Progress of Single-Crystal Nickel-Cobalt-Manganese Cathode Research

Abstract

The booming electric vehicle industry continues to place higher requirements on power batteries related to economic-cost, power density and safety. The positive electrode materials play an important role in the energy storage performance of the battery. The nickel-rich NCM (LiNi_xCo_yMn_zO₂ with x + y + z = 1) materials have received increasing attention due to their high energy density, which can satisfy the demand of commercial-grade power batteries. Prominently, single-crystal nickel-rich electrodes with s unique micron-scale single-crystal structure possess excellent electrochemical and mechanical performance, even when tested at high rates, high cut-off voltages and high temperatures. In this review, we outline in brief the characteristics, problems faced and countermeasures of nickel-rich NCM materials. Then the distinguishing features and main synthesis methods of single-crystal nickel-rich NCM materials are summarized. Some existing issues and modification methods are also discussed in detail, especially the optimization strategies under harsh conditions. Finally, an outlook on the future development of single-crystal nickel-rich materials is provided. This work is expected to provide some reference for research on single-crystal nickel-rich ternary materials with high energy density, high safety levels, long-life, and their contribution to sustainable development.

1. Introduction

Since the economic crisis in 2008, the global energy crisis and environmental pressures are becoming increasingly serious. In order to improve industrial competitiveness, and maintain sustainable economic and social development, the major automobile producing countries (e.g., the United States, Germany, Japan) have adopted the development of electric vehicles as a major strategy [1]. At present, the electric vehicle industry is one of the strategic emerging industries pursued by many countries [2,3]. After decades of development, ever-increasing requirements for energy storage devices have been identified, such as, higher energy density, better safety and longer service life, etc. [4,5]. However, realization of these goals mainly depends on the cathode material in the power batteries [6,7,8].

Ternary layered transition metal oxide, LiNi_xCo_yMn_zO₂ (NCM, x + y + z = 1), was first proposed by J. R. Dahn’s group in 2001 [9]. According to the report, Li[Ni_xCo_1−2xMn_x]O₂ with x = 1/4 or 3/8 could be prepared by the ‘‘mixed hydroxide’’ method which combines a two-step calcination progress. The obtained samples have a layered α-NaFeO₂-type structure, and deliver a stable capacity above 150 mAh g⁻¹ at the current density of 40 mA g⁻¹ in the voltage range of 2.5–4.4 V vs. Li. In particular, the capacity retention behavior of Li[Ni_3/8Co_1/4Mn_1/4]O₂ was close to that of LiCoO₂ in the same potential window (2.5–4.2 V) and the thermal stability was better. As mentioned above, NCM is based on the hexagonal crystal system of the α-NaFeO₂-type layered structure, which belongs to the R$\overline{3}$m space group and can be regarded as a solid solution of three compounds: LiCoO₂, LiNiO₂ and LiMnO₂. The layered structure and compositional phase diagram of an NCM cathode are shown as Figure 1. the NCM cathode combines the advantages of LiCoO₂, LiNiO₂ and LiMnO₂, exhibiting high operating voltage, large energy density and relatively good cycling performance [10,11,12,13,14,15,16]. In 2021, the Ministry of Industry and Information Technology of the People’s Republic of China officially released the “Lithium-ion Battery Industry Specification Conditions (2021)”, which states that the energy density of ternary materials-based batteries should not be lower than 210 Wh kg⁻¹, the energy density of battery packs should not less than 150 Wh kg⁻¹, and the specific capacity of ternary materials cathode should not less than 165 Ah kg⁻¹. There has been some research investigating how to improve the stability, safety, and meanwhile reduce the cost of NCM material while ensuring its high energy density [15,17,18].

It is well known that nickel, cobalt and manganese in NCM materials have obvious synergistic effects. Cobalt can stabilize the layered structure, improve electric conductivity, and thus promote the cycle and rate capability for NCM. However, excessive cobalt content leads to more serious economical and environmental problems due to its cost and toxic nature. Nickel can improve the volume energy density of the NCM, while ternary materials with high nickel content lead to cation mixing and cause various problems including capacity loss, structure deterioration and poor thermal stability. Manganese can reduce the costs and improve the safety and structural stability of the NCM. However, a higher manganese content leads to reduced electrochemical activity in the charging/discharging process, and lower specific capacity of the NCM [4,19,20,21,22,23,24,25,26,27,28,29]. For these reasons, numerous scholars have improved the electrochemical properties of NCM by adjusting the elemental ratios of Ni-Co-Mn in order to obtain ternary materials with better performance in all aspects. The relevant NCM materials that have been studied include LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111) [30,31,32,33,34,35], LiNi_0.4Co_0.2Mn_0.4O₂ (NCM424) [30,31,32,33,34], LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) [36,37,38,39,40,41] and other NCM materials with non-stoichiometry ratios of Ni-Co-Mn elements [42,43,44,45,46,47,48,49,50,51,52], etc. In particular, the nickel-rich NCM materials LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) [52,53,54,55,56,57,58,59,60,61], LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) [7,62,63,64,65,66,67,68,69,70,71] and other NCM with more than 90% nickel content [69,72,73,74,75,76,77,78], even the cobalt-free ternary materials [79,80,81,82,83,84,85,86,87,88,89,90,91], have become increasingly popular active cathode materials because of the higher potential vs. Li, higher energy densities, less toxicity, and lower priced raw materials, which can better meet the needs of electric vehicles. Nevertheless, there still are many remaining challenges in the commercialization of NCM cathodes with high-nickel content (Ni ≥ 60%) including performance deterioration and potential safety concerns. The poor lithium storage and safety of nickel-rich NCM cathodes mainly originate from the following aspects [14,15,16,28,92,93,94,95,96,97,98,99]. (i) During the cycling, heavy Ni²⁺ and Li⁺ cation mixing and more vacancies accompanied by oxygen release occur at the stage of the deep delithiation, resulting in irreversible phases transformation from the original layered structure into the spinel-like phase or inactive rock-salt phase, thus leading to poor kinetics, structural stability, thermal stability and cycling performance. (ii) Predomination of the highly oxidized Ni⁴⁺ ions at the end of charge process leads to a list of issues including the dissolution of transition metal ions, electrolyte decomposition, undesired side interfacial reactions, and the formation of cathode solid electrolyte interfaces (cSEI), which result in low coulombic efficiencies and rapid capacity reduction. (iii) During the lithiation and delithiation process, large lattice volumetric change brings about stress accumulation inside nickel-rich NCM materials, which results in secondary particle microcracks along the grain boundaries, and thus induces further structural degradation and sustained capacity loss.

