. The synthesis of hollow LFP
by Yang et al. delivered a reversible capacity of 101 mAh/g at
20 C and showed capacity retention of 80% after 2000 cycles [59].
Moreover, Yoo et al. [51] investigated special morphology by syn-
thesizing porous supraparticles of LFP nanorods and carbon
through spray-drying a mixture of LFP nanorods and glucose and
subsequent heat treatment, as shown in Fig. 3(c). The electrochem-
ical properties of the porous LFP/C supraparticles were better com-
pared with LFP/C nanoparticles at a higher current density; the
discharge capacity at 10 C was reduced only by 9% and, even at
50 C, the supraparticle-based electrode showed a surprisingly high
capacity of ∼126.58 mAh/g, which was almost 80% of the initial
capacity of ∼160.15 mAh/g. Also, the packing density of LFP/C
supraparticles (1.07 g/cm
3
) was higher than that of the nanoparticles
(0.78 g/cm
3
) due to their spherical shape.
Another special morphology of LFP is shown in Fig. 4, Wang et al.
[60] fabricated ultrafine ∼6.5 nm LFP quantum dots (LFP-QDs)
co-modified by graphene and amorphous carbon through a novel
micro-reactor strategy. The nanocomposite prepared by this strategy
had the properties of a large surface area, abundant active sites for
faradic reactions, and ideal kinetics for electron as well as ion trans-
port; and therefore, had outstanding electrochemical performance,
including ultrahigh-rate performance (78 mAh/g at 200 C) as well
as excellent cycling stability (99% over 1000 cycles at 20 C).
Fig. 3 (a) Charge/discharge profiles of LFP and LFP/C of starfish-like micro-nanoparticles for the first cycle at 0.2 C, 0.5 C,
1.0 C; (b) discharge capacities at various C rates of LFP/C micro-nanostructure starfish-like porous LFP/C supraparticle [50];
and (c) schematic illustration of porous LFP/C (LFP/C) supraparticle synthesis [51]
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In another study, Zhang et al. prepared high-energy quantum
dots, embedded in mesoporous biocarbon coated LFP nanosheet,
through a biotemplate and carbon thermal reduction method
(using high-energy biomolecule ATP). This exhibited the best
first reversible capacity of 197 mAh/g at 0.1 C beyond the theoret-
ical value of LFP and ultrahigh coulombic efficiency ∼100%. Even
after 100 cycles, it still delivered a high reversible capacity of 180
mAh/g at various current rates of 0.1 C, 1 C, 5 C, and 10 C. This
outstanding performance was ascribed to the quantum tunneling
of high-energy QDs in LFP nanoparticles and improved percolation
of the coating network structures [61]. The interfaces of high-
energy QDs (<10 nm) act as pathways for fast Li
+
ions transport
and also provide more active sites for Li- ions storage in the
LiFePO
4
, and hence, enhance the Li-ion insertion and extraction
capability, leading to extra storage of Li
+
ions and a higher capacity.
From the above discussion, it can be concluded that the Li-ion dif-
fusion can be improved to some extent by reducing the particle size of
LFP to the nanoscale, fabricating porous structures, or through mor-
phology control (shorter Li
+
ion diffusion length along the [010]
direction) due to the shortened length for Li-ion transfer within the
particles. Consequently, these strategies to overcome ionic diffusion
limitations improve the high-rate capacity and cycling life of LFP
materials. Reducing particle size to nanoscale is a commonly used
strategy to improve the rate capability, but it directly causes a
decrease in volumetric energy density, which is not desirable as
the theoretical density (3.68 g/cm
3
) of LFP is lower compared with
other cathode materials (e.g., 5.1 g/cm
3
for LiCoO
2
, 4.8 g/cm
3
for
LiNiO
2
), so novel methods (not solely relying on nano routes)
need to be explored which enhance the rate performance while
keeping the tap density at an acceptable value [62].
3 Surface Coating and Modification
3.1 Carbon-Coated Cathode Material. The basic purpose of
using a carbon coating layer is to cover the LFP particles with a
uniform layer of coating to facilitate the movement of electrons in
order to improve the surface conductivity and utilize the active
material at high rates [63]. Coating of LFP improves its electrical
conductivity, prevents surface degradation, prevents active material
from participating in a chemical reaction by avoiding the direct
contact of the electrolyte with the active material, and restricts the
growth of the crystal [64]. Carbon is the most commonly used mate-
rial for coating LFP cathodes, for its high electronic conductivity
and stability, as well as low cost. Moreover, the addition of
carbon can play the role of the nucleating agent in the LFP
cathode material to suppress the growth of LFP grains, which
results in enhancement of the ionic conductivity by narrowing the
diffusion path of Li
+
. Also, adding or coating with a conductive
carbon can also play the role of reducing agent to prevent the oxi-
dation of Fe
2+
[65–67].
The carbon addition process includes the mixing of battery mate-
rials with various carbon precursor, followed by heat treatment at
high temperatures. The performance of carbon-coated LFP strongly
depends on three parameters: (1) thickness, (2) degree of graphitiza-
tion, and (3) porous structure of the coating layer. It is difficult to
control the thickness of the coating layer and improve carbon
quality. Too thick of a carbon coating layer hinders the diffusion
of Li
+
and decreases the volumetric energy density of battery mate-
rials, while too thin a carbon coating does not uniformly cover the
active material. Commonly, the carbon sources used for coating
include organic (e.g., citric acid, lactose, and glucose) and inor-
ganic (e.g., acetylene black (AB), super P, carbon nanotubes
(CNTs), and graphene) precursors. Along with inorganic carbon
materials such as CNTs and graphene which can form a 3D con-
ductive network in LFP, organic carbon materials may still be
needed to form local conductive paths between particles in the
LFP electrode (particle surfaces). Usually, organic compounds
offer the advantage of a uniform coating layer structure (homoge-
neity, thickness, full coverage) on LFP and can be easily trans-
formed into carbon during the pyrolysis process, but the carbon
quality (including conductivity and graphitized degree) is difficult
to control. Inorganic carbon sources offer the opposite advantage
and disadvantages [65–68].
The combination of both organic as well as inorganic carbon mate-
rials is more promising for high-performance C/LFP composites. It is
also hard to find the optimum amount of carbon, as this can vary with
geometrical shape, size, and the chemical structure of LFP electrodes
even for the same carbon source [69–71]. Based on the idea of using
both organic and inorganic sources, Ding et al. synthesized a dual-
carbon network-based LFP via the solvothermal method followed
by postsynthesis heat treatment. LFP/G–C delivered a capacity of
160 mAh/g at the rate of 0.5 C, and discharge capacities of 124
and 104 mAh/g at very high rates of 30 C and 50 C, respectively.
