Synthesis and Modification of Single-Crystal High-Nickel Ternary Cathode Materials

Hanfeng Wu; Jiushuai Deng; Jinli Liu; Yingqiang Wu; Li Wang; Xiangming He

doi:10.7536/PC231112

Progress in Chemistry >

2024 , Vol. 36 >Issue 6: 827 - 239

DOI: https://doi.org/10.7536/PC231112

Review

Synthesis and Modification of Single-Crystal High-Nickel Ternary Cathode Materials

Hanfeng Wu ¹^,² ,
Jiushuai Deng ^,¹^,^* ,
Jinli Liu ²^,³ ,
Yingqiang Wu ^,²^,⁴^,^* ,
Li Wang ² ,
Xiangming He ^,²^,^*

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¹ School of Chemistry and Environment, China University of Mining and Technology-Beijing, Beijing 100083, China
² Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
³ School of Materials, Nanjing University of Science and Technology, Nanjing 210094, China
⁴ School of Materials, Hainan University, Haikou 570228, China

* e-mail: dengshuai689@163.com (Jiushuai Deng);

yqwu@hainanu.edu.cn(Yingqiang Wu);

hexm@tsinghua.edu.cn (Xiangming He)

Received date: 2023-11-15

Revised date: 2024-03-01

Online published: 2024-04-16

Supported by

National Natural Science Foundation of China(U21A20170)

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Abstract

With the rapid development of portable electronic products and electric vehicles,the demand for high energy density lithium-ion batteries is increasing.High-nickel ternary materials with nickel content higher than 0.6(include)(e.g.,LiNi_0.6Co_0.2Mn_0.2O₂,LiNi_0.8Co_0.1Mn_0.1O₂and LiNi_0.9Co_0.05Mn_0.05O₂),which can deliver a high reversible specific capacity of more than 200 mAh·g^-1at an upper cut-off voltage of 4.3 V vs Li⁺/Li,are an important development direction of cathode material with high specific capacity.However,the weak mechanical strength,low compaction density of polycrystal ternary materials and the anisotropy of primary grains lead to intergranular cracks in the polycrystal particles during the charging and discharging process.The electrolyte will penetrate into the polycrystal particles along the intergranular cracks,thus aggravating the side reaction between the electrode and electrolyte and deteriorating the cycle performance and safety of the battery.The design of single crystal material without grain boundary can reduce the formation of intergranular cracks,effectively suppress the side reaction at the interfaces and improve the cycle stability.In this study,the advantages and problems of single-crystal high-nickel ternary materials are reviewed,and their synthesis methods and modification strategies are analyzed.Finally,the application prospects and challenges of single-crystal high-nickel ternary materials are reviewed and prospected.

Contents

1 Introduction

2 Performance difference between monocrystalline and polycrystalline materials

3 Synthesis methods of single-crystal high-nickel ternary materials

3.1 Coprecipitation method

3.2 Molten salt synthesis

3.3 Corrosion method

3.4 Spray pyrolysis

3.5 Hydrothermal method

4 Modification strategies of single-crystal high-nickel ternary materials

4.1 Problems in single-crystal materials

4.2 Surface coating

4.3 Ion doping

5 Conclusion and outlook

Key words： lithium-ion battery; high-nickel ternary cathode material; single crystal; polycrystalline

Cite this article

Hanfeng Wu , Jiushuai Deng , Jinli Liu , Yingqiang Wu , Li Wang , Xiangming He . Synthesis and Modification of Single-Crystal High-Nickel Ternary Cathode Materials[J]. Progress in Chemistry, 2024 , 36(6) : 827 -239 . DOI: 10.7536/PC231112

1 Introduction

Lithium-ion batteries have been widely used in portable electronics,electric vehicles,grid energy storage and other fields because of their high energy density,low cost and long cycle life^[1,2]。 As a key indicator of battery performance,energy density plays a decisive role in space utilization and endurance,and countries around the world attach great importance to the research and development of higher energy density lithium-ion batteries.In 2020,Europe formulated the"2030 Battery Innovation Roadmap",which aims to narrow the gap between battery energy density and theoretical value by at least 1/2 under the condition of ensuring service life and safety.The United States has developed the National Development Blueprint for Lithium Batteries 2021–2030,which plans to further reduce the cost of batteries and increase the energy density of lithium batteries to 500 Wh·kg⁻¹in 2030.South Korea has also recently released the 2030 Secondary battery(K-Battery)Industry Development Strategy,which will provide financial support and large-scale research and development for all kinds of secondary batteries,and plans to increase the energy density to 400 Wh·kg^-1in 2028^[3,4]。 The Chinese government has also supported the development of lithium-ion batteries as a strategic key industry in the 14th Five-Year Plan,and the overall development goal is expected to achieve an energy density of 300~600 Wh·kg^-1^[5]。 It can be seen that high energy density lithium-ion batteries will remain one of the important development directions in the field of new energy for a long time to come^[3⇓~5]。

