Ni/NHC-catalyzed cross-coupling of methyl sulfinates and amines for direct access to sulfinamides

Electrochemical Energy Reviews

https://doi.org/10.1007/s41918-019-00053-3

REVIEW ARTICLE

Degradation Mechanisms and Mitigation Strategies of Nickel‑Rich

NMC‑Based Lithium‑Ion Batteries

Tianyu Li^1,3· Xiao‑Zi Yuan¹· Lei Zhang¹· Datong Song¹· Kaiyuan Shi²· Christina Bock²

Received: 25 April 2019 / Revised: 2 July 2019 / Accepted: 13 September 2019

Abstract

The demand for lithium-ion batteries (LIBs) with high mass-specifc capacities, high rate capabilities and long-term cycla-

bilities is driving the research and development of LIBs with nickel-rich NMC (LiNi_xMn_yCo_1−x−yO₂, x ⩾ 0.5) cathodes and

graphite (Li_xC₆) anodes. Based on this, this review will summarize recently reported and widely recognized studies of the

degradation mechanisms of Ni-rich NMC cathodes and graphite anodes. And with a broad collection of proposed mechanisms

on both atomic and micrometer scales, this review can supplement previous degradation studies of Ni-rich NMC batteries.

In addition, this review will categorize advanced mitigation strategies for both electrodes based on diferent modifcations

in which Ni-rich NMC cathode improvement strategies involve dopants, gradient layers, surface coatings, carbon matrixes

and advanced synthesis methods, whereas graphite anode improvement strategies involve surface coatings, charge/discharge

protocols and electrolyte volume estimations. Electrolyte components that can facilitate the stabilization of anodic solid

electrolyte interfaces are also reviewed, and trade-ofs between modifcation techniques as well as controversies are dis-

cussed for a deeper understanding of the mitigation strategies of Ni-rich NMC/graphite LIBs. Furthermore, this review will

present various physical and electrochemical diagnostic tools that are vital in the elucidation of degradation mechanisms

during operation to supplement future degradation studies. Finally, this review will summarize current research focuses and

propose future research directions.

Keywords Lithium-ion battery · Ni-rich NMC · Degradation · Mitigation · Graphite · Diagnostic tools

1 Introduction

LiNi_xMn_yCo_1−x−yO₂(NMC) layered oxides have received

increasing attention due to their enhanced specifc capacity

and thermal stability [1]. As compared with LiMO₂(M=Ni,

Mn, or Co) cathodes, NMC-based cathodes also possess the

combined merits of three transition metals in which nickel

can ofer high specifc capacities, whereas Co and Mn can

provide layered structures and enhanced structural integrity.

In addition, NMC-based cathodes can outperform lithium

iron phosphate (LFP) cathodes in many areas, particularly

in terms of operational voltage in which LFP-based LIBs

can only output voltages below 3.4 V [2] and sufer from

high rates of self-discharge. Moreover, materials such as

Li₂MnO₃and lithium titanate do not possess great indus-

trial potential as cathodes due to short lifespans and high

costs. Here, NMC-based cathode materials are much cheaper

to produce (~ USD 23 per kg for NMC111) as suggested

by researchers using a co-precipitation method [3], and

the increase in NMC-based material prices from January

2017 to March 2018 (~ 43% for NMC532 and NMC622,

and~27% for NMC811) [4] indicates that more research is

Due to signifcant advantages such as high energy densi-

ties, high galvanic potentials, wide temperature ranges, no

memory efects and long service lifespans, lithium-ion bat-

teries (LIBs) have been widely employed in various appli-

cations, including in the felds of communication, aviation

and transportation. Overall, the chemistries of LIBs are

constantly evolving, and in terms of cathode materials,

* Lei Zhang

lei.zhang@nrc.gc.ca

1

Energy, Mining and Environment Research Center, National

Research Council Canada, 4250 Wesbrook Mall, Vancouver,

BC V6T 1W5, Canada

2

Automotive and Surface Transportation Research Centre,

National Research Council Canada, 75 Boulevard de

Mortagne, Boucherville, QC J4B 6Y4, Canada

3

Department of Chemistry, University of Victoria, 3800

Finnerty Rd, Victoria, BC V8W 3V6, Canada

Vol.:(0123456789)

1 3

Electrochemical Energy Reviews

being directed to the feld of NMC-based cathode materials

due to their commercial potential.

coating as recent research progress, whereas Myung et al.

[29] in their review only provided a brief developmental

history of NMC and NCA-based LIBs with a focus on the

categorization of gradient layers and core–shell structures

as remedies. A more recent and detailed review conducted

by Hou et al. [30] covered more mitigation topics varying

from ion substitution and electrolyte additives to gradient

layers; however, to the best of our knowledge, no review has

presented a comprehensive collection of mitigation methods

or has categorized them based on characteristics. Similarly,

controversies in mitigation strategies and trade-ofs need to

be considered, such as the selection of dopants or coating

materials, all of which have yet to be discussed. Based on

this, these vacancies in the literature will be covered in this

review to assist in the further investigation of Ni-rich NMC-

based LIBs.

Following the initial commercial success of

LiNi_1/3Mn_1/3Co_1/3O₂(NMC333 or NMC111), NMC-based

LIBs have become the mainstream with gradual improve-

ments in NMC technology through the steady increase

in the nickel content in each generation of cathode mate-

rials. And because Ni can cycle between Ni²⁺/³⁺and

Ni³⁺/⁴⁺redox couples without large variations in voltage

during the charge/discharge process, layer-structured Ni-

rich NMC (LiNi_xMn_yCo_1−x−yO₂, x ⩾ 0.5) cathodes [e.g.,

LiNi_0.5Mn_0.3Co_0.2O₂(NMC532), LiNi_0.6Mn_0.2Co_0.2O₂

(NMC622) and the most recent LiNi_0.8Mn_0.1Co_0.1O₂

(NMC811)] have been developed. Here, the main advan-

tage of Ni-rich materials is their high discharge capacity

(200–220 mAh g⁻¹), which represents a large increase in

energy density (~ 800 Wh kg⁻¹) as compared with con-

ventional LiCoO₂(~ 570 Wh kg⁻¹) and LiMn₂O₄spinel

(~440 Wh kg⁻¹) materials [5]. In addition, increasing the Ni

content is the most efective method to enhance capacity in

the current state-of-the-art LIBs without requiring advanced

and novel battery chemistries. As a result, researchers are

reviewing recent progress achieved in the development of

Ni-rich cathode materials in an attempt to provide important

clues for the future design of Ni-rich cathodes [6].

Apart from the cathode, graphite—the anode material

that has received the most attention in LIBs due to its high

mass-specifc capacity and economic advantages [26, 31,

32]—can function collaboratively with NMC-based cat h-

odes to provide promising performances in coin cells [20,

33], pouch cells [9, 34], pouch bags [11] and T-cells [35 ].

Based on this, this r eview will also provide a complete pic-

ture of the aging of graphite anodes that can afect both elec-

trodes in battery cells based on degradation mechanisms and

efective mitigation strategies. Furthermore, the solid elec-

trolyte interface (SEI) is a flm that forms on graphite anode

surfaces during initial charging [36–39] and thickens in sub-

sequent cycles, which can protect and impede anodes and

is vital in the discussion of the aging behaviors of graphite

anodes. Therefore, detailed mechanisms of anode SEIs are

also reviewed to supplement previously published reviews.

To further investigate the degradation mechanisms of

LIBs, multiple physical methods have been used in both

electrochemical tests and physical characterizations [e.g.,

electrochemical impedance spectroscopy (EIS) to charac-

terize the resistance of electron transfer and surface flms].

Conventional and modern physical characterization methods

have also been utilized to reveal surface morphology, ele-

ment compositions, crystal lattice dimensions and reaction

processes, and are also important to the interpretation of the

degradation of Ni-rich NMC/graphite LIBs. Based on this,

these diagnostic tools will also be presented in this review.

To fully present the evolution of electrode performances

in Ni-rich NMC/graphite LIBs, degradation mechanisms

based on various experiments and recent mitigation strate-

gies in which cell conditions were evaluated separately or

as a whole are presented in this review. And overall, this

review will provide a summary of the recent advances in

Ni-rich NMC-based batteries and can supplement pioneer-

ing reviews published previously.

However, although increasing Ni percentages can allow

for enhanced Ni-rich NMC-based battery performances

as compared with other NMC combinations in terms of

capacity, capacity fading becomes severely pronounced. To

address this, signifcant eforts have been made to elucidate

the mechanisms of capacity f ading and associated growth

of impedance [7, 8], as well as to fnd remedies [9–12]. And

in recent decades, numer ous studies and reviews ha ve been

published worldwide [13–15] in which various testing proto-

cols [16], research methodologies, models [17–19] and phys-

ical characterizations [9, 20] have been used t o interpret the

behavior of NMC-based cathodes with or without modifca-

tions. Here, studies based on microscopy [21], crystallog-

raphy [22 ], mass spectroscopy [23], gas measurements [24]

and calorimetry [8] have revealed that parasitic reactions [7 ],

cation mixing (leading to restructured surface regions) [25 ],

active material dissolution [26] and oxygen release [23 , 25]

are primary factors responsible for cathode degradation at

the atomic scale. In addition, scanning electr on microscopy

(SEM) [13] together with computational studies [27] has

revealed that intergranular cracking can weaken connec-

tions between primary particles. Despite these fndings, the

majority of recently published studies are insufcient and

only partially cover the processes through which degradation

occurs. For example, Liu et al. [28] reviewed surface lay-

ers and surface chemistries during Ni-rich NMC-based LIB

operations and presented cationic doping and metal oxide

1 3

Electrochemical Energy Reviews

2.1.1 Surface Degradation During Cell Operation

2 Degradation Mechanisms

Ni-rich NMC-based materials are some of the most prom-

ising cathode candidates for next-generation LIBs due to

high capacities and large voltage windows. Despite impres-

sive progress in the development of advanced materials and

fabrication processes, the development of high-capacity

electrodes with long-term stability and prolonged cycle

lifespans remains an important challenge. To address this,

the understanding of underlying degradation mechanisms

presented in this review can serve as a starting point to learn

and improve the performance of Ni-rich NMC-based cath-

odes. In addition, the information included in this review can

provide valuable insights into the creation of new devices

with improved efciency and productivity. And for the sake

of completeness in the understanding of degradation mecha-

nisms in LIB full cells, parasitic reactions on NMC-based

cathode surfaces, the aging of graphite anodes as well as SEI

formation and aging are also briefy presented in this section.

̄

Lattice Expansion/Contraction NMC oxide possesses an R3m

structure (rhombohedral symmetry) with a Li-layer on the

3a site, an NMC layer on the 3b site and an oxygen layer

on the 6c site [7] and can usually be indicated in the split-

ting of (110) and (108) peaks and of (006) and (102) peaks.

For example, the expansion and contraction of the c-axis

(14.21 Å) of NMC811 lattices can be observed by using

in situ XRD measurements in the cell voltage windows of

3.0–4.0 V and 4.0–4.4 V, in which (003) peaks can decrease

from 18.96° (3.0 V) and subsequently rise to 19.09° (4.4 V)

[7, 10, 43]. This can also be observed for the movement

of (104), (015) and (108) peaks. Here, researchers suggest

that the expansion of the c-axis is due to the repulsive force

generated from MO₆slabs, which are positively charged in

a highly delithiated state, whereas the loss of electrons can

shrink the radius of transition metal ions to reduce the a-axis

[44]. In addition, the (110) peak can shift to higher angles in

experiments, suggesting that the a-axis can contract during

charging due to the smaller radius of transition metal ions

at elevated valences [45]. Researchers have also reported

that the a-axis of NMC811 lattices remains constant at

potentials exceeding 4.3 V [10] and that a steep drop of

the c-axis occurs at potentials exceeding 4.2 V in whic h the

drop becomes steeper as the Ni content increases [7, 20, 43].

Here, researchers proposed that this drop is due to t he f act

that the repulsion between O²⁻layers decreases with mor e

covalent M–O bonding at higher delithiated states [46, 47]

(Fig. 1a, b). Furthermore, studies have also confrmed two

pathways (Fig. 1c2, c3) invol ved in the delithiation of Ni-

rich NMC-based cathodes [40, 48], including oxygen dumb-

bell hopping and tetrahedral site hopping, in which at the

beginning of charging, Li can exit through oxygen dumbbell

hopping until a certain degree of delithiation is reached and

continue through tetrahedral site hopping due to increased

energy barriers as caused by Li–O bonds during delithiation.

2.1 Ni‑Rich NMC‑Based Cathodes

The degradation of Ni-rich NMC LIBs involves differ-

ent types of capacity loss, including initial capacity loss

(ICL), sudden capacity loss (SCL) and gradual capacity

loss (GCL), all of which are accompanied by impedance

growth. And in the case of GCL, which is the most fre-

quently investigated process, a major cause is the loss of Li

and transition metals on cathode surfaces. In general, the

majority of degradation mechanisms for Ni-rich NMC-based

cathodes can be indexed to the same mechanism independ-

ent of Ni/Mn/Co ratios because they share similar crystal

and microspherical structures. Here, adjustments (typically

increases in Ni) to the relative amounts of Ni, Mn and Co

can result in augmented performances as well as various

negative features. For example, enhanced mass-specifc

capacity can be achieved but at the expense of rate capabil-

ity and structural stability. In addition, researchers have also

reported higher electronic conductivity and reduced polari-

zation [40] in which the varied polarization as compared

with the portion of Ni is caused by the higher e_gorbital of Ni

over the t_2gorbital of Co [41]. Researchers have also shown

that the reduced Mn content can r esult in faster degradation

through surface reconstruction [42] and that the increased

Ni content can result in increased cation mixing due to the

bulk difusion of Ni²⁺into the Li-layer and increased para-

sitic reactions as the valence of surface Ni increases toward

highly reactive Ni⁴⁺. Overall, cathode performances can be

impacted by various factors that interact with each other, and

therefore, this section will discuss degradation mechanisms

in detail.