The high-rate performance of LFP/G–C was ascribed to the synergis-
tic effect of two interconnected electron-conductive networks, a
primary transport network of connected graphene sheets that func-
tioned as an extended current conductor and a secondary transport
network of continuous nanoscale carbon coating that rendered
rapid charge insertion and extraction from all nanocrystal surfaces
[72]. Jiang et al. [73] fabricated LFP@C/reduced graphene oxide
(rGO) composites via an EG assisted solvothermal method with a
subsequent carbon coating. The composite exhibited discharge
capacities of 148 mAh/g and 129 mAh/g at 1 C and 20 C, respec-
tively, and cycling stability of 100% after 200 cycles at 10 C. The
improved electrochemical performance was attributed to the syner-
gistic effect of the 3D conductive network architecture of the rGO
and the carbon coating layer; see Fig. 5(a).
Fig. 4 Schematic synthesis procedure for G/LFP-QDs@C via a novel micro-reactor approach [60]
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Lei et al. [76] also synthesized a 3D LFP/CNT/graphene compos-
ite by the solid-state method and its discharge capacities were
168.9 mAh/g at 0.2 C and 115.8 mAh/g at 20 C, close to the theo-
retical value. In a study to compare the effect of carbon sources on
coating film quality and thickness, Meng et al. [77] coated LFP
using ionic liquid (IL) [BMIm][N(CN)
2
] and glucose as carbon pre-
cursors. It was found that the electrode material synthesized using
IL as carbon precursor exhibited superior electrochemical perfor-
mance, with a reversible capacity of 163.8 mAh/g at 0.1 C and
capacity retention of 160.6 mAh/g after 50 cycles, compared with
glucose as carbon source, which was ascribed to uniform, highly
graphitized, and ultrathin carbon films (1–2 nm).
Yi et al. [78] prepared a 3D porous graphene conductive net-
work with confined (010) oriented LFP nanoflakes by a facile
template-free concentrated gel method. LFP@G composite exhib-
ited improved Li-storage properties, high-rate reversible capacity
of 129 mAh/g at 20 C, long cycling stability with efficiency
∼99.8% over 600 cycles up to 10 C and even the capacity of
163 mAh/g was restored when rate was reduced to 0.2 C. The supe-
rior electrochemical performance was attributed to improved con-
ductivity of Li-ions and electrons due to 3D porous structure of
graphene and exposed (010) planes of LFP. Lithium ions preferen-
tially migrate along the (010) facet in LFP because of the lower
lithium insertion and extraction potential, and short Li-ion diffusion
distance, which allows electrons and the lithium ions to move faster
along this facet. Wang et al. [79] studied the effect of carbon content
on preferred crystal orientation and found a transformation of
crystal planes from the (010) to the (100) with the increase in
carbon from 1.65 to 6 wt%. The sample coated with the optimum
value of carbon content (1.65 wt%) showed the best initial revers-
ible capacity of 162 mAh/g and efficiency of 95.4% at 0.1 C,
which was ascribed to the low R
sf
value and the preferential orien-
tation of (010) plane.
Carbon addition can control the particle size, growth mechanism,
and structure during the synthesis and increase electronic conduc-
tivity, thus higher capacity and better rate capacity of cathode mate-
rials can be achieved [80]. In order to achieve better rate capacity,
Guan et al. [81] prepared a hierarchical porous architecture by com-
positing capacitive behavior material, activated carbon (AC), with
LFP and graphene (G). This LFP/AC/G cathode ensured high-rate
performance (both abundant Li
+
diffusion pathways and fast elec-
tron transfer) with high-efficiency capacitive-battery characteristics
and porous structure arising from AC and conductive graphene
network. The LFP/AC/G cathode exhibited a remarkably high
capacity of 66 mAh/g at a very high discharge rate of 100 C, and
high cycling stability of 82% after 3000 cycles, assuring higher
power density.
Carbon coatings have the drawbacks of high processing cost and
low tap density, which may cause reduced energy density of the full
battery cells. To avoid a decrease in tap density, Cao et al. [82] pro-
posed the in situ synthesis of special morphology of 3D porous
C@LFP/G composite microspheres modified from carbon-coated
LFP nanoparticles along with embedded graphene via the one-pot
solvothermal method followed by heat treatment. C@LFP/G deliv-
ered a high reversible capacity of 163.7 at 0.1 C and good rate capa-
bilities of 137.3, 107.1, and 94.9 mAh/g at the current rate of 1 C,
5 C, and 10 C, respectively. In conclusion, it is highly desirable to
prepare carbon coating or composite with a porous structure,
uniform thickness, and a high degree of graphitization.
Hu et al. [29] modified LFP with RGO, the capacity of which
went beyond the theoretical limit of specific capacity (i.e.,
208 mAh/g at 0.1 C). In this case, the additional capacity was
ascribed to the reversible redox reaction between the Li
+
of electro-
lyte and RGO. The conductive graphene flakes wrapped around the
carbon-coated LFP also assisted electron transport during the
charge/discharge processes, diminished the irreversible capacity
loss, and hence resulted in ∼100% coulombic efficiency at varied
C rates. However, the additional capacity from the interfacial
lithium storage did not contribute to the capacity at high rates due
to sluggish diffusion of Li-ions of LFP. Owing to this, Zhang
et al. reported (010) orientated LFP nanorods (LFPNR) encapsu-
lated with conformal double layers consisting of N-doped carbon
and RGO with capacity beyond the theoretical value of 170 mAh/
g along with ultrahigh-rate performances (147.3 mAh/g at 10 C,
Fig. 5 (a) Schematic illustration of a 3D electron transport network of LFP@C/rGO composite [73]; (b) schematic of
LFP with N +B type co-doped carbon coating. Reprinted with permission from Ref. [74] Copyright 2015 American
Chemical Society; and (c) schematic illustration of electronic/ionic hybrid coating for LFP cathode material during
charge and discharge phenomenon [75].
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which is a higher than any already reported carbon-modified LFP)
and ultra-long cyclability (retaining 95.8% capacity after 1000
cycles at 10 C) [83].
Recently, Zhao et al. [28] synthesized a glucose-derived carbon
(GC) encapsulated LFP composite with internal carbon (IC)
sheets, GC/IC/LFP, by using phytic acid (Phy A) not only as a phos-
phorus source but also an IC precursor, via a hydrothermal–alcohol
washing–calcination process. The cathode material GC/IC/LFP
showed an excellent cyclability, an ultrahigh capacity of
192 mAh/g at 0.1 C and a high-rate performance of 140 mAh/g at
10 C. The extra capacity was attributed to reversible redox reaction
between the Li-ions of electrolyte and the oxygenic groups at the
defects in the ICs. This means that the oxygenic groups act as inter-
facial storage sites, which explains the reason for the excess
capacity.
Table 1displays the electrochemical performances of LFP with
various carbon sources (both organic and inorganic).