After more than 30 years of development,the energy density of lithium-ion batteries has continuously increased from 80 Wh·kg^-1in 1991 to nearly 300 Wh·kg^-1due to the continuous development and progress of electrode materials and battery structures^[6⇓⇓~9]^[10,11]。 The first generation of cathode materials for lithium-ion batteries appeared in the 1970s,represented by embedded cathode materials such as NbS₂,TaS₂,and TiS₂.However,the working potential of these sulfides is generally low.Considering that oxygen ions are more electronegative than sulfur ions,which can improve their redox potential when coordinated with transition metal ions,Goodenough et al.Developed 4.0 V（vs Li⁺/Li）LiCoO₂layered oxide cathode in 1980^[8]^[12]。 Since then,a series of lithium batteries based on LiCoO₂have been discovered,and the cathode materials that have been widely used now mainly include LiCoO₂,LiMn₂O₄,LiFePO₄and LiNi_xCo_yMn₁₋_x−yO₂（x>0,y<1).Among them,the ternary cathode material（LiNi_xCo_yMn₁_−x−yO₂）has become one of the important research objects of high energy density lithium-ion batteries because of its advantages such as high working voltage(average voltage>3.8 V vs Li⁺/Li）)and high theoretical specific capacity（270~285 mAh·g^-1）.Compared with LiCoO₂materials,the use of Ni and Mn instead of Co greatly reduces the cost of raw materials^[2,12⇓~14]; On the other hand,Mn exists stably in the valence of+4 and does not participate in the electrochemical reaction during the charge-discharge process,which is beneficial to the improvement of the thermal stability of the material;Ni mainly exists in the form of Ni²⁺and Ni³⁺,which is the main contributor to the capacity of ternary oxide positive electrode due to redox reaction during charge and discharge;Co mainly exists in the form of Co³⁺,and its stable electronic structure is beneficial to improving the electronic conductivity of the material and inhibiting the mixing of Li⁺/Ni²⁺cations^{[15⇓⇓~18]}^[19,20]。 Obviously,there is an important synergistic effect of Ni,Co and Mn in the material^[15,21,22]。

The LiNi₁_−x−yCo_xMn_yO₂can be divided into three groups according to the nickel content.The first type is symmetric ternary materials with low nickel content,mainly represented by LiNi_1/3Co_1/3Mn_1/3O₂,LiNi_0.4Co_0.2Mn_0.4O₂.The layered oxide has the advantages of good thermal stability and stable crystal structure,but has low reversible specific capacity at a low voltage（<4.3 V vs Li⁺/Li）^[23,24]; The second type is the material system with medium nickel content,which is typically represented by LiNi_0.5Co_0.2Mn_0.3O₂.This kind of material combines the advantages of low cost,high capacity and crystal structure stability,and is the most widely used ternary layered oxide cathode at present^[25,26]; The third category is the high-nickel ternary material system with nickel content greater than 0.6(inclusive),such as LiNi_0.6Co_0.2Mn_0.2O₂,LiNi_0.8Co_0.1Mn_0.1O₂,LiNi_0.9Co_0.05Mn_0.05O₂,etc.This kind of high nickel ternary oxide positive electrode is characterized by（<4.3 V vs Li⁺/Li）,high reversible specific capacity and（>200 mAh·g^-1）at low potential,which is an important development direction of high specific capacity positive electrode in the future 。

However,the electronic structure of Ni³⁺in high nickel ternary oxides is less stable,and the NiO₆octahedron is prone to structural distortion(Jahn-Teller effect)during cycling,resulting in poor stability of the material^{[27⇓⇓⇓⇓~32]}。 To solve these problems,researchers have carried out a large number of modification studies,including morphology control(polycrystalline secondary spheres and single crystal particles),bulk doping,surface coating and other technologies^{[25,33⇓~35]}。 In terms of material morphology,the polycrystalline secondary ball has the advantage of good fluidity,which is conducive to uniform pulping and pole piece coating,and the smaller primary particles are also conducive to the rate performance^[36]。 However,there is anisotropy between the primary grains in the secondary sphere,and the large volume change during the deintercalation of lithium ions can easily cause the secondary sphere to crack^{[37⇓⇓~40]}。 by regulating the orderly arrangement of the primary particles in the secondary sphere,the stress caused By the volume change can be relieved^[41]。 In recent years,researchers have proposed to make high-nickel ternary cathode materials into dispersed single crystal particles.Monocrystalline materials have more stable internal microstructure,higher compaction density,better safety and superior cycling stability than polycrystalline materials,and are therefore more suitable for use under high pressure^{[42⇓⇓~45]}。 On the one hand,the active area of the single crystal particle in direct contact with the electrolyte is small,which can effectively reduce the interfacial electrochemical side reaction;On the other hand,single crystal materials do not have interparticle grain boundaries,thus avoiding cracking at grain boundaries,which is more conducive to showing excellent cycling stability.However,single crystal high nickel ternary cathode materials also face many challenges.For example,the synthesis process usually requires a high sintering temperature,but the thermal stability of high nickel ternary materials is poor,and the sintering at high temperature easily leads to serious lattice defects and Li/Ni mixing^[29,46]。 In addition,too large grain size can also lead to stress accumulation during charging and discharging,and then produce intragranular cracks^[47]。 Therefore,it is still a big problem to explore an environmentally friendly and low-cost preparation method to synthesize single-crystal high-nickel ternary cathode materials with less lattice defects,highly ordered structure and moderate size。