To address or alleviate the above issues and improve the electrochemical performance of nickel-rich NCM materials, some feasible strategies have been proposed. First, ionic doping is an effective method to stabilize the structure of a nickel-rich NCM electrode. The purpose of doping is to make the dopant ions enter the lattice, replace some of the ions in the raw lattice, stabilize the raw material structure and improve the cycling stability during the charging and discharging process [100,101,102]. In particular, about ten years ago, John B. Goodenough, and Arumugam Manthiram had conducted an in-depth study into the effects of element doping, cation ordering and lithium content on the conductivity and electrochemical characteristics of high-voltage spinel transition metal oxide cathode materials [26,27]. The relationship between the conductivity, phase transformation mechanisms and the content of Mn³⁺, and the degree of cation ordering were investigated. It was found that Mn³⁺ content and ordering of spinel were not critical to the phase transformation behavior but benefited the high rate capability. Studies of ionic doping in nickel-rich NCM materials include a multitude of dopants such as Mg [102,103], Al [4,102,104,105], Zr [106,107,108], Ti [44,109,110,111], Nb [112,113], Mo [114,115], Cr [116], F [117,118,119,120], B [76,121,122], etc. Proper doping can also boost the cycling performance of the conductivity and lithium ion migration rate of the NCM electrode [15,123]. In addition, the design of the concentration-gradient structured materials can effectively improve the rate and cycling performance [124,125]. Second, surface coating is considered an effective method to improve the capacity retention, rate capability, and thermal stability of an NCM cathode. With the surface coating, the cathode and electrolyte are mechanically separated, and the dissolution of transition metal ions and the interfacial side reaction between cathodes and liquid electrolytes are effectively suppressed, improving the cycling performance of the NCM materials. The surface coating can also reduce the collapse of the material structure during long charging and discharging processes, which is beneficial to the cycling and thermal stability of the NCM materials [15,16,28,92,93,94,95,126,127]. Various coating materials applied on the NCM cathode can be divided into the following categories, such as oxides (SiO₂ [128,129,130], Al₂O₃ [131,132,133,134,135], ZrO₂ [136,137,138,139,140,141], TiO₂ [138,142,143,144,145,146], etc.), phosphates (AlPO₄ [147,148], FePO₄ [149,150], Ni₃(PO₄)₂ [151], CaHPO₄ [152], Mn₃(PO₄)₂ [153,154], ZrP₂O₇ [155,156], etc.), Li-containing compounds (Li₃PO₄ [157,158,159], Li₂ZrO₃ [160,161,162,163], Li₂TiO₃ [160,164,165], Li_1.5Al_0.5Ge_1.5(PO₄)₃ [166], etc.), electron conducting coatings (graphene or reduced graphene oxide (rGO) [167,168,169,170,171], permeable poly (3,4-ethylenedioxythiophene) (PEDOT) [172,173], polyaniline (PANI) [174,175], poly-pyrrole (PPy) [176,177], etc.), etc. Combining a concentration gradient and surface coating, the NCM materials with a core-shell structure represent the advantages of the high capacity and high thermal and mechanical stability [178,179]. Third, electrolyte optimization is another effective strategy toward improved performances. As mentioned earlier in the text, the cSEI film reconstruction occurs easily between the solid cathode and organic liquid electrolytes during the cycling, degrading the electrochemical properties of NCM cathodes, especially at high voltages. Different electrolyte additives have been considered to adapt to the high operating voltage of NMC, restrain undesired side reactions and stabilize the surface structure [18,30,46,68,126,180]. With the addition of LiBOB dopamine [181], metal-organic framework (MOF) [182], acetonitrile (AN) [183], vinylene carbonate (VC) [184], fluoroethylene carbonate (FEC) [185] or other tailoring electrolyte additives, the lithium-ion transference number has been effectively increased, the rate capability has been enhanced, and the cycling life has been prolonged [186,187,188]. Typically, the solid-state electrolyte also attracts much attention because of the higher safety and cycling stability compared with combustible organic electrolytes. The solid-state interface engineering strategy is a simple promising method for NCM batteries to meet the ever-increasing requirements of safe power vehicles [189,190,191,192,193,194,195].

Nevertheless, the research about NCM materials summarized above, whether on normal ternary materials or nickel-rich, nickel-ultra-rich or cobalt-free materials, mostly focuses on macro-size polycrystalline secondary spheres composed of nano-size primary grains with random orientations. The continuous growth and expansion of deep-rooted cracking along weak internal grain boundaries still cannot be thoroughly eliminated due to the ever-present anisotropic strain among the randomly oriented primary grains during the charge–discharge process. Although the increasing specific surface area improves the lithium ion conductivity, it aggravates the undesired side reactions between the cathode and electrolyte, increasing the degradation of capacity retention and further reducing the cycling stability. In addition, the internal kinetics of polycrystalline materials are more unstable and the stress distribution is more uneven, especially for nanoscale NCM materials with a higher Ni content or operating voltage (≥4.5 V), which makes the materials highly susceptible to structural collapse and capacity decay during prolonged cycling, largely reducing thermal stability and safety [36,67,94,196,197].

Recently, another effective strategy for single-crystal nickel-rich NCM has been put forward to address the issues of safety and cycle stability of nickel-rich NCM materials [36,198,199,200,201,202,203,204]. Compared with a polycrystalline NCM, the primary particles of single-crystal nickel-rich NCM materials without grain boundaries are composed of a large single grain in micron size, which can effectively mitigate the particle microcracking. Simultaneously, the basic structure of single-crystal with excellent mechanical integrity of the grain, defined as consistent lattice orientations and ordered crystal plane arrangements, possesses low specific surface area. This can greatly reduce the interface reaction, maintain a more complete layered structure, and reduce the side reaction with the electrolyte and the generation of lithium dendrites, thus improving the structural stability, cycling performance and thermal stability, especially the electrochemical performance under high temperature or operating voltage [36,205,206,207,208,209,210]. The remaining issues and modification strategies for nickel-rich NCM are summarized in Figure 2. Single-crystal NCM materials have drawn much intense attention in academia and industry. However, some problems still need to be solved including their unsatisfactory capacity retention, inferior rate characteristics and the industrialization for nickel-rich, nickel-ultra-rich (≥90%) or cobalt-free NCM materials. This might be owing to the sluggish Li⁺ diffusion kinetics, and complex and expensive synthesis methods [107,206,211,212,213,214,215,216,217,218,219,220]. To address these existing issues, significant research strategies have been proposed in recent years.

Up to now, there have been several reports reviewing single-crystal NCM materials or polycrystalline nickel-rich NCM materials, but the studies have not involved single-crystal nickel-rich NCM materials [215,221,222,223]. In this review, the remaining issues and modification strategies of normal nickel-rich NCM materials are summarized in brief. The research related to the single-crystal nickel-rich NCM are summarized and discussed in-depth. This paper aims to provide a reference for the electrochemical performance improvement of single-crystal nickel-rich NCM materials, especially for the research of performance and stability modification at high cut-off voltage or high temperature.

2. Synthesis

At present, various synthesis strategies have been developed, including co-precipitation combined with the multi-calcination method (solid-state method) [36,200,224,225,226,227,228,229,230,231,232,233], the molten salt assistant method [199,211,218,234,235,236,237,238,239,240,241,242,243] and the solvothermal method [244,245,246,247]. The main synthesis strategies of single-crystal nickel-rich NCM materials are shown in Figure 3.