3.2 Modifying Carbon Coating Layer. Carbon coating is
considered one of the most effective ways to improve the electro-
chemical performance of electrode materials. However, the excel-
lent electrochemical performance of LFP materials cannot be
achieved only through pure carbon coating [96]. Further modifica-
tion into the carbon-coating layer via nitrogen, sulfur, phosphorus,
and boron doping provides a feasible approach to optimize the
carbon coating of LFP [97]. Because of the hydrophilic properties
of nitrogen, the nitrogen-doped carbon layer provides better disper-
sion of carbon materials and avoid LFP aggregation; thus, treated
LFP has proven to exhibit superior rate performance and cycle sta-
bility and enhanced electric conductivity of LFP. So, nitrogen
doping can improve the electronic conductivity by providing elec-
tron carriers for conduction band and enhancing the Li-ion diffusion
by inducing defects, thus lowering the energy barrier for Li-ion dif-
fusion kinetics [98,99]. Similarly, Feng and Wang first used boron-
doped carbon coating to improve the performance of LFP. The
LiFePO
4
@B
0.4
-C composite exhibited outstanding properties
including the reversible specific capacity of 164.1 mAh/g at 0.1C,
enhanced Li storage property, and carbon conductivity by increas-
ing the number of hole-type charge carriers [97]. Since F atom has a
higher electronegativity (4.0) than the other anions N (3.0), S (2.5),
B (2.0), and Cl (3.0), therefore F-doping would show great potential
for enhancing the electrochemical performance of LFP, especially
by combining carbon-coating and F-doping to form C-F covalent
bonds. Due to the presence of LiPF
6
in the electrolyte, C-F covalent
bonds are beneficial for the permeation of electrolyte. In this regard,
most recently Wang et al. [100] reported the fluorine-doped carbon
(FC) coating on pristine LFP (LFP@FC), using polyvinylidene
fluoride (PVDF) as a C and F source via ball-milling assisted rheo-
logical phase method combined with a solid-state reaction [94],
which manifests highly desirable electrochemical performance,
including a high discharge capacity of 174.3 mAh/g at 0.1 C
higher than the theoretical capacity, an attractive cycling stability
over 1000 cycles, and an impressively high-rate performance of
100.2 mAh/g at 20 C. The unique structure of FC and the presence
of defects could offer some redox-active sites for lithium storage
(this storage phenomenon is reversible) and enhance the rate capa-
bilities of LFP [29].
Li et al. successfully prepared boron and nitrogen individually
doped and co-doped LFP/C materials via the simple ball-milling
process with subsequent heat treatment. The boron and nitrogen
co-doped LFP/C materials showed that electrochemical perfor-
mance was susceptible to the addition sequence of nitrogen and
boron. LFP with N +B type co-doped carbon coating exhibited
significant improvement in performance regarding high current
rates and the retention rates as well [74]. The schematic of LFP
with N +B type co-doped carbon coating is shown in Fig. 5(b).
Table 2summarizes electrochemical performances of LFP coated
with a doped carbon layer.
3.3 Electronic and Ionic Conductive Coating Material.
Carbon coating mainly contributes to electronic conductivity, but a
charge balance is required, so it is necessary to improve ion diffusion
Table 1 Comparison of electrochemical performances of LFP (both organic and inorganic sources)
Carbon- coated
cathode Synthesis process Coating material/wt%
Electrochemical performance
(mAh/g) Capacity retention Ref.
C-LFP DAP Sucrose/3 wt% 167 at 0.1 C 98% after 50 cycles at 0.1 C [84]
LFP@C/rGO Solvothermal Sucrose/10 wt% 148 at 1 C Almost 100% after 200 cycles at 10 C [73]
LFP/C Hydrothermal IL[BMIm][N(CN)
2
]/glucose 163.8 at 0.1 C 160.6 after 50 cycles [77]
LFP/C Solvothermal Glucose 147 at 0.1 C 100% after 50 cycles [85]
LFP/C Hydrothermal
stripping synthesis
Glucose/2.46% 154.8 at 0.2 C 99.1% after 50 cycles at 0.2 C [86]
LFP/C Colloidal N-Methylimidazole (NMI)/2.53% 164 at C/20 99% after 35 cycles at C/20 [63]
C-L
1.05
FP Sol-gel Oleic acid 155 at C/30 70 at 45 C Excellent cycling stability after 100
cycles
[87]
LFP/C Hydrothermal Fructose/calcium lignosulfate (1.91 wt%) 162.5 at 0.1 C 100% after 100 cycles [88]
LFP/G Hydrothermal Graphene (3 wt%) 159 at 0.1 C Approx. 90% after 45 cycles at 0.2 C [89]
C@LFP/CNTs Hydrothermal 6.17 wt%
2.78 wt% CNTs
155 at 0.2 C 73 at 60 C 98% after 1000 cycles at 10 C [90]
C@LFP/G Solvothermal 5 wt% GO
20 wt% glucose
163.7 at 0.1 C 97.8% over 500 cycles at 1 C [82]
LFP-CNT-G Solid state 3 wt% CNTs
1 wt% G
168.9 at 0.2 C 115.8 at 20 C 98% after 100 cycles at 0.2 C [76]
LFP@G Solvothermal/
freeze-drying
∼6.2% 163 at 0.2 C 139 at 10 C 99.8% after 600 cycles at 10 C [78]
LFP/GO Solution
combustion/
colloidal
4 wt% G 162 at 0.1 C 96% after 40 cycles at 0.2 C [91]
LFP/G Solid state 5 wt% 161 at 0.1 C 70 mAh/g after 44 cycles at 50 C [92]
LFP/C/CPO Hydrothermal Ascorbic acid 156 at 17 mAh/g 97% after 50 cycles at 1 C [93]
LFP@C/G Rheological phase/
solid state
Glucose 10%
Graphene 3.97%
163.8 at 0.1 C 80 at 20 C 92% after 500 cycles at 10 C [94]
LFP-G-CNT Hydrothermal 5% 168.4 at 0.1 C 103.7 at 40 C 100% after 100 cycles at 0.1 C [95]
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along with electronic conductivity. Therefore, it is useful to use
an extra agent to contribute to the ionic conductivity of LFP.
Another modification strategy is co-coating the carbon along with
metal oxides and ionic conductors. The combination of different
coating materials could create an ideal coating layer for better Li
transport and electron conduction in LFP described as a hybrid
coating [102,103]. Shu et al. reported a spherical LFP hybrid
coated with electron-conductive carbon and fast Li-ion conductive
La
0.56
Li
0.33
TiO
3
perovskite oxide. The LFP/(C +La
0.56
Li
0.33
TiO
3
)
composite material with spherical morphology prepared via an
ammonia-assisted hydrothermal method showed improved electro-
chemical properties. These improved electrochemical properties
were attributed to the hybrid coating layer, which facilitated fast elec-
tron and Li
+
transport and prevented HF erosion [104]. In a similar
study, Yang et al. synthesized an LFP composite via a hybrid
coating, i.e., Li
1.4
Al
0.4
Ti
1.6
(PO
4
)
3
(LATP) via the sol-gel method
and a graphene nanosheets (GNS) layer via an in situ wet chemical
process. The high-rate performance and cyclability of the LFP/
C@LATP@GNS composite materials were significantly enhanced
at deviating temperatures of (55 °C) and (−20 °C) simultaneously
[105]. Li et al. [106] co-coated LFP with novel ionic conductor
GdPO
4
and carbon simultaneously via a hydrothermal-assisted solid-
phase method. The LiFePO
4
/C and 0.03GdPO
4
exhibited the most
excellent electrochemical performance with discharge capacities of
158, 141.6, 134.9, 104.9, and 86.7 mAh/g at 0.1, 0.5, 1, 5, and 10
C rates. The excellent performance was attributed to enhanced
ionic and electronic conductivities and inhibited interfacial reactions
between the materials and electrolytes.
