Degradation mechanisms and modification strategies of nickel-rich NCM cathode in lithium-ion batteries

Ni-rich Lithium Nickel Cobalt Manganese Oxide (NCM) cathode materials have garnered attention for their high specific capacity, but they grapple with issues of cycling stability, thermal performance, and safety. This concise yet comprehensive review embarks on an exploration, commencing with an examination of fundamental characteristics, including crystallographic structures and electrochemical properties. It delves into the intricate failure mechanisms contributing to capacity degradation and thermal instability. The review places emphasis on major material-focused modification techniques, encompassing surface coatings and multifunctional additives, all scrutinized for their potential to enhance both performance and safety. Furthermore, it spotlights pivotal research domains, notably novel synthesis methods, positioned to reshape the landscape of Ni-rich NCM technology. The review also emphasizes future development directions, aiming for simplified and cost-effective methodologies to tackle the complexities of nickel-rich cathodes. Ultimately, this review offers a forward-looking analysis, envisioning a future marked by safer, higher-capacity lithium-ion batteries, underscoring an enduring commitment to scientific and technological progress.


Introduction
The persistent global environmental challenges posed by carbon dioxide and other greenhouse gases have spurred individuals to actively seek alternatives to fossil fuel-based energy sources like oil, in favor of cleaner energy solutions.In this regard, it's important to acknowledge the pivotal contributions of John Goodenough, who is credited with pioneering the lithium-ion battery in 1980, and Sony, which successfully commercialized the rechargeable lithium-ion battery in 1991.This battery technology has since emerged as the predominant power source for the next generation of electric vehicles (EVs) due to its remarkable attributes, including high energy density and extended cycle life [1][2][3].According to the 'Net Zero Emissions Scenario 2050' projections, it has been forecasted that by 2030, there will be an estimated 300 million electric vehicles (EVs) in circulation, constituting over 60% of new vehicle sales, whereas their market share was just 4.6% in 2020 [4].Although it's only 4.6%, in 2020 it still achieved a daily saving of 0.5 million barrels of diesel and gasoline.Furthermore, by 2030, the widespread adoption of electric vehicles over internal combustion engine vehicles is expected to lead to a substantial reduction of approximately 3.5 million barrels per day in oil consumption [5].In many modern electric vehicles, lithium-ion batteries commonly employ LiNi x Co y Mn 1-x-y O 2 (NCM) as their primary electrode material.To meet the growing demand for electric vehicles with extended range and enhanced capabilities, scientists have developed nickel-rich NCM materials, denoted as LiNi x Co y Mn 1-x-y O 2 (x 0.8).This material offers several advantages, including cost-effectiveness, reduced toxicity, a high practical specific capacity exceeding 200 mAh g −1 , and a high operating voltage [6][7][8].However, increasing nickel concentration, while boosting energy density, introduces challenges such as reduced battery life and compromised thermal stability.These issues arise from the intricate interplay of factors involving structural and chemical transformations (figure 1(A)) [9].The United States Advanced Battery Consortium (USABC) has set a specified calendar life of 15 years for electric vehicles operating within a temperature range of 30 °C to 52 °C.Achieving this duration exceeds the electrochemical capabilities of lithium-ion batteries using cathodes composed of Ni-rich NCM materials.To address the inherent challenges associated with Ni-rich NCM materials and elevate their performance to meet the demands of energy storage systems, a comprehensive understanding of the material, including its degradation mechanisms and modification strategies, is essential.Furthermore, with the swift rise in market demand for nickel-rich cathodes, the cost of nickel sulfate has doubled over the past two years.Tackling challenges related to nickel-rich cathodes and extending their lifespan will be a crucial factor in achieving substantial cost savings within the electric vehicle industry.This review paper will primarily examine the failure mechanisms of Ni-rich NCM batteries, starting with an exploration of the fundamental characteristics of Ni-rich cathode materials.It will subsequently delve into a comprehensive analysis of the failure mechanisms associated with Ni-rich cathodes.Finally, the review will discuss the latest material-based advancements in modification techniques, concluding with a prospective analysis of the future evolution of Nirich NCM technology.Future development directions, aiming for simplified and cost-effective methodologies to tackle the complexities of nickel-rich NCM cathodes is also provided.