Based on the above problems,this paper discusses the performance differences between single crystal materials and polycrystalline materials,and systematically introduces the research progress of single crystal materials in synthesis methods and modification strategies in recent years。

2 Property difference between monocrystalline and polycrystalline materials

at present,the commercial high nickel ternary cathode materials are mainly polycrystalline materials,which are agglomerated by nano-scale primary particles.In the process of charging and discharging,because of the low mechanical strength and compaction density of the material,obvious intergranular cracks usually occur between the primary particles.in the process of repeated intercalation/deintercalation of lithium ions,the anisotropic lattice will expand and contract,resulting in anisotropic stress and distortion in the material,resulting in intergranular cracks,which further hinder the migration of ions and electrons in the secondary particles.In addition,the electrolyte permeates into the secondary particles along the intergranular cracks,which significantly increases the area of the electrode/electrolyte interface,intensifies the side reactions At the electrode/electrolyte interface and the phase change of the cyclic electrode material.As a result,the cracks gradually spread along the gaps or grain boundaries between the primary particles,which makes the primary particles fall off and the secondary particles break,further reducing the electrochemical stability of the material^{[40,47⇓⇓⇓⇓⇓~53]}。 Single crystal materials have stable internal microstructure,higher compaction density and superior cycling stability,which are more suitable for use at high voltage than polycrystalline materials^{[42⇓⇓~45]}。 monocrystalline ternary cathode materials with high compaction density can effectively alleviate the formation of microcracks and maintain the integrity of the structure.Due to the small specific surface area of the single crystal,the active material/electrolyte interface area is usually small,which can effectively reduce the erosion of the electrolyte on the electrode and the side reactions inside the battery.in addition,the surface of the monocrystalline particles is smooth,which can fully contact with the conductive agent to promote the transfer of electrons.the changes of the particle interface during the charge-discharge process of monocrystalline and polycrystalline materials are shown In Figure 1^[54,55]。 Compared with the secondary agglomerated particles,the single crystal particles have better dispersion,and the gap between grains can adapt to the volume change,which can avoid the stress caused by volume distortion.Xu et al.Carried out 300 charge-discharge cycles on polycrystalline LiNi_0.8Co_0.1Mn_0.1O₂and monocrystalline LiNi_0.8Co_0.1Mn_0.1O₂cathode materials,and then compared the scanning electron microscope images of the two materials.It can be clearly observed that after 300 cycles,the secondary spherical particles of the polycrystalline cathode materials were broken,the primary particles fell off,and the particle structure was seriously deformed^[56]。 In contrast,after 300 cycles,the shape of the single crystal particles remains intact without obvious cracks,and the capacity retention rate is 95.5%,which is significantly higher than that of the polycrystalline material 84.5%。

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图1 多晶颗粒和单晶颗粒界面变化^[57]

in addition,monocrystalline materials have better safety performance than polycrystalline materials.Because there are few intragranular gaps,the lithium ion transmission impedance of monocrystalline materials is lower than that of polycrystalline materials,the ion diffusion coefficient is higher,and the heat generated In the cycle process is less than that of polycrystalline materials^{[31,58⇓~60]}。 Sun et al.studied the thermal runaway behavior of lithium-ion batteries with high-nickel ternary materials as cathodes,and found that monocrystalline materials have better thermal stability than polycrystalline materials under the same nickel content,mainly due to the lower cation mixing rate and higher structural stability of monocrystalline materials^[31]。 On the other hand,the continuous oxygen released from the lattice will Also react with the electrolyte,resulting in an exothermic phenomenon,which is also one of the main causes of thermal runaway in batteries.the monocrystalline material has a solid shape structure,which can inhibit the micro-cracks inside the particles during the charge-discharge process,reduce the side reaction with the electrolyte,and alleviate oxygen evolution,thereby improving the cycle stability and thermal stability.Zhong et al.also found that monocrystalline materials have higher electrochemical stability and higher thermal decomposition temperature than polycrystalline materials^[61]。