Ionic conductors (Li
3
PO
4
and CePO
4
) provide extra sites for
lithium storage and allow faster diffusion of Li-ions. Along with
a highly electronically conductive carbon coating layer, this type
of hybrid coating layer improves the rate capacity and performance
of LFP. A schematic illustration of the electronic/ionic hybrid
coating for LFP material is shown in Fig. 5(c). The improved elec-
trochemical performances are compared in Table 3. However, the
optimized ratio of the electronic and ionic conductive materials
and the controllable hybrid coating technique remains challenging
for practical LFP fabrication.
Hence, electronic conductivity can usually be improved by
coating or compositing LFP particles with conductive materials.
In addition to providing acceptable conductivity, surface modifica-
tion (both coating and composite) improves the diffusion of lithium
ions in the lattice of LFP and so maximizes the utilization of LFP. A
nitrogen-doped carbon layer provides additional electrons, and
hence better electrical contact/conductivity and prevention of LFP
aggregation. In a similar study, the sulfur-doped carbon surface
coating layer has a significant effect on improving the electronic
conductivity and the defect degree of carbon which also contributes
to ionic diffusion of the electrode as well [110]. Hence, the overall
performance of sulfur-doped carbon-coated LFP increases without
destroying the crystal structure of the bulk LFP material. Along
with electronic conductive materials (e.g., carbon, metal, conduc-
tive polymers), ionic conductive materials (e.g., CePO
4
, LaPO
4
)
offer the advantage of high capacity for Li-ion storage, as well as
improve the ionic conductivity of LFP dramatically. The capacity
of LFP can be enhanced by making a composite with the high
capacity cathode active materials like NCM but it will result in two-
phase composite active materials and affect the lithium insertion
and extraction potential. However, the mixing of LFP with
carbon does not affect its structure and contribute to enhanced
Table 2 Comparison of electrochemical performances of carbon-coated LFP (with carbon coating layer modification)
Cathode composite Synthesis method Carbon content (wt%)
Electrochemical performance
(mAh/g) Capacity retention Ref.
LFP@B
0.4
-C Sol-gel Citric acid 164.1 at 0.1 C
126.8 at 10 C
98.2% after 100 cycles
at 10 C
[97]
F-LFP@NBFC Hydrothermal IL[BMIM]BF
4
162.2 at 0.1 C
71.3 at 15 C
100% after 40 cycles at
0.1 C
[101]
LFP
NR@N-C@RGO
Surfactant-assisted synthesis CTAB 172 at 0.1 C
143.7 at 10 C
95.8% after 1000 cycles
at 10 C
[83]
LFP/C-N +B Hydrothermal/ball milling Glucose 166.8 at 0.1 C 121.6 at 20 C 96.9% after 50 cycles at
10 C
[74]
N-C@LFP Hydrothermal plus chemical
polymerization
P123/polydopamine 162.1 at 1 C
107.5 at 30 C
100% after 100 cycles
at 10 C
[99]
LFP/CN Microwave heating route Polydopamine 160 at 0.2 C
118 at 10 C
97.9% after 50 cycles at
0.1 C
[98]
LFP/C–P Hydrothermal Glucose/
triphenylphosphine
165.5 at 0.1 C
124 at 20 C
91.4% after 50 cycles [96]
Table 3 Comparison of electrochemical performances of various hybrid-coated LFP
Hybrid/dual-coated
cathode Synthesis processes
Hybrid coating
material (wt%)
Electrochemical
performance (mAh/g) Capacity retention Ref.
LFP/C-CPO Liquid precipitation/solid state Superfluous citric acid
(5 wt%)
166.1 at 1 C 77.5% after 650 cycles at
10 C
[107]
LFP/C-LaPO
4
LPP/carbothermal reduction Superfluous citric acid
(5 wt%)
150.7 at 1 C 98.3% after 100 cycles at
10 C
[75]
LFP@C@Ag Hydrothermal Glucose (20 wt%) 152 at 0.1 C 97.4% after 5 cycles at
0.1 C
[108]
LFP/C +
La
0.56
Li
0.33
TiO
3
(LLT)
Ammonia-assisted hydrothermal Citric acid (5 wt%) 162.1 at 0.1 C
126.1 at 5 C
99.6% after 100 cycles [104]
LFP/C-rGO Polymerization restricted
precipitation method
Sucrose/graphene 168 at 0.05 C
72 at 60 C
Approx. 100% after 45
cycles at 0.1 C
[109]
LFP/C@LATP@GNS Solid state/sol-gel/wet chemical Glucose 7.33% 160 at 0.1 C
and at 55 °C
91% for 500 cycles at
10 C
[105]
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electronic and ionic conductivity as well as the capacity. Because
carbon sources like graphene, CNTs, etc. contribute to capacity
by offering sites for Li storage. But too much carbon content
lowers the cathode potential which negatively affects the overall
performance of Li-ion battery. So an optimized amount of carbon
(<5%) should be added which should stabilize the cell performance
without affecting it negatively.
4 Element Doping
In addition to the morphology/size control and carbon-coating,
doping is considered to be another significant strategy to improve
the intrinsic electronic/ionic conductivity of LFP. The replacement
of a small amount of Li
+
,Fe
2+
,orO
2−
sites with other ions is
expected to enhance electronic conductivity, Li-ion diffusion, and
the charge/discharge performance at high current densities. The
doping strategy can be simply classified into single-element
doping and multielement doping.
4.1 Single-element Doping
4.1.1 Lithium-site Doping. Li-sites can be replaced with ions
with a small ionic radius, such as Na, Nb, Al, K, Zr, and Nd.
Doping with Li-sites with small ions will increase the width of
1D diffusion channels of Li-ions and decrease charge transfer resis-
tance, which enhances the electrochemical properties of LFP cath-
odes. Chung and Chiang [111] reported that Nb-doped LiFePO
4
showed higher electronic conductivity compared with undoped
samples by a factor of ∼10
6
at room temperature. However, in
recent research, Nb-doped LiFePO
4
showed an improved perfor-
mance, where at 1% Nb-doped sample achieved enhanced revers-
ible capacities of 110 mAh/g at 10 C and 156 mAh/g at 0.5 C
[112]. Usually, cationic substitution to the Fe site in LFP results
in higher ionic mobility due to cell volume expansion and weaken-
ing of the Li–O interactions, which lowers the charge transfer resis-
tance and thus improves the lithiation and delithiation reversibility
[113–115]. However, few reports also suggested the probability of
cationic doping at the Li site can result in the production of Li
vacancies and enhancement in the capacity of LFP. Chiang et al.