Surface Reconstruction to Fm3m Rocksalt Surface reconstruc-

tion of the particles of NMC-based cathodes can initiate

if delithiation exceeds 70% as shown in NMC111 [49]. In

addition, increasing the Ni content can lead to increased

portions of Ni³⁺(usually Ni ions with higher valences) in

the total amount of Ni. Furthermore, elemental Ni partially

located in the transition metal layer as Ni²⁺(0.69 Å) pos-

sesses a similar radius to Li⁺(0.76 Å) [50], and Li⁺and

Ni²⁺can exchange positions in a delithiated state through

Ni²⁺difusion to the octahedral sites of Li⁺(energetically

favo rable) through a neighboring tetrahedral site (Fig. 1c)

[42, 51]. Here, the degree of Ni²⁺migration can be evalu-

ated b y calculating the c/a ratio of the NMC lattice because

this migration reduces the c/a ratio. And based on XRD

1 3

Electrochemical Energy Reviews

measurements, ratios of I₀₀₃to I₁₀₄can decrease to less

than 1.2 after signifcant cation mixing and can therefore

serve as an indicator of the degree of cation mixing [52].

Moreover, researchers reported that cation disorder can

lower spacing between atomic layers in the lattice of the

NMC-based cathodes, hinder Li⁺movement and reduce the

amount of active Ni and Li [53]. In the case of higher Ni

percentage cathodes, Mn⁴⁺can partially be replaced by Ni,

which increases the valence of Ni and therefore lowers the

possibility of Ni atoms migrating to 3a sites [8]. However,

because the total amount of Ni also increases, Ni²⁺migra-

tion still occurs to a larger extent. To minimize Ni²⁺migra-

tion, researchers reported that mitigation methods can be

used to modify cathodes [54]. Furthermore, a disordered

spinel structure (Fd3m) due to non-ideal cation mixing [55]

can f or m if LIBs are charged to high voltages (e.g., 4.8 V)

[21, 56–60] and consists of a LiM₂O₄spinel structure and a

M₃O₄spinel structure if Co migration into tetrahedral sites

occurs [42] (Fig. 2a). Xiong et al. [61] and Eom et al. [62]

also reported t hat spinel phases can be formed on charged

cathodes stored at 90 °C for a week. Moreover, research-

ers found that migrated Mn and Co possess high valences

and smaller radii for movement and more vacancies in the

Li-layer in a highly delithiated state [63]. Researchers also

reported that long-term cycling to 4.2 V (NMC 622) [64]

can result in phase changes to an Fm3m structure consisting

of Ni²⁺, Mn²⁺and Co²⁺with low ered valence states as com-

par ed with a spinel structure [63, 65 ] and ion-insulating [56,

63, 66] (a cubic rocksalt phase) (F ig. 2b). Here, the conver-

sion from a layered structure to a cubic rocksalt phase (usu-

ally confrmed by HRTEM/EELS [9]) can release O₂into

the cell, which can react with electrolytes to generate CO₂

and result in increased electrode interfacial resistances due

to kinetic barriers in the Li⁺insertion/desertion process [50].

To address this, researchers reported that NMC811 cathodes

pretreated with ramping to 4.5 V limited the growth of the

rocksalt layer in subsequent cycles and can serve as a pillar

structure to stabilize the lattice and isolate the surface of

NMC particles [67]. Furthermore, researchers reported that

the oxidation states of Mn and Co decrease in the sur face

layer/region of NMC811 and NMC442 after cycling [9, 63].

Phase Transformations Two hexagonal phases of NMC811

can coexist on the surface of particles if freshly prepared

in which the evolution of one phase is irreversible and van-

ishes after initial charging (with lattice constants remaining

at the initial value), whereas the second phase is reversible

(except for a small diference between charging and dis-

charging above 4.2 V). Here, the fading of the (110) peak at

65.12° can track the conversion of the region in which the

two hexagonal phases coexist and are converted to a single

phase [7]. In addition, the broadening of the (003) peak dur-

ing cycling as recorded by using in situ XRD measurements

is of great interest in which researchers [25] suggested that

this phenomenon was due to the creation of a second phase

in the potential range of 3.8–4.0 V (H2, evolved from the

monoclinic phase in contrast to the phase H1 following

the stoichiometric composition) that possesses less Li.

Researchers also reported that this process occurred earlier

with higher Ni portions (Fig. 2c) and that at higher volt-

ages, the conversion from H2 to H3 occurs with impedance

growth (volume contraction) only for NMC811 (or NMCs

Fig. 1 The c-axis (a1), the a-axis (a2) and potential (a3) of an

NMC 811 cell as a function of specifc capacity. The c-axis (b1)

and the a-axis (b2) as a function of cell potential during the second

cycle and dQ/dV of the second cycle as a function of cell potential

Author(s) 2015. Published by ECS). c1 Lattice of an NMC layered

structure. c2 Tetrahedral site pathway and c3 oxygen dumbbell path-

way for Li-ion difusion in an NMC layered structure (Reprinted with

GmbH & Co. KGaA, Weinheim)

1 3

Electrochemical Energy Reviews

with Ni portions larger than 80%) [25]. Researchers have

also found that the phase transition from H2 to H3 can lead

to (003) peaks shifting to higher degrees [68] and that Li-

reordering (Li difusing into Ni⁴⁺-rich sites) can further

convert H3 to H3-2 [69] in which the fast shrinkage of the

c-axis at potentials above 4.15 V is a clear indication of

this phase transition. Moreover, the generated microregion

of NiO₂in the intralayer of the NMC811 lattice as a result

of the phase transition from H2 to H3 can lead to the irre-

versible transformation of the NMC811 structure [45] and

this phase transition becomes severer as Ni portions further

increase to extreme numbers as indicated by rising peaks

of the diferential capacity curve centered at 4.15 V [68].

Here, the fading of this peak leads to capacity fade and is

referred to as the “deleterious efect.” Furthermore, phase

changes to a second hexagonal phase for other NMC-based

cathodes have yet to be clarifed. (Fig. 2d illustrates crystal

lattice evolution during delithiation.)

can lead to more hydroxide (e.g., LiOH) on NMC622 [72]

and that surface impurities can afect cell impedances in sub-

sequent cycles through reactions with electrolytes to form a

Li⁺difusion inhibiting layer.

x x

Li(Ni, Mn, Co)O₂+ CO₂+ O₂

2

4

x

(1)

→ Li_1−x(Ni, Mn, Co)O₂+ Li₂CO₃

2

NMC Dissolution The dissolution of NMC materials can

also result in capacity attenuation [26, 74] because it can

decrease Li⁺insertion sites and is trigg ered by the products

of electrolyte decomposition such as HF from the reaction of

PF₅with H₂O (Eqs. 2, 3) [31]. In addition, higher voltages

can accelerate the dissolution of NMC111 due to the release

of more acidic components as a result of rapid electrolyte

decomposition [75]. Despite these fndings, efects on Ni-

rich NMC have yet to be reported. The dissolution of NMC

materials can also cause a concurrent issue involving the

production of resistive MF_xas a side product on the surface

of NMC particles. Moreover, the dissolution of transition

met als can pose a threat to anodes through electrodeposition

[32, 76], catalysis of solvent reduction [77] and formation

of inor ganic layers in SEIs [78 ], all of which can impede

Li⁺intercalation and reduce capacity [79]. Here, Mn³⁺dis-

proportion into Mn²⁺and Mn⁴⁺was proposed to explain

Mn dissolution [26 , 75 ] as previously reported in Li–Mn–O

spinel structures [80 , 81] and various electrolytes have been

utilized to form protective surface flms containing carbon

on electrodes to isolate surfaces from electrolytes [33].

Surface Stabilizer In general, NMC-based cathodes with

more Mn content tend to possess better cycling stability [13,

70], whereas NMC-based cathodes with more Ni content

are believed to be incapable of reaching high cutof volt-

ages due to the lack of Mn⁴⁺as a structure stabilizer [65].

Apart from Ni²⁺, structural instability also applies to Mn³⁺,

although with the high Ni content, Mn migration is not con-

sidered to be signifcant. Here, due to low octahedral site

stability energy, NMC-based cathodes possess a tendency to

form spinel-structured LiMn₂O₄on surfaces [71] and Mn⁴⁺

possesses a similar tendency to migrate to the Li-layer [33,

71] and decrease capacity. Nevertheless, difused Mn⁴⁺on

par ticle surfaces can stabilize structures during long-term

cycling [50].

LiPF₆→ LiF + PF₅

(2)

2.1.2 Impurities and Parasitic Reactions

LiPF₆+ 2e⁻+ 2Li⁺→ LiF + Li PF

(3)

x

y

Surface Impurities To maintain a layered structure, excessive

Li is used for Ni-rich NMC-based cathodes. However, side

reactions of Li with water vapor and CO₂in air can lead

to LiOH and Li₂CO₃in which exposure to air can result

in a reconstructed surface layer 3 nm thick (evidenced by

STEM) that thickens if cycled [9]. A surface layer with simi-

lar impacts can also form as cathodes come into contact

with electrolytes [65]. Here, researchers suggest that this

reconstructed layer can be tuned t hrough diferent synthesis

methods of the cathode powder [63] and that in the case of

NMC111, surface impurities can also grow on top of cath-

ode particles as resistive flms over the same time period for

uncycled Ni-rich NMC-based LIBs [35] that contain mainly

insulating hydroxides and carbonates as revealed through

Raman spectroscopy (Fig. 3a). Here, the circumstances in

whic h Li₂CO₃(Eq. 1) or NiCO₃is favored remain unclear

[35, 72, 73]. Researchers also reported that high humidity

Self‑redox Reaction At highly delithiated states, the valence

of transition metals increases, and because the low-spin

Co^3+/4+:t_2gband overlaps with the 2p band of O²⁻(Fig. 3b),

electron transfer from O²⁻to Co^3+/4+can occur and reduce

oxidized transition metal ions, thus releasing O₂into bat-

tery cells [82]. Oxygen release based on this also occurs

at the frst charging period in which O²⁻reduces transition

metals oxidized to 4+ (similar efects on Ni⁴⁺also exist and

dominate oxygen release in Ni-rich NMC-based cathodes)

[14, 42].

Parasitic Reaction: Efects of Ni⁴⁺Highly delithiated (highly

charged) states tend to generate large amounts of Ni⁴⁺that

can react with electrolytes. (No reaction schemes have been

proposed.) This side reaction can signifcantly thicken cath-

ode–electrolyte interfaces (CEIs) and reduce the number

1 3

Electrochemical Energy Reviews

Fig. 2 a Schematic of phase transition and possible TM cation migra-

tion pathways in charged NMC cathode materials during thermal

decomposition (Reprinted with permission from Ref. [42]. Copyright

LiNi_0.5Co_0.2Mn_0.3O₂after cycle tests under two upper cutof voltage

conditions (4.5 V and 4.8 V) (Reprinted with permission from Ref.

Weinheim). c Diferential capacity plot of three diferent NMC/

graphite cells recorded at 0.1 C-rate (3rd cycle) with marked phase

transformations (Reprinted with permission from Ref. [25]. Copy-

right © The Author(s) 2017. Published by ECS). d Schematic of H1–

H2–H3 phase transformation from the perspective of local environ-

ments in diferent views along the c-axis (up) and the a-axis (bottom)

of available Li⁺[14, 83], thus increasing impedance [13].

Researchers have also in vestigated gas evolution (CO₂) from

NMC532 cathodes using DEMS [84], and isothermal calo-

rimetry (IMC) results have shown that at highly delithiated

states above 4.2 V, the further removal of Li from NMC811

cathodes can result in decreased entropy and endothermic

heat fow [7] in which rapidly increasing heat fow indicates

that the highly delithiated cathode is very reactive with the

electrolyte and can therefore cause capacity fading. In addi-

tion, researchers also found that remaining at a highly del-

ithiated state can cause signifcant electrolyte oxidation as

well as other side reactions [9].

higher Ni content sufer more from parasitic reactions with

Ni⁴⁺that are a source of emitted CO₂. In addition, because

highly reactive oxygen can be produced through O²⁻from

self-redox reactions, surface reconstructions and highly del-

ithiated cathodes (ca.>80%) [86], CO and CO₂can also be

traced back to the reaction be tween O₂and alkyl carbon-

ate electrolytes (Eq. 4) [24, 25]. Alternatively, sweeping to

higher voltages can also caus e the electrochemical decom po-

sition of electrolytes to produce CO₂(Eq. 5, Fig. 3c) [23, 24,

87]. Furthermore, Gasteiger et al. [88] have exper iment ally

shown three other mechanisms that may contribute to CO₂

emission, including solvent hydrolysis (if trace amounts of

OH⁻exist), electrolyte impurity oxidation and HF reacting

with lithium carbonate as a surface impurity. Moreover, the

decomposition of lit hium carbonate has also proven to be a

source of CO₂[89, 90]. Researchers have also suggested that

Parasitic Reaction: CO₂generation Large Ni⁴⁺portions

at higher voltages can cause increased CO₂evolution at

increased upper cutof voltages [85] in which cathodes with

1 3

Electrochemical Energy Reviews

Fig. 3 a Raman spectra of fresh and stored NMC111 and NMC811

electrodes before cycling, measured in air within ~1 h with the sam-

ples not treated prior to measurements (Reprinted with permission

b Positions of various redox couples relative to the top of t he oxy-

gen 2 p band (Reprinted with permission from Ref. [14], https://pubs.

can Chemical Society. Further permissions related to this fgure

should be directed to ACS). c Proposed electrooxidation and chemi-

cal oxidation pathways for ethylene carbonate (EC) and their potential

dependence (Reprinted with permission from Ref. [25]. Copyright

ferent issues facing Ni-rich NMC materials: (1) SEI formation and

reactivity of Ni⁴⁺; (2) Li/Ni displacement and formation of disordered

phases; (3) microcracks in secondary particles (Reprinted with per-

by ECS)

NMC-based cathodes with Ni-rich surfaces behave difer-

ently than other cathodes with diferent portions of Ni [7].

For example, the degradation of NMC811 cells was studied

by using diferent electrolyte additives including VC and

PES211 to suppress side reactions and it was revealed that

NMC442/graphite cells and NMC111/graphite cells showed

better performances with PES211, whereas NMC811/graph-

ite cells aged slower with VC (with slower capacity fading

and impedance growth) [79]. Here, PES was also reported

to be a viable additive for the suppression of gas evolution

[91] and impedance growth during cycling to higher volt-

ages (above 4.3 V), and rocksalt surface layers formed with

the use of VC but not with PES211, concluding that elec-

trolyte additives can signifcantly afect the rate of parasitic

reactions and that par asitic reactions were the main reason

for capacity fading [7, 92].