The traditional synthesis strategy of co-precipitation combined with multi-calcination for single-crystal nickel-rich NCM materials usually includes the general co-precipitation method for preparing the precursor and multi-step calcination of the obtained precursor and lithium salt mixture. In the co-precipitation process, a variety of transition metal salts, such as nitrates, sulfates, and hydrochlorides, are commonly used to prepare precursors. In the calcination process, the ratio of the lithium to transition metal (Li/TM) and the temperature are crucial. J. R. Dahn’s group introduced co-precipitation (the transition metal sulfates as the source of Ni, Co and Mn) combined with multi-calcination (the as-obtained precursor mixture with Li₂CO₃ were sintered in a box furnace at 930, 950, 970, 990 or 1020 °C for 12 h unless specified, otherwise in air) to synthesize the single-crystal Li[Ni_0.5Mn_0.3Co_0.2]O₂ material (SC-NCM523) with a grain size of ∼2–5 μm. They also discussed the respective effects of the Li/TM ratio, sintering temperature, precursor size and sintering time. The results showed that, the obtained SC-NCM523 displayed good electrochemical performance. Among them, the single-crystal samples prepared at the lowest temperature and minimal Li/TM ratio possessed the highest energy density [200]. At the same time, the group made a comparative study of the synthesized SC-NCM523, the conventional polycrystalline NCM523 and polycrystalline Al₂O₃-coated NCM523 by assembling pouch cells and coin cells with graphite as the negative electrode via multiple advanced means. The long-term cycling tests showed that cells with SC-NCM523 exhibited much better capacity retention but slightly lower specific capacity than that of the cells with polycrystalline cathodes when tested to an upper cut-off potential of 4.4 V [36]. Xinming Fan’s group exploited a one-step calcination method, which is a more simplified and lower cost process than the traditional multi-calcination method. As summarized in Figure 4, a single-crystalline LiNi_0.6Co_0.1Mn_0.3O₂ (NCM613) was synthesized with an excess lithium source (an Li/M ratio of 1.08:1), under 930 °C in air. According to the morphology characterization and electrochemical test results, the as-prepared NCM613 had a suitable micron-size particle with a robust and stable primary particle grains. The NCM613/graphite full cell delivered a capacity retention of 73.9% after 900 cycles at 1 C at the working temperature of 45 °C with the cut-off voltage of 4.2 V [248]. In-depth studies indicated that the energy density, electrochemical performance and thermal stability were significantly improved at 55 °C under a high charging voltage of 4.4 V. The single-crystal NCM622 material (SC-NCM622) obtained with same method showed a much more robust particle structure without crack formation during cycling. The SC-NCM622/graphite pouch cells with a cathode areal capacity of 6 mAh cm⁻² exhibited an excellent capacity retention of 83% after 3000 cycles, demonstrating the effectiveness of a single-crystal approach in mitigating degradation in the lithium-ion battery cathode [249].

However, the calcination temperature required for the preparation of single-crystal nickel-rich NCM materials is too high, basically above 900 °C, a temperature which would destroy the layered structure. To overcome this problem, Yang-Kook Sun’s group developed a calcination method under an oxygen atmosphere, during which the sintering temperature was 50 °C lower than the optimal calcination temperature The reduction of annealing temperature in this work was realized by Ce doping, which is thought to promote the formation of single-crystals. As a result, a single-crystal Ce-doped Li[Ni_0.9Co_0.05Mn_0.05]O₂ (SC NCM90) material with ultra-high nickel content was obtained at 800 °C. The material delivered excellent electrochemical performance in terms of a high initial discharge specific capacity of 199.7 mAh g⁻¹ at 0.1 C, high capacity retention (80.5% of the initial capacity after 100 cycles at 0.5 C) and better cycling stability [250]. Yet the single-crystal pristine Li[Ni_0.9Co_0.05Mn_0.05]O₂ (Pri-NCM90) synthesized at 850 °C demonstrated comparatively inferior capacity retention and thermal stability. Moreover, the group conducted an in-depth analysis on the structural instability of charged single-crystal Li[Ni_0.9Co_0.05Mn_0.05]O₂ (SC-NCM90) and Li[Ni_0.7Co_0.15Mn_0.15]O₂ (SC-NCM70) without any dopants. The results showed that there was no irreversible structural damage in SC-NCM70 when charged to 4.3 V, while the significant intra-particle submicroscopic cracks appeared at the multiple phase boundaries of charged SC-NCM90. The analysis emphasized that the internal strain generated by the phase transition is aggravated by inhomogeneous distribution of Li, causing the fundamental structural instability [233]. In addition, J. R. Dahn’s group further prepared Co-free Ni-rich single-crystal cathode materials based on a multi-step calcination method during which a preheated process was introduced [251,252,253]. The relevant reports analyzed the effects of heating temperature, Li/TM ratio and lower temperature steps on the properties of the composite materials.

The molten salt assistant method is another commonly used strategy to synthesize high-quality nickel-rich NCM materials at lower temperatures. Hyun-Soo Kim and his co-workers prepared a single-crystal NCM material with ultra-high nickel content by a flux method with LiCl-NaCl as a molten salt [242], which was denoted as LiNi_0.91Co_0.06Mn_0.03O₂ (SNCM91). When used as the cathode for a lithium battery, SNCM91 displayed favorable morphology retention during cycling and a high initial discharge capacity of 203.8 mAh g⁻¹ at 0.1 C in the voltage range of 3.0–4.3 V, demonstrating excellent initial specific capacity as a nickel-ultra-rich NCM material. Similarly, Yuzong Gu’s group synthesized nickel-ultra-rich single-crystalline LiNi_0.92Co_0.06Mn_0.02O₂ powder (SC-780) by a novel LiOH-LiNO₃-H₃BO₃ ternary molten-salt method with a calcination temperature of 780 °C for 20 h under an oxygen atmosphere [235]. When tested as the cathode in a pouch-type full cell at 45 °C, the as-obtained SC-780 exhibited excellent long-term cycling performance in terms of a superior initial discharge capacity of 214.8 mAh g⁻¹ at 0.5 C in the voltage range of 2.7–4.2 V, and a high-capacity retention of 86.3% over 300 cycles. The flux of H₃BO₃ can regulate crystal growth to improve the particles’ uniformity and monodispersity. Meanwhile, a small amount of boron doping with stronger B-O covalent bonds may promote the structural stability and expand the layered distance, ensuring the enhanced electrochemical kinetics.

The advantages of melting characteristics and excellent flowability for molten salt assistants provokes much attention and thinking. For example, Wuwei Yan and his co-workers prepared single-crystal LiNi_0.92Co_0.06Mn_0.01Al_0.01O₂ (NCMA) cathode materials with ammonium metatungstate and ammonium molybdate as the co-doping fluxing agents and doping materials [240]. Due to the co-doping fluxing strategy, the as-prepared NCMA had a smaller particle size, less cationic mixing, a more stable phase structure and less internal resistance, displaying superior electrochemical performance. The optimal NCMA contained 1000 ppm W and 1000 ppm Mo (NCMA-B), displaying a higher initial discharge capacity of 221.4 mAh g⁻¹ at 0.1 C in the voltage range of 3.0–4.3 V, and the corresponding capacity retention after 100 cycles at 25 °C and 45 °C was 95.7% and 94.9%, respectively. Notably, Xiaobo Ji’s group successfully designed and prepared a single-crystalline Co-free Ni-rich LiNi_0.95Mn_0.05O₂ (SC-NM95) layered cathode without any cracks, as shown in Figure 5a, during which the LiOH and LiNO₃ were mixed homogeneously as the molten salt system and lithium source [237]. When tested at 0.1 C with an operating voltage of 2.7–4.3 V, the obtained SC-NM95 cathode achieved a high initial discharge capacity of 218.2 mAh g⁻¹, a high energy density of 837.3 Wh kg⁻¹, and an outstanding capacity retention (84.4%) after 200 cycles at 1 C. Even in the extended voltage range of 2.7–4.5 V, SC-NM95 showed a superior initial capacity of 230.1 mAh g⁻¹ with a Coulomb Efficiency of 86.48%. A capacity retention of 81% was achieved after 100 cycles, indicating the reinforced electrochemical reversibility and cycling stability at a high cut-off voltage (Figure 5b–d).