[116] investigated the mechanism of improving the electrochemical
properties of a bare LFP by fabricating nanometer-sized cathodes
with V doped into the LFP crystal structure preferentially at the
Li site using a modified sol−gel synthesis process. The electrode
delivered a better capacity of 155 mAh/g and better conductivity
of 1.9 × 10
−2
S/cm for the V-added compound Li
1−x−v
V
x
FePO
4
rel-
ative to pristine LiFePO
4
(capacity of 138 mAh/g and conductivity
of 4.75 × 10
−4
S/cm without V addition). The enhanced properties
were associated with the substitution of V at the Li site and the sub-
sequent generation of Li vacancies. Chung et al. [117] presented the
benefits of aliovalent (Mg
2+
,Al
3+
,Ti
4+
,Zr
4+
,Nb
5+
,orW
6+
)
doping into the LFP crytal structure and consequently dramatic
inscrease in electronic conductivity with a very low dopant concen-
tration. They proposed that cation doping on the Li-sites allows the
stabilization of solid solutions with a net cation deficiency and the
generation of Li vacancies. As reported by the work of Wagemaker
et al. [118] that the aliovalent dopant charge is often balanced by the
generation of Li vacancies. However, the immobile dopant may
influence Li-ion mobility and even hinder Li-ion diffusion, as it
causes a small increase in the Li-ion diffusion channel by only
0.3%.
4.1.2 Iron-site Doping. Doping of alkali metal ions in Fe-sites
is the most actively studied doping site. Doping at iron-site will
facilitate Li-ion diffusion along the 1D pathway, so increase both
electronic and ionic conductivity of LFP cathodes. Wang et al. ver-
ified the enhanced speed of lithium storage because graphene mod-
ification provides active sites for nuclei and limits the crystallite
size, along with improved cycling stability due to Mg
2+
doping.
The Mg-ions preferred to occupy the Fe-sites in Mg-doped LFP,
while Mg
2+
doping improved the intrinsic electronic and ionic
transport of LFP crystals [119,120]. Örnek and Efe reported that
LiFe
0.96
Mg
0.04
PO
4
-C synthesized by sol-gel–assisted CTR synth-
esis exhibited high discharge capacities of 167, 155, 141, 103,
and 92 mAh/g at discharge rates of 0.1 C, 0.5 C, 2 C, 10 C, and
20 C, respectively, while discharge capacity remained above 97%
after 300 cycles [121].
V-doping can refine LFP particle size, by inducing lattice dis-
tortion to weaken the Li–O bonds, and improve electronic conduc-
tivity by increasing Li-ion diffusion, hence resulting in enhanced
electrochemical performance of LFP/C composites. Johnson et al.
verified that V ions can be substituted in the Fe-sites of LFP. The
LiFe
0.95
V
0.05
PO
4
was synthesized by a continuous hydro-
thermal flow pilot-scale synthesis process. The combined effect of
carbon coating, nanosizing, and vanadium doping was analyzed
and found to dramatically improve the rate performance. The
LiFe
0.95
V
0.05
PO
4
exhibited excellent discharge capacities of 119
and 169 mAh/g at the rates of 9 C and 0.3 C, respectively. In addi-
tion, X-ray absorption spectroscopy results and hybrid-
exchange density function theory (DFT) analysis suggested that
vanadium ions replaced both iron as well as phosphorous ions in
the structure, thus contributing the Li
+
diffusion due to Li
+
vacancy generation and modifying the crystal structure [122].
Many other reports also show that vanadium doping has attracted
much attention due to its benefits including various oxidation
states, electrochemically active phases (e.g., Li
3
V
2
(PO
4
)
3
,
LiVOPO
4
, and V
x
O
y
) and evasion of impurity phases. Also,
V-doped LFP has significantly improved electrochemical perfor-
mance, especially at high current rates. Most recently, Jiang and
Wang [123] reported vanadium doped LFP and obtained LiFe
1−x-
V
x
PO
4
@C composites via the sol-gel route. The electrode showed
improvement in electrical conductivity and lithium storage perfor-
mances of vanadium doped LFP/C compared with pristine LFP/
C, exhibiting a high discharge capacity of 162.9 mAh/g at 0.1C
(which is almost 95.8% of the theoretical capacity); a high revers-
ible capacity of 112 mAh/g at a high rate of 10C after 200 cycles
was still achieved. Lv et al. [124] investigated the synergistic
effect of V
3+
and F
−
co-doping on the electrochemical performance
of LFP at low temperatures and found that co-doping is better than
single ion doping. The V
3+
and F
−
co-doped LFP/C cathode mate-
rials prepared via facile wet milling-spray drying-carbothermal
Table 4 Comparison of electrochemical performances of iron-doped LFP
Doping element Synthesis method Best doping product Electrochemical performance Ref.
Mg
2+
Rheological phase G/Mg-doped LiFePO
4
(LFMP) composite 164 mAh/g at discharge rate of 0.2 C [119]
Mg
2+
Sol-gel LiFe
0.96
Mg
0.04
PO
4
-C 167 mAh/g at discharge rate of 0.1 C [121]
V Continuous hydrothermal flow LiFe
0.95
V
0.05
PO
4
, 169 mAh/g at discharge rate of 0.3 C [122]
Ni Solid state Ni-doped LFP/C 161 mAh/g at discharge rate of 0.1 C [130]
Nb
5+
Continuous hydrothermal flow LiFe
0.99
Nb
0.01
PO
4
110 mAh/g at discharge rate of 10 C [112]
Ti
4+
Solid state LiTi
0.08
Fe
0.92
PO
4
160 mAh/g at discharge rate of 0.2 C [131]
Ru
3+
Rheological phase reaction LiFe
0.99
Ru
0.01
PO
4
/C 156 mAh/g at discharge rate of 0.1 C [132]
Ce
3+
Solid state LiFe
0
.
9
Ce
0.1
PO
4
/C 160.1 mAh/g at discharge rate of 0.1 C [133]
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reduction (WSC) process showed excellent cycle performance and
discharge capacities of 136 mAh/g (10 C) at 20 °C and of 86 mAh/
g (10 C) at 0 °C was obtained, which is 20 mAh/g higher than
undoped LFP/C. The improved performance of V
3+
and F
−
co-doped LFP/C was ascribed to lower charge transfer resistance,
improved electrical conductivity, and generation of holes on the
surface which can provide more ion transport channels.
The nickel doped LFP is a promising choice in the future as it
offers the advantage to stabilize the LFP structure. Örnek et al.
[125] demonstrated that Ni-doped LiNi
x
Fe
1−x
PO
4
/C cathode mate-
rial prepared by sol-gel assisted carbothermal reduction method
showed more stable lattice structure, enhanced conductivity, and
diffusion coefficient of Li
+
ions, in which Ni
2+
is regarded to act
as a column in a crystal lattice structure to prevent the collapse
during cycling. The prepared LiNi
0.05
Fe
0.95
PO
4
/C sample showed
a high discharge capacity of 155 mAh/g at 0.2 C and no obvious
capacity fade with a stable cycle life at 0.2 C was observed.