2.1.3 Other Issues

Initial Capacity Loss Initial capacity loss is strongly linked

to the loss of available Li and the growth of impedance

in anodes due to SEI formation. Here, researchers have

reported that NMC-based cathodes cannot return to their

fully lithiated states after initial cycling [26] in which sepa-

rate mechanisms have been pr oposed, including parasitic

reactions consuming Li⁺[93] and the sluggishness of Li⁺

difusion into the few vacancies of t he Li-layer [94] (cir-

cumstantially proved by Gasteiger et al. [26 ]). In addition,

researchers have also suggested that initial capacity loss

is related to the C-rate because it is kinetically infuenced.

Furthermore, partial losses of available cathodic Li occur

to compensate for losses of anodic Li⁺to SEIs [26], and

other losses of Li in the cycling period can be attributed to

parasitic reactions including the immobilization of Li in the

SEI and enlarged polarization, which become signifcant at

higher cutof potentials [26, 95].

EC + 2O₂→ 2CO₂+ CO + 2H₂O

(4)

EC → CO∕CO₂+ R-H⁺+ e⁻

(5)

1 3

Electrochemical Energy Reviews

Cracking of Secondary Particles Reduced performances due

to the cracking of spherical secondary particles have been

extensively reported and are of signifcant concern [96].

Here, the expansion and contraction of the c-axis during

repeated cycling at above 4.2 V can lead to microstrains on

particles in cathodes, leading to the generation of microc-

racks in the core of primary particles with some initially

generated cracks being able to close in subsequent cycles

[68]. These cracks can cause poor connection between par-

ticles and micropores and even the cracking of secondary

particles [13], all of which contribute to degradation at the

microscale. To address this, Lim et al. [27] have used frst-

principle calculations to study these cracks generated by

gaps between primary particles and found that they were a

result of anisotropic and average contractions. In addition,

researchers reported that these cracked particles possessed

enlarged surface areas and therefore increased the possibil-

ity of parasitic reactions [13]. Moreover, these microcracks

were also found to partially originate from the gas evolution

of NMC particles [13] (Fig. 3d), and recent studies on coated

NMC (76 14 10) suggested that the infltration of liquid

electrolytes into the gap between primary particles can also

cause cracking [97]. Alternatively, various NMCs have also

been found wit h no cracking by using SEM [7, 20]. Here,

Dahn et al. [20] suggested that the shell of core–shell struc-

tured NMC cathodes (a Mn-rich shell and a Ni-rich core)

was resistant to cracking due to the absence of compressive

stress on the core and tensile stress on the shell. Based on

all of this, numerous mitigation methods including carbon

matrixes and gradient layers have been proposed to resolve

cracking at the microscale (further details in Sect. 4.1).

2.2 Graphite Anode

Similar to the cracking of NMC secondary particles, crack-

ing can also occur in graphite anodes due to Li intercalation/

deintercalation, which can expand and contract the distance

between graphite layers [102]. In addition, recent research

indicated that unit cell expansion (13.2% in total) during

Li inter calation is neither continuous nor linear, but staged

(Fig. 4a–e) [103]. Here, Li intercalation from C₆(Li-free

graphite) to LiC₂₄(Stage 2L) showed no in-plane ordering

of intercalated Li, whereas from 2L to LiC₁₂(Stage 2), the

ordering of Li appeared and the lattice can be assigned to the

space group P6/mmm. Furthermore, in the transition from

Stage 2 to LiC₆(Stage 1), total volume change suddenly

increases as the amount of x exceeds 0.6 in Li_xC₆(where

Stage 1 appears as Li presents in every interlayer). These

results showed that optimal composition was between 20 and

80% charged in which Stage 2 stably exists or extends into.

Furthermore, “dead lithium,” which is Li-ions electro-

chemically plated onto the surface of graphite anodes, is

the principal reason for capacity fading, especially at fnal

stages [104]. This phenomenon occurs on graphite surfaces

that possess limited mass transport, slow difusion within

the graphite or low charge transfer speeds [105] and is

strongly promoted by low temperatures/large polarizations

(including high electrolyte resistances induced by high tem-

peratures) during lithiation as well as the properties of the

graphite electrode (e.g., morphology [102], loading). And

as Li⁺intercalation is replaced by Li plating, issues such

as dendrite growth become more pronounced, leading to

increased impedance and more importantly, potential safety

hazards due to internal short circuiting. To avoid this, the

limits of charging currents and the state of charge need to

be established to maintain polarization in optimal ranges

[106]. However, large polarizations also need to be adopted

to minimize charging durations to meet industrial applica-

tion demands. Here, researchers reported that the reduction

of graphite thicknesses and improvements in porosity can

decrease charge transfer impedances [105, 107].

The Effects of Temperature Studies have shown that cell

degradation can accelerate at elevated temperatures above

30 °C and that at low C-rate cycling, cell degradation is

mainly afected by the time in which cells are placed under

high temperatures and not by cycling [98]. Long-term

operation of LIB cells can produce large amounts of heat.

And although this released heat may even be useful in cold

weather conditions, heat emissions are a major issue of Ni-

rich NMC-based LIBs due to thermal instability [33] in

which thermal runaways can occur if NMC-based cathodes

react with LiPF₆from the electrolyte and limit the com-

mercial and practical use of these battery packs. To improve

thermal stability, coatings such as SiO₂[62], TiO₂[99] and

Li₂ZrO₃[100] have been chosen to prevent cont act between

cathodes and electrolytes, and an ALD-coated NMC532

with Al₂O₃has even been reported [101] to resolve the issue

of uneven coatings and prevent the failure of cathode separa-

tion from electrolytes.

2.3 SEI

Electrolytes used in LIBs contain two main components

including Li salt and organic solvents. Here, the use of

additives as flm-forming agents can allow for stable SEIs

and, in general, the concentration of Li salt is maintained

at 0.8–1.3 M to optimize electrolyte viscosity and ion con-

ductivity. As compared with cathodes, the potential of LIB

anodes is low and can trigger electrolyte reduction (SEI

formation), which can cause Li loss during initial charg-

ing (Fig. 4f) [31]. However, because SEIs are electronically

insulating, fur ther reductions of electrolytes do not occur

1 3

Electrochemical Energy Reviews

in which formed SEIs are usually 10–50 nm thick and are

conductive to Li⁺ions. Researchers have extensively stud-

ied the constituents of passivating SEIs on graphite anodes

and have reported that lithium ethylene dicarbonate (LEDC,

Eq. 6, Fig. 5a–d) [108] was the main component of SEIs

formed wit h ethylene carbonate (EC) solvent [109] apart

from electronically insulating LiF and gaseous products

(H₂and C₂H₄). Propylene carbonate (PC) as a solvent has

also been well investigated due to its thermal stability and

wide voltage windows. Furthermore, DFT calculations have

shown that stable SEI flms can be produced by oligom-

ers of SEI flm components (SFCs) [108], which are frst

created through solvent decomposition. LiF is also a prod-

uct of almost all common Li salts such as LiPF₆(the only

commercially available salt), LiBF₄, LiTFSi, LiTSi, etc., in

which researchers reported that the amount of LiF produced

from LiBF₄was large and would result in the formation of a

grainy flm instead of a smooth flm.

deposited onto graphite anodes, both of which lower stor-

age capacity [38]. Researchers have also proposed another

mechanism of Li⁺that suggests that even monolayer Mn²⁺

can impede the movement of Li⁺through the graphite and

the SEI [111]. As for PF₅, it is produced from the decompo-

sition of various electrolytes and can trigger the open-ring

polymerization of EC, which further increases SEI decom-

position along with the production of HF [112]. Moreover,

the damage/repair process caused by gaseous PF₅consumes

active Li-ions until Li-ions are depleted in the system [113].

Researchers have reported, however, that this degradation

can be mitigated through the addition of Lewis-basic addi-

tives. The presence of H₂O in electrolytes can also reduce

the production of C₂H₄(usually used as an indicator of EC

reduction) and promote H₂production. Here, the existence

of SEIs can reduce the reduction of trace amounts of water

that usually generate H₂and CO₂[114].

An automotive scale NMC pouch cell was also analyzed

by Dahn group using ultra-high precision cycling technolo-

gies, and it was revealed that the performance of graphite

anodes was decisive and that corresponding capacity fading

(proportional to t^1/2) was related to SEI thickness and was

inversely proportional to the rate of SEI growth, suggesting

that the gradual lo wering of SEI thickening rates can lead to

stable SEIs [115, 116]. Here, alternative relationships pro-

posed for SEI gr o wth need to take stability as a factor into

consideration [117].

2EC + 2Li⁺+ 2e⁻→ Li₂EDC + C₂H₄

(6)

As aging continues during cycling, researchers believe

that the inorganic portions of SEIs also increase as LiF

(can be used as an indicator of SEI maturation) is produced

through LiPF₆reacting with lithium carbonates (Eqs. 7, 8)

[37].

2RCO₃Li + LiPF₆→ 2LiF + 2RF + LiPO₂F₂+ 2CO₂

(7)

RCO₃Li + LiPF₆→ 2LiF + RF + POF₃+ CO₂

(8)

2.4 Summary

After SEI formation, the resistance of anodes/SEIs toward

divalent transition metal ions such as Ni²⁺, Co²⁺, Cu²⁺and

Mn²⁺(relatively more impactive) is signifcantly enhanced,

preventing the reduction of EC solvent by these ions. How-

ever, formed SEIs remain targets to many reductive sub-

stances in which reductive decomposition can degrade SEIs

(Li₂CO₃and C₂H₄are produced as a result) and expose ther-

modynamically unstable graphite anodes to reactive environ-

ments [38]. Here, if the exposure of graphite anodes occurs,

a series of damage can occur on the graphite anode, includ-

ing the intercalation of solvated Li⁺, which leads to larger

sizes and the potential risk of the breaking of the graphite

layer. In addition, Mn²⁺has been reported to be able to pen-

etrate SEIs and reside in the inner area between the SEI and

the anode through the exchange of positions with Li⁺ions

located in the SEI [110]. Furthermore, the recurrent genera-

tion of C₂H₄suggests that Mn²⁺is a destructive substance

to both solvents and graphite anodes in which accumulated

Mn²⁺near graphite can function as a catalyst, suggest-

ing that it is reduced at the surface of the anode but can

reduce EC afterward, leading to thickened SEI flms and the

immobilization of active Li (Fig. 5e) [110]. In addition, the

reduction of Mn²⁺can cause the gradual transfer of active

Li⁺to SEIs or the conversion of active Li⁺into compounds

Overall, the degradation of Ni-rich NMC-based cathodes

originates from the insertion/desertion of Li⁺during charge/

discharge in which the expansion and contraction of crys-

tal lattices (primary particles) gradually generates tension,

leading to the cracking of secondary particles (Fig. 6). Elec-

trolyte infltration as well as oxygen release can further dete-

riorate this issue. On an atomic scale, Ni⁴⁺can accumulate

during charging because delithiation can elevate the valence

state of Ni together with high-valence Mn and Co ions. And

due to smaller radii, easier difusion can lead to surface

reconstruction (from rhombohedral to rocksalt), which can

further inhibit Li⁺insertion/desertion. Moreover, increased

voltage levels of high-valence ions can trigger side reactions

such as the oxidization/decomposition of electrolytes and

the further dissolution of NMC materials. Electrochemical

potentials of electron-depleted Co and Ni can also fall below

the fermi level of O²⁻, leading to the transfer of electrons

from O²⁻to transition metal ions and the release of O₂into

systems, which can also cause electrolyte decomposition.

Throughout the operation of Ni-rich NMC-based LIBs, four

types of unwanted substances exist on the surface of cathodic

particles, including other phases, surface impurities (carbon-

ates and hydroxide), rocksalt structures and surface flms. To

1 3

Electrochemical Energy Reviews

Fig. 4 Crystal structures of a

graphite (space group P63/mmc)

and two major Li intercalation

compounds, b LiC₁₂and c LiC₆

(space group P6/mmm). Difer-

ent stacking of graphene layers

with d the AB sequence in

graphite and e the AA sequence

in LiC₁₂and LiC₆(Reprinted

with permission from Ref.

American Chemical Society). f

Schematic of SEI formation and

degradation on graphite anodes

(Reprinted with permission

2005, Elsevier B.V. All rights

reserved.)

sum up, aging efects on both micrometer and atomic scales

can result in two common outcomes in which the frst is

the chemical (through chemical reactions) and mechanical

(through the breakage of crystallites) damage/loss of active

materials and the second is the production of detrimental

substances on electrode particle surfaces, including surface

flms (both SEIs and CEIs), surface impurities and surface

layers. And although many diferences in the performance of

various Ni-rich NMC-based cathodes have been discovered,

these remain unexplained. Therefore, future studies should

focus on the interpretation of diferent aging behaviors in

NMCs with or without mitigation methods.

As for graphite anodes, degradation primarily occurs due

to the plating of Li dendrites and the cracking of layered

graphitic structures. In addition, SEIs tend to thicken dur-

ing cell operation as EC solvent is reduced electrochemi-

cally. Furthermore, transition metals, especially Mn²⁺from

dissolved NMC, can cause signifcant capacity loss due to

massive damage to SEIs. A comprehensive list of identifed

degradation mechanisms for Ni-rich NMC-based cathodes,

graphite anodes, SEIs and CEIs is presented in Table 1.

Here, the degradation mechanisms of SEIs are categorized

based on anodes and the degradation mechanisms of CEIs

1 3

Electrochemical Energy Reviews

3 Diagnostic Tools

3.1 Electrochemical Techniques

Throughout the development of LIBs, a variety of electro-

chemical methods have been employed to test, analyze and

diagnose LIBs. To provide a comprehensive overview, these

methods are categorized into three groups, including charge/

discharge tests, cyclic voltammetry (CV) and electrochemi-

cal impedance spectroscopy (EIS).