Fundamentals of Ni-rich NCM cathode materials
The Ni-rich NCM cathode material, known as LiNi x Co y Mn 1-x-y O 2 , shares a crystal structure isomorphic to NaFeO 2 .NaFeO 2 possesses a rhombic crystal structure with an O3-type layered oxide arrangement.This structure comprises a cubic dense stacking lattice surrounded by oxygen atoms, as illustrated in figure 1(B) [10].The distinctive layered structure, characterized by the alternating arrangement of transition metal and lithium layers, facilitates the intercalation and de-intercalation of lithium ions during the charging and discharging processes.The development of Ni-rich NCM cathode materials can be traced back to the widespread adoption of LiCoO 2 (LCO) as an efficient cathode material in commercially available lithium-ion batteries.However, it is important to highlight that LCO has a limited practical capacity range of 140-170 mAh g −1 and is burdened by significant costs, primarily due to the high expense of its primary constituent, cobalt [11].Subsequent research endeavors focused on elemental substitution, leading to the discovery that LiNiO 2 (LNO) exhibited favorable reversible capacity at low potentials and shared a layered structure akin to LCO.However, when the upper limit of the cut-off charge voltage was raised, a rapid decline in LNO's capacity during cycling became evident, resulting in shortened cycle life [12].Researchers then embarked on a quest to identify alternative elements to substitute for Ni, ultimately culminating in the development of a ternary compound characterized by enhanced capacity and stability.Through the process of substituting Ni partially with Co and Mn, a cathode material known as LiNi x Co y Mn 1-x-y O 2 (NCM) was synthesized, showcasing improved capacity and stability during cycling.Each of the three elements, Ni, Co, and Mn, plays a distinct and crucial role in the performance of NCM cathode materials.The reversible capacity of the system is primarily attributed to the redox pairs Ni 2+ /Ni 3+ and Ni 3+ /Ni 4+ .The inclusion of Co enhances the ordering of the lamellar structure and enhances the system's rate capability.Additionally, Mn plays a pivotal role in maintaining a stable tetravalent state and stabilizing the local structure, particularly in the highly excited states [13].
The electrochemical properties and structural stability of NCM materials are significantly influenced by their Ni content.As Ni content increases, so does the material's capacity.However, this capacity increase comes at the expense of reduced thermal stability and capacity retention.As depicted in figure 1(Cb) [14], the chargedischarge curves suggest that Ni-rich NCM cathodes, characterized by higher Ni-content, yield a greater specific capacity but exhibit compromised structural stability (figure 1(Ca)).This trade-off is a result of structural and chemical evolution of Ni-rich cathode materials subject to high charging and deep de-lithiation states.The structural and chemical evolution could be attributed to a complex interplay of factors.This includes the aging of the Ni-rich layered structural characteristics of the materials, the lattice strain generated during lithium de-/ intercalation processes, and phase transitions occurring during charge and discharge cycles.Additionally, the complex interactions occurring at the surface-electrolyte interface also play a pivotal role in the structural and chemical evolution.Direct electrochemical observation of this evolution can be achieved through the analysis of dQ/dV curves for a series of NCM cathode materials with varying nickel contents during charge and discharge cycling (figure 1(D)) [15].Initially, the LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM111) material exhibits a single set of redox peaks, signifying the phase transition from hexagonal (H1) to monoclinic (M).However, with increasing nickel content, additional redox peaks emerge.When the nickel content reaches 80%, four distinct redox peaks become apparent, indicating a more intricate structural and chemical evolution during charging.This evolution involves multiple phase transitions from hexagonal (H1) to monoclinic (M) and further to hexagonal (H2 and H3) phases.The advantages of high capacity in Ni-rich NCM cathodes are offset by the challenges posed by the relatively low potential and thermodynamic instability of the H2 →H3 phase transition [16].Additionally, at a high state of charge, the generation of highly active Ni 4+ ions leads to parasitic reactions at the electrolytecathode interfaces, further compromising the cathode's stability.These structural and chemical evolutions in Ni-rich cathode materials give rise to several underlying performance degradation mechanisms, including exacerbated Li/Ni cation disorder [17,18], oxygen evolution [19,20], layered-spinel-rocksalt phase transition [21][22][23], intergranular and intragranular microcracking [24][25][26], and transition metal dissolution [27] (as depicted in figure 2).Collectively, these mechanisms contribute to performance degradation and a shortened cycle life for Ni-rich NCM cathode materials [28][29][30][31].The degradation pathways of Ni-rich NCM cathode materials, contributing to these challenges, are complex and interconnected.Comprehensive investigations are imperative to fully comprehend the underlying mechanisms and devise more targeted modification strategies.

Degradation mechanisms of Ni-rich NCM cathode materia
Nickel-rich NCM cathodes have garnered significant attention in both industry and academia owing to their impressive capacity and remarkable energy density.However, as nickel content increases, critical challenges have emerged, notably a decline in capacity retention and thermal stability.In this section, we embark on a comprehensive exploration of the degradation mechanisms that afflict these Ni-rich cathodes.Our goal is to shed light on the origins and interrelationships of the intricate factors responsible for degradation, including Li/ Ni disorder, transition metal dissolution, irreversible phase transitions, surface reconstruction, as well as O 2− oxidation and oxygen release.The repercussions of these phenomena, such as intergranular and intragranular microcracking, will also be incorporated in discussion.

Li/Ni disorder
Cation disorder in Ni-rich NCM materials is primarily associated with Li/Ni disorder, characterized by structural occupancy irregularities.Specifically, this occurs when Ni 2+ ions, originally situated in the 3b site, intrude into the 3a site typically occupied by Li + ions.The prevailing consensus in the scientific community attributes this phenomenon to the close radii of Ni 2+ (0.069 nm) and Li + (0.076 nm) ions, which promote the intercalation of Ni 2+ into the Li layer.This displacement of Li + ions and compresses the layer spacing due to the smaller Ni 2+ radius within the Li layer, causing a localized collapse of the Li layer space.Consequently, this poses challenges during the lithium intercalation in the discharge process [32,33].As the charge and discharge processes progress, the severity of Li-Ni mixing increases.
Characterizing Ni/Li disordering involves the utilization of a range of techniques, including x-ray diffraction (XRD), neutron powder diffraction (ND), transmission electron microscopy (TEM) imaging, and scanning transmission electron microscopy (STEM).Among these methods, XRD takes center stage as a frequently employed approach for assessing Ni/Li disordering, relying on two vital parameters to quantify the extent of Ni and Li ion intermixing: the I(003)/I(104) integrated intensity ratio and the merging of peak splits for (018) and (110) pairs [34,35].A ratio of integrated intensities I(003)/I(104) less than 1.2, along with the merging of the (018) and (110) peaks, signifies the occurrence of Ni and Li ion mixing.Furthermore, researchers frequently employ the Rietveld refinement technique to analyze XRD patterns and determine the concentration of Ni 2+ at the 3b sites.During the Rietveld refinement process, the disordered arrangement model can be applied.Alternatively, ND also stands out as a highly effective method for directly quantifying the content of Ni/ Li exchange [36].
Cationic mixing near the surface of stacked LiTMO 2 particles can be readily observed using bright-field TEM imaging and by analyzing related electron diffraction patterns [11,37].Liu et al [38] conducted cycling tests on NCM811 at cut-off voltages of 4.3 V and 4.6 V, revealing that the battery capacity experienced more severe decay under a voltage cycle of 4.6 V (see figure 3(A)).Later, through using STEM to observe the electrode cycled at 4.6 V, they found it had severe lithium-nickel mixing disorder.The presence of transition metal (TM) atoms within lithium planes is directly visible as lighter-gray-intensity dots (indicated by red arrows) within the darker Li layers in the magnified STEM image of figure 3(Ba).The disordered phase of TM/Li is characterized by the presence of tiny domains randomly distributed throughout the bulk grains.The intensity profile depicted in figure 3(Bb) reveals additional peaks between the two TM plane peaks, as indicated by the red arrows.These additional peaks have lower intensities, offering further evidence of TM atoms within the Li layer.Figure 3(C) illustrates an atomic model depicting the migration of TM.In the presence of TM atoms, the original Li sites become occupied by these atoms, preventing Li + ions from returning to their initial positions.Consequently, this leads to a reduction in Li content within the disordered domains.
The effective management of Li/Ni disorder can be achieved through the identification of the optimal sintering temperature or by adjusting the precursor type [39,40] and lithium salt ratio [41,42].Ronduda et al [43] delved into the impact of calcination temperature on cathode materials, specifically LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NCM622).Their findings revealed that at higher temperatures, the material exhibited a well-ordered structure, evident from the powder XRD results (figures 3(D) and (E)).However, it was also observed that materials subjected to elevated calcination temperatures displayed an increased level of Li/Ni disorder, leading to a decline in their electrochemical performance.In a separate study by Zheng et al [44] , a unique ion-exchange technique was employed to manipulate Li/Ni disorder in layered materials.This approach successfully yielded NCM materials with an organized layered structure, minimal Li/Ni mixing, and exceptional performance.The LiNi 0.85 Co 0.06 Mn 0.09 O 2 (NCM85) material was synthesized through the ion-exchange process of a sodium-ion layered oxide (NaNi 0.85 Co 0.06 Mn 0.09 O 2 ) with Li + ions.The ion-exchange reaction took place at a notably low temperature of 300 °C, employing a mixture of molten salts, specifically LiCl and LiNO 3 , as the lithium source.The deliberate selection of this lower temperature, 300 °C, was made to ensure that the lithium salt remains in a molten state, thus meeting the required thermodynamic conditions for the reaction.This temperature selection also balances kinetic factors, favoring a faster diffusion rate for cations over anions.As a result, the anionic sublattice can be effectively preserved, leading to the stabilization of the crystal structure during the cationexchange reaction.This controlled temperature regulation plays a pivotal role in managing Li/Ni disorder, as illustrated in figure 3(F).