Additionally, in order to alleviate the environmental impact of waste lithium-ion batteries and the rising cost of cathode materials, the molten salt assistant methods are also commonly used to extract transition metal elements from spent polycrystalline layered cathode materials [238,239]. For instance, Weixin Zhang’s group developed a simple and effective strategy to recycle spent polycrystalline ternary cathode materials into single-crystal cathodes which is based on an alkaline LiOH-LiNO₃ molten salt [238]. The Li-based molten salt system repaired the lithium defects and the damaged structures generated during repeated lithium and (de)lithiation process. The as-obtained plate-like single-crystal NCM622 with exposed (010) planes delivered a high capacity of 155.1 mAh g⁻¹ at 1 C in the operating voltage range of 2.8–4.3 V and a superior long cycling stability of about 94.3% capacity retention, even after 240 cycles. Significantly, the recycling method can be expanded to other waste Ni-Co-Mn ternary cathode materials or their mixtures for producing high-performance single-crystal cathode materials to utilize the large amounts of waste lithium-ion batteries, thus facilitating green and sustainable development.

The solvothermal method, including hydrothermal method, is a synthesis method for the reaction of the original mixture in a closed system, using organic matter or a non-aqueous solvent (the solvent for the hydrothermal reaction is water) under a certain high-temperature and high-pressure environment. In this method, the phase formation, particle size and morphology of the prepared samples can be well controlled, resulting in better dispersion of the product. The reaction process is greatly affected by solvent type, reactant ion concentration and reaction temperature and time. The solvothermal method is commonly used to prepare micro or nano materials with specific morphology. For NCM materials, single-crystal nickel-rich NCM materials with a specific crystal plane orientation can be obtained by the solvothermal method. Jianguo Duan’s group prepared a hexagonal morphology Ni_0.8Co_0.1Mn_0.1(OH)₂ precursor with the preferred orientation of (001) lattice plane [244]. After the sintering process at a lower reaction temperature (780 °C) and lower excess lithium/transition metal ratio (1.03:1), the highly-crystalline micrometer-sized Ni-rich single-crystal LiNi_0.8Co_0.1Mn_0.1O₂ (SC-NCM) with the retained precursor hexagonal morphology was obtained (the preparation process and corresponding SEM image are illustrated in Figure 6a,b). All the primary particles of SC-NCM displayed smooth surfaces with legible grain boundaries, a lower specific surface area, stable crystal plane exposure, unimpeded Li⁺ transport structure, larger C-axis, and lower sulfur impurities. The electrochemical test results conducted in a CR2025 coin-type cell showed an initial capacity of 186.2 mAh g⁻¹ at 1 C during 2.8–4.3 V and a capacity retention of ~93.4% after 100 cycles (Figure 6c). Even when the rate was increased to 10 C, the SC-NCM cathode achieved a capacity of 130.4 mAh g⁻¹, representing an excellent rate performance (Figure 6d). Youxiang Zhang’s group also prepared single-crystal LiNi_0.8Co_0.1Mn_0.1O₂ by the simple solvothermal method combined with a lower calcination temperature. However, the obtained sample displayed a different rod-like morphology [245]. When the excess lithium content was 50%, the LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) material showed uniform mono-dispersed rod particles with a micrometer-scale and good crystallinity and showed an excellent discharge capacity and cycling stability (a high initial discharge capacity of 226.9 mAh g⁻¹ with a Coulombic Efficiency of 91.2% at 0.1 C in the voltage range of 2.8–4.3 V). When the current density increased to ten times of the original, the discharge capacity could still reach 178.1 mAh g⁻¹ with a capacity retention of 95.1% after 100 cycles. The excellent high-rate cycling performance was mainly attributed to the lower specific area, weaker cation mixing, and uniform particle distribution.

As is well-known, the pH value is another important parameter influencing the solvothermal method, which is critical to the lattice structure and morphology of products. Ke Du and his co-workers prepared single-crystalline LiNi_0.6Co_0.2Mn_0.2O₂ (SC-NCM) with hexagonal slabs by the hydrothermal method [246]. In this hydrothermal process, NH₃·H₂O was used as the precipitant, and the value of pH was adjusted to 9.6. The morphology and electrochemical characterization demonstrated that SC-NCM displayed micro-sized particles with a highly-ordered layer structure, and had excellent capacity retentions of 93.2% and 89.6% at 1 C after 100 cycles at the cut-off potentials of 4.3 V and 4.5 V, respectively. The lower cation mixing and fine grain size of the SC-NCM primary particle inhibited structural collapse and promoted lithium ion transport, thus elevating the rate capability.

The electrochemical properties of single-crystal nickel-rich NCM materials synthesized via different strategies are reported in

Table 1. Electrochemical properties of single-crystal nickel-rich NCM materials synthesized via different strategies. (CR = Capacity Retention).

Material Components	Synthesis Methods	Electrochemical Performance	Ref.
LiNi0.6Co0.1Mn0.3O2 (SC-NCM613)	One-step calcination method	CR of 73.9% after 900 cycles at 1 C, 45 °C, 2.75–4.2 V, pouch full cell	[248]
LiNi0.6Co0.2Mn0.2O2 (SC-NCM622)	One-step calcination method	CR of 82.6% after 3000 cycles at 1 C, 25 °C, 3.0–4.2 V, pouch full cell	[249]
Ce-doped Li[Ni0.9Co0.05Mn0.05]O2 (SC-Ce-NCM90)	One-step calcination method with lower temperature	CR of 80.5% after 100 cycles at 0.5 C, 30 °C, 2.7–4.3 V, half cell	[250]
Li[Ni0.7Co0.15Mn0.15]O2 (SC-NCM70)	Calcination method	CR of 91% after 100 cycles at 0.5 C, 30 °C, 2.7–4.3 V, half cell	[233]
LiNi0.91Co0.06Mn0.03O2 (SNCM91)	Molten salt assistant method	initial discharge capacity of 203.8 mAh g−1 at 0.1 C, 3.0–4.3 V, half cell	[242]
LiNi0.92Co0.06Mn0.02O2	Molten salt assistant method	CR of 86.3% after 300 cycles at 0.5 C, 25 °C, 2.7–4.2 V, pouch full cell	[235]
LiNi0.92Co0.06Mn0.01Al0.01O2 (NCMA)	Solid-phase sintering method	221.4 mAh g−1 at 0.1 C, 3.0–4.3 V, CR of 94.9% after 100 cycles at 45 °C, half cell	[240]
LiNi0.95Mn0.05O2 (SC-NM95)	Molten salt assistant method	CR of 81% after 200 cycles at 1 C, 25 °C, 2.7–4.5 V, half cell	[237]
LiNi0.6Co0.2Mn0.2O2 (SC-NCM622)	Molten salt assistant method	155.1 mAh g−1 at 1 C, CR of 94.3% after 240 cycles at 1 C, 25 °C, 2.8–4.3 V, half cell	[238]
LiNi0.8Co0.1Mn0.1O2 (SC-NCM811)	Hydrothermal method	186.2 mAh g−1 at 1 C, CR of 93.4% after 100 cycles at 1 C, 25 °C, 2.8–4.3 V, half cell	[244]
LiNi0.8Co0.1Mn0.1O2 (SC-NCM811)	Solvothermal method	226.9 mAh g−1 at 0.1 C, CR of 91.2% after 100 cycles at 1 C, 25 °C, 2.8–4.3 V, half cell	[245]
LiNi0.6Co0.2Mn0.2O2 (SC-NCM622)	Hydrothermal method	184.2 mAh g−1 at 0.1 C, CR of 89.6% after 100 cycles at 1 C, 2.8–4.5 V, half cell	[246]

.

In addition to the methods summarized above, the synthesis strategies of ion anchoring [254], sol-gel [255], etching [256], and other advanced methods [257,258,259,260] are usually employed to produce single-crystal nickel-rich NCM materials.