3.1.1 Charge/Discharge Tests

Charge/discharge tests include basic electrochemical char-

acterizations that can be used to determine the voltage at

which a certain reaction (including Li⁺insertion/desertion

reactions that generate current in external circuits and side

reactions that negatively impact cell performance) occurs

and that can be used to evaluate the reversibility of cycling

performances. Data from these charge/discharge tests can

be analyzed by using many methods and can provide useful

information on cell performance, reaction and reversibility,

degradation and associated mechanisms.

Voltage versus capacity Charge/discharge curves are com-

mon and useful plots contained in almost every published

study in the feld of batteries. These plots are usually dis-

played in the format of voltage versus capacity (normalized

by the mass of the active material) for a clear understanding

of total capacity within a voltage range and capacity loss

for certain cycles, including initial capacity loss (ICL), sud-

den capacity loss (SCL), gradual capacity loss (GCL) and

voltage versus time. In some studies, the horizontal axis is

changed to the content of Li in NMC compounds for easier

comparison with physical characterization results in terms of

lattice parameters/structures. Furthermore, the reversibility

of cycling can be interpreted through these curves in which

the presence of a reaction that can reconstruct the cathode

at a certain voltage can be clearly predicted if the curve does

not repeat itself in the next cycle after reaching that point

[35]. Moreover, a small but important feature (a small peak)

often found in the charge/discharge curve at the beginning of

the initial charging can indicate the existence of impurities

(most probably nickel hydroxide and nickel carbonate) dur-

ing storage (a typical example showing the reaction of sur-

face impurities of NMC811 af ter long-term storage during

cycling is illustrated in Fig. 7a) [35]. Finally, the polarization

of battery cells can be estimated by comparing the voltage

levels of diferent cycles at fxed capacities.

Fig. 5 DFT-MD snapshots of electrode/electrolyte interphases with

Li₂EDC: a adhesion structure of one Li₂EDC monomer on a graph-

ite electrode, b dissolution structure of one Li₂EDC molecule in EC

solvent, c adhesion structure of 12 Li₂EDC molecule aggregates on

a graphite anode, d dissolution structure of 12 Li₂EDC molecule

aggregates in EC solvent (Reprinted with permission from Ref. [108].

mechanism for the continuous decomposition of SEI and electro-

lytes as monitored by C₂H₄evolution for a preformed electrode with

a Mn²⁺-containing electrolyte: (1) absorption of Mn²⁺ions into the

SEI; (2) reduction of Mn²⁺ions in the SEI and deintercalation of Li⁺

from graphite; (3) re-oxidation of Mn⁰to Mn²⁺; (4) recurrent elec-

trolyte reduction; (5) the catalytic cycle of electrolyte decomposi-

Author(s) 2018. Published by ECS)

are categorized based on cathodes because certain degrada-

tion mechanisms may involve both electrode materials and

interfaces.

Diferential Capacity Curve Diferential capacity curves are

another kind of plots derived from charge/discharge curves [7,

1 3

Electrochemical Energy Reviews

Fig. 6 Summary of degradation

mechanisms of Ni-rich NMC-

based cathodes on the microm-

eter and atomic scale

20, 118]. These curves are also referred to as the incremental

discharge curves can reveal the impact of C-rates on cell deg-

radation. In addition, discharge capacities at each time slot

during discharge can be plotted versus corresponding C-rates

and a trendline ftting of all data points taken at the same volt-

age can be drawn and extrapolate to the Q-axis, thus revealing

the relationship between maximum capacity Q_maxand voltage.

Moreover, maximum capacity fading plots based on EMF

curves can be used to determine capacity loss (irreversible

dQ

capacity and can be obtained by plotting versus voltage.

dV

(For similar reasons mentioned above, the vertical axis is con-

dx

verted to in some cases, as shown in Fig. 7b.) The main use

dV

of these curves is to cross-check features already shown in

charge/discharge curves at certain voltages but can also pro-

vide other information. Here, the most important feature

revealed based on diferential capacity curves for Ni-rich

NMC-based cathodes is the phase transition in which three

peaks can be identifed in both charging and discharging pro-

cesses, indicating phase transition from H1 (hexagonal) to M

(monoclinic), and subsequently to H2 and H3 (see Sect. 2.1.1)

as Li is extracted from the cathode [45]. In addition, dif eren-

tial capacity curves can be plotted with in situ X-ray difrac-

tion results to correlate phases to corresponding lattice param-

eters [119]. Moreover, the loss of available Li between

graphite layers or in the Li-rich layers of cathodes can be

revealed through the fading of these peaks. And by integrating

peak areas and calculating the ratio of these peak areas to the

total charge/discharge capacities for each cycle, trends in the

pathways/steps of Li transfer can be implied (e.g., more domi-

nating, remaining the same or fading). Furthermore, the posi-

tion of peak potential is indicative of cell impedance in which

positive shifting during cycling implies an increasing cell

capacity loss ∆Q_ir) at diferent C-rates. Diferential voltage

dV_EMF

curves can also be applied to EMF, and

can be regarded

as a nondestructive method in which pea^dk^Qshifting is indica-

tive of cell degradation and can reveal anode material decay

and voltage slippage efects [121].

Capacity Fading Capacity fading curves can provide more

macroscopic views of capacity loss during long-term cycling

and can be derived from capacity fading plots in which cell

capacity at diferent conditions is displayed. And because

electrode materials and aging conditions vary signifcantly in

diference battery systems, the speed of capacity fading can

also vary signifcantly. Therefore, the degree of cell aging

needs to be normalized in which the vertical axis of capac-

ity fading curves is sometimes converted to represent nor-

malized capacity. A modifed version has also been recently

presented [16] to investigate the efects of the amount of

Li⁺ions entering/exiting Li-layers in cathodes rather than

focusing on the number of alterations. Here, an accumula-

tive discharge capacity fading plot can be drawn and used

to compare capacity fading in diferent currents, which can

provide insights into the possible relationships between the

amount of Li⁺transferred and structure deformations.

dV

impedance. Similarly, a diferential voltage plot ( vs. Q) can

dQ

be used to show the fuctuation of voltage during charge/dis-

charge and is also linked to phase transitions [98].

Maximum Capacity To determine maximum capacity and

associated fading, electromotive force (EMF) data [120] can

be derived from the discharge curve of the same cell at difer-

ent C-rates. Here, comparisons of EMF curves with charge/

1 3

Electrochemical Energy Reviews

Table 1 List of identifed degradation mechanisms of Ni-rich NMC/graphite LIBs

No. Condition

Degradation mechanism

Electrode

References

1

2

3

4

N/A

Initial SEI formation consuming active Li

Li difusion into the NMC layer

Sluggish Li difusion back into the cathode

Anode

Cathode

[108]

[26]

[94]

High voltage, high temperature Sudden capacity loss caused by surface flm formation and thickening under Anode/cathode [31]

harsh conditions

5

6

High SOC

N/A

Ni²⁺migrating to vacant Li⁺sites (cation mixing), reducing the number of

Cathode

[51]

[64]

Li sites and blocking Li difusion pathways

Structural changes from rhombohedral (layered) to spinel and later to rock- Cathode

salt (NiO, nickel carbonate and minor quantities of hydroxides) as a result

of massive cation mixing

7

Long cycling

Mn³⁺forms a spinel structure from a layered (rhombohedral) structure

because of low octahedral site stability energy (based on studies of Mn-

rich NMC cathodes)

Cathode

[71]

8

9

High SOC

Storage in air

Ni⁴⁺oxidizing electrolytes and thickening CEI (self-discharge)

Cathode

[83]

[35]

Surface impurities formed on particle surfaces due to long-term storage in

air

10 N/A

NMC dissolution in HF (less active sites and higher charge transfer resist-

ances)

Cathode

[74]

[33]

[25]

11 Extreme high temperature

12 N/A

Thermally unstable NMC reacting with LiPF₆and causing thermal runa-

ways (fre and explosion)

Release of reactive oxygen due to surface reconstruction causing chemical

oxidation of electrolytes to produce CO and CO₂

13 High voltage

14 High voltage (above 4.6 V)

CO₂produced from the electrochemical oxidation of electrolytes

[87]

[82]

Oxygen reducing metal ions due to the hybridization of electrons and orbit- Cathode

als (self-redox reaction)

15 Low voltage

16 N/A

17 Long cycling, high DOD

Disproportion of Mn³⁺(based on studies of Mn-rich NMC cathodes)

Carbonate layers reacting with electrolyte

Cathode

[75]

[35]

[13]

Secondary particles cracking along grain boundaries and enlarging surface

areas for more side reactions to occur

18 N/A

19 High voltage

Loss of active Li metal to SEI as immobilized during cell operation

Anode

[110]

[38]

SEI layer decomposition facilitated by the precipitation of transition metals Anode

into the SEI (huge capacity loss)

20 N/A

21 N/A

22 N/A

Volume expansion and contraction of graphite during operation cracking the Anode

graphitic layer

[102]

[111]

[38]

Metal ions from the cathode inserting into the anode and blocking active

Anode

sites

Mn²⁺reacting at the surface of the anode to generate Li₂CO₃, which can

crack graphite surfaces

Anode

23 N/A

24 N/A

25 N/A

Solvents getting into the holes of graphite

Anode

Anode/cathode [104]

[31]

[110]

Solvated Li⁺cracking graphite during Li intercalation

Li dendrite plating (may trigger short circuiting if growth becomes too

large)

26 N/A

Continuous thickening of surface flms during cell operation

Anode/cathode [37]

Note: N/A in the condition column indicates a common degradation mechanism occurring in many conditions; Only one representative reference

is provided for each degradation mechanism because many of them have been reported extensively in many studies

Based on capacity fading plots, the number of electrons

consumed by side reactions (both at the cathode and at

the anode) can be measured by subtracting the discharge

capacity by the corresponding charge capacity. Here, a

charge efficiency (CE) plot focusing on the degree of side

it describes the percentage of electrons charged to the

anode that can be used to release Li⁺(remaining electrons

consumed by side reactions during charging), whereas the

number of electrons that can be discharged from the cath-

Q_charge,n+1

ode can be calculated by

. And in general, the two

Q_discharge,n

reactions is commonly used in which the charge effi-

percentages should be around 100% (CE_anodeis a bit less,

Q_discharge,n

ciency of an anode can be calculated by

, because

Q_charge,n

1 3

Electrochemical Energy Reviews

Fig. 7 a Charge/discharge curves of the 1st, the 2nd and the 3rd

cycles of NMC811-Li (Reprinted with permission from Ref. [35].

tice constant and diferential capacity as a function of cell voltage for

NMC811 (LiNi_0.8Mn_0.1Co_0.1O₂) (3–4.4 V) (Reprinted with permis-

ety). c Cyclic voltammetry of the frst three cycles of an NMC 811/

811 (The inset: the equivalent circuit model used for data ftting). e

The Nyquist plot of SiO₂-coated NMC 811 (Reprinted with permis-

reserved.)

1 3

Electrochemical Energy Reviews

but CE_cathodeis a bit more); however, the existence of

kinetic barriers may cause enlarged differences.

varied capacitance from point to point. To address this, a

resistor in parallel with a constant phase element (CPE) can

be used to model each process. As for the long tail attached

to the two semicircles in the EIS curve, this is related to

mass-transfer resistance and therefore can be represented

by using a Warburg element. During aging, both semicircles

commonly increase (Fig. 7d, e), indicating large cell polari-

zations and thickened CEIs; however, with remedial eforts,

the increase in both resistances (linked to both semicircles)

can usually be reduced. For example, researchers have

reported that impedance growth of surface flms and charge

transfer can be signifcantly reduced through a SiO₂-layer

coating of NMC811 (Fig. 7e) [123].

To study impedance growth during cycling, average

polarization plots combined with discharge voltage decay

plots can be obtained and can reveal capacity retention

behaviors by measuring ∆V_aveincreases, which can indi-

cate impedance growth through cell degradation as well

as potential kinetic barriers of Li⁺insertion/desertion [9].

3.1.2 Cyclic Voltammetry

As a supplementary method, cyclic voltammetry (CV) can

be used to determine the reversibility of charge/discharge

in electrodes by measuring the diference in charge/dis-

charge peak potentials, which is r elated to major delithi-

ation/lithiation processes (Fig. 7c) [33]. As such, CV can

assist in the investigation of transition metal oxidation and

reduction in which the plot of the initial CV can difer

from subsequent cycles due to the formation of SEIs dur-

ing the frst delithiation. In addition, upper cutof voltages

can also be determined by using CV [122].

3.2 Physical Techniques

3.2.1 Crystal Lattice and Particle Morphology

X‑ray Difraction In situ and ex situ XRD methods are the

most frequently used physical characterization methods to

determine lattice parameters and crystal structures. Here,

in situ XRD can be used to measure the evolution of lattice

parameters (both a-axis and c-axis) of NMC-based cathodes

during cycling [43]. For example, the use of in situ XRD

can evidence the shrinking of the a-axis to a constant value

and the expansion and rapid shrinking of the c-axis dur-

ing delithiation in Ni-rich NMC cathodes through the shift-

ing of (003) and (006) peaks to a lower angle together with

the shifting of the (101) peak to a higher angle (Fig. 8a).

In addition, in situ XRD can allow for the comparison of

the lattice shrinkage percentage of the a-axis to that of the

c-axis, leading to the conclusion that lattice shrinkage is

anisotropic and that lattice parameters are linked to corre-

sponding phases (H1, M, H2, H3). In situ XRD can also be

used to test the thermal stability of NMC cathodes and show

temperature-resolved (TR) results [42] similar to voltage-

resolved results. For example, TR-XRD results of heated

NMC-based cathodes can be used to examine thermal sta-

bility in which researchers found that the structures of all

NMC cathodes tested experienced surface reconstruction

from rhombohedral to spinel (frst the LiMn₂O₄type and

later the Mn₃O₄type) and subsequently to rocksalt structure

(Fig. 8b). In another study, in situ XRD plots were combined

with gas chromatography–mass spectroscopy (GC–MS) data

to reveal the relationship between the Li content and evolved

gas from NMC-based cathodes [90].