Transition metal dissolution
Transition metal (TM) ions are expected to remain within the cathode during successive Li + intercalation and deintercalation cycles.However, trace amounts of TM elements have been detected in the electrolyte and on the anode after extended periods of cell cycling, particularly at elevated temperatures [45,46].These TM ions dissolve from the cathode, traverse through the electrolyte, and ultimately deposit on the anode.This dissolution-migration-deposition (DMD) process is closely linked to the capacity degradation of the battery [47,48].Understanding the underlying mechanisms, causes, conditions, processes, influencing factors, and potential control strategies related to the dissolution of transition metals is of paramount importance for improving the electrochemical performance of nickel-rich ternary lithium-ion batteries.
The Jahn-Teller effect (J-T) in NCM materials has been linked to the dissolution of transition metals (TM) in these materials.Manthiram et al [49] offered an explanation for the dissolution of NCM transition metals in connection with the Jahn-Teller effect.In most transition metal oxides (TMOs), the TM cation adopts a configuration with six oxide ligands symmetrically arranged along the vertical axis, forming an octahedral coordination structure referred to as O h .Within this O h coordination field, the d-orbitals of the transition metal cation undergo splitting, resulting in the formation of the triple-simplex t 2g orbitals (d xy , d xz , and d yz ) and the double-simplex e g orbitals (d z 2 and d x 2-y 2 ).Configurations with entirely filled, half-filled, or vacant e g orbitals (e.g., t 2g 3 e g 0 , t 2g 3 e g 2 , and t 2g 6 e g 0 ) are more favorable for transition metal cations that exhibit a triple symmetric electron density.However, in the case of the Ni 3+ ion in a low spin state, electron densities deviate from a cubic distribution due to the occupation of a single orbital within the double simplex e g (e.g., e g 1 ).The Jahn-Teller theorem suggests that this unfavorable state can be mitigated by elongating the two axial TM-O bonds while constricting the other four flat-voltage links.This alteration shifts the TMO 6 symmetry from cubic to tetragonal, facilitating the relaxation process.Consequently, the lattice containing Ni 3+ is prone to local lattice deformation.When local distortion occurs, changes in the lengths of individual TM-O bonds do not significantly impact the overall macrostructure, resulting in a state of dynamic lattice instability.This dynamic lattice instability exhibited by transition metal oxides during local Jahn-Teller distortion makes them highly reactive towards acids, ultimately leading to the solvation of transition metal ions.
The dissolution of transition metals from the cathode is intricately linked to the generation of hydrofluoric acid (HF) within the organic electrolyte.The production of hydrogen fluoride (HF) occurs through the reaction of a salt containing polyfluoroanions in the electrolyte with a small amount of water, or it can result from the decomposition of the organic electrolyte.Consequently, hydrogen ions (H + ) within the electrolyte initiate an attack on the cathode's surface, ultimately leading to the dissolution of transition metals.This phenomenon, along with the repetitive charge and discharge cycles, triggers structural transformations in the NCM material, shifting it from a layered configuration to spinel-and rocksalt-like structures on the surface.These structural changes can contribute to the performance degradation of the material over time.
Anisotropic lattice variations significantly influence the dissolution of transition metals.Yoon and his coworkers [50] discovered that the anisotropic lattice variations between in-plane (a lattice) and out-of-plane (c lattice) directions are crucial factors leading to intergranular microcracks among primary particles.The severity of lattice deformations intensifies as lithium ions are extracted from the host structures at high states of charge.Experimental confirmation of intergranular microcrack formation was obtained using a stoichiometric full-Nicontent LiNiO 2 cathode.Lattice strains in the a and c directions exhibited nonlinear variations with the vacancy content in the Ni-rich layered oxide, with the c lattice strain playing a dominant role in anisotropic lattice variations.The cumulative anisotropic lattice variations and rapid structural collapse during phase transitions were observed to induce severe intergranular microcracks at the grain boundaries among primary particles.The presence of microcracks within the cathode material leads to an expanded contact area between the interior of the particles and the electrolyte.This expansion accelerates the degradation of the cathode material and exacerbates the dissolution of transition metals [51].When comparing particles that have undergone cracking and pulverization with those in their pristine state, as illustrated in figure 4(A), it becomes apparent that there is a noticeable increase in the absence of transition metals as a results of more TM dissolution.This observation implies that transition metal ions tend to be released more readily from sites that have undergone fracturing.
The correlation between the dissolution of transition metals and the charging cutoff voltage is particularly noteworthy.Taylor Gasteiger et al [52] conducted in situ x-ray absorption spectroscopy (XAS) studies on NCM622-graphite cells to observe the deposition of transition metals on the anode.Changes in metal content at the graphite surface were tracked during both the charging and discharging processes, as shown in figure 4(B).Notably, there is no substantial increase in transition metal concentration at the graphite surface during the initial two cycles, up to 4.6 V.However, as the voltage rises to 4.8 V, there is a significant upsurge in the concentration of deposited transition metal on the graphite surface.This clearly illustrates the correlation between the charge state and the dissolution of transition metals.Subsequently, simulations were conducted involving the dissolution of transition metals by introducing nickel-manganese-cobalt salts into the electrolyte.Charge-discharge test results indicated that the inclusion of nickel and cobalt salts had minimal impact on battery performance.Conversely, the introduction of manganese salt in the electrolyte resulted in a reduction in battery capacity and an accelerated rate of decay.Moreover, this detrimental effect intensified with an increase in the manganese salt content (refer to figure 4(C)).