The Mn
2+
substitution into olivine LFP is also an effective
approach to modify the electrochemical performance of the olivine
compound. Novikova et al. [126] showed that the manganese substi-
tuted LFP samples with low manganese content exhibited improved
charge/discharge capacity as compared with pristine LFP, especially
at high current densities. Ding et al. [127] demonstrated that the
Mn-composites can improve both electronic and ionic conductivity
as well, suggesting that Mn substitution is a feasible way to
improve the conductivity in olivine LFP cathode materials and
enhance the battery performance. Recently, Budumuru et al. [128]
studied the influence of Mn on lithium diffusion, rate capability,
and cyclic stability of carbon-coated single-crystalline nanotubular
(NT) and nanoparticular LiFe
1-x
Mn
x
PO
4
(x=0, 0.2, and 0.5) fabri-
cated by the template-assisted method. NT cathodes exhibited excep-
tionally good discharge capacities ∼60 (∼165) mAh/g, ∼32 (∼110)
mAh/g, and ∼22 (∼82) mAh/g at 25C (1C) rate for x=0, 0.2, 0.5,
respectively. The results predicted that single-crystalline nanotubu-
lar LiFePO
4
are suitable for high-power density applications,
optimal Mn substitution (x∼0.2) in LFP with slightly compromised
capacity is ideal for cathodes with increased cyclic stability and rate
capability. In a recent study, Zhang et al. [129] calculated the
mechanical stability and electronic properties of LFP doped with
Mn, Co, Nb, and Mo through the first principle and revealed that
the doping of Co effectively improved the mechanical stability of
the material by reducing the risk of microcracking and deformation
of the material. Table 4shows the comparison of the electrochemical
performances of some iron-site doped LFP.
4.1.3 O-site Doping. O-site doping can significantly increase
the conductivity of LiFePO
4
even more than Fe-site doping, as pre-
dicted by first-principle calculations. The improved conductivity is
attributed to the accurate ion doping suppressing antisite defects in
LFP. Anion/O-site doping can especially change the lattice param-
eters aand bwhile keeping the cparameter unchanged, which is not
observed with cation doping. Significantly enhanced high-rate per-
formance and cycling stability have been reported for anion-doped
LFP. Okada et al. fabricated sulfur-doped LFP nanoparticles
∼100 nm in diameter via the solvothermal method using thioaceta-
mide as a sulfur source. Due to the larger ionic radius of sulfur than
oxygen, the lattice parameters aand bexpanded by 0.2% while
there was no significant increase in c(0.03%). Also, a decrease in
antisite defects was observed, which enabled the easy migration
of lithium ions along the expanded diffusion paths without block-
age. Owing to the suppression of Fe−Li antisite defects and the
lattice expansion, sulfur-doped LFP nanoparticles delivered
improved high-rate performance with a high reversible capacity
of ∼113 mAh/g at the high current rate of 10 C [134].
F-doping can significantly improve the electrochemical perfor-
mance of LFP, contributing to the enhanced high-rate performance
and cycling stability. Recently, Li et al. reported the synthesis of
F-doped LiFePO
4-x/3
F
x
/C via a wet mechanical agitation-assisted
high-temperature ball-milling method. The results showed that
LiFePO
3.98
F
0.06
/C delivered the best initial reversible capacities
of 162.6, 156.6, 150.2 144.5 131.6, and 115.8 mAh/g at 0.1 C,
0.5 C, 1 C, 2 C, 5 C, and 10 C, respectively, and moreover cycle
stability of 95.4% at 10 C after 100 cycles [135]. In a similar
study, Gao et al. synthesized F-doped LiFePO
4-x
F
x
/C (x=0.05
0.10, 0.15, 0.25) materials via co-precipitation method with sub-
sequent high-temperature treatment using hydrofluoric acid (HF)
source. The LiFePO
4-x
F
x
/C (x=0.15) sample showed the highest
rate capability and an excellent cycling stability; its discharge
capacities were 165.7, 161.1, 150.8, 140.3, 129.8, and
115.7 mAh/g at rates of 0.1 C, 1 C, 5 C, 10 C, 20 C, and 30 C,
respectively. The reason for improved electrochemical performance
is that F-doping increased electronic conductivity as well as Li
+
ion
diffusion coefficient and enhanced structural stability [136].
4.2 Multielement Doping. Recently, multielement doping has
attracted much attention; this means doping with more than one
kind of element. Multielement co-doping may contribute to better
electrochemical performance than single-element doping. It can
Fig. 6 Crystal structure of the Zr-Co co-doped LFP system [62]
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be divided into two kinds of co-doping: one is doping with different
elements in different sites and the other is doping with different ele-
ments in the same sites of LFP (Fig. 6).
Tu et al. studied Mg-Ti co-doping of LiFe
1-x-y
Mg
x
Ti
y
PO
4
via a
CTR reaction combined with a spray-drying process for the devel-
opment of fast charging process in Li-ion batteries that led to
improved high-rate performance with discharge capacities of
161.5, 160.3, 156.7, 147.5, and 139.8 mAh/g at 0.2 C, 0.5 C,
1 C, 3 C, and 5 C, respectively. Also, the porous Mg-Ti co-doped
LFP microspheres showed excessive improvement in the electronic
conductivity (1.58 × 10
−3
S/cm), diffusion coefficients (5.97 ×
10
−9
cm/s for charge and 4.30 × 10
−9
cm/s for discharge) and high-
rate cycling stability with capacity retention of 99.6%, 97.4%,
and 92.9% at rates of 0.5 C, 1 C, and 5 C, respectively, after 100
cycles [137].
Tian et al. investigated the synergistic effect of supervalent cation
co-doping and the conductive graphene-coating on the structure and
performance of LFP. Li
0.99
Nb
0.01
Fe
0.97
Ti
0.03
PO
4
/G cathode was
prepared via a sol-gel method in which graphene (G) coating and
cation doping were attained simultaneously, and it delivered a
reversible capacity of 163 mAh/g at 0.1 C, with excellent cycling
stability (99.1% after 30 cycles) and rate performance (140 mAh/g
after discharge at 0.1 C, 2.0 C, and 5.0 C stepwise). This enhanced
electrochemical performance was ascribed to the 3D conductive
network of graphene and the electronic compensation ability by
Nb
5+
and Ti
4+
co-doping [138]. Yuan et al. [139] investigated the
individual and combined effect of Ni and Mn doping on the struc-
ture, morphology, and electrochemical properties of the LFP
cathode material. The Ni and Mn co-doped LiNi
0.02
Mn
0.03
Fe
0.95-
PO
4
/C sample delivered a higher initial discharge capacity of
164.3 mAh/g at a rate of 0.1 C and better cyclic stability with a
capacity retention of 98.7% cycled at 1 C after 100 cycles.
Table 5shows the comparison of the electrochemical properties
of multi-doped LFP cathode materials.
From all the doping cases mentioned above, it can be conluded
that doping with certain elements is an effective way to improve
the electrochemical performance of LFP. The Li-site doping
increases the width of the 1D diffusion channels of lithium ions
and decreases the charge transfer resistance, which results in good
electrochemical performance of doped LFP. The Fe-site doping
modification is mainly carried out to enhance the electronic conduc-
tivity of LFP. The O-sites doped with the correct element improve
the intrinsic conductivity of LFP. Moreover, the combination of
these three kinds of doping methods, called multielement
co-doping, enhances the electrochemical performance of LFP cath-
odes. In the future, modification through co-doped LFP would
develop a new olivine-structured cathode material with enhanced
electrochemical properties.