3.1.3 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is another

common method to investigate cell impedance evolution and

degradation mechanisms and involves the application of AC

sinusoidal waves (voltage or current) to target cells. Here,

AC sinusoidal waves are applied at an amplitude with dif-

ferent frequencies because semicircles in resulting Nyquist

plots can overlap with each other if represented processes

occur in the same frequency range. In addition, half-cell EIS

can be carried out with either 2-electrode symmetric cells

(e.g., Swagelok 2-electrode symmetric confguration cells)

or 3-electrode cells by installing a reference electrode in

which the resulting EIS curve often involves a vertical short

tail, two depressed semicircles (only if a half cell is tested)

and a long tail at a certain degree to the horizontal axis from

high to low frequencies. Here, the short tail usually appears

at extremely high frequencies, indicating the inductance of

cables, which is rarely seen in test results. Solution resist-

ances can also be measured at the intercept of the short tail

with the real (Z) axis and therefore can be represented by a

resistor. As for the frst semicircle at high frequencies (usu-

ally a small one), this is indicative of the resistance of the

CEI, and as the CEI continuously reacts (thickening and

decomposing), the impedance cannot be taken as merely

resistive. The second semicircle can be attributed to charge

transfer at the cathode. (Double-layer capacitance is gener-

ated because of voltage decay across the interface.) In typi-

cal EIS curves, these semicircles can appear as depressed

due to surface roughness (unevenness), which can cause

Ex situ XRD with Rietveld refnement is easier to conduct

than in situ XRD but conveys less information because it can

only illustrate the crystal structure of NMC-based cathodes.

For ex situ XRD data, more peaks can be seen if a bulk

phase of impurities or coating materials is formed on the

1 3

Electrochemical Energy Reviews

surface of cathode materials. Here, the ratio of I₀₀₃–I₁₀₄in

the ex situ XRD data indicates the degree of cation mixing

and can be used to determine the ameliorative performance

of a mitigation method in which ratios lower than 1.2 rep-

resent signifcant cation mixing. Moreover, the peak split-

ting of (110)/(108) and (006)/(102) represents a well-layered

hexagonal structure (Fig. 8c) [45] and the evolution of the

full width at half maximum (FWHM) of peaks represents

change in crystallinity. Surface species can also cause peak

shifting [e.g., (003) of NMC622] if stored [72], or generate

new peaks, and therefore, ex situ XRD can also be used to

cross-check the existence of impurities.

will show two strong difraction spots that indicate the for-

mation of rocksalt phases with high symmetry. Phase con-

version can also be tracked and compared by using HAADF-

STEM results [9].

High-angle annular dark-field scanning transmission

electron microscopy (HAADF-STEM) measurements can

provide array images of transition metal atoms located

from the bulk to the surface of NMC particles and spectra

of electron energy loss spectroscopy (EELS). Here, orderly

arrays of atoms can clearly display lay ered structures of the

crystal lattices of NMC (Fig. 9b) [63, 124, 125]. Here, the

degree of surface reconstruction can be de termined based

on changes in atom patterns and a clear boundary between

reconstructed and unreconstructed phases can be traced. In

addition, region thicknesses can also be measured based on

HAADF-STEM images.

Microscopy and Electron Diffraction Scanning electron

microscopy (SEM) and transmission electron microscopy

(TEM) can be used to reveal microscopic structures of

NMC-based cathodes and assess structural damage [124].

In addition, diameters of primary and secondary particles

can be measured from obtained images together with the

thickness of any surface coating layers [100]. Here, aggre-

gates of primary particles, i.e., secondary particles, are usu-

ally observed in electron microscopy images possessing

elongated or even needle-like shapes before degradation,

whereas microcracks on secondary particles can usually be

seen after repeated cycling, which can cause the breakage

of particles. As a result, the spherical shape of secondary

particles is usually distorted, and subfgures of SEM images

obtained from samples aged under diferent conditions are

usually displayed together for comparison. Another indis-

pensable microscopic method is high-resolution transmis-

sion electron microscopy (HRTEM), which can reveal

microregions of Fm3m NiO and spinel structures and allow

for the distance between atomic layers to be clearly marked

(Fig. 9a shows a surface reconstruction region on a NMC-

based cat hode and an electron difraction pattern of selected

spots) [56, 70]. A drawback of HRTEM, however, is that it is

unable to di stinguish between phases with similar structures

and lattice parameters [50].

3.2.2 Chemical Analysis

Elemental Analysis Elemental analysis is necessary for the

physical characterization of NMC-based LIBs and the com-

position of NMC-based cathodes as well as the investigation

of impurities and remedial coatings/dopants. In addition, the

semiquantitative properties of various elemental analyses are

useful in the quantifcation of relative increases/decreases of

certain elements during operation or storage. Here, energy-

dispersive spectroscopy (EDS) mapping analysis can illus-

trate elemental distributions (Li, transition metals, C, etc.)

semiquantitatively on cathode particle surfaces. For exam-

ple, the dissolution of these surfaces through attack with HF

can be evidenced by the increase in F element signals and

the loss of transition metals.

Alternatively, electron-probe X-ray microanalysis

(EPMA) accompanied with SEM can allow for the exami-

nation of transition metal element concentrations within

diferent regions of NMC particles with a spatial resolution

of ~ 0.3 μm and can often be used to determine the con-

centration gradient layer of modifed-NMC particles [126].

Researchers have also reported that the average composition

of Ni, Mn and Co can be measured using atomic absorption

spectroscopy (AAS) [126].

Electron difraction is a useful tool to characterize phases

and microregions in NMC lattice crystals and includes

selected area electron difraction (SAED) and nano-beam

electron difraction (NBED, in which a condenser lens and

an upper objective lens are used to achieve nano-beams of

electrons). Here, Kang et al. [56] have used electron difrac-

tion to measure the space group of NMC-based cathodes

with the zone axis at the bulk and surface regions of a NMC

particle. In addition, electron difraction is usually conducted

separately on diferent regions of particles such as surface

regions, transitional regions and bulk regions. Here, the

existence of secondary phases can be confrmed based on

obtained patterns in which spinel structures can lead to an

additional spot (Fig. 9a2–a5), whereas Fm3m space groups

X-ray photoelectron spectroscopy (XPS) is another ele-

mental analysis method that can detect the chemical and

electronic states of transition metals and oxygen in NMC

cathodes as well as elemental carbon from current collec-

tors and binders [73]. Here, the 2p orbitals of Ni, Mn and

Co and the 1s orbital of O can be captured to reveal oxida-

tion st ates by checking the peak positions of 2p_1/2and 2p_3/2

(Fig. 9c) [127]. For example, the binding energies of Ni 2p_3/2

in NiO and in Ni₂O₃were found to be located at 854.0 and

856.0 eV, respectively [50]. In addition, the shift in oxidation

states can be clearly shown by changes in peak positions,

1 3

Electrochemical Energy Reviews

Fig. 8 a In situ XRD patterns of an NMC 811 cathode over a voltage

range from 3.330 to 4.298 V during the frst charge (Reprinted with

GmbH & Co. KGaA, Weinheim). b Contour plots of TR-XRD pat-

terns at the selected 2θ range for charged b1 NMC433, b2 NMC532,

b3 NMC622 and b4 NMC811 (Reprinted with permission from Ref.

patterns of NMC-811 and Li₂MnO₃-coated (10% and 30%) NMC-811

ican Chemical Society)

and changes in the amount of Ni can be deduced from the

increase in peak area and intensity. Moreover, the use of

cycle-dependent XPS can allow for the change of valence

states to be attributed to a certain range of cycles, whereas a

decrease in XPS peaks can indicate the loss of elements or

the coverage of elements with surface impurities. Further-

more, XPS can enable the identifcation of new substances

on cathode surfaces that evolve during storage or operation

[35]. As for organic elements including C (1s), O (1s), F

(1s), N (1s) and S (2p) that are contained in the components

of SEI layers, XPS can also be used to track the existence of

these elements in diferent molecules versus voltage [128].

To validate XPS data, X-ray absorption spectroscopy

(XAS) can be used as a supplementary technique to moni-

tor transition metal valence changes during charge/dis-

charge and include extended X-ray absorption fne struc-

ture (EXAFS) and X-ray absorption near edge structure

(XANES) [129]. For example, stronger Ni²⁺signals over

Ni⁴⁺in NMC532 at 4.5 V reveal that the redox reaction of

Ni⁴⁺with electrolytes can occur, which reduces Ni⁴⁺to Ni²⁺

[130]. And due to the viability of XAS in the examination

of transition metal concentrations and oxidation states, XAS

has also been widely utilized for anodic elemental analysis

(e.g., transition metals deposited on graphite anodes) under

various cycling conditions [113], leading to the discovery

that the massive dissolution of Mn can occur if NMC111/

graphite cells are charged above 4.6 V [76].

Electron energy loss spectroscopy (EELS) is another

elemental analysis method that can investigate elemental

energy loss and possesses three modes, including auger elec-

tron yield (AEY), total electron yield (TEY) and total fuo-

rescence yield (TFY), each of which provides information

from diferent depths (1–2 nm, 2–5 nm and 50 nm, respec-

tively). Due to smaller probing depths, EELS signals refect

information on the top layer of cathodes [63], and the shift in

energy loss peaks from time to time or from the bulk to the

surface can indicate shifts in oxidation states of elements,

in which lower energy losses translate to lower oxidation

states of transition metals [9]. In general, each EELS curve

consists of two peaks, including a pre-edge peak and a pri-

mary peak. Here, the pre-edge peak of oxygen is thought to

be caused by the transition of electrons from the 1s to the

unoccupied 2p state, which can hybridize with the 3d orbit-

als of transitional metals [66]. Alternatively, the primary

1 3

Electrochemical Energy Reviews

Fig. 9 HRTEM images and FFTs after 50 cycles under 3.0–4.8 V

conditions. a1 Lattice image of the surface region where a2–a5 cor-

respond to the FFTs of regions 1–4, respectively. (11-1)_Cis the dif-

fraction spot of the rocksalt phase of the metal monoxide (Reprinted

Verlag GmbH & Co. KGaA, Weinheim). b1 Low-magnifcation

STEM-HAADF image of one surface primary particle of the x=0.10

sample. b2 Corresponding X-ray spectrum from the highlighted area

in (a). b3 High-resolution STEM-HAADF image showing the lattice

structure from the highlighted area in (b1) (Reprinted with permis-

reserved.). c XPS data for lithiated FCG-Mn-F Li(Ni_0.60Co_0.15Mn_0.25

)

O₂: c1 Ni 2p, c2 Co 2p and c3 Mn 2p (Reprinted with permission

peak originates from the transition of electrons from the 1s

to the 2p state of O or to the 4sp state of transition metals.

Moreover, area-integrated EELS data can be compared to

check the homogeneity of NMC particles [63].

provide relatively precise inf ormation on the amounts of Li

[132], transition metals [133–135] and impurities in exam-

ined samples in which concentr ations can be indicated by

the intensity of characteristic waves emitted from excited

atoms or ions. In addition, changes in the elemental com-

position of electrodes or powder samples due to operation

or modifcation can be refected in ICP-AES data and the

results (in ppm) are often compared with theoretical values

or results obtained by using other elemental analysis tools

to observe diferences. Researchers have also combined ICP

with mass spectroscopy in studies. For example, ICP-MS

was used to reveal signifcant increases in the Ni content

on graphite anodes related to the dissolution of Ni from

NMC 811 cathodes [136] and was also used to conduct a

series of Li content measur ements in sampled electrolytes,

including sample preparation procedures [137], in which the

Electron paramagnetic resonance (EPR) spectroscopy

can also be applied to NMC materials such as NMC622 to

investigate the efects of doping with Al and Fe on cation

mixing in which the change of the Ni²⁺line indicates the

redistribution of Ni within the structure. Hubert Gasteiger’s

group have also demonstrated Li plating onto graphite

anodes quantitatively and in real time by using operando

EPR [106] quantifed the amount of Li-ions not available

for future cycling. Despite these uses, EPR cannot provide

more detailed structural information, which requires other

physical characterization methods [131].

Inductively coupled plasma–atomic emission spectros-

copy (ICP-AES or ICP) is a cutting-edge technique that can

1 3

Electrochemical Energy Reviews

results were pre-calibrated with known Li concentrations

and proved the credibility of the ICP-MS method.

3.2.3 Thermal Stability

Diferential scanning calorimetry (DSC) is a method that

utilizes microcalorimeters to measure heat fow from cells

(W g⁻¹) in which the thermal stability of NMC-based cath-

odes with/without electrolyte contact is recorded to reveal

heat fow from the NMC -based cathode as it is heated to a

certain temperature [33, 140]. For example, in two NMC-

based cathodes and LiNi_0.7Mn_0.3O₂with a concentration

gradient layer, one major peak can be found for each DSC

curve centering at a certain temperature in which a higher

onset temperatur e of the DSC peak indicates better thermal

stability (Fig. 10b) [126]. In addition, exothermic peaks can

be integrated to calculate the amount of heat released, which

can translate into the degree to which corresponding reac-

tions occur [99]. Here, the application of common mitigation

methods usually results in smaller exothermic peak areas and

higher onset temperatures [100]. However, small endother-

mic peaks can also be seen for some cases, indicating the

decomposition of Li salt in the electrolyte [101]. In addition,

TG is often combined with DSC to record mass loss during

the heating of NMC-based cathodes (Fig. 10c), including

processes such as the addition of ca rbon matrixes to remedy

cathode degradation [141 ] or the release of oxygen due to

surface reconstruction [140], or through the loss of surface

impurities and adsorbed species [72].

Gas Measurement Ex situ gas measurements have been

adopted by many researchers in the study of LIBs in which

the volume of evolved gas is measured as the cell is charged

to certain voltage levels [9, 85]. Gas evolution under other

conditions can also be measur ed, including the addition of

electrolyte additives or under diferent compositions of Ni,

Mn and Co. And if combined with mass spectroscopy, the

content of emitted gas can be identifed (usually including H₂,

C₂H₄, CO and CO₂) and allows for volume or moles versus

voltage to be plotted with various curves related to diferent

gases [24]. Furthermore, online electrochemical mass spec-

troscopy (OEMS) is a tool that can facilitate the detection of

gas in real time and can quantify the amount of H₂, CO₂and

O₂from diferent mass to charge ratios [23, 90] in which gas

evolution (e.g., O₂) is related to cathode sur face reconstruc-

tion (if aimed at cathodic processes) through comparisons of

voltage profles (or SOC) (Fig. 10a). Similarly, anodic pro-

cesses with gas evolution can also be tracked by using OEMS.