Oxygen release
The primary safety concern associated with lithium-ion batteries is oxygen release [53].This occurs as O 2− anions undergo oxidation during charging, resulting in the production of either O 2 or reactive peroxide species.The release of oxygen can be ascribed to the structural instability of multilayer oxide cathode materials [54].These chemical processes linked to oxygen release are typically exothermic, setting off a chain of successive events that can ultimately pose a significant hazard, potentially leading to the catastrophic self-ignition of the battery [55].
Oxygen release is a complex phenomenon that occurs at distinct stages during the charging and discharging cycles of battery materials.In a study conducted by Wei and his colleagues [56], they delved into the structural transformations of LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) during the overcharge process.To unravel this intricate process, they harnessed a range of in situ spectroscopic and electron microscopy techniques.Notably, they employed isotopic labeling with 18 O to trace oxygen atoms on the surfaces of secondary particles, aiding in pinpointing the source of oxygen emissions at different stages of the charge-discharge cycle.By correlating the variations in O 2 evolution with the overcharge curve (see figures 5(A) and 5(B), the researchers delineated the process into three distinct stages: (I) No oxygen evolution was observed below the standard cutoff voltage of 4.3 V. (II) A minor amount of O 2 was released within the voltage range of 4.3-5.5 V. (III) A significant quantity of O 2 was discharged, aligning with the overcharge plateau around 5.5 V.These comprehensive spectroscopic characterizations, including in situ XRD, in situ Raman spectroscopy, and solid-state nuclear magnetic resonance (NMR) results, unveiled a profound structural transformation occurring in stage II (figure 5(C)).This transformation was attributed to the excessive extraction of lithium and oxidation of transition metals (TM) [57].In contrast, there was no obvious structural evolution in the ultrahigh-voltage state of stage III.This apparent discrepancy with the significant O 2 evolution eliminates the possibility of oxygen release from the core or the surface of secondary particles.Further investigation using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis (as illustrated in figure 5(D)) revealed the presence of notable fractures and microcracks following high-voltage charging and discharging.These microcracks were found to be distributed both between and within the grains of the material.Consequently, the oxygen release observed in stage III was attributed to substantial internal micro-stress within the lattice, induced by the accumulation of lithium vacancies.This, in turn, led to rapid oxygen evolution, accompanied by the initiation and propagation of fractures or microcracks, ultimately impacting the overall material integrity.
The oxygen release phenomenon in NCM cathode materials is influenced by various factors.The correlation between O 2 release and state of charge (SoC) is notable, as these materials tend to become increasingly unstable at high degrees of delithiation [58].In a study by Wandt et al [59], conducted using operational emission spectroscopy, it was observed that both NCM111 and NCM811 cathodes exhibit oxygen release when subjected to an SOC exceeding 80% (figure 6(A)).Another illuminating investigation by Sun et al [60] focused on the secondary particles of various LiNi x Co y Mn z O 2 cathodes.They delved extensively into the generation and aggregation of oxygen vacancies on the cathode surface, conducting a quantitative analysis of oxygen vacancy distribution (figure 6(B)).Their findings shed light on the role of high nickel content in facilitating the diffusion of oxygen vacancies from the cathode's surface into its interior, increasing their concentration within secondary particles and boosting oxygen release.When nickel content in LiNi x Co y Mn z O 2 exceeds 0.8, both the material's surface layer and bulk show higher concentrations of these vacancies (figure 6(C)).
Numerous studies have investigated the correlation between oxygen release and the concurrent sequence of transition metal reductions.Ligand field theory (depicted in figure 6(D)) explains how the transition metal 3d orbitals split into e g and t 2g orbitals owing to an octahedral ligand field created by anions.In layered rocksalt oxides, Mn 4+ (3d 3 ) typically exists in a high spin state, while Co 3+ (3d 7 ) and Ni 2+ (3d 8 ) are in low spin states [61].The t 2g orbitals of Ni 2+ and Co 3+ , along with the up-spin t 2g orbitals of Mn 4+ , are fully occupied.For NCMs where Ni valences exceed +2, such as NCM523 (Ni 2.4+ ) and NCM622 (Ni 2.67+ ), the Fermi level E F lies within the partially filled Ni up-spin eg band.Assuming a rigid band, both the reduction process that results in oxygen release and the oxidation process derived from delithiation cause shifts in the Fermi level of transition metals in NCM.Specifically, it moves upward during reduction and downward during oxidation.As a result, the reduction of Ni 3+ occurs first, initiating the subsequent release of oxygen and other transitions.Subsequently, we observed the reduction of Co 3+ , suggesting that the Co up-spin e g band is positioned above the Ni up-spin e g band.The stability of Mn 4+ during both reduction and oxidation indicates that the energy gap between certain occupied and unoccupied bands can accommodate other specific bands.This electronic structure model aligns with reported magnetic properties, spectroscopic data, and explains the sequence of transition metal reductions in NCMs (Ni 3+ > Co 3+ > Mn 4+ ) [62].Nakamura et al [63] conducted an investigation focusing on the charge compensation mechanism associated with the oxygen release behavior of Ni-rich NCM materials, arriving at similar conclusions.In their study, they combined oxygen coulometric titration and soft synchrotron x-ray absorption spectroscopy (XAS) techniques to delve into this phenomenon.Through empirically analyzing oxygen release patterns, it was found that the energy needed for Ni 3+ reduction, resulting in oxygen release, falls within the lower range of approximately 0.5-1.5 eV.In contrast, Co 3+ reduction demands a higher energy range, typically ranging from 1.8-2.7 eV (refer to figure 6(E)).