the stability of single crystal materials during storage is also significantly better than that of polycrystalline materials.Kong et al.Carefully studied the differences in electrochemical properties and material structures of monocrystalline and polycrystalline high-nickel ternary materials with the same nickel content after storage for a certain period of time^[62]。 the SEM cross-sectional view of the monocrystalline and polycrystalline high-nickel material After storage for a period of time is shown in Fig.2.They found that during storage,the impurities of the polycrystalline material could enter the interior of the material along the gaps of the primary particles,while the impurities on the surface of the monocrystalline material would not affect the interior of the material.after the same storage time,the single crystal material can maintain better structural and electrochemical properties than the polycrystalline material.in practical applications,this advantage helps to reduce the costs incurred in production and transportation.It can be seen that the development of high-performance single crystal materials is an important direction for the future development of ternary materials。

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图2 (a) 单晶NCM622，(b) 多晶NCM622，(c) 单晶NCM811，(d) 多晶NCM811材料储存一段时间后的SEM横截面图^[62]

3 Ynthesis method of monocrystal high nickel ternary material

Although single crystal materials have advantages in many aspects,the preparation of single crystal materials with good structure and morphology still faces challenges.Firstly,the synthesis temperature of monocrystalline materials is usually 60~100℃higher than that of polycrystalline materials,and the higher sintering temperature makes monocrystalline high-nickel ternary cathode materials prone to lattice defects and serious Li/Ni mixing;Secondly,if the grain size is too large,the accumulation of stress during the long charge-discharge cycle will also cause intragranular cracks.Therefore,the development of simple,low-cost and environmentally friendly synthesis methods to obtain single-crystal high-nickel ternary materials with less lattice defects and excellent electrochemical properties is a current research hotspot^[29,46]。

the synthesis methods of single crystal high nickel ternary materials can be roughly divided into two categories:one is indirect synthesis of single crystals from secondary particles,and The other is direct synthesis of single crystals^[63]。 Indirect synthesis usually uses chemical or physical methods to break polycrystalline precursors or cathodes into single crystal materials,such as coprecipitation,molten salt synthesis,etching,etc.,which generally introduce some inactive impurities^[64]^[63]。 Most of the impurities can be removed by washing,but they can also lead to the loss of lithium.the direct synthesis method refers to the direct synthesis of single crystal precursors by spray pyrolysis,hydrothermal and other methods,which can significantly reduce the introduction of impurities^[51,65]^[66]。 Direct synthesis methods usually have complex synthesis conditions and require volatile toxic solvents,which limit their wide application。

3.1 Coprecipitation method

Coprecipitation method is a key technology for the synthesis of high-performance ternary oxide cathode materials in industry and academia.Precursor materials with different shapes,sizes and degrees of agglomeration are prepared by adjusting the concentration of complexing agent,pH value,synthesis temperature,stirring speed and reaction time^[67]。 At present,NaOH is mainly used as a precipitant,and NH₃·H₂O is used as a complexing agent to control the speed of the precipitation reaction,thereby controlling the crystal orientation and morphology of the hydroxide precursor.The obtained precursor is mixed with lithium salt,ground or ball-milled,and then sintered to obtain the monocrystalline material.After obtaining the homogeneous precursor,the solid state reaction between(Ni₁_−x−yCo_xMn_y)(OH)₂and lithium salt(such as LiOH or Li₂CO₃）)is as follows:

(Ni₁_−x−yCo_xMn_y)(OH)₂+LiOH+1/4O₂↔LiNi₁_−x−yCo_xMn_yO₂+3/2H₂O

the precursor prepared by the method has the advantages of uniform components,adjustable size structure,simple operation,industrial production and the like,but ammonia water used as a complexing agent is corrosive and irritating,and the cost for meeting the environmental protection requirement is increased。

Zhang et al.Firstly prepared Ni_0.6Co_0.1Mn_0.3(OH)₂precursor by co-precipitation method,and then mixed Ni_0.6Co_0.1Mn_0.3(OH)₂with LiOH·H₂O(Li∶TM=1.08∶1),and calcined at 930°C in air for 12 H using one-step calcination method to obtain LiNi_0.60Co_0.10Mn_0.30O₂single crystals with particle size of 2–5μm^[54]。 Meanwhile,polycrystalline LiNi_0.60Co_0.10Mn_0.30O₂material was prepared by sintering at 850℃in air for 12 H with the ratio of Li∶M=1.05∶1 as a control.The initial specific discharge capacities of the two materials are similar in the voltage range of 2.75~4.3 V at 1 C and 25℃.After 150 cycles,the capacity retention of the single crystal cathode is as high as 90.0%.However,the capacity retention rate of polycrystalline cathode is only 72.8%,indicating that the capacity retention ability of monocrystalline NCM battery is better.After 150 cycles at a high cut-off voltage of 4.4 V and a high temperature of 55℃,the capacity retention of the single crystal cathode is as high as 87.4%,which is much better than that of the polycrystalline cathode(63.1%after 65 cycles )。