Researchers have also combined diferential electrochemical

mass spectrometry (DEMS) with diferential electrochemi-

cal infrared spectroscopy (DEIRS) to identify and quantify

gases emitted from NMC 532/graphite cells [84] and found

that potential-resolved gas evolution partially originates from

SEI formation. And if combined with thermogravimetry (TG),

the mass loss of cells can be correlated to oxygen evolution

from NMC-based cathodes heated to a certain temperature,

showing that the majority of weight loss can be attributed to

oxygen release as they both rapidly rise to around 175 °C [85].

Accelerating rate calorimetry (ARC) is another method

adopted by researchers t o measure self-heating rates (SHRs)

versus temperature [8, 142]. For example, in the compari-

son of t he SHR of 4 types of Ni-rich NMC-based cathodes

(Fig. 10d), the onset temperature of a rapidly increasing

SHR marks the upper limit of the temperature range for the

safe operation of a cell [7] in which cathodes with higher

SHR onset temperatures are considered to be better LIB

cathodes for industrial applications. Isothermal microcal-

orimetry (IMC) has also been used to examine heat fow

induced by polarization, entropy and parasitic reactions

versus the state of charge of NMC811 and has revealed that

heat fow originating from parasitic reactions can increase

sharply if the r elative SOC rises from 0 to 1 at certain volt-

ages (Fig. 10e) [7].

Impurities and Organics Raman spectroscopy can be used to

determine the existence of surface impurities that can cause

capacity fading such as nickel carbonate, lithium carbonate

and nickel hydroxide [35]. Because of this, Raman spectros-

copy is a useful tool in the identifcation of impurities pro-

duced during long-term storage (Fig. 3a). Similarly, Fourier

transformed infrared spectroscopy (FT -IR) can be used to

identify impurities on cathode surfaces [72] or organic coat-

ings [138]. For example, the O–H stretching mode from LiOH

possesses a characteristic band between 3650 and 3200 cm⁻¹

in corresponding FT-IR spectra, whereas 1450 and 870 cm⁻¹

are regarded as characteristic peaks of the C=O mode origi-

nating from Li₂CO₃[61]. Raman spectra of NMC-based cath-

odes can also be voltage dependent to monitor band shifts of

A_1gand E_gof Ni, Mn and Co. Moreover, FT-IR and Raman

spectroscopy are widely used in the characterization of anodic

graphite and SEIs because they can identify substances form-

ing SEI layers such as lithium alkyl carbonates and therefore

can reveal reaction mechanisms involved in the SEI forma-

tion process and present diferences in mechanisms if multiple

electrolyte additives are used [139].

3.2.4 Advanced Physical Characterization

Many advanced physical characterization methods have also

been used by researchers to study LIBs. For example, 3D

transmission X-ray tomography mapping [92] can be used to

determine the distribution of transition metals in NMC parti-

cles in which 3D mappings can allow for elemental analysis

within cross-sections at diferent depths. Surface potential

mappings of NMC-based cathodes can also be conducted by

using Kelvin probe atomic force microscopy (AFM) [143] in

which smaller surface potentials can allow for easier electron

1 3

Electrochemical Energy Reviews

Fig. 10 a Specifc charge and

CO₂and O₂evolution rate

profles versus cathode potential

during the frst and the second

charges of four NMC-based

cathodes (Reprinted with

permission from Ref. [90].

Chemical Society). b DSC

traces and c TGA curves of

electrochemically delithiated

FCG Li_0.25(Ni_0.54Co_0.16Mn_0.30

O₂, Li_0.25(Ni_0.5Co_0.2Mn_0.3)O₂

and Li_0.27(Ni_1/3Co_1/3Mn_1/3)O₂

charged to 4.3 V (Reprinted

with permission from Ref.

)

WILEY-VCH Verlag GmbH

& Co. KGaA, Weinheim). d

Self-heating rate as a function

of temperature for delithi-

ated NMC electrode samples

reacting with electrolytes in

accelerating rate calorimeter

experiments (Reprinted with

permission from Ref. [8].

Chemical Society). e Isothermal

calorimetry measurements of

heat fow of extracted polariza-

tion, entropy and parasitic reac-

tions (Reprinted with permis-

The Author(s) 2015. Published

by ECS)

transfer from cathodes and therefore smaller polarizations.

The use of nano-secondary-ion mass spectrometry (nano-

SIMS) analysis can provide good spatial resolution (less

than 50 nm) for elemental mapping [144]. And with higher

resolution, diferent phases can be identifed by comparing

the ratios of Mn to Ni along a direction across the parti-

cle. In situ neutron powder difraction (NPD) can also be

used to obtain more accurate results of lattice parameters

as compared with in situ XRD [145], in which although

the energy of neutrons is lower than X-rays, the penetration

1 3

Electrochemical Energy Reviews

depth is higher. In situ NPD can also be used to determine

the d-spacing of anodic graphite during Li intercalation and

therefore the amounts of LiC₆, LiC₁₂and LiC₂₄[44]. Despite

these advantages, researchers have reported that hydrogen-

containing species can attenuate neutrons. As for prompt

gamma activation analysis (PGAA), this method can be used

to quantify the amount of transition metals deposited onto

graphite anodes by placing the sample in a neutron beam and

collecting signals [26]. The use of PGAA allowed research-

ers to discover that cycling and high cutof voltages can

promote transition metal deposition onto graphite anodes.

water. Furthermore, Li salts can also be used to improve

NMC-based cell performances. For example, LiPF₆can

react with excessive alkaline Li on the surfaces of cathode

particles to produce chemically stable Li₃PO₄and LiF, and

signifcantly suppress parasitic reactions caused by exces-

sive Li [152]. Surface coating is another commonly used

strategy to separate NMC cathode surfaces from electro-

lytes [15], and atom substitution has also been reported to be

efective in enhancing the performance of mitigated Ni-rich

NMC-based LIBs. Based on these strategies, this review sec-

tion will present techniques applied to improve the cycling

performance of NMC cathodes in the most recent decade,

in which cited studies are divided into categories based on

the type of modifcation, including inner surfaces (doping/

gradient layers) and outer surfaces (coating/matrixes) of

Ni-rich NMC-based particles. New synthesis methods of

Ni-rich NMC cathodes that enhance performance without

introducing new substances are also presented.

4 Mitigation Strategies for Ni‑Rich

NMC‑Based Cathodes and Graphite

Anodes

4.1 Ni‑Rich NMC‑Based Cathodes

Ni-rich NMC-based cathodes sufer signifcantly more from

capacity loss and impedance growth as compared with low

Ni content NMC-based cathodes such as NMC111. This is

because large amounts of Ni⁴⁺on cathode surface regions

can trigger side reactions and Ni²⁺can cause cation mixing.

In addition, excessive Li is usually added in the prepara-

tion of NMC-based cathodes and tends to accumulate on the

surface of particles and react with electrolytes to convert to

lithium carbonate and hydroxide. Furthermore, the continu-

ous charging and discharging of these cathodes can cause the

cracking of secondary particles.

4.1.1 Inner Surface

Doping Doping can minimize side reactions on cathode par-

ticle surfaces and enhance the rate of Li difusion and charge

transfer. In addition, active material costs can be reduced if

Co is substituted with other elements. To achieve doping/

atom substitution, researchers often use co-precipitation,

solid-state reaction, self-combustion and co-calcination,

in which the two types of doping include cationic doping

and anionic doping. And in general, few dopants can afect

the layered structure of electrodes, but some can form new

phases (e.g., the presence of a Li₂SnO₃phase in Sn-doped

NMC622 [153] and the presence of Li₂ZrO₃in Zr-doped

NMC532¹⁵⁴).

To address these issues, researchers have proposed many

strategies. One common method is the mixing of LMO with

NMC [45, 118, 124], which can enhance capacity reten-

tion (not review ed in this review because this method does

not directly modify cathode particles) and another method

involves the addition of electrolyte additives. Here, organic

additives can be sacrifced at high voltages (4.3–4.5 V)

because their HOMOs are higher than the HOMOs of

organic solvents to preferentially lose electrons and be oxi-

dized to prevent the oxidative decomposition of solvents

[146]. In addition, the electropolymerization of electrolyte

additives can also provide electrons to counteract the decom-

position of solvents [147]. Moreover, the use of electrolyte

additives can allow for t he f ormation of more even CEIs

on cathode surfaces [148, 149], which can reduce contact

between electrode surfaces and electrolytes to prevent side

reactions and lower impedance. One representative electr o-

lyte additive is fuoroethylene carbonate (FEC) [150, 151]

in which vinyl fuoride and Li₂CO₃are produced as o xida-

tion products during cell operation. Wang et al. [146] have

also reported that 3-IPTS as an electrolyte additive can con-

sume trace amounts of water in electrolytes to minimize the

amount of HF produced from reactions between LiPF₆and

Cation-doped surfaces of Ni-rich NMC particles can ben-

eft from the lowered Ni content and therefore suppressed

cation mixing [155]. Here, the selection of dopants is based

on the bond energy of M–O and the radius of dopant ions in

which trade-ofs between structural stability and Li⁺move-

ment need to be considered. For example, strongly bonded

dopants to oxygen can improve structur al stability and reduce

the amount of oxygen released [156, 157], but in most cases

tend to withdraw more electron density from the M–O bond,

leading to the reduction in oxygen repulsion and t he verti-

cal contraction of the crystal lattice (Fig. 11a) [153], which

negatively a fects Li⁺movement due to incr eased kinetic

barriers [158]. For easier lithiation/delithiation, increased

distances between atomic layers in the lattice structure are

desired and can be achieved by using dopants with larger

radii. For example, dopants such as Fe³⁺and Sn⁴⁺(with radii

larger than substituted Co³⁺) can expand Ni-rich NMC crys-

tal lattices horizontally [145] and Ca²⁺(with a radius larger

than Ni²⁺) can expand lattices horizontally by substituting

1 3

Electrochemical Energy Reviews

Ni at 3a sites [53]. Researchers have also reported that dop-

ing with Al can contr act cathode lattices slightly due to the

smaller radius of Al³⁺[159]. Here, reported results of dif-

ferent cationic dopants can sometimes be contradictory in

which enlarged lattices due to cationic dopants with larger

radii can either promote (mostly reported) or hinder (block-

ing efect, rarely reported) Li⁺difusion in diferent studies.

Because of this, these results need to be comprehensively

reviewed in the future [154, 160].

Gradient Layer Modifcations of Ni-rich NMC-based cath-

ode particles have been investigated by many studies in

which a typical modifcation is a concentration gradient layer

that can cover a Ni-rich NMC core [126, 173–175]. This

concentration gradient layer design originates fr om a simple

core–shell structure in which the shell (single or multiple)

is composed of other NMC compositions (lower Ni content

and higher Mn content) to stabilize the layered structure.

And to overcome drawbacks of voids generated between the

shell and the core during long-term cycling and mismatches

of diferent lattice parameters, gradient layers were invented

to replace core–shell structures. Here, the composition of the

gradient layer is diferent from the core material, in which

Ni atoms are gradually replaced by Mn atoms from the inter-

face to the outer edge, resulting in a surface with higher

structural integrity (Fig. 12a). Furthermore, concentration

gradient layers can reshape primary particles into a rod-like

structure, which can promote Li⁺movement by minimiz-

ing difusion pathways [140]. This feature can be further

magnifed in needle-shaped s tructur es for some cases and

turned into plates if calcined [175, 176]. Modifcations with

concentration gradient layers can also prevent the separa-

tion of cores and shells caused by continuous expansion and

contraction. Moreover, gradient layers can be e xtended into

full concentration gradient (FCG) layers [175, 177, 178] and

multiple gradient layers with diferent slopes can also be

obtained to maximize the Ni content at the core as compared

with single-sloped gradient layers [179]. For example, Noh

et al. [127] reported a NMC (60 25 15) particle with a con-

stant Mn concentration from the center to the surface and

a gradually decreasing Ni and increasing Co concentration

and found that the composition of the surface layer did not

change signifcantly after long-term cycling.

To achieve better cycling stability, capacity is sometimes

sacrifced because some dopants such as Al³⁺are chemi-

cally inactive [28 ]. Cr doping can improve cycling stability

without capacity decr ease [161], and Zr doping can enhance

cycling performance [154, 162] but will reportedly produce

a Li₂ZrO₃phase. In addition, diferent dopants possess dif-

ferent valences and stabilize crystal structures at diferent

sites. For example, Mg²⁺possesses a similar radius to Li⁺

and will enter the Li-layer rather than the transition metal

layer on the surface region of Li-rich NMC particles [125,

163], allowing for the suppression of cation mixing due

to the repulsion of Mg²⁺to transition metal ions and the

delaying of the surface transformation to a spinel structure

(Fig. 11b, c). Alternatively, Na⁺can substitute for Li⁺at

3a sites [52, 164] to enlarge the distance between layers,

and benef tting from the larger radius of Na⁺, cation mix-

ing can be suppressed [165] and weakened Li–O bonds can

enhance potential capacity. However, because Na⁺doping

can enlarge primary particles, optimal amounts are limited

to ensure a connection between primary particles. Chen et al.

have also suggested that the standard of choosing appropri-

ate cationic dopants for NMC cathode improvements had

not been established [53], whereas others have reported

that ions with low valences were favored for Li⁺difusion

[166]. New methods of cation doping have also been pro-

posed in which Mn doping into NMC71515 can provide an

outer region with Fm3m Ni²⁺(reduced from Ni³⁺) residing

in Li slabs (pillar structure) and can be used to consolidate

the layered structure of cathodic crystal lattices and avoid

particle degradation [167]. As for anionic doping, fuorine

is usually used [168 ] and can expand both lattice parameters

by reducing transition metal ions (mainly Ni and Mn) [169]

and increase mean charging voltages due to incr eased bond

energy of Li–F as compared with Li–O [170]. In addition,

fuorine substitution can protect cathode materials from

HF. Furthermore, although bromine and sulfur atoms can

be used to enhance NMC cathode performances, few stud-

ies have shown their efect on Ni-rich NMC-based cathodes.