Surface reconstruction and irreversible phase transition
The surface structure of cathode materials plays a pivotal role in influencing battery performance throughout its application.This influence encompasses critical aspects such as redox reaction kinetics, as well as the overall structural and cycling stability of the battery [64][65][66].As the battery undergoes charge and discharge cycles, the Ni-rich cathode surface experiences chemical changes and structural transformations at the cathode/electrolyte interface.This phenomenon is often referred to as an in situ surface reconstruction process.During this process, the cathode surface evolves from a layered structure to a spinel-type surface reaction layer (SRL).These changes directly contribute to the degradation of the battery's overall performance [21,[67][68][69][70][71][72][73].Gaining a more profound insight into the surface phase transformation processes helps in unraveling the underlying causes of degradation in Ni-rich cathodes.Furthermore, it provides valuable insights into how surface modifications can be harnessed to improve cyclability [74,75].
The formation of the rock-salt phase during surface reconstruction hampers Li + ion diffusion pathways and subsequently reduces ionic conductivity.This rock-salt phase, resembling NiO in crystal structure, epitaxially grows on the periphery of cathodes with high nickel content during surface reconstruction.Grey et al [76] employed in situ synchrotron-radiation powder x-ray diffraction (SR-PXRD) to propose a degradation process in NCM811, driven by surface reconstruction.Figures 7(Aa) and (Ab) provide a comparison of distinct structural changes occurring at the surface of NCM particles.In instances where a thin or absent rock-salt surface layer is formed, it facilitates unhindered expansion and contraction of the NCM lattice, allowing the active material to achieve the complete state of charge (SoC) range.Conversely, the formation of a thick rock-salt layer impacts the lattice planes within the layered structure at the interface.This effect, attributed to pronounced interfacial lattice strain, particularly along the c-direction, pins the lattice planes due to the presence of the rocksalt phase.Consequently, this impedes further contraction of the layers, which is essential for subsequent delithiation.
Surface reconstruction occurs more rapidly at higher voltage and temperature conditions.Liu et al [77].conducted a study on the surface reconstruction of 75% delithiated NCM811 at various temperatures.It was observed that as the temperature increases, NCM powders swiftly transition from a layered structure to a spinellike structure, ultimately transforming into a rock-salt structure, as depicted in figure 7(B).Liu et al [38].established charging cutoff voltages of 4.3 V and 4.6 V, respectively.They then investigated the impact of voltage on cathode surface reconstruction by examining the surface morphology of the cathode subjected to various voltage levels during cycling.The formation of the rock-salt phase resulting from surface reconstruction is evident on the cathode material's surface during both 4.3 V and 4.6 V cycles.Upon closer examination, it is observed that the material retains a layered structure with a well-organized arrangement of transition metal and lithium atoms within the inner measurement of the yellow line in figure 7(C)a.However, on the outer side of the yellow line, the previously low-intensity lithium surface is replaced by a more intense transition metal surface, resulting in the formation of a rock-salt phase characterized by significant lithium-oxygen depletion.The rocksalt phase resulting from a 4.6 V cycling process exhibits a thickness of at least 15 nm, whereas the rock-salt phase created by a 4.3 V cycling process is predominantly less than 5 nm in size, as depicted in figure 7(C).Therefore, it can be concluded that the degree of surface reconstruction intensifies with an increase in the cutoff voltage of the cycle.

Improvement strategies
Addressing the intrinsic challenges outlined in the preceding section concerning Ni-rich NCM cathode materials, significant research endeavors have been directed towards exploring modification strategies.These strategies aim to not only enhance specific capacity but also extend cycle life and ensure safety [78,79].In the forthcoming section, we will provide an extensive overview of these modification strategies for Ni-rich cathode materials.We will explore various aspects, including doping and coating, structural modifications such as single-crystal structure fabrication, concentration gradient structure design, and the introduction of a second phase.