5 Cathode Prelithiation Additives
Loss of lithium in the initial charge process of a Li-ion battery
due to the formation of an SEI layer on the anode surface reduces
the energy density. This process consumes 5−20% of the lithium
from the cathodes (e.g., LiCoO
2
and LiFePO
4
) and thus reduces
Table 5 Comparison of electrochemical performances of multi-doped LFP
Doping elements Synthesis method Best doping product Electrochemical performance Ref.
Ni
2+
and Mn
2+
Solid state LiNi
0
.
02
Mn
0.03
Fe
0.95
PO
4
/C 164.3 mAh/g at discharge rate of 0.1 C [139]
Na and V Solid state Li
0.97
Na
0.03
Fe
0.97
V
0.03
PO
4
/C 156.5 mAh/g at discharge rate of 0.1 C [140]
Zr and Co Solid state Li
0.99
Zr
0.0025
Fe
0.99
Co
0.01
PO
4
139 mAh/g at discharge rate of 0.1 C [62]
V
3+
and F
−
WSC process LiFe
0.95
V
0.05
(PO
4
)
0.97
F
0.03
/C 136 mAh/g at discharge rate of 10 C at 20 °C [124]
Mg and F Solid state LiFe
0
.
92
Mg
0.08
(PO
4
)
0.99
F
0.03
/C 121 mAh/g at discharge rate of 10 C and −20 °C of temperature [141]
Nb and Ti Sol-gel Li
0
.
99
Nb
0.01
Fe
0
.
97
Ti
0.03
PO
4
/g 163 mAh/g at discharge rate of 0.1 C [138]
Fig. 7 (a) Schematic illustration of Li
2
S/Co additive synthesis and release of Li
+
during initial cycle
[145]; (b) schematic of required potential of prelithiation additive [160]; and (c) synthesis of Li
3
N material
via nitridation of lithium metal foil in N
2
atmosphere and subsequent annealing (200 °C, 24 h) [161]
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the first-cycle coulombic efficiency and creates a high initial irre-
versible loss of the total battery capacity. To address this problem,
electrochemical prelithiation has been explored [142,143]. The pre-
lithiation materials should have high capacities (both specific and
volumetric), as well as high stability. This stability includes compat-
ibility with the electrolyte and chemical stability with the solvent
and binder. The prelithiation materials need not provide electro-
chemical reversibility, as they are only required to compensate for
the lithium loss in the first cycle [144,145].
Sacrificial Li salt additives (e.g., Li
2
C
4
O
4
,Li
2
C
2
O
4
, and
Li
2
C
3
O
5
) exhibited Li compensation effects for the first irreversible
capacity loss. However, the use of these additives was accompanied
by the evolution of undesired gaseous N
2
, CO, or CO
2
[146]
.
Now
stabilized lithium metal powders and lithium silicide particles in
small amounts are being studied as anode prelithiation additives
due to their high specific capacity and high prelithiation efficiency
[147–153]. However, these anode prelithiation materials have lim-
itations of poor stability and poor compatibility with electrolytes,
common solvents, binders, and fabrication processes due to their
low potential and high chemical reactivity.
Lately, cathode prelithiation materials have attracted attention in
providing an alternative approach to compensate for the initial
lithium loss in the Li-ion battery. Cathode prelithiation materials
offer the advantages of relatively high open-circuit voltage (OCV)
and stability. Previously, lithium-rich compounds such as
Li
6
CoO
4
,Li
2
NiO
2
, and Li
2
MoO
3
have been studied as
lithium-ion-donating additives [154–157]. However, the applica-
tions of such additives are limited due to their low specific capacity
(∼300 mAh/g) and stability issues. It is desirable for cathode pre-
lithiation additives to have high specific capacities (e.g.,
>400 mAh/g or >1200 mAh/cm
3
), high stability, and alignment
with the potential of the working electrode.
Most recently, Cui’s group has explored conversion reaction-
based nanoscale composites (Li
2
O/metal, LiF/metal, Li
2
S/metal
composites) as candidates for cathode prelithiation material.
These nanocomposites offer much higher theoretical capacities
(>400 mAh/g), high stability, and good compatibility with existing
battery-processing applications [62,63,76]. Li
2
O/metal offers a high
specific capacity (e.g., 609 mAh/g for the Li
2
O/Co composite,
612 mAh/g for the Li
2
O/Fe composite, and 495 mAh/g for the
Li
2
O/Ni composite) for first-cycle Li-ion compensation. Among
the cathode prelithiation additives, Li
3
N offers the highest theoret-
ical capacity (≈2308.5 mAh/g based on the reaction formula: 2Li
3
N
→6Li
+
+6e
–
+N
2
) and low theoretical decomposition potential
(∼0.44 V versus Li
+
/Li) compared with other binary additives
(e.g., 6.1 V for LiF, 3.5 V for Li
2
S). The mechanism by which
Li
2
O becomes electrochemically active is not fully understood
yet. So, compositing with electrode active material could catalyze
the irreversible decomposition of Li
2
O into Li
+
and O
2
[158]
.
More-
over, no residue is left after delithiation of Li
3
N, but it has limita-
tions of poor electronic conductivity, reactivity with moisture and
solvents like N-methylpyrrolidione (NMP), and the need to be
handled in an argon atmosphere [159]. To overcome these chal-
lenges, Pei et al. modified the additive surface with a passivating
layer of Li
2
O and Li
2
CO
3
to ensure high stability by allowing the
operation of the additive in the air [160]. Li
3
N material was synthe-
sized via the nitridation of Li metal foil inside the nitrogen-filled
glovebox for 3 days, followed by an annealing process at 200 °C
for 24 h and Li
3
N powder obtained after grinding. Also, Li
3
N
film was fabricated by melting Li metal foil on a conducting Cu sub-
strate followed by nitridation in the same way (i.e., inside nitrogen-
filled glovebox) (Fig. 7(c))[160]. Table 6shares more details about
the structure and electrochemical properties of Li
3
N and other addi-
tives such as Li
2
O, LiF, and Li
2
S.
LiF suffers the limitations of poor electronic and ionic conductiv-
ity, a high decomposition potential of 6.1 V, and the emission of
toxic fluorine gas. However, it has the advantages of high lithium
content and stability in the ambient atmosphere and is used as a
coating layer for low-stability cathodes [164].
Li
2
S also exhibits limitations of instability in the ambient atmo-
sphere, poor ionic and electronic conductivity, and reactivity with
carbonate-based electrolytes. To solve the Li
2
S compatibility limi-
tations, Zhan et al. mixed Li
2
S with KB (ketjenblack) and PVP
poly(vinylpyrrolidone) without using NMP in the cathode fabrica-
tion process [165]. Sun et al. investigated this issue by mixing
metal sulfide (e.g., CoS
2
or FeS
2
) with molten Li in the inert
(argon) atmosphere. Afterward, this composite was stirred for
20 min at 185 °C and for 2 h at 220 °C, resulting in the formation
of Li
2
S/Co [161] (Figs. 7(a)and 7(b)).
Table 7shows the increase in the capacity for the first cycle of
various cathode materials after the addition of cathode prelithiation
additives.