And currently, studies into the surface modifcation of Ni-

rich NMC-based cathodes have been insu f cient and the

e f ects of dopants such as Ti [171], Zn [160], Cl and Br

[172] and their infuences on Ni-ric h NMC-based cathode

cycling stability need to be further explored.

4.1.2 Outer Surface

Li_xM_yO_zcoating Li_xM_yO_zcoatings can be applied to the sur-

face of primary particles to enhance performance and are

composed of two subgroups. One group involves layered

NMCs with diferent compositions/structures as compared

with core Ni-rich NMCs and usually possesses higher Co

or Mn content. Here, a Mn-rich NMC shell can signif-

cantly enhance the chemical and structural stability of Ni-

rich NMC cores [180, 181] in which studies have shown

that the coating of Mn-r ich spinel oxide onto Ni-rich NMC

can signifcantly impr ove chemical stability in both opera-

tion and storage [144, 182]. The second group includes all

other types of Li_xM_yO_zcoatings (diferent transition met-

als or structures) and also includes pioneering works with

disparate coating materials and methods in which Si, Zr,

Ti and Al are usually used. For example, Li₂TiO₂with a

rocksalt structure can be coated onto NMC532 due to its

superior structural integrity and strong adhesion to the core

1 3

Electrochemical Energy Reviews

Fig. 11 a Lattice parameter trends of Co and Sn-doped NMC-622

cathode materials in fully lithiated (black squares) and 60% delithi-

ated (red squares) states (left) along with corresponding schemat-

ics of Li-interlayer movement along the c-axis due to doping efects

bare and Mg²⁺pillared Li-rich cathode materials. b Mn migration

in bare Li-rich materials from the TM slab to the Li slab through

the adjacent tetrahedral site. c Prohibition of Mn migration in

Mg²⁺-doped layered oxide by electrostatic repulsion between Mg²⁺

and TM ions (Reprinted with permission from Ref. [125]. Copyright

1 3

Electrochemical Energy Reviews

[183] and an ionically conductive Li₂ZrO₃coating layer on

NMC71515 can form large islands of the coating material,

protect the core and facilitate Li movement [184]. In addi-

tion, Li₂SiO₃is also a promising coating material because it

is a fast ionic conductor that can provide two pathways for

Li⁺movement [(010) and (001) planes] [185]. Furthermore,

a recent study using LPO infusion to coat NMC particles

has gained much attention due to the creation of solid–solid

interfaces in the gap between primary particles to replace

liquid–solid interfaces, thus reinforcing the microstructure

by shielding of liquid electrolytes and eliminating side reac-

tions within secondary particles (Fig. 12b) [97]. Researchers

have also suggested that other types of cat hode materials

with more cycling stability can be used as coating materials

such as LFP [186].

deposition (usually used to enhance coating uniformity

through the precise control of coating thicknesses to several

nanometers or even thinner) of Al₂O₃on NMC532 [101,

130] can suppress parasitic reactions and phase transitions in

which one study by Dahn group suggested that performance

stabilizations were due to the protection of cathode surfaces

to surface-oxidized species rather than the prevention of

electrolyte oxidation [91]. Furthermore, researchers have

also reported that the crystallization of amorphous TiO₂

applied on NMC811 surfaces can result in Li₂TiO₃, which

can further inhibit phase transformation from H2 to H3 [96].

The use of metal oxide layers has also been reported to be

able to function as mediators of HF attacks due to the protec-

tion they can provide to NMC cores from corrosion [194].

The application of Li-free coatings can also lead to issues

in Ni-rich NMC-based cathodes. For example, Park et al.

have reported that excessive Li on cathode surfaces as caused

by the coating of metal phosphate [187] can cause gas evolu-

tion due to its possible transformation into lithium carbonate

and hydroxide as well as further side reactions with elec-

trolytes [61]. However, researchers have also reported that

the use of phosphate can lead to the formation of lithium

phosphate [195] and lithium transition metal phosphate,

both of which can maintain cell performance. Alternatively,

FePO₄coatings have been shown to be resistive to cathode

dissolution in electrolytes [192] and V₂O₅coatings [189]

applied through a wet coating method by using NH₄VO₃

were found to be capable of removing excessive Li. Moreo-

ver, the introduction of acids or metal oxides can allow for

excessive Li on the surface of cathodic particles to trans-

form into Li_xM_yO_zcoatings at certain temperatures [196].

For example, researchers suggested that the use of excessive

surface Li on NMC811 to form a protective layer consisting

of Li⁺-conductive Li_xAlO₂and superconductive Li_xTi₂O₄

[197] was a cost-saving and efcient method to resolve the

issue of excessive Li and improve conductivity.

Li‑Free Coating Li-free coatings on the outer surface of

NMC cathodes have been extensively investigated due to

a unique advantage over doping in which the valence of

transition metal ions remains unafected and so does the

theoretical capacity. Here, materials used for Li-free coating

include transition metal phosphates [187], metal o xides [99,

188, 189], fuorides [143 , 190 ], silicon oxide [91, 100 ] and

organic coating layers [138 , 191] in which the gener al con-

cept is to prevent cathode sur faces containing highly reactive

Ni⁴⁺from contacting the electrolyte and therefore to reduce

side reactions. Here, coating layer thicknesses are usually

between 5 and 40 nm and similar to doping, XRD meas-

urements usually do not show new characteristic peaks and

instead, existing peak widths/positions may change slightly.

In addition, the metal ions in coating layers tend to difuse

into the cathode lattice and can change lattice parameters

[192]. To meet the basic demands of coating materials, high

conductivity (coatings tend to possess higher resistances and

are even insulating, however) and frm adherence to NMC

cathodes are preferred [138]. And to optimize cycling sta-

bility, the weight ratios of coating precursors need to be

carefully estimated because excessive coating can block Li

transport pathways and result in large polarization and low

Li-ion difusivity. This careful estimation of coating precur-

sor weight ratios can also improve the uniformity and disper-

sity of precursor particles at the micrometer scale [141, 188].

Organic coating layers possess a unique advantage of

being stretchy, which allows for the absorption of internal

microstrains and the mitigation of primary particle crack-

ing [191]. And recently, a novel coating method using the

tr ansesterifcation of organophosphates has been reported

[198] in which surface coatings were achieved through the

replacement of M–OH bonds of NMC622 with M–O–P

bonds. Here, this novel coating difers from others because

it can be achieved through the chemical deactivation of sur-

face groups. Researchers have also reported that like doped

fuorine, f uoride coatings can also be used to prevent HF

attacks [190]. For example, YF₃-coated NMC111 can exhibit

enhanced electron transfer due to smaller surface potentials

and reduced oxidation of electrolytes due to smaller work

functions [143]. Despite these promising results, the num-

ber of studies into metal fuoride coatings on Ni-rich NMC-

based cathodes is limited. Alternatively, sulfated ZrO₂as a

Among various metal oxide coatings studied, Al₂O₃has

received the mos t attention due to multiple benefcial fea-

tures (Fig. 12c) [12]. For example, Chen et al. have found

that coating layers of Al₂O₃and nano-TiO₂can suppress

cation mixing (Li⁺and Ni²⁺) as indicated by the ratio of

I₀₀₃–I₁₀₄in XRD [99, 188] and that Al₂O₃layers can reduce

self-redox reactions b y introducing new energy levels to pre-

vent the reduction of Ni ions by O²⁻. In addition, researchers

have also found that Al₂O₃surface coatings can maintain

stabler CEI structures [188, 193] and that the atomic layer

1 3

Electrochemical Energy Reviews

Fig. 12 a SEM and EPMA results of precursor hydroxide (left) and

fnal lithiated oxide Li(Ni_0.64Co_0.18Mn_0.18)O₂(right) (Reprinted with

STEM-HAADF images, EDS maps and schematic of an LPO-infused

Ni-rich NMC layered cathode (Reprinted with permission from Ref.

Al₂O₃-coated LiNi_0.6Co_0.2Mn_0.2O₂powder (Reprinted with permis-

reserved.). d SEM image of an NMC 532 cathode obtained through

fltration with 5% CNTs (Reprinted with permission from Ref. [202].

coating material has shown particular promise [193]. This

is because diferent from regular/ultrathin ZrO₂coatings

[199], sulfated ZrO₂coatings can incorporate sulfate groups

into coating layers and functionalize coatings by forming

surface impurities due to the coating separating the core

from the atmosphere [200].

Carbon Matrixes A novel method to enhance the perfor-

mance of Ni-rich NMC-based cathodes is to build carbon

matrixes containing active material particles. Examples of

this method include studies involving graphene nanosheets

for NMC811,¹⁴¹carbon n anotube conductive networks for

NMC532 (Fig. 12 d) [201, 202], and interwoven carbon fb-

ers for NMC622 [203 ]. Her e, the matrixes chosen are usually

highly conductive carbon materials that can enhance charge

−

–SO₃– to stabilize CEIs on cathode surfaces, indicating

that these coatings to a large extent are modifable and func-

tional groups can be added to gain additional functionalities.

Researchers have also reported that graphene oxide-coated

NMC532 does not possess a carbon matrix but a thin layer

covering primary particles, allowing for the reduction of

1 3

Electrochemical Energy Reviews

transfer due to 3D connections. In addition, these matrixes

can prevent side reactions and stabilize particle structures

by fully covering primary cathode particles with a conduc-

tive matrix. Furthermore, carbon matrixes can enhance

fexibility and strengthen adhesion between primary par-

ticles. Numerous unconventional and cutting-edge studies

have also emerged in recent years involving the reshaping

of aggregated NMC primary particles through the use of

carbon matrixes. For example, multi-shelled alginate hollow

fbers were used to template NMC primary particles, which

allowed for the accommodation of Ni, Mn and Co atoms

within an organic fber matrix that possessed pinholes to

ensure Li⁺exchange [204].

4.1.4 Synergy of Multiple Mitigation Methods

Recently, many studies have focused on the synergy of two

or more mitigation methods to achieve better overall Ni-rich

NMC-based cathode performances in which the simplest

examples involve multiple dopants or coatings. For example,

Al and Fe can be co-doped into NMC622 [131] to combine

the polarization efects of Fe with the lower chemical poten-

tial attributed to Al. As for multiple coatings, this can be

achieved with the help of organic coating layers, which can

supplement the insufciencies of single-type coatings. For

example, polypyrrole is a fexible and conductive polymer

that is typically used in this type of modifcation in which

researchers have reported a dual-conductive coating layer of

polypyrrole on NMC811 primary particle surfaces that can

convert excessive Li into Li₃PO₄[191]. In another study,

researchers coated a Zr-doped NMC532 wit h polypyrrole to

form a network that prevented HF attacks [157].

4.1.3 Advanced Preparation Methods

Aside from materials for NMC development, preparation

methods are also being investigated due to similar infuences

on the cycling/thermal stability of Ni-rich NMC materials,

commonly allowing for the reduction of cation mixing. For

example, Du et al. [154] have reported that NMC532 sintered

in oxygen can exhibit lower degrees of cation disordering,

less oxygen defects and higher discharge capacities. These

researchers also reported that if Ni²⁺was partially replaced

by Li⁺, the remaining Ni existed as Ni³⁺, leading to capac-

ity loss [154]. In addition, researchers have also reported

that the atomization co-precipitation method can be used to

obtain homogeneous spherical shaped primary NMC parti-

cles by forming an aerosol of precursors with an ultrasonic

nebulizer [205] and that a similar efect can be achieved by

using a self-combustion method, which produces massive

heat fow to accelerate synthesis and therefore improve the

uniformity of NMC particles [206]. Furthermore, nanoscale

surface treatments utilizing Co and Li precursors combined

with high-temperature sintering have been used to transform

NMC622 surfaces into a Co-rich area with a NiO rocksalt

phase that can act as a pillar layer to prevent inter-slab col-

lapse [129]. Moreover, a hydrothermal procedure following

the regular co-precipitation synthesis of NMC (70 15 15)

can reportedly accelerate Li⁺entering and exiting Li slabs

by making use of PVP to reduce crystal growth along the

(001) plane and favor crystal growth along t he (010) plane,

which is an unimpeded pathway [207]. Dahn et al. [208]

have also used a washing and reheating method to synthesize

single crystal NMC622 in which superior performances as

compared with multi-crystal NMC622 can be achieved if

a remedial electrolyte additive was added. Carbonate co-

precipitation and impregnation methods can also reportedly

be combined to synthesize porous NMC622 microspheres

that can efectively precipitate Ni²⁺and shorten Li difusion

pathways [209].

Apart from coatings and dopants, a newly developed

trend is to combine surface modifcations with gradient lay-

ers, which can prevent phase transitions of NMC811 and

NCA particles [210]. In addition, the cathodic doping of

Al into NMC (61 27 12) with FCG has been reported to be

able to consolidate and reduce grain boundaries and improve

mechanical properties [211]. Moreover, researchers have

combined gradient layers (the core) with Mn-rich surface

coatings on NMC532 and reported that the valences of Ni

and Co can be kept at lower states than regular NMC532

particles. Similarly, NMC811 with an exterior concentra-

tion gradient layer was uniformly coated with Al₂O₃through

freeze-drying and displayed better capacity retention as com-

pared with untreated samples [212]. Furthermore, LiPON as

a conductive material used to coat anodes can also report-

edly be applied to NMC811 with a surface gradient layer

to maintain rate capability and improve capacity retention

[213].

4.1.5 Summary

Despite novel synthesis methods for NMC particles (which

usually require more demanding conditions and extra pro-

cedures), modifcations are usually applied to diferent parts

of Ni-rich NMC primary particles to protect Ni-rich NMC

cores. Among these, dopants and coatings are the most

extensively studied methods. Here, dopants can afect sur-

face elemental compositions and lower the Ni content, which

is reactive and can generate spinel phases during cycling. In

addition, dopants can expand/contract LiNi_xMn_yCo_1−x−yO₂

lattices and infuence Li⁺movement through bulk particles.

Furthermore, dopants possessing diferent valances from

substituted ions can afect the valences of Ni, Mn and Co

and therefore increase/decrease their capacity as the capac-

ity is achieved by the change of valence of these elements.

1 3

Electrochemical Energy Reviews

Additional efects such as the repulsion of cation disorder

and the prevention of HF corrosion can also be achieved

through doping. As for coatings, these can impact NMC

performances by separating surfaces from electrolytes and

hindering side reactions in which optimal performances can

be achieved through the accurate estimation of the amount

of coating applied.