Element doping and protective layer coatings
Element doping and surface coating represent two widely adopted and effective strategies for modifying Ni-rich NCM cathode materials.Doping involves the introduction of cations into either the Li site or transition metal site within the cathode material.In cases where the Li site is targeted, elements such as Mg [80][81][82] and Na [83] are commonly used as 'pillar ions'.Their role is to mitigate Li/Ni mixing, expand lithium-ion pathways, alleviate bulk structure phase transformations, and enhance charge and discharge reversibility.On the other hand, when the transition metal site is the focus, elements like Ta [84,85], Zr [86][87][88][89], and Al [90][91][92].are often employed to stabilize crystal structure and enhance electronic conductivity, thus reducing the degradation of the cathode material and leading to improved cycle life and overall performance.In addition to cation doping, anion doping, such as F − and Cl − [93][94][95][96].can enhance the binding energy between the cations and anions, thereby stabilizing the structure and guarding against transition metal dissolution, oxygen release, and associated phase transitions and intergranular cracking in the cathode during charge and discharge [66].Both cation and anion doping have shown promising results.
M Stanley Whittingham et al [118], introduced a dual modification approach for enhancing LiNi 0.8 Mn 0.1 Co 0.1 O 2 , utilizing lithium-free NbO y .The procedure involved blending NCM811 powders with niobium ethoxide in a flask, followed by the addition of ethanol.After overnight stirring, the mixture underwent ethanol evaporation at 80 °C.Subsequently, both pristine NCM811 and Nb-modified NCM811, in the specified molar ratio, were subjected to sintering in a pure oxygen atmosphere for 3 h, spanning temperatures from 400 to 800 °C.This approach involves both doping and coating processes.Notably, the coating layer comprises LiNbO 3 /Li 3 NbO 4 , and Nb substitution occurs at transition metal sites, leading to the incorporation of a portion of the Mn into the surface niobate layer.The Nb coating serves to stabilize the cathode surface, reduce initial capacity loss, and enhance cycling performance.On the other hand, Nb doping contributes to improved capacity retention during extended cycling by providing lattice stabilization (as illustrated in figure 8(A)).
Liu et al [119], employed a dual modification approach on the NCM811 involving the application of a La 2 Li 0.5 Al 0.5 O 4 coating and Al doping.This dual modification had a substantial impact on reducing gas emissions during the electrochemical processes.In particular, when voltage increased, the unmodified cathode (LLA 0) released both CO 2 and O 2 , while the modified cathode (LLA 3) exhibited minimal CO 2 release, as depicted in figures 8(Ba) and 7(Bb).Throughout electrochemical cycling, figure 8Bc and 7Bd depict the schematic evolution of morphology and structure for LLA 0 and LLA 3. The direct interaction between the electrolyte and NCM811 resulted in irreversible phase transitions and detrimental side reactions, facilitating the dissolution of transition metals, the release of CO 2 and O 2 , and the precipitation of lattice oxygen.These processes contributed to the formation of intragranular cracks.The analysis above clearly demonstrates that the dual modification approach on NCM811 effectively mitigated performance degradation in several key aspects.First, it prevented direct contact between NCM811 and the surrounding electrolyte.Meanwhile, it impeded the migration and dissolution of transition metal ions, while also preventing irreversible phase transitions.In addition, it efficiently inhibited the formation of cracks and oxygen vacancies.

Structural modifications
In response to the inherent challenges related to the structural instability of Ni-rich cathodes during cycling, researchers have delved into various structural modification techniques.These approaches encompass the preparation of single crystals, the establishment of concentration gradients, and the incorporation of a secondary pillar phase.These endeavors are geared towards bolstering the structural stability of cathode materials when subjected to repetitive lithium de/intercalation processes.The ultimate goal is to enhance capacity retention, extend cycle life, and improve thermal stability.
Compared to Ni-rich cathode materials with a polycrystalline structure, single-crystal structures offer several notable advantages.In polycrystalline nickel-rich cathode materials, primary particles aggregate to form secondary particles, but this connection is not very robust.Consequently, during cycling, internal cracking tends to occur, exacerbating side reactions between the electrode material and the environment.In contrast, singlecrystal cathode materials consist entirely of micron-sized primary particles, with an optimal particle size of around 3 μm.These materials do not suffer from intergranular cracking due to anisotropic lattice distortion and stress concentration.Moreover, single crystals have larger dimensions and smaller specific surface areas, resulting in a reduced effective exposed area and minimized contact with the electrolyte.This characteristic helps mitigate side reactions between the material and the electrolyte.Additionally, single-crystal materials exhibit greater resistance to stress compared to polycrystalline materials, allowing for a higher charging cut-off voltage.In research conducted by Wei and his colleagues [46], it was observed that under overcharge conditions, oxygen primarily originates from the grain boundary region.Because single-crystal materials have fewer grain boundaries and a more intact crystal structure, the development of single-crystal NCM811 (SC-NCM811) effectively suppresses overcharge-induced oxygen generation.Transmission electron microscopy (TEM) results confirm that SC-NCM811 maintains a more intact morphology after the completion of overcharging.Furthermore, SC-NCM811 can reduce capacity decay following overcharging and suppress the increase in interfacial impedance.
To mitigate the challenges posed by interfacial issues arising from the high nickel content in nickel-rich NCM materials, which include surface reconstruction, transition metal dissolution, oxygen release, and the subsequent issues of poor capacity retention and thermal stability, researchers have pursued the design and fabrication of NCM cathode materials with a concentration gradient.This design involves having a lower Ni content on the surface and a higher Ni content in the core of the particles.For instance, Niu and his colleagues [120], developed a Ni-rich cathode, NCM811, using a nickel-based MOF (Metal-Organic Framework) as the precursor.During high-temperature sintering, a stable oxygen-depleted protective layer forms on the surface of the primary particles.Simultaneously, a gradient of nickel concentration is established within the secondary particles, gradually decreasing from the interior to the exterior.The resulting Ni-rich NCM cathode with a concentration gradient, referred to as D-NCM811, demonstrated significantly improved cycling stability compared to the unmodified cathode.It achieved an impressive energy density of 216.4 Wh kg −1 in a 300 mAh pouch cell while retaining 84.1% of its initial capacity after 500 cycles at a 1.0 C magnification rate, as illustrated in figure 9(A).
A novel approach to mitigating the inherent structural evolution and associated issues arising from lattice strain buildup during repeated lithium de/intercalation in Ni-rich NCM cathode materials involves the incorporation of a secondary pillar phase.The primary culprit behind phenomena like microcracking and irreversible phase changes is the accumulation of strain within the lattice.
This strain results from the redox reactions of transition metals, causing the oxygen octahedra to contract, thereby shortening the lattice's a-axis [121,122], and the Coulombic repulsion among oxygen atoms altering the c-axis distance [123,124].As the lattice undergoes continuous charge and discharge cycles, it experiences cycles of expansion and contraction until it eventually succumbs to degradation.Many contemporary approaches aim to alleviate the detrimental effects of lattice strain by increasing the dimensions of the a and c axes and employing microscopic modulation to reduce stress concentrations.However, these methods can only mitigate the adverse effects of lattice strain without addressing the core issue [125,126].To address this problem at its root, Lu and his colleagues [127].introduced a layered chalcogenide variant phase known as La 4 [LiTM]O 8 (LLMO, TM = Ni, Co, and Mn) as a strain-delaying component within the layered structure.This stable perovskite structure, which combines a layered structure with a rock-salt structure, is ideally suited for a layered cathode structure that promotes the strain-retardant strategy (as depicted in figure 9(B)).Moreover, the similarity in lattice parameters between LLMO and layered oxides ensures the potential for coherent growth.Compared to conventional materials, the modified material exhibits a remarkable reduction of nearly 70% in lattice strain evolution per cycle.This substantial improvement enhances morphological integrity and significantly boosts cell cycling performance, as evidenced in figures 9(C) and (D).