the one-step sintering process is relatively simple,and the prepared single crystal material has good thermal stability.this method is commonly used in ternary material systems With nickel content not higher than 0.6.with the increase of nickel content,the structure and stability of the material will decrease,and long-term sintering at high temperature will easily lead to agglomeration,which usually requires ball milling to improve the dispersion of the material,but This will further introduce surface defects^{[27⇓⇓⇓⇓~32]}^[42,68]。 Based on this problem,Zhu et al.Prepared single crystal LiNi_0.83Co_0.12Mn_0.05O₂cathode materials by co-precipitation method and multi-step sintering method,placing the ball milling process between the two sintering steps^[69]。 The prepared precursor was first mixed with LiOH in a molar ratio of 1∶1.05,and then the mixture was sintered at 500℃for 5 H and calcined at 880℃in oxygen atmosphere for 10 H.Subsequently,the material was ball-milled at a speed of 300 r·min^-1for 4 H,and then calcined again at 750°C for 10 H to prepare the single crystal LiNi_0.83Co_0.12Mn_0.05O₂material.The specific discharge capacity of this single crystal material was found to be 209 mAh·g⁻¹at 0.1 C by electrochemical test,which is slightly lower than that of the polycrystalline material with the same composition,because the ion transmission path of the single crystal material is longer,resulting in the loss of initial capacity 。

Long time high temperature calcination is easy to cause particle agglomeration,and the multi-step sintering method introduces the ball milling and resintering process after the first calcination.the resintering process can repair the surface defects introduced by ball milling and refuse the fallen fragments,which is the key to obtain fine single crystal particles with excellent electrochemical properties^[70]。 Although the introduction of ball milling and resintering process can slightly improve the electrochemical performance of single crystal materials,it increases the complexity of the preparation process and is not conducive to large-scale production。

3.2 Molten salt synthesis

Direct solid phase sintering,either one-step sintering or multi-step sintering,usually requires a higher sintering temperature,which leads to agglomeration of single crystal particles and intensifies cation mixing.It is an effective way to reduce the sintering temperature of single crystal by using low melting point materials as the medium of ion diffusion and crystal growth^[64]。 the molten salt synthesis method can avoid the co-precipitation of related impurities,which is beneficial to the formation of large particle size and high phase purity monocrystalline.molten salt is used to induce the formation of flux during calcination,so that it can remain liquid At high temperature,which is the main difference between molten salt synthesis and coprecipitation synthesis.at the same time,the molten salt synthesis method also has the disadvantages of slow growth rate,long cycle and small crystal size,and many molten salts have different degrees of toxicity,and their volatiles usually pollute the environment。

high nickel layered single crystal materials are sensitive to water.When using molten salt synthesis method to prepare High nickel single crystal,the washing process usually leads to the lack of oxygen in the surface layer of the material and the loss of capacity^[64,71,72]。 In order to solve these problems,Lv et al.Synthesized high nickel single crystal LiNi_0.92Co_0.06Mn_0.02O₂single crystal materials by a new LiOH-LiNO₃-H₃BO₃ternary molten salt method,adding a proper amount of lithium salt of boric acid（H₃BO₃）to reduce the melting point and omit the high temperature calcination and washing process^[64]。 In addition,the introduction of boric acid enhances the B-O bond by boron doping,which is very effective in stabilizing the surface structure of the layered structure.Electrochemical tests show that the initial discharge capacity of the sample is 209.2 mAh·g^-1at 2.8~4.25 V and 0.2 C,which is slightly lower than that of the polycrystalline material prepared by the traditional solid state method（212.1 mAh·g^-1）.This is because the polycrystalline material has more full contact with the electrolyte and shorter lithium ion diffusion path than the single crystal.However,after the first cycle at 0.2 C and then 100 cycles at 0.5 C and 45°C,the discharge specific capacity of the single crystal material is 200.3 mAh·g^-1,and the capacity retention rate is 88.5%.Under the same circumstances,the discharge capacity of the polycrystalline material is 177.2 mAh·g^-1with a capacity retention of 76.9%.The addition of boric acid not only saves the washing process,but also plays a key role in improving the structural stability of the material 。