4.3 SEI

4.3.1 Electrolyte Additive

LIBs need to be safely transported and operated. Here, the

addition of relatively small amounts of electrolyte additives

can improve overall cell safety without the introduction of

any detrimental products. For example, vinylene carbonate

(VC) is one of the most extensively utilized electrolyte addi-

tives because it can improve compatibility between SEIs and

graphite anodes by forming a thin flm to separate electro-

lytes from graphite anodes and enhance the performance

of ionic liquid-based electrolytes [128]. Researchers have

also hypothesized that VC can reduce SEI thicknesses by

inhibiting electron transfer from graphite anodes to sol-

vents. Despite many positive infuences of VC as reported

by Dahn group [107], researchers have also suggested that

the highly resis tive flm formed with VC can facilitate Li

plating [217] and that the polymerization of VC can limit

LIB lifespans. Here, many additives have been tested to

replace V C to resolve polymerization issues in which FEC

[139], PES [218] and VEC have been used to reinforce SEIs

because they possess higher LUMOs that can prevent the

reductive decomposition of VC by sacrifcing themselves

(Fig. 13a, b). In addition, FEC can form an SEI that is dura-

ble to fas t charging, thus eliminating Li plating on graphite

anodes [219]. Researchers have also reported that the use

of FEC can suppress [139] or slow down [220] SEI for-

mation (formation of lithium alky l carbonate) to result in

a thinner SEI. The use of co-solvents can also lower the

viscosity and the melting point of solvents and therefore be

useful in practical cases (Fig. 13c). For example, 2% PES

with 1% DTD and 1% TTSPi (PES211) can enhance the

cycling performance of NMC cells based on its composi-

tion [142]. Researchers have also reported that PTSI as a

less investigated additive can generate better SEIs with less

inorganic components in which due to the presence of S=O

groups, PF₅can be suppressed as delocalized N cores can

act as weak base sites [112]. And more recently, 3-sulfolene

(3SF) as a f lm-forming agent was used to reduce electrolyte

reduction [221]. Additives that stabilize SEI can be divided

into tw o subgroups including SEI formers and SEI modifers

[222] and phenyl carbonate, regarded as a “modifer” addi-

tive, can outperform VC by maintaining low impedance and

gas production in NMC111/graphite pouch cells and there-

fore can be used to stabilize cells [222]. In addition, phenyl

carbonate can remain stable until 4.2 V and possess good

compatibility with 2% vinylene carbonate (VC). And since

electrolyte additives can be multi-functional because certain

types of additives can react with electrodes to produce gas

if cells are overcharged to unsafe voltages, which results in

pressurized internal environments that can deactivate cells

[222], the ability to reduce gas production makes phenyl

4.2 Graphite Anode

4.2.1 Testing Protocols

The rate of charge, as limited by the production of plated Li,

can be increased by applying the constant-current constant-

voltage (CCCV) protocol and the pulse charging protocol

[105] in which Li concentration gradients are alternatingly

enlarged and reduced during charging with the CCCV pro-

tocol and concentration diferences between anode surface

regions and bulk electrolytes are minimized in the pulse

charging protocol.

4.2.2 Surface Coating

Aside from testing protocols, substantial barriers involv-

ing materials being applied to graphite surfaces through

different means can fully separate graphite anodes from

electrolytes, leading to significant reductions in the

reductive decomposition of solvents, and subsequent

conversion and thickening as well as the suppression of

exfoliation. For example, LiPON as a recently investi-

gated material can be evenly coated onto graphite anodes

and can protect the anode from high-temperature storage

aging [214].

4.2.3 Electrolyte Volume Adjustment

The performance of full cells is strongly dependent on the

amount of accessible active material. And because both

electrodes and the separator in full cells are porous, they

need to be sufciently wetted. In addition, electrolyte vol-

umes can also be increased to maximize the amount of

accessible active material; however, higher electrolyte

volumes do not directly translate into higher capacities in

which researchers have reported that excessive electrolytes

tend to decrease performance because higher fractions of

SEIs can dissolve into increased electrolyte volumes [215].

This principle is also valid in pouch cell studies showing

the lowering of impedance and increase in cycling stabil-

ity [216].

1 3

Electrochemical Energy Reviews

Fig. 13 a HOMO and LUMO energy levels of EC, FEC and VC. b

Schematic showing the efects of VC- and FEC-derived SEIs on the

fast charging capability of graphite anodes (Reprinted with permis-

reserved.). c Up: capacity versus cycle number for NMC111/graph-

ite pouch cells (unclamped) containing select additives or additive

blends. Cycling was conducted between 2.8 and 4.2 V at 55 °C at

80 mA. Bottom: capacity versus cycle number for NMC442/graph-

ite pouch cells (unclamped) containing select additives or additive

blends. Cycling was conducted between 3.0 and 4.4 V at 45 °C at

The Author(s) 2015. Published by ECS)

carbonate a promising type of electrolyte additives. Further-

more, PBF and PPF as electrolyte additives have also been

reported to be able to positively enhance SEIs [36].

of SEIs with capping agents such as oxalate in which the

capping agents can reductively decompose electrolytes and

prevent LiF from aggregating into large particles, thus pro-

ducing LiF nanoparticles 5 nm in diameter uniformly in

SEIs [224]. Furthermore, the reduction in LiF aggregation

and the formation of nanostructured LiF can form uni-

form regions for Li⁺difusion and reduce lithium plating

(Fig. 15). Researchers have also reported that LiTFSi can

outperform conventional LiPF₆salts due to its resistance

to trace amounts of water and therefore minimize the pro-

duction of harmful species [225].

4.3.2 Lithium Salt

A simple method to enhance the quality and durability

of anodes and SEIs is to raise the concentration of Li

salts to above 3 M [223], which can signifcantly reduce

polarization that can lead to negative efects such as den-

drite growth and slow charging. In addition, the resulting

solvation of Li-ions possesses a disparate structure from

regular Li-ions in which the planar shape of the solvated

Li-ions benefts from Li intercalation and can mitigate

graphite exfoliation with smaller spaces being occupied

during operation [223]. Advanced Li salts have also been

widely investigated to obtain better performances with

promising results; however, few have been commercial-

ized. In addition, a new group of Li salts (Fig. 14) have

been reported to signifcantly enhance the performance

4.3.3 Solvent

The polarity and the viscosity of solvents also need to be

considered in the selection of appropriate solvents for Li

intercalation/deintercalation in which solvents with lower

viscosity can facilitate Li⁺transportation and decrease

impedance, whereas the polarity of a solvent can deter-

mine the degree of solvation of Li⁺inside [225]. Here,

ethyl methyl carbonate and dimethyl carbonate are usually

1 3

Electrochemical Energy Reviews

toward cathode degradation. Alternatively, graphite anodes

can function collaboratively with NMC-based cathodes

to provide promising performances at cell levels. How-

ever, the aging of graphite anodes due to the formation

of dendrites as caused by the massive plating of Li⁺onto

graphite surfaces remains the main reason for increased

impedance. Furthermore, side reactions involving elec-

trolyte reduction (SEI formation) on graphite anodes can

cause Li loss after initial charging, and the thickening and

decomposition of SEIs in subsequent cycles can cause

aging behaviors.

To gain a deeper understanding of degradation mecha-

nisms of Ni-rich NMC/graphite LIBs, emerging testing

protocols and characterization methods have emerged in

which currently, research on LIB degradation is focused on

the elucidation of the degree of capacity fading in certain

mechanisms so as to distinguish major factors and allow

for the design of specifc remedies. And as a result of this

research, mitigation strategies, functioning individually or

synergistically, have been proposed to resolve long-term

cyclability issues, most of which have proven to be promis-

ing. Among these, dopants, gradient layers, coatings (both

Li-free and Li-contained), carbon matrixes and advanced

synthesis procedures have become mainstream methods to

enhance the performance of Ni-rich NMC-based cathodes.

In addition, coatings, advanced protocols and the use of new

electrolyte components can be applied to ameliorate anodic

performances (SEIs and graphite). And with these mitiga-

tion strategies, the morphology, element composition and

reactivity of electrodes can be improved.

Fig. 14 Comparison of a Coulombic efciency versus cycle number

and b the total sum of reversibly cycled Li over 50 cycles obtained

from LiFePO₄/Cu cells (Reprinted with permission from Ref. [224].

Although differences in the performance of various

Ni-rich NMC-based cathodes have been discovered, these

remain unexplained and the future of degradation mecha-

nism investigations needs to rely on more advanced physi-

cal techniques such as X-ray tomography, operando neutron

difraction and online electrochemical mass spectroscopy,

all of which can provide more comprehensive information

in operando and be less destructive to cells. And for the

deeper understanding of mitigation strategies for Ni-rich

NMC/graphite LIBs, trade-ofs in modifcation techniques

and controversies need to be taken into considerations. Here,

several prospective research directions for Ni-rich NMC/

graphite LIBs are proposed:

considered [219]. In addition, ionic liquids as substituents

of conventional solvents can form thicker SEIs with high-

capacity retentions at 90 °C [226].

5 Summary and Prospective

LIBs using Ni-rich NMC-based cathodes such as NMC811

can produce high specifc capacities (200–220 mAh g⁻¹)

and promising energy densities (~ 800 Wh kg⁻¹) in com-

parison with conventional LiCoO₂(~ 570 Wh kg⁻¹) and

LiMn₂O₄spinel (~ 440 Wh kg⁻¹) materials. Because of

this, these materials have received extensive attention from

researchers. However, due to low portions of manganese

as structural stabilizers, large amounts of Ni⁴⁺on cath-

ode surface layers/regions can trigger side reactions and

Ni²⁺can cause cation mixing. As a result, enhanced mass-

specifc capacity comes at the expense of rate capability

and structural stability in these Ni-rich cathode materials,

leading to severe capacity fading. In addition, other factors

such as active material dissolution, oxygen release and the

intergranular cracking of primary particles also contribute

1. Future experiments should focus more on the quantita-

tive interpretation of diferent aging behaviors in NMC-

based cathodes with or without mitigation methods as

well as the understanding of correlations between dif-

ferent degradation mechanisms.

2. Eforts should be made toward profling the performance

of LIBs under harsher conditions (stressors) that more

closely represent practical operational conditions.

1 3

Electrochemical Energy Reviews

Fig. 15 a, b Proposed mechanisms of LiDFOB acting as a capping

agent for LiF nanoparticle generation; c, d models of SEIs from c

LiDFOB and d LiBF₄+LiBOB electrolytes; and e, f schematics of

difusion felds at Li plated from each electrolyte. Each Li plate pos-

sesses active and inactive areas on its surface (Reprinted with permis-

istry)

Open Access This article is distributed under the terms of the Crea-

tive Commons Attribution 4.0 International License (http://creativeco

mmons.org/licenses/by/4.0/), which permits unrestricted use, distribu-

tion, and reproduction in any medium, provided you give appropriate

credit to the original author(s) and the source, provide a link to the

Creative Commons license, and indicate if changes were made.

3. Studies should be conducted to investigate and optimize

the synergistic efects of multiple mitigation methods

on Ni-rich NMC-based cathodes that can outperform

the efects of single mitigation methods and enhance

capacity retention to higher levels.

4. Inexpensive and nondestructive characterization tools

need to be developed to decrease research costs and pre-

vent cell destruction, especially in the inline detection

of cells. In terms of EIS that is time-consuming, fast

impedance needs to be developed to accelerate measure-

ment processes.

References

1. Iturrondobeitia, A., Aguesse, F., Genies, S., et al.: Post-mortem

analysis of calendar-aged 16 Ah NMC/graphite pouch cells for

EV application. J. Phys. Chem. C 121 , 21865–21876 (2017).

https://doi.org/10.1021/acs.jpcc.7b05416

5. Time-dependent EIS in combination with diferential

specifc capacity measurements should be used as a

characterization tool to analyze surface phases/structures

versus impedance growth.

2. Julien, C.M., Mauger, A., Zaghib, K., et al.: Comparative issues

of cathode materials for Li-ion batteries. Inorganics 2, 132–154

(2014). https://doi.org/10.3390/inorganics2020132

3. Ahmed, S., Nelson, P.A., Gallagher, K.G., et al.: Cost and energy

demand of producing nickel manganese cobalt cathode material

for lithium ion batteries. J. Power Sources 342 , 733–740 (2017).

https://doi.org/10.1016/j.jpowsour.2016.12.069

Acknowledgements We wish to thank the Energy Storage Program

of the National Research Council Canada (NRC) for its fnancial sup-

port as well as the support from the Energy, Mining and Environment

Research Center of the NRC, Vancouver. Open discussions with team

members Shawn Brueckner, Ken Tsay, Svetlana Niketic and Khalid

Fatih and assistance from Tamara McLaughlin on literature searches

are also highly appreciated.

4. Wentker, M., Greenwood, M., Leker, J.: A bottom-up approach to

lithium-ion battery cost modeling with a focus on cathode active

materials. Energies 12, 504 (2019). https://doi.org/10.3390/

en12030504

5. Zheng, J.M., Yan, P.F., Zhang, J.D., et al.: Suppressed oxygen

extraction and degradation of LiNi_xMn_yCo_zO₂cathodes at high

charge cut-o f voltages. Nano Res. 10 , 4221–4231 (2017). https

://doi.org/10.1007/s12274-017-1761-6

Compliance with Ethical Standards

6. Xia, Y ., Zheng, J.M., Wang, C.M., et al.: Designing principle for

Ni-rich cathode materials with high energy density for practi-

cal applications. Nano Energy 49 , 434–452 (2018). https://doi.

org/10.1016/j.nanoen.2018.04.062

Conflict of interest The authors declare no conficts of interest.

1 3

Article Doi

Ni/NHC-catalyzed cross-coupling of methyl sulfinates and amines for direct access to sulfinamides

DOI: 10.1016/j.tetlet.2019.151260

Source and publish data:

Authors:

Article abstract of DOI:10.1016/j.tetlet.2019.151260

Full text of DOI:10.1016/j.tetlet.2019.151260

Products guided by the article

R&D Labs maybe for 31401-17-9

Relevant to this article

Hot Product