Summary and outlook
This review conducts an examination of the current state of nickel-rich NCM cathode materials within the context of lithium-ion batteries.It explores the intricate degradation mechanisms that afflict Ni-rich cathodes and evaluates a diverse range of strategies aimed at enhancing their performance.The challenges faced by Nirich NCM cathodes are multifaceted.Factors such as cation disorder, bulk phase transformation, transition metal dissolution, surface reconstruction, and oxygen release are intricately linked to the structure characteristics of these cathodes and the lattice strain generated during lithium de-/intercalation cycles and phase transitions.Additionally, the complex interactions occurring at the surface-electrolyte interface also contribute to a cascade of degradation processes.These processes manifest primarily as capacity loss, reduced cycle life and compromised safety.In response to these multifaceted challenges, researchers have proposed a spectrum of strategies aimed at ameliorating the performance of Ni-rich cathodes.These strategies encompass various approaches, including doping, coating, and microstructure optimization, all geared towards mitigating the adverse effects that Ni-rich cathodes endure during cycling.These endeavors aim to extend the operational lifespan of Ni-rich NCM cathode materials.While these solutions mark substantial progress, the pursuit of greater capacity and enhanced performance from nickel-rich cathodes remains an ongoing challenge.Additionally, the real-world scale-up production of Ni-rich NCM materials introduces complexities, including challenges in the material preparation process, stringent preparation environments, inconsistent product quality, and heightened risk of lithium volatilization, further impacting material performance.Addressing these challenges necessitates exploration of simpler and more cost-effective methodologies for nickel-rich cathodes.Potential avenues for future research include: 1. Mechanistic Understanding: A deeper exploration of the interrelationships among degradation mechanisms is vital.Identifying the most pivotal mechanism contributing to capacity decline and addressing it effectively is key.
2. Multifunctional electrolyte additives: multifunctional electrolyte additives are indispensable in enhancing the overall performance, safety, and durability of Ni-rich NCM cathode materials in lithium-ion batteries.
Ni-rich NCM cathodes are prone to undesirable side reactions with the electrolyte, which can lead to capacity fade, reduced cycling stability and damaged thermal stability.Multifunctional additives can form protective layers on the cathode surface, inhibiting these side reactions.This helps maintain a stable electrodeelectrolyte interface over multiple charge-discharge cycles, improving capacity retention and increasing battery safety.
3. Solid-State Electrolytes (SSE): Solid-state electrolytes provide a more stable and secure interface between the cathode and anode, mitigating the risk of thermal runaway and fire hazards associated with traditional liquid electrolytes.This innovation addresses safety concerns linked to the use of nickel-rich cathodes, offering a promising avenue for safer and more reliable lithium-ion battery technology.Additionally, solid-state electrolytes contribute to improved energy density and overall performance, making them a key focus in advancing the safety and efficiency of next-generation battery systems.
4. Cobalt-Free Alternatives: Recognizing the environmental and scarcity concerns associated with cobalt mining, researchers are actively working on reducing cobalt content in NCM cathodes.Cobalt-free NCM variants are gaining attention for their potential to mitigate supply chain challenges and environmental impact.The future may witness widespread adoption of such cobalt-free cathode materials.

Artificial intelligence (AI):
The complexity of understanding intricate material behaviors, degradation mechanisms, and optimal compositions makes AI a valuable tool for data analysis and pattern recognition.Machine learning algorithms enable researchers to analyze vast datasets, identify correlations, and predict material properties, accelerating the discovery and optimization of nickel-rich cathodes.Additionally, AI facilitates the simulation of electrochemical processes, aiding in the design of cathode materials with enhanced performance and durability.As the field continues to evolve, the integration of AI promises to revolutionize the exploration and development of advanced materials for next-generation lithium-ion batteries.
6. Advanced Characterization Techniques: State-of-the-art characterization equipment encompasses a wide array of powerful tools and methodologies.These tools are instrumental in providing comprehensive insights into the complex processes occurring within nickel-rich cathodes during charge and discharge cycles.
7. Fast-Charging Capabilities: As the demand for fast-charging solutions grows, NCM cathode materials are under intense scrutiny to facilitate rapid charge and discharge rates.Innovations in NCM formulations, particle morphology, and electrode designs will be crucial for meeting the demands of high-power applications that require swift charging capabilities.
8. Sustainability and Recycling: With a growing emphasis on sustainability, future NCM cathodes may incorporate recycled materials and employ eco-friendly manufacturing processes.Recycling strategies for lithium ion batteries, including cathode materials, will play a pivotal role in reducing environmental impact and conserving critical resources.
In summary, this review underscores the intricate nature of Ni-rich NCM cathode materials in lithium-ion batteries, emphasizing the imperative of ongoing research and innovation.Through a comprehensive analysis of failure mechanisms and a deep exploration of innovative material-based modifications, this paper offers a guiding path toward unlocking the complete potential of Ni-rich NCM technology.The future outlook holds the potential to revolutionize the energy storage arena, with the ultimate aim of delivering lithium-ion batteries that are not only safer but also capable of higher capacity and enhanced efficiency.