3.3 Etching method

Etching method is a method to prepare single crystal by directly using dilute acid to etch polycrystalline materials and then adding lithium salt to calcine at high temperature.Ma et al.Put polycrystalline NMC111 and NCM622 materials into dilute sulfuric acid at a ratio of 0.05 g·mL^-1and stirred^[63]。 After etching,NMC111 and NCM622 were washed with deionized water and dried.The dried powder was mixed with Li₂CO₃（(5%excess),and then sintered in air at 450℃for 5 H and 800℃for 18 H,respectively,with the heating and cooling rates kept at 2℃·min^-1,to obtain single crystal NCM111 and single crystal NCM622.Through the electrochemical test,it is found that the first cycle discharge specific capacity of single crystal NMC111 is 163.9 mAh·g^-1at 0.1 C,and the first cycle discharge specific capacity of single crystal NMC622 is 176.0 mAh·g^-1,which are higher than those of polycrystalline materials under the same conditions,and the rate performance is also greatly improved compared with polycrystalline materials.The corrosion method has the advantages of easy control and removal of impurities,simple operation,and reuse of dilute sulfuric acid,but the disadvantage is that the cost of sewage treatment of dilute sulfuric acid is high.This method provides a feasibility for the industrial production of single crystal materials 。

3.4 Spray pyrolysis

Spray pyrolysis is a method of capturing precursors by atomizing and heating nickel,cobalt and manganese salt solutions in oxygen atmosphere.The spray pyrolysis method has the advantages of simple process and short production cycle,and does not need crystal crushing,flux addition and multiple sintering in the preparation process.However,spray pyrolysis has serious energy consumption and high cost,which hinders its industrial development.Zhu et al.Used spray pyrolysis(SP)to synthesize NiO-MnCo₂O₄-Ni₆MnO₈microparticles as the precursor of single crystal LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）,and successfully prepared submicron single crystal NCM811 cathode material with excellent electrochemical properties by optimizing the sintering temperature and lithium source.In the experiment,LiNO₃was selected as both lithium source and flux^[52]。 According to the electrochemical test,the initial discharge capacity of the sample is 216 mAh·g^-1at 0.1 C and 3.0–4.4 V,and the coulombic efficiency is 84.7% 。

Wang et al.Found that when LiNO₃was used as the main lithium source for the preparation of high nickel(>0.8)cathode materials by spray pyrolysis,the grain size was sometimes out of control,and too large or too small grain size had adverse effects on the electrochemical performance^[65]。 Therefore,in the preparation of single crystal LiNi_0.9Co_0.055Mn_0.045O₂by spray pyrolysis method,LiOH·H₂O was selected as the main lithium source,and a small amount of LiNO₃was introduced to adjust the particle size.After the precursor and lithium source were mixed evenly,the LiNi_0.9Co_0.055Mn_0.045O₂single crystal material with a particle size of 1μm was obtained by calcining at 500℃for 5 H and at 800℃for 12 H.According to the electrochemical test,the sample has a specific discharge capacity of 220 mAh·g^-1at 0.1 C and 2.8–4.3 V,and a specific discharge capacity of 173.8 mAh·g^-1at 10 C 。

3.5 Hydrothermal method

Hydrothermal method is a chemical synthesis method using water as solvent in supersaturated liquid solution at high temperature and high pressure^[73⇓~75]。 The ternary material prepared by the hydrothermal method has the advantages of small particle size,high crystallinity,uniform and controllable morphology,easy control of the reaction process and low production cost.However,the hydrothermal reaction equipment is expensive and the safety performance of the preparation process is poor,so that the industrialization degree is still low.Guo et al.Synthesized LiNi_0.8Co_0.1Mn_0.1O₂single crystal microrods using a hydrothermal method^[66]。 Compared with the traditional synthesis method,these LiNi_0.8Co_0.1Mn_0.1O₂rods were calcined with excess lithium source at low temperature,which not only reduced the sintering temperature,but also ensured the monodispersion of micron-sized particle distribution.Preparation process Dissolve Ni(CH₃COO)₂·4H₂O,Co(CH₃COO)₂·4H₂O in 100 mL of a mixture of absolute ethanol and deionized water(1:1 by volume),and then add an appropriate amount of CO(NH₂)₂under continuous stirring conditions.Then H₂C₂O₄·2H₂O was added to the above mixed solution,and the solution was transferred to a reaction kettle and heated at 180℃for 12 H.The heated sample was filtered and washed to obtain the oxalic acid Ni_0.89Co_0.11C₂O₄·2H₂O precursor.The synthesized oxalic acid precursor was dissolved in ethanol with Mn(NO₃)₂and 50%excess of LiOH·H₂O to form a suspension,which was evaporated to dryness.Finally,the LiNi_0.8Co_0.1Mn_0.1O₂single crystal microrod material was obtained after heating at 500°C for 6 H and sintering at 750°C for 12 H.The sample was found to have a first cycle specific discharge capacity of 187.2 mAh·g^-1at 2.8–4.3 V and 1 C and a capacity retention of 95.1%after 100 cycles by electrochemical testing,and it can provide a high discharge capacity of 140 mAh·g⁻¹even at a discharge rate of 2.8–4.4 V and 5 C 。