3. Issues and Recent Progress for Single-Crystal Nickel-Rich NCM Materials

3.1. Issues

As described in the introduction, compared with polycrystalline structure nickel-rich NCM materials, single-crystal structural nickel-rich NCM materials exhibit more robust structure and mechanical stability, higher discharge specific capacity and compaction density, and better rate performance. However, the development of single-crystal nickel-rich NCM materials is still in the research stage, and there are still some challenges in terms of comprehensive electrochemical performance enhancement and industrial application. In summary, the current problems that need to be solved for single-crystal nickel-rich NCM materials include slow lithium ion diffusion, internal microcracking in the single crystal, and phase transitions [212,219,220,233,261,262]. The phase transition issue is outlined in Figure 7.

Most of the reported single-crystal nickel-rich NCM materials display micro-sized morphology. It is known that a larger single-crystal particle size leads to increased diffusion pathways and reduced diffusivity of lithium ions, resulting in poor rate properties [212,213,233]. Feng Wang’s group conducted a careful study on the redox reactions and ion transmission during the charging and discharging of individual NCM811 particles with TXM-XANES and quantitative calculation [263]. The redox kinetics were directly related to the state of charge (SOC). At the beginning of charging or the end of discharging, a single particle of single-crystal NCM811 was in the lithium-rich state, and the diffusivity of lithium ion was three orders of magnitude lower than that in the (de)intercalation state, leading to sluggish diffusion dynamics and large charge transfer resistance, and for a larger primary grain at a micron scale, the longer diffusion pathway may exacerbate the slow diffusion of lithium ions. Likewise, Yang-Kook Sun’s group conducted a systematic capacity decay mechanism analysis using in situ XRD and TEM for single-crystal NCM materials with a particle size of about 3 μm and corresponding polycrystalline NCM materials. It was found that the capacity decay mechanism differed between the two types. For single-crystal NCM materials, the lithium ion concentration became spatially inhomogeneous during cycling; this phenomenon intensified with increasing current rate and nickel content, leading to the existence of multiple phases with different grain size within the single-crystal NCM particles. The coexistence of these two phases caused inhomogeneous stresses, generated structural defects, hindered the diffusion of lithium ions, and eventually led to rapid capacity decay [220].

The intergranular crack is an important physical behavior of nickel-rich materials, which is effectively eliminated in single-crystal nickel-rich NCM materials due to the absence of primary grain boundaries. However, uneven stresses can still distribute and accumulate inside the single crystal at high current density and high cut-off voltages, which results in internal microcracking and propagation during long-term cycling [219]. Jie Xiao’s group has conducted a comprehensive and in-depth study on the structural changes inside the single crystal LiNi_0.76Mn_0.14Co_0.1O₂ (NMC76) electrode during the charging and discharging process at high and low cut-off voltages, during which in situ atomic force microscopy (AFM) and theoretical calculations were applied [262]. The results showed that a lithium concentration gradient is formed along the direction of the lithium ion diffusion in the grain, which can generate stress. This local tensile stress triggered the grain to slip along the (003) plane during the charging process, leading to microcracks along the direction perpendicular to the (003) plane within the grain. It is worth noting that this grain slipping phenomenon returned to its initial state during the discharge process, proving that the microcrack generation inside the single crystal is a reversible process. However, this reversible process will not keep the grain structure in a perfect initial state as the long cycle proceeds. Meanwhile, with the increase in nickel content in single-crystal NCM materials, operating voltage and charging current, this intracrystalline grain surface slippage is further aggravated, which in turn leads to more severe lithium concentration gradients and internal strains, eventually leading to structural degradation and performance deterioration [212,215,218]. Furthermore, J. R. Dahn’s group studied the morphology and structural changes in commercial grade single-crystal LiNi_0.5Mn_0.3Co_0.2O₂ (SC532), LiNi_0.6Mn_0.2Co_0.2O₂ (SC622) and LiNi_0.8Mn_0.1Co_0.1O₂ (SC811) materials after pro-longed cycling (over 1000 cycles) of pouch cells at 0.05 C and 4.3 V [264].

The phase transition has been proved to be an unavoidable issue for nickel-rich NCM material by J. R. Dahn via in situ XRD [96]. In the literature, the relationship between the capacity retention and the available capacity is also given. For nickel-rich NCM materials, the higher nickel contents led to partial nickel dissolution and the transformation of the layered hexagonal structure to spinel and rock salt phases caused by the lithium and nickel cations mixing persisted as the charging and discharging proceeded. This process led to the surface reconstruction of single-crystal particles, which hindered lithium ion transport and led to capacity and rate performance degradation. In addition, although the single-crystal particles have a small specific surface area, the side reactions still occur at the interface between the electrolyte and cathode, causing phase transitions, cSEI film formation and eventually capacity decay and cathode structure damage. In addition, the phase transition during charging was more likely to occur at the lithium ion diffusion channel, which is because Ni²⁺ is more likely to occupy the Li⁺ vacancy, leading to Ni²⁺/ Li⁺ mixing [212,216]. Fang Zhang and his co-workers prepared an t-NCM622 material with an adjustable surface [261]. In this surface modification process, the unstable rock salt phase Ni²⁺ on the surface of single-crystal NCM622 was oxidized to layered Ni³⁺ by recalcining the mixture of single-crystal NCM and LiOH in oxygen, which ensures the stability of the layered structure on the surface of single-crystal NCM. The progressive operando X-ray spectroscopy imaging and nano-tomography were used to investigate the correlation of the surface structure, internal strain, and capacity deterioration for the rock salt phase and layered structure. It was found that, in the unstable rock salt phase, phase transition can lead to uneven stresses within the grains, inducing a collapse of the grain structure and generating microcracks, which can reduce the cyclic stability. After the recalcining process with LiOH, the surface regulated to a stable layered structure, which could produce a uniform phase distribution within the single crystal and further improve the surface chemical stability and capacity retention stability.

Additionally, the complex preparation process and high cost are also obstacles to the market application of single-crystal nickel-rich NCM materials. Although there are various preparation strategies, as described in the synthesis section, most methods possess some shortcomings, such as process complexity and strict conditions. Therefore, optimizing the synthesis process of materials and reducing the preparation cost and energy consumption are also focus issues of current research.

3.2. Modification Progress

Similar to the polycrystalline NCM materials, the research directions for modifying the single-crystal nickel-rich NCM materials mainly focus on three aspects: ion doping, surface coating and electrolyte optimization.

3.2.1. Ion Doping

Ion doping is an effective method for both cycling stability and capacity enhancement of polycrystalline and single-crystal nickel-rich NCM materials. As for the ion doping mechanism mentioned in the introduction, the doped ions can stabilize the monolithic structure and improve the material performance by replacing or forming new covalent bonds with existing ions, thus improving the operating voltage, discharge capacity, cycling and safety stability of the single-crystal nickel-rich NCM. The ions used for doping polycrystalline materials are also applicable to single crystal materials. In addition, the doping mechanism for both single crystal and polycrystalline materials is basically the same.