Figure 2 .
Figure 2. Current challenges arising in Ni-rich cathode-based LIBs and emerging solutions.

Figure 4 .
Figure 4. (A) Elemental mapping of Ni, Co, and Mn in LiNi 0.87 Co 0.09 Mn 0.04 O 2 particles at pristine, cracked, and broken states [51].(Reprinted from Nano Energy, Copyright (2019), with permission from ScienceDirect) (B) Concentration changes of Ni, Mn, and Co on the graphite anode with the cycling of NCM622/graphite cell.(Reproduced from [52].CC BY 4.0) (C) The charge-discharge curve of batteries containing various transition metal ions in the electrolyte.(Reproduced from [52].CC BY 4.0).

Figure 5 .
Figure 5. (A) Charge-discharge curves before overcharge cycle (blue line), overcharge curve (red line), and after overcharge cycle (green line), and obvious electrochemical property degradation could be observed after this overcharge behavior [56].(Reprinted from Advanced Energy Materials, Copyright (2022), with permission from Wiley) (B) OEMS results for O 2 evolution along overcharge process [56].(Reprinted from Advanced Energy Materials, Copyright (2022), with permission from Wiley) (C) Revealing evolution of the bulk structure using in situ and ex situ diffraction/spectroscopy techniques [56].(a) In situ XRD and (b) related refinement crystal data for the whole overcharge process.The initial pattern could be indexed as the O3-type structure c) In situ Raman d) ex situ 7 Li ss-NMR scans at different stages.(e)-(g) Ni, Co, and Mn K-edge XANES scans for different stages.D Spatial and temporal evolution of structural degradation of NCM811 [56].(Reprinted from Advanced Energy Materials, Copyright (2022), with permission from Wiley) (a) Cross-sectional SEM images showing the structural evolution during the overcharge process.Images of the NCM811 after stage I, 4.7 V, stage II, and stage III are shown from left to right.(b) Representative HAADF-STEM image and the corresponding elemental maps of NCM811 after stage III.(d) HAADF-STEM image of NCM811 after the overcharge process.The yellow arrow indicates the gliding marks within the particle.(e)-(g) High-resolution STEM images of NCM811 after the overcharge process.Images of (f) and (g) correspond to the regions enclosed by the white and yellow squares in (e), respectively.(Reprinted from Advanced Energy Materials, Copyright (2022), with permission from Wiley).

Figure 6 .
Figure 6.(A) Release of Oxygen as a function of SOC for NCM811, NCM111, HE-NCM, and LNMO [59].(Reprinted from Materials Today, Copyright (2018), with permission from ScienceDirect) (B) Cross-sectional scanning electron microscopy images of different NCM secondary particles at 50 °C [60], (Reprinted from Energy Storage Materials, Copyright (2022), with permission from Science Direct) (a) NCM 111, (b) NCM 523, (c) NCM 622, (d) NCM 811, revealing the inhomogeneous distribution of nanovoids in the secondary particles.(e) Estimation results of porosity in different areas in the cross-section of four NCM cathodes.(f) Estimation results of total porosity on the cross-section of four NCM cathodes.(C) Schematic diagram shows the distribution differences of oxygen vacancies in different NCM cathode secondary particles under thermal induction [60].(Reprinted from Energy Storage Materials, Copyright (2022), with permission from ScienceDirect) (D) Schematic illustration of the proposed electronic structure of NCM523.The black dashed line represents the Fermi level E F [63]. (Reprinted from ACS Energy Letters, Copyright (2022), with permission from ACS) (E) Calculated partial molar enthalpy of oxygen, −(h O -h O ), as a function of oxygen vacancy concentration δ in NCM111 (gray), NCM523 (yellow), and NCM622 (orange).The upper color bars represent reduction species during oxygen release.−(h O -h O ) with Ni 3+/2+ reduction and that with Co 3+/2+ reduction are depicted in circular and square markers, respectively.Reported oxygen vacancy formation energy (or oxygen binding energy) of fully lithiated LiTMO 2 are shown by the lines in green (LiNiO 2 ), blue (LiCoO 2 ), and violet (LiMnO 2 ), respectively [63].(Reprinted from ACS Energy Letters, Copyright (2022), with permission from ACS).

Figure 7 .
Figure 7. (A) Illustrations of the structural evolution of (a) the active phase and (b) the fatigued phase during delithiation [76].(Reprinted from Nature Materials, Copyright (2021), with permission from Nature) (B) In-situ x-ray diffraction patterns of 75% delithiated NCM 811 heated to 500 °C.(Reproduced from [77].CC BY 4.0) C Surface evolution of cycled NCM 811 cathodes.HAADF-STEM images of NCM 811 cathodes after: (a)-(c) 4.3 V UCV cycling, and (d)-(f) 4.6 V UCV cycling.Yellow lines mark the boundary of the observed SRLs.The inset of (f) shows a Fourier transform diffractogram of the region marked in (f) [38].(Reprinted from Small, Copyright (2022), with permission from Wiley).

Figure 8 .
Figure 8. (A) Electrochemical behavior of pure andNb-modified NCM 811 in the voltage range of 2.8-4.6 V for (a) first chargedischarge profiles, (b) rate behavior, and (c) cycling performance (the first 3 cycles are at a C/10 rate) and for 2.8-4.4V cycling (d) capacity and (e) capacity retention [118].(Reprinted from ACS Energy Letters, Copyright (2021), with permission from ACS) (B) (a) (b)Gas release during charging of LLA 0 and LLA 3. (c)(d) Schematic diagram of crack difference and structure evolution [119], (Reprinted from Energy Storage Materials Copyright (2022), with permission from ScienceDirect).