Among the cathode additives, binary additives are superior to
lithium-rich compounds such as Li
6
CoO
4
,Li
2
NiO
2
, and Li
2
MoO
3
regarding their capacities, except for Li
5
FeO
4
(LFO) with a theoret-
ical capacity equal to 867 mAh/g [167]. However, prelithiation
additives may add unwanted weight to positive electrodes if not
fully decomposed upon delithiation and there is the possibility of
unwanted side reactions. Also, cathode prelithiation additives
have the problems of requiring strict preparation conditions and
safety issues involving Li melting.
In the last part of this review, we have discussed the role of cathode
prelithiation additives in compensating for the capacity loss during
the first cycle and how specific capacity beyond the theoretical
value can be achieved. We know that capacity is lost during the
first cycle due to the SEI formation at the anode surface. To solve
this problem, anode prelithiation materials were usually used to over-
come the capacity loss. Unfortunately, less attention was paid to the
Table 6 Comparison of structure and electrochemical properties of binary cathode prelithiation additives
Additive
Theoretical capacity
(mAh/g)
Space
group De-lithiation reaction
Theoretical decomposition
potential
Energy barrier of Li migration
(eV)/band gap (eV) Ref.
Li
3
N 2308.8 P6/mmm 2Li
3N
→6Li
+
+N
2
+6e
−
0.44 V 0.007–0.0038/1.1 [159,162]
Li
2
O 1794.2 Fm-3m 2Li
2
O→4Li
+
+O
2
+4e
−
˗˗ 0.15/4.7 [158,162]
Li
2
S 1166.7 Fm-3m Li
2
S→2Li
+
+S+2e
−
3.5 V 0.37/3.865 [161,163]
LiF 1033.4 Fm-3m 2LiF 2Li++F2+2e−6.1 V 0.73/8.9 [144,162]
Table 7 Comparison of gravimetric capacities of various
prelithiation additives for the positive electrode
Additive/
active material
Gravimetric
capacity
(mAh/g)
Additive (%) in cathode
increases capacity by
(% increase) Ref.
Li
3
N 1399.3 2% in LiCoO
2
(LCO) delivers
178.4 mAh/g at 0.1 C (19%)
[159]
Li
2
O/Co 583 8% in LFP delivers 195 mAh/g
at 0.1 C (19%)
[145]
Li
2
S/Co 683 4.8% in LFP delivers 204 mAh/
g at 0.1 C (26%)
[161]
LiF/Co 520 4.8% in LFP delivers 197 mAh/
g at 0.1 C (20%)
[144]
Li
5
FeO
4
700 7% in LCO delivers 233 mAh/g
at 0.1 C
[166]
Li
6
CoO
4
318 15% in LCO delivers 149 mAh/
g at 0.1 C
[157]
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cathode prelithiation additives which have their advantages of rela-
tively high OCV and stability. If the electrochemically reversible
cathode prelithiation additives with a capacity greater than
400 mAh/g are added into the LFP, then the overall specific capacity
beyond theoretical value can be achieved, as reported especially by
using binary compounds (e.g., Li
2
O/M, LiF/M, Li
2
S/M). On the con-
trary, if the cathode prelithiation additives are not reversible and
added as a compound (e.g., LiF), the initial capacity can be compen-
sated only with the Li source provided.
6 Conclusions and Perspectives
6.1 Conclusions. It is well known that the olivine-type
LiFePO
4
(LFP) has the advantages of safety, lifetime, and cost as
one of the most interesting cathode materials for lithium-ion batte-
ries. In order to keep these advantages and obtain a greater energy
density, a lot of efforts have been made to increase the LFP specific
capacity and rate capability to increase the driving range of electric
vehicles and fast charging. The improvement of capacity provided
with four strategies has been reviewed by this work, and the detailed
summaries corresponding to each of four strategies are given as
follows:
(1) Tailoring particle size and morphology of the LFP effectively
improves its high-rate capacity and cycling life due to
increased electrode/electrolyte contact area, shortened
Li-ion diffusion distance, and better stress release.
(2) The coating of the LFP particles with nanostructured carbon
contributes to enhanced electronic and ionic conductivity as
well as the extra capacity by providing interfacial storage
sites for Li-ions.
(3) The doping contributes to the electrochemical performance
of LFP via enhanced electronic and ionic conductivity and
reduced charge transfer resistance. Especially, the structural
modification through co-doped LFP would develop a new
olivine-structured cathode material with enhanced electro-
chemical properties.
(4) The addition of the cathode prelithiation additives can com-
pensate for the Li losses due to SEI layer formation and sig-
nificantly enhance the specific capacity beyond theoretical
value.
6.2 Perspectives. After reviewing the recent research pro-
gress, including the newest articles published in 2020, we created
a chart summarizing the benefits of each of the four strategies in
order to give a clear vision about the LFP capacity enhancement.
The chart in Fig. 8is made according to typical capacity data
taken from the published results and shows a rising trend in how
these strategies contribute to the capacity (first charge/discharge).
This trend shows that the surface coating and modification on
the LFP nanoparticles seems a more effective approach as it
can achieve a greater capacity than the theoretical value of
170 mAh/g by providing interfacial active sites for extra storage
of Li-ions. Moreover, the combination of LFP nanoparticles and
nano-carbon coating can enhance the transport of electrons, and
thus improve the rate capacity performance. The prelithiation strat-
egy is a good way to compensate for high 1
st
cycle active lithium
losses caused by solid electrolyte interphase (SEI) formation.
Using the cathode prelithiation additives can significantly enhance
the specific capacity beyond theoretical value.
In order to achieve and exceed its theoretical capacity of
170 mAh/g when discharged at a high current density, many
efforts are needed to tailor the particle size and shape, nanostruc-
tural hybrid coating, and new cathode prelithiation additives of
the LFP material. The following research directions should be
emphasized in the future:
(1) The particle size reduction is a very important way to
improve the specific/ rate capacity due to Li diffusion path
and kinetics optimization. The nanostructured LFP is a key
to approach to the theoretical value.
(2) Hybrid coating and interface construction needs to be
explored. The hybrid coating materials improve the interfa-
cial contact between electrolyte and LFP surface, and the
desirable interface structure can provide more active sites
for Li storage, resulting in the extra capacity beyond the the-
oretical value.
(3) New type of cathode prilithiation additives needs to be devel-
oped which are most suitable for the LFP material. The
cathode prelithiation will be mandatory to compensate the
active lithium losses in the first charge discharge cycle in
the future, and helpful for the LFP material to possess ultra
charge capacity of the first cycle, which will benefit to the
full cell application, enhancing the energy density of the
full cell.
Acknowledgment
This research is financially supported by Anhui Natural Science
Foundation (No. 1908085ME151), Anhui Province High-end
Talent (DT18100044), National Level Foreign Expert Introduction
Plan Project (G20190219004), and the School-Enterprise Coopera-
tion Project (RD18200058).
Conflict of Interest
There are no conflicts of interest.
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Fig. 8 A trend illustration of LFP capacity enhancement with
various strategies
010801-14 / Vol. 18, FEBRUARY 2021 Transactions of the ASME
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