In recent years, single ion doping modification for single crystal nickel-rich NCM materials was reported using ions involving F [265,266,267], B [268], Ta [269], Nb [270,271], Zr [272], Ba [273], Ce [250], Br [274], etc. Xinming Fan’s group and his colleagues in the School of Metallurgy and Environment of Central South University have conducted more research on dopants. With B doping, B-O covalent bonds are formed in nickel-rich single-crystal NCM materials (LiNi_0.83Co_0.05Mn_0.12O₂), which exhibited superior discharge capacity (205 mAh g⁻¹ at 0.1 C with the cut-off voltage up to 4.4 V) and outstanding cycle stability (91.35% capacity retention after 500 cycles at 0.5 C in the voltage range 2.75–4.2 V) (Figure 8a,b) [268]. The doping of Zr enabled single-crystal NCM95 (NCM with a nickel content of ~95%) materials to form a crystalline surface with highly exposed (010) planes, which showed outstanding cycling stability and rate characteristics at a high temperature of 45 °C (Figure 8c–e) [272]. Other researchers have also reported that single-doped Ta or Nb formed a concentration gradient distribution in single-crystal NCM811 materials, which can effectively improve the discharge specific capacity and cycling stability [269]. For example, Shujuan Bao’s group synthesized a Nb-doped single-crystal NCM811 (SNCM@Nb) electrode [271]. Among them, the optimized SNCM@Nb-2 not only generated a strong Nb-O bond inside, but also generated a Li/Ni disordered robust layer on the surface. This collaborative structure not only stabilizes the single crystal integrity, but also shortens the lithium ion transport path so that the SNCM@Nb-2 exhibited faster redox reaction kinetics during charging and discharging processes. The electrochemical results of SNCM and SNCM@Nb-2 showed a lower capacity loss of 7.46% and 13.3% after 100 cycles at 1 C with the operating voltage range of 2.7–4.3 V at a normal temperature and 55 °C, respectively. For SNCM@Nb-2, even with a cut-off voltage up to 4.5 V, the initial discharge capacity of 285.2 mAh g⁻¹ with retention of 79.8% could be reached at 0.5 C, 25 °C. It delivered remarkably enhanced electrochemical characteristics and stability at high voltages or temperatures.

More strikingly, the strategy of multiple ions co-doping derived from single ion doping has also been widely proposed. The synergistic effect of multiple ions has more obvious effects in promoting lithium ion diffusion, stabilizing structural integrity and improving the high temperature or high operating-voltage cycling stability. Recently, ions used in NCM co-doping include Al/Zr [275,276], Zr/Ti [277] and W/Mo [240], etc. Yougen Tang’s group selected Al/Zr co-doping to further upgrade the thermal and high-voltage cycling stability of single-crystal NCM622 materials (Figure 9) [275]. Interestingly, Al and Zr not only entered into the NCM622 lattice, but also appeared on the NCM622 grain surface in the form of Li₂ZrO₃. The performance optimization of single-crystal NCM622 actually benefitted from the cooperative effect of the double-ion co-doping and surface coating. On the one hand, the double ion co-doping inside the lattice can largely inhibit the phase transition initiated by cation mixing, thus stabilizing the single crystal structural integrity. On the other hand, the outer lithium salt coating can profit the diffusion of lithium ions without dissolving in the electrolyte. These phenomena allowed the single-crystal NCM622 to exhibit excellent high-temperature and high-voltage cycling stability with a capacity retention of 92.1% at 1 C at a testing temperature of 50 °C and voltage range of 3.0–4.4 V for 100 cycles. Xinming Fan’s group also employed Al/Zr co-doping, but the studies were conducted on the LiNi_0.88Co_0.09Mn_0.03O₂ (SNCM) materials with higher nickel content [276]. In this case, Al adequately incorporated in the SNCM lattice, and Zr aggregated on the SNCM surface. Results of the studies further confirmed the synergistic effect of double ions co-doping on the electrochemical performance of the nickel-rich single-crystal NCM materials. To further demonstrate the practical application of SNCM, the group assembled a pouch cell with a capacity of 10.8 Ah using lithium metal as negative electrode and SNCM as positive electrode for electrochemical testing. Surprisingly, the initial specific energy at 0.1 C and at normal temperature reached 504.5 Wh kg⁻¹, which provides a strong basis for the commercial application of double ion co-doping improving electrochemical performance of nickel-rich single-crystal NCM materials. To further demonstrate the above prediction, Xinming Fan’s group further hoisted the electrochemical characteristics of single-crystal LiNi_0.6Co_0.1Mn_0.3O₂ to the material-level via an in situ Zr/Ti co-doping method [277]. The experimental analysis and theoretical calculation results comprehensively explained the mechanism of double ion doping and the macroscopic effect results. The obtained single-crystal LiNi_0.598Co_0.08Mn_0.3Zr_0.002Ti_0.002O₂ (Z/T@SC-NCM-0.2) cathode achieved an initial specific energy of up to 715 Wh kg⁻¹ at 0.3 C in the operating voltage range of 2.75–4.6 V and a satisfactory capacity retention of 88.5% after 150 cycles. Impressively, the Zr/Ti@SC-NCM||graphite cell with an actual area capacity of 4.96 mAh cm⁻² still achieved a capacity retention of 80.6% after 4000 cycles at 1 C in the voltage range of 3.0–4.2 V. It is expected to be the most promising cathode material for marketable applications.

Inspired by the findings of Yougen Tang’s group, Dong Yang’s group prepared Zr/Y co-doped LiNi_0.83Co_0.12Mn_0.05O₂ materials (ZrY-NCM@Al) with an Al₂O₃ coating. The full cell assembled with graphite as the negative electrode also showed excellent high-temperature prolonged cycle stability [278]. The combination of different modification strategies can give full play to the advantages of various methods and effectively improve the performance of nickel-rich single-crystal NCM materials. Increasingly, researchers are also focused on the “synergistic strategy”. The optimization of ion doping and surface coating is described in the following section. The electrochemical properties of single-crystal nickel-rich NCM materials with different ion doping are reported in

Table 2. Electrochemical properties of single-crystal nickel-rich NCM materials with different ion doping. (CR = Capacity Retention).

Doping Ion	Materials Components	Electrochemical Performance	Ref.
F	LiNi0.8Co0.1Mn0.1O2	202.7 mAh g−1 at 0.1 C, CR of 86.6% after 100 cycles at 1 C, 2.8–4.3 V	[265]
B	LiNi0.83Co0.05Mn0.12O2	205 mAh g−1 at 0.1 C, 2.75–4.4 V, CR of 91.35% after 500 cycles at 0.5 C, 2.75–4.2 V	[268]
Ta	LiNi0.8Co0.1Mn0.1O2	211.2 mAh g−1 at 0.1 C, CR of 90.4% after 100 cycles at 0.5 C, 3.0–4.3 V	[269]
Nb	LiNi0.8Co0.1Mn0.1O2	226 mAh g−1 at 0.1 C, CR of 92.54% after 100 cycles at 1 C, 2.7–4.3 V	[271]
Zr	LiNi0.948Co0.03Mn0.02Zr0.002O2	121.4 mAh g−1 at 10 C, CR of 81.8% after 250 cycles at 10 C, 3.0–4.3 V	[272]
Ba	LiNi0.6Co0.2Mn0.2O2	CR of 81.85% after 200 cycles at 1 C, 2.8–4.3 V	[273]
Br	LiNi0.6Co0.2Mn0.2O2	CR of 92.1% after 100 cycles at 1 C, 50 °C, 3.0–4.4 V	[274]
Ce	Li[Ni0.9Co0.05Mn0.05]O2	CR of 80.5% after 100 cycles at 0.5 C, 30 °C, 2.7–4.3 V	[250]
Al/Zr	LiNi0.88Co0.09Mn0.03O2	208.4 mAh g−1 at 0.5 C, CR of 90.3% after 200 cycles at 0.5 C, 25 °C, 2.75–4.55 V, pouch-type full cell	[276]
Zr/Ti	LiNi0.6Co0.1Mn0.3O2	CR of 80.6% after 4000 cycles at 1 C, 25 °C, 3.0–4.2 V, pouch full cell	[277]
Zr/Y	LiNi0.83Co0.12Mn0.05O2	CR of 89.2% after 1000 cycles at 1 C, 45 °C, 2.8–4.25 V	[278]
W/Mo	LiNi0.92Co0.06Mn0.01Al0.01O2	221.4 mAh g−1 at 0.1 C, 3.0–4.3 V, CR of 94.9% after 100 cycles at 45 °C, half cell	[240]

.