4 Modification Strategy of Single Crystal High Nickel Ternary Material

4.1 Problems of single crystal materials

Single crystal particles endow the electrode with ultra-high stability under long-term cycling,but in the presence of electrolyte,HF erosion and volume change cause changes in the c-axis of the d lattice and surface damage,making intragranular cracks inevitable.intragranular cracks,similar to intergranular cracks in polycrystalline materials,expose the activity of the electrode,thereby increasing the area of the electrode in contact with the electrolyte,resulting in increased side reactions on the particle surface,thickening of the CEI layer,release of lattice oxygen,and dissolution of transition metals.There are several hypotheses for the cause of intragranular cracking:oxygen vacancies,sliding of the(003)plane,and H2/H3 phase transformation^[76]^[46]^[20,77]。 the oxygen vacancy originates from the high-temperature calcination process and the interfacial side reaction between the cathode material and the electrolyte,resulting in oxygen release and structural degradation.During high voltage and long cycling,the lithium ion concentration gradient produces radial stress driving,and the surface energy of(003)crystal plane is low,which produces planar sliding and exposes new crystal planes.at the same time,the sliding of the(003)crystal plane leads to a sharp contraction of the c-axis and a typical H2/H3 phase transformation,which causes lattice parameter changes,resulting in the formation of intergranular slip and intergranular microcracks,thus leading to a significant decrease in its cycle stability.Aiming At a series of problems of single crystal materials,there are many improvement methods,including surface coating,element doping and so on^[29]。 the surface coating can inhibit the side reaction between the electrode and the electrolyte,thereby improving the mechanical strength of the particle.Doping elements on the surface of the single crystal particle can not only prevent the formation of intragranular cracks,but also prevent the precipitation of oxygen on the surface。

4.2 Surface coating

The surface coating method is to coat the electrode surface with a heterogeneous layer to reduce the loss of active materials and alleviate the side reactions at the electrolyte/cathode interface during charge and discharge,so as to improve the structural stability and cycle performance of the material.Metal oxide coatings generally act as inhibitors of HF,but also hinder Li⁺migration due to metal oxide electrochemistry and electronic inertness^[78]。 On the other hand,lithium reaction coating is a good choice because it can form a coating by consuming residual lithium,which is beneficial to Li⁺migration while achieving interface protection^[79]。 In addition,the researchers also prepared a composite coating combining the characteristics of both the metal oxide coating and the lithium reaction coating,which can enhance the Li⁺migration efficiency while acting as an HF inhibitor.The electrochemical performance of the single crystal high nickel ternary material with different coatings is shown in Table 1^[80]。

表1 Comparison of electrochemical properties of single crystal high nickel ternary materials with different coatings

Table 1 Comparison of electrochemical performance of single-crystal nickel-rich samples with different coatings

Cathode material	Coating material	First-cycle specific discharge capacity /(mAh·g⁻¹)	Voltage range/V (vs Li⁺/Li)	T/℃	C-rate/C	Capacity retention (cycles)	Ref
LiNi_0.6Mn_0.2Co_0.2O₂	Al₂O₃	158.6	2.8~4.3	25	1	92.6% (100)	78
LiNi_0.6Co_0.2Mn_0.2O₂	LiNbO₃	175.7	2.1~3.68	40	0.1	-	81
LiNi_0.6Co_0.2Mn_0.2O₂	LiNbO₃	-	2.1~3.68	40	0.5	91.3% (100)	81
LiNi_0.88Co_0.09Mn_0.03O₂	Li_1.4Y_0.4Ti_1.6PO₄	202.3	2.75~4.4	25	0.5	86.5% (200)	82
LiNi_0.88Co_0.09Mn_0.03O₂	Li_1.4Y_0.4Ti_1.6PO₄	-	2.75~4.4	55	0.5	82.6% (200)	82
LiNi_0.8Co_0.1Mn_0.1O₂	AlPO₄-Li₃PO₄	201.6	3.0~4.3	25	0.1	-	80
LiNi_0.8Co_0.1Mn_0.1O₂	AlPO₄-Li₃PO₄	-	3.0~4.3	25	1	88.9% (200)	80
LiNi_0.83Co_0.07Mn_0.1O₂	Li_1.25Al_0.25Ti_1.5O₄	167.2	2.7~4.3	25	1	88.9% (400)	83
LiNi_0.83Co_0.07Mn_0.1O₂	Li_1.25Al_0.25Ti_1.5O₄	-	2.7~4.3	25	5	92.6% (200)	83

