Single-Crystal Nickel-Cobalt-Manganese Cathode Research: Comparison
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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 (LiNixCoyMnzO2 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.

  • single-crystal
  • nickel-rich NCM materials
  • cathode

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][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][4][5]. However, realization of these goals mainly depends on the cathode material in the power batteries [6,7,8][6][7][8].
Ternary layered transition metal oxide, LiNixCoyMnzO2 (NCM, x + y + z = 1), was first proposed by J. R. Dahn’s group in 2001 [9]. According to the report, Li[NixCo1−2xMnx]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[Ni3/8Co1/4Mn1/4]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 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][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][15][17][18].
Figure 1. Illustration of the ordered layer structure and phase diagram of NCM materials: (a) the hexagonal crystal system layered structure; (b) NCM composition phase diagram of several typical Ni-Co-Mn ratios, reproduced from Ref. [15], Copyright 2008, The Royal Society of Chemistry.
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][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 LiNi1/3Co1/3Mn1/3O2 (NCM111) [30[30][31][32][33][34][35],31,32,33,34,35], LiNi0.4Co0.2Mn0.4O2 (NCM424) [30[30][31][32][33][34],31,32,33,34], LiNi0.5Co0.2Mn0.3O2 (NCM523) [36,37,38,39,40,41][36][37][38][39][40][41] and other NCM materials with non-stoichiometry ratios of Ni-Co-Mn elements [42[42][43][44][45][46][47][48][49][50][51][52],43,44,45,46,47,48,49,50,51,52], etc. In particular, the nickel-rich NCM materials LiNi0.6Co0.2Mn0.2O2 (NCM622) [52[52][53][54][55][56][57][58][59][60][61],53,54,55,56,57,58,59,60,61], LiNi0.8Co0.1Mn0.1O2 (NCM811) [7,62,63,64,65,66,67,68,69,70,71][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[69][72][73][74][75][76][77][78],76,77,78], even the cobalt-free ternary materials [79,80,81,82,83,84,85,86[79][80][81][82][83][84][85][86][87][88][89][90][91],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][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][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][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[102][103],103], Al [4,102[4][102][104][105],104,105], Zr [106,107,108][106][107][108], Ti [44[44][109][110][111],109,110,111], Nb [112[112][113],113], Mo [114[114][115],115], Cr [116], F [117[117][118][119][120],118,119,120], B [76[76][121][122],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][15][123]. In addition, the design of the concentration-gradient structured materials can effectively improve the rate and cycling performance [124,125][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][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[128][129][130],129,130], Al2O3 [131[131][132][133][134][135],132,133,134,135], ZrO2 [136[136][137][138][139][140][141],137,138,139,140,141], TiO2 [138[138][142][143][144][145][146],142,143,144,145,146], etc.), phosphates (AlPO4 [147,148][147][148], FePO4 [149[149][150],150], Ni3(PO4)2 [151], CaHPO4 [152], Mn3(PO4)2 [153[153][154],154], ZrP2O7 [155[155][156],156], etc.), Li-containing compounds (Li3PO4 [157,158,159][157][158][159], Li2ZrO3 [160,161,162,163][160][161][162][163], Li2TiO3 [160[160][164][165],164,165], Li1.5Al0.5Ge1.5(PO4)3 [166], etc.), electron conducting coatings (graphene or reduced graphene oxide (rGO) [167[167][168][169][170][171],168,169,170,171], permeable poly (3,4-ethylenedioxythiophene) (PEDOT) [172[172][173],173], polyaniline (PANI) [174[174][175],175], poly-pyrrole (PPy) [176[176][177],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][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][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][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][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][36][67][94][196][197].

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[36][198][199][200][201][202][203][204][205][206][207][208],228,229,230,231,232,233], the molten salt assistant method [199,211,218,234,235,236,237,238,239,240,241,242,243][209][210][211][212][213][214][215][216][217][218][219][220][221] and the solvothermal method [244,245,246,247][222][223][224][225]. The main synthesis strategies of single-crystal nickel-rich NCM materials are shown in Figure 32.
Figure 32. A schematic diagrams of the main synthesis strategies for single-crystal nickel-rich NCM materials, reproduced from Ref. [222]. Copyright 2021, Elsevier.
A schematic diagrams of the main synthesis strategies for single-crystal nickel-rich NCM materials, reproduced from Ref. [226]. Copyright 2021, Elsevier.
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[Ni0.5Mn0.3Co0.2]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][198]. 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 43, a single-crystalline LiNi0.6Co0.1Mn0.3O2 (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][227]. 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][228].
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Figure 43. Research content reproduced from Ref. [248][227] of Xinming Fan’s group: (a) schematic diagram of preparation process and (b) SEM image for single-crystal NCM613, (c) the cycling performance of NCM613/graphite full cell, Copyright 2021, Elsevier.
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[Ni0.9Co0.05Mn0.05]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][229]. Yet the single-crystal pristine Li[Ni0.9Co0.05Mn0.05]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[Ni0.9Co0.05Mn0.05]O2 (SC-NCM90) and Li[Ni0.7Co0.15Mn0.15]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][208]. 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][230][231][232]. 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][220], which was denoted as LiNi0.91Co0.06Mn0.03O2 (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 LiNi0.92Co0.06Mn0.02O2 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][213]. 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 LiNi0.92Co0.06Mn0.01Al0.01O2 (NCMA) cathode materials with ammonium metatungstate and ammonium molybdate as the co-doping fluxing agents and doping materials [240][218]. 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 LiNi0.95Mn0.05O2 (SC-NM95) layered cathode without any cracks, as shown in Figure 54a, during which the LiOH and LiNO3 were mixed homogeneously as the molten salt system and lithium source [237][215]. 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 54b–d).
Figure 54. Research content reproduced from Ref. [237][215]: (a) SEM image of fresh SC-NM95 materials; (b) the first discharge/charge curves, and the cycling performance at difficult cut-off voltage of (c) 4.3 V and (d) 4.5 V of comparison between SC-NM95 and PC-NM95 materials, Copyright 2022, Elsevier.
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][216][217]. 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][216]. 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 Ni0.8Co0.1Mn0.1(OH)2 precursor with the preferred orientation of (001) lattice plane [244][222]. 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 LiNi0.8Co0.1Mn0.1O2 (SC-NCM) with the retained precursor hexagonal morphology was obtained (the preparation process and corresponding SEM image are illustrated in Figure 65a,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 65c). 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 65d). Youxiang Zhang’s group also prepared single-crystal LiNi0.8Co0.1Mn0.1O2 by the simple solvothermal method combined with a lower calcination temperature. However, the obtained sample displayed a different rod-like morphology [245][223]. When the excess lithium content was 50%, the LiNi0.8Co0.1Mn0.1O2 (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.
Figure 65. Research content reproduced from Ref. [244][222]. (a) The schematic diagram for the preparation process and (b) SEM image for SC-NCM. The comparison of (c) cycling performance and (d) rate capability between SC-NCM and PC-NCM, Copyright 2022, American Chemical Society.
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 LiNi0.6Co0.2Mn0.2O2 (SC-NCM) with hexagonal slabs by the hydrothermal method [246][224]. 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.
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][227]
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][228]
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][229]
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][208]
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][220]
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][213]
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][218]
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][215]
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][216]
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][222]
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][223]
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][224]
In addition to the methods summarized above, the synthesis strategies of ion anchoring [254][233], sol-gel [255][234], etching [256][235], and other advanced methods [257,258,259,260][236][237][238][239] are usually employed to produce single-crystal nickel-rich NCM materials.

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