3.2.2. Surface Coating

Surface coating is a universal modification method to improve the stability and conductivity of electrode materials for lithium ion batteries. Its mechanism in NCM materials has been described in detail in the Introduction part and will not be repeated here. In recent years, surface coating layers for single-crystal nickel-rich NCM materials have been reported for LiV₂O₄ [279], Li₂CO₃ [280], Li₂TiO₃ [281], Li−Si−O [282], H₃PO₄·12MoO₃ [283], BaTiO₃ [284], Li_1.4Y_0.4Ti_1.6(PO₄)₃ (LYTP) [285], Li_1.8Sc_0.8Ti_1.2(PO₄)₃ (LSTP) [286], La₂Li_0.5Co_0.5O₄ [287], PMMA [254], etc. Interestingly, Xinming Fan’s group has proposed an ingenious strategy to construct an in situ ion/electron conduction network that can interconnect particles of single-crystal nickel-rich materials. As a result, an intermediate phase film of ion/electron conduction can be formed between the cathode and electrolyte. This film itself had good lithium ion and electron conductivity, which can improve the rate capability of the single-crystal nickel-rich materials. In terms of stability, it not only protected the single-crystal particles from electrolyte corrosion, which triggers undesirable interface reactions leading to structural collapse, but also safeguarded the stability inside the single grain and reduced the possibility of microcrack generation. The good connection between the conductive network and the single crystal particles allowed the material to exhibit excellent electrochemical properties, even under severe conditions. The pouch cells assembled with the prepared Li_1.4Y_0.4Ti_1.6(PO₄)₃ (LYTP) modified single-crystal LiNi_0.88Co_0.09Mn_0.03O₂ (SC-NCM88) and Li_1.8Sc_0.8Ti_1.2(PO₄)₃ (LSTP) modified single-crystal-layered LiNi_0.6Co_0.1Mn_0.3O₂ (SC-NCM) materials as positive electrodes and graphite as a negative electrode exhibited ultra-long cycle (1000 cycles and above) stability at high cut-off voltages (4.4 V and above) [285,286]. Most impressively, the LSTP-modified SC-NCM material delivered an initial discharge specific capacity of 144.3 mAh g⁻¹ at a high cut-off voltage of 4.6 V at 5 C. A high-capacity retention of 90.27% was achieved after 500 cycles. The assembled full battery could still reach 89.6% capacity retention after 1700 cycles at 1 C in the voltage range of 2.75–4.2 V. The prolonged cycle stability was quite excellent.

Additional studies have reported about ion doping and surface coating methods, which can be mainly categorized as follows: (i) ion doping combined with the same ionic oxide coating [288,289,290], (ii) ion doping combined with the other oxide or lithium salt coating [278,291,292,293], and (iii) other surface coating combined with electrolytes optimization, especially combined with solid-state electrolytes [294,295,296].

In this field, Xing Ou’s group from Central South University has done more concentrated research on ion doping and surface coating, which included the use of Li₃BO₃ as the surface coating material and the selection of different types of ions as dopants to improve the performance of single-crystal nickel-rich materials. The prepared single-crystal nickel-rich cathodes have a considerably high voltage, high rate capacity or ultra-long cycle stability. Among the literature reports, the group treated the LiNi_0.7Co_0.2Mn_0.1O₂ (NCM721) material with H₃BO₃ by a simple wet chemical method to form a unique structure of B₂O3/Li₃BO₃ coating layer on the surface of NCM721 doped with B in its bulk phase. This structure gave full play to the multiple advantages of ion doping and surface coating, allowing the NCM721 material to exert excellent multiplicative properties with a reversible capacity of up to 162.7 mAh g⁻¹ at 10 C in the voltage range of 3.0–4.5 V [290]. In addition, the latest results proposed a strategy of Zr ion doping combined with Li₃BO₃ coating, which enabled single-crystal LiNi_0.83Co_0.11Mn_0.06O₂ (NCM83) to exhibit excellent high-voltage and prolonged cycling stability. The prepared single-crystal NCM83 (Zr-NCM83@B) was assembled with graphite employed as an anode into a pouch full cell for testing, delivering a cycling capacity retention of 83.5% after 1400 cycles in the range of 2.8–4.4 V at 1 C. This was mainly attributed to the fact that Zr ion can stabilize the lattice structure of internal grains. In addition, the Li₃BO₃ coating layer can reduce the interfacial impedance between the electrode and electrolyte, promoting lithium ion diffusion [293]. According to the above research, the universality of the multi-method synergistic strategy is further verified.

The synergistic strategy of the coating approach includes not only the ion doping, but also the electrolyte optimization methods, especially with solid-state electrolytes. Zhaoze Fan and his co-workers assembled sulfide-based all-solid-state lithium ion batteries (ASSLBs) with single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ (S-NCM622) coated by Li_0.35La_0.55TiO₃ (LLTO) (denoted by S-NCM622@LLTO) as the cathode [296]. The core-shell structure of S-NCM622@LLTO was fabricated by the sol-gel method, with a 6 nm LLTO shell uniformly coated on the S-NCM622 grain surface. The high ionic conductivity of LLTO effectively solved the problems of interfacial side reactions and the space charge layer. Experiments were conducted to assemble all-solid-state cells with Li₆PS₅Cl as the electrolyte and Li/In alloy as the anode. The electrochemical performance of S-NCM622 with and without LLTO coating was compared, which proved that LLTO played a key role in enhancing the capacity and cycling stability of S-NCM622. The electrochemical properties of single-crystal nickel-rich NCM materials with different surface coatings are reported in

Table 3. Electrochemical properties of single-crystal nickel-rich NCM materials with different surface coatings. (CR = Capacity Retention).

Surface Coating	Material Components	Electrochemical Performance	Ref.
LiV2O4	LiNi0.5Co0.2Mn0.3O2	a reversible discharging capacity of 131.2 mAh g−1 and CR of 80% after 100 cycles at 1 C, 3.0–4.5 V	[279]
Li2CO3	LiNi0.5Co0.2Mn0.3O2	CR of 91% after 250 cycles at 1 C, 25 °C, 2.7−4.4 V, full coin cell	[280]
Li2TiO3	LiNi0.83Co0.12Mn0.05O2	155.7 mAh g−1 at 1 C, CR of 88.5% after 100 cycles at 1 C, 2.7–4.3 V	[281]
Li−Si−O	LiNi0.6Co0.2Mn0.2O2	CR of 90.6% after 100 cycles at 1 C, 3.0–4.5 V	[282]
H3PO4·12MoO3	LiNi0.8Co0.1Mn0.1O2	CR of 92% after 200 cycles at 0.5 C, 3.0–4.3 V	[283]
BaTiO3	LiNi0.8Co0.1Mn0.1O2	138.5 mAh g−1 at 1 C, CR of 53.8% after 100 cycles at 5 C, 2.75–4.5 V	[284]
Li1.4Y0.4Ti1.6(PO4)3 (LYTP)	LiNi0.88Co0.09Mn0.03O2	CR of 85% after 1000 cycles at 0.5 C, 25 °C, 2.75–4.4 V, full pouch cell	[285]
Li1.8Sc0.8Ti1.2(PO4)3 (LSTP)	LiNi0.6Co0.1Mn0.3O2	CR of 89.6% after 1700 cycles at 1 C, 25 °C, 2.75–4.2 V, full pouch cell	[286]
La2Li0.5Co0.5O4	LiNi0.8Co0.1Mn0.1O2	CR of 81.15% after 200 cycles at 1 C, 2.8–4.3 V	[287]
PMMA	LiNi0.8Co0.1Mn0.1O2	181.1 mAh g−1 at 1 C, CR of 91.2% after 100 cycles at 1 C, 2.8–4.3 V	[254]
B2O3/Li3BO3 coating and B3+ doping	LiNi0.7Co0.2Mn0.1O2	162.7 mAh g−1 at 10 C, CR of 87.4% after 150 cycles at 1 C, 3.0–4.5 V	[290]
Li3BO3 coating and Zr doping	LiNi0.83Co0.11Mn0.06O2	169 mAh g−1 at 4 C, 3.0–4.3 V, CR of 83.5% after 1400 cycles at 1 C, 2.8–4.4 V, full pouch cell	[293]
sulfide-based all-solid-state lithium ion batteries coated by Li0.35La0.55TiO3	LiNi0.6Co0.2Mn0.2O2	179.7 mAh g−1 at 0.05 C, CR of 84.5% after 100 cycles at 0.1 C, 2.2–3.7 V	[296]

.

3.2.3. Electrolyte Optimization

In fact, there are still some problems with single-crystal nickel-rich NCM materials, such as unsatisfactory cycling performance. The primary reason is related to the instability of the electrode–electrolyte interface, and the influence mechanism was stated in the first part of the introduction. Moreover, it needs to be emphasized again that the negative influence of the electrode–electrolyte interface on the performance of single-crystal nickel-rich NCM materials is more obvious and severe under harsh conditions such as high rates, high cut-off voltages and high temperatures. Back in 2018, J. R. Dahn’s group conducted an in-depth study on the effect of electrolyte on the performance of single-crystal NCM materials [201,202]. In particular, a comprehensive and systematic analysis of the long-cycle characteristics of six different electrolyte additives on NCM811 materials at different temperatures and cut-off voltages were conducted using advanced characterization combined with various mathematical and statistical methods for graphite-negative pouch cells assembled with a single-crystal and polycrystalline NCM811 cathode [68]. The results of the study provided a good research basis for the development of single-crystal nickel-rich NCM batteries, especially for the development and tailoring of electrolytes.

Based on the above studies, some advances have been achieved in electrolyte optimization in recent years [294,297,298,299]. Yue Zou and his colleagues added a certain amount of 1,2,4-1H-Triazole (HTZ) to the electrolyte for conventional ternary electrodes and investigated the effects of different HTZ additives on the performance of a commercial single-crystal LiNi_0.9Co_0.05Mn_0.05O₂ (NCM90) cathode [279]. The optimal battery system was determined through electrochemical tests and theoretical calculations. When the concentration of HTZ was 0.3%, the battery exhibited the optimal capacity retention, which was 11.8% higher than that of the additive-free system. This was due to the fact that HTZ can prevent the pyrolysis of LiPF6 and the generation of HF. In addition, the HTZ could be easily oxidized to form a dense CEI film, which was conducive to stabilizing the interfacial structure and inhibiting the occurrence of interface reactions. To improve the performance of nickel-rich NCM electrodes, Xinming Fan’s group has conducted extensive research involving ion doping, surface coating and electrolyte optimization. The group used tris(2-cyanoethyl) borate (TCEB) as an electrolyte additive to enhance the electrochemical performance of high-voltage lithium metal batteries with LiNi_0.88Co_0.09Mn_0.03O₂ (SC-NCM) as the cathode. The corresponding modification mechanism is displayed in Figure 10 [299]. TCEB is believed to be able to induce the formation of an N/B-rich CEI film on the electrode and electrolyte interfaces with strong B-F and B-O bonds. This can effectively suppress interfacial side reactions under high voltage and thus improve the high voltage stability of the SC-NCM||Li-metal battery and enable it to retain 80% capacity after 150 cycles at 0.1 C with a 4.7 V cut-off voltage.

In particular, all-solid-state lithium-ion batteries with single-crystal nickel-rich NCM materials as the cathode have been widely developed due to the introduction of solid-state electrolytes. Compared with liquid electrolytes, solid-state electrolytes have become a research hotspot in recent years due to their high safety, good mechanical and thermal stability, and positive compatibility with different kinds of electrode materials. To promote the development and application of the LIBs with high-energy density and high-safety, nickel-rich all-solid-state batteries with nickel-rich cathodes have attracted increasing attention [300,301,302,303,304,305]. Typically, Jiajun Wang’s group has creatively developed a sulfide-based all-solid-state lithium-ion battery with single-crystal LiNi_0.6Mn_0.2Co_0.2O₂ (DC-TNO@SCNCM) as the cathode, a Ti-dopant and a TiNb₂O₇ coating layer [304]. The DC-TNO@SCNCM combined the triple synergistic effects of ion doping, surface coating and electrolyte optimization from the surface to bulk phases, and achieved better improvements in terms of lattice stability, ionic conductivity and interfacial side reaction suppression. The prepared DC-TNO@SCNCM exhibited superior cycling characteristics, with a capacity retention of 92.2% at 0.1 C and a high cut-off voltage of 4.4 V for 140 cycles. It provided a new perspective on the design of high-performance all-solid-state batteries with single-crystal nickel-rich NCM as the cathode.

4. Conclusions and Outlook

In summary, single-crystal nickel-rich NCM materials show greater advantages and potential in meeting the national requirements for high energy density of power batteries. They are expected to be the most competitive candidates for the cathodes of next generation power batteries. Considering the present research, single-crystal nickel-rich materials still have some problems in lithium ion diffusion, internal microcracking and phase transitions. Some suitable preparation methods have been proposed to solve these problems, including ion doping, surface coating and electrolyte optimization. Single-crystal nickel-rich materials can be prepared in various ways. The preparation method, reaction temperature, and synthesis time can affect the single crystal size, selective orientation, and crystal shape of the materials. In addition to the traditional co-precipitation, multi-step calcination, and solvothermal methods, advanced routes such as spray pyrolysis, sol-gel, selective leaching, and etching have been developed recently. Instead of using only one optimization strategy, more researchers are bringing together multiple optimization approaches to play a synergistic role in enhancing the multifaceted properties of the material all at once, especially the ultra-long cycle and thermal stability of the material under high rate, high cut-off voltage and high temperature conditions. In order to further improve the energy density, reduce the cost of raw materials and promote the commercial safety of single-crystal nickel-rich NCM materials, scientists have developed nickel-ultra-rich single-crystal materials, studied the significance of the presence of Co, developed cobalt-free nickel-rich materials, and built all-solid-state batteries with the application of solid-state electrolytes, respectively.

In the future, the research into all-solid-state batteries will become more mature, which will lead to further extensive research and the preliminary application of nickel-ultra-rich or cobalt-free ternary sulfide-based all-solid-state batteries with lithium metal canodes. Especially, the batteries assembled by the combination of various optimization methods will present higher energy density, better high-rate ultra-long cycle stability and high temperature characteristics. They will greatly increase the possibility of the industrial application of single-crystal nickel-rich NCM in electric vehicles and large energy storage equipment in the future. The development of multifunctional, self-supporting electrodes for flexible electronic devices in combination with new foldable materials with good electrical conductivity is also a relatively popular research direction [306,307]. Moreover, the recovery of valuable metal ions in spent power batteries and the regeneration of the existing polycrystalline materials into single-crystal NCM materials will also be an important aspect for the future development of the power battery industry, contributing substantially to energy saving and emission reductions, recycling and sustainable development. However, there is still a long way to go for the large-scale mass production of single-crystal nickel-rich materials because there are still technical barriers in the preparation process.