In terms of oxide coating research,Ma et Al.Prepared single-crystal nanosheets of NCM622 cathode material by hydrothermal method,and coated them with a Al₂O₃layer with a thickness of 8–16 nm(Al content of 0.05%,0.11%and 0.22%,respectively )^[78]。 As a typical stable oxide,Al₂O₃can effectively prevent the electrode surface from contacting with the organic electrolyte,which slows down the electrolyte decomposition and reduces the oxygen evolution.After electrochemical test,the sample with Al content of 0.11%has the best cycle performance and rate capability with an initial capacity of 158.6 mAh·g^-1and a capacity retention of 92.6%.The specific discharge capacity of the sample with Al content of 0.22%is the lowest at all rates,because when the thickness of Al₂O₃coating reaches 16 nm,too thick coating will hinder the diffusion of lithium ions,resulting in a significant decrease in discharge capacity.Therefore,it is important to determine the optimal Al₂O₃coating thickness to ensure the stability and reliability of electrochemical cycling performance 。

In all-solid-state batteries,the interface problem between the cathode and the electrolyte is regarded as one of the key factors for performance degradation.In order to achieve high-performance solid-state cathode,it is particularly important to construct a stable interface between the active material and the electrolyte.In addition,the electrochemical performance of all-solid-state batteries can also be significantly improved by using monocrystalline materials instead of polycrystalline materials to eliminate grain boundaries within particles.Due to the small particle size of single crystal materials,the internal stress can be effectively released,thus avoiding the problem of mechanical cracking caused by excessive volume change.Li et al.Used LiNbO₃as the buffer layer of the cathode material for all-solid-state batteries and coated it on the LiNi_0.6Co_0.2Mn_0.2O₂single crystal,and the weight ratio of LiNbO₃in the coating was 1%^[81]。 Combining the advantages of the single crystal structure and the buffer layer,the discharge capacity of the single crystal coated sample reaches 175.7 mAh·g^-1at 0.1 C,and the initial coulombic efficiency is 88.7%（0.2 mA·cm^-2）.The capacity retention was 91.3%after 100 cycles at a high current density of 1.0 mA·cm^-2at 40°C.In addition,the single crystal coating sample still shows excellent electrochemical performance at high mass loading of 23.2 mg·cm^-2and high current density,showing an areal capacity of 4.0 mAh·cm^-2.In this experiment,the solid electrolyte with high thermal stability and high energy/power density was selected,and the LiNbO₃was used as the buffer layer to inhibit the interfacial side reaction between the cathode material and the solid electrolyte,which solved the problems of instable solid electrolyte interface and short cycle life.The method effectively improves the electrochemical performance of the monocrystalline cathode material,solves the problems of unstable solid electrolyte interface and short cycle life,and effectively improves the electrochemical performance of the monocrystalline cathode material in an all-solid-state battery 。

Fan et al.Constructed a Li_1.4Y_0.4Ti_1.6PO₄（LYTP）layer on the surface of LiNi_0.88Co_0.09Mn_0.03O₂（SC-NCM88）single crystal particles,and the synthesis schematic diagram is shown in Fig.3.LYTP is a NASICON-type compound with high Li⁺conductivity,which helps to promote the diffusion of lithium ion（Li⁺）between cathode material particles^[82]。 The electrochemical performance of the samples before and after coating is shown in Figure 4,and it is found that the sample with a mass ratio of LYTP:SC-NCM88 of 0.01:1 has the best electrochemical performance.After 200 cycles at 0.5 C and 2.75~4.4 V,the 1%LYTP sample achieved a reversible capacity of 175 mAh·g^-1at 0.5 C,with a corresponding capacity retention of 86.5%and a coulombic efficiency of about 99.9%,while the corresponding capacity retention of the pristine SC-NCM88 cathode was only 80.3%.After 200 cycles at 55℃,the capacity retention of the 1%LYTP sample can reach 82.6%,which is higher than the capacity retention of the initial SC-NCM88 positive electrode of 78.0%after 100 cycles.The 1%LYTP sample was higher than the pristine SC-NCM88 at different current densities from 0.1 to 5 C.The excellent electrochemical performance is mainly attributed to the multiple modification effects of the LYTP coating:(1)The LYTP protective layer and a small amount of Ti doping synergistically inhibit the formation of disordered spinel/rocksalt phase and the lattice mismatch between the ordered layered structure and the disordered structure,which greatly alleviates the shrinkage of c-lattice parameters and improves the reversibility of H2/H3 phase transformation.(2)Conformal LYTP modification has reduced c-axis shrinkage and better mechanical properties,which significantly alleviates intragranular/intergranular cracks.(3)The LYTP modification layer is beneficial to the transport of Li⁺,and the sample has a high reversible capacity due to the high Li⁺conductivity of the 3D network between the cathode particles 。

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图3 LYTP修饰SC-NCM88正极的合成方法示意图^[82]

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图4 原始SC-NCM88和1% LYTP@SC-NCM88在0.5 C下以锂金属为负极且测试温度为(a) 25 ℃和(b) 55 ℃下的循环稳定性。(c) SC-NCM88和(d) 1% LYTP@SC-NCM88在55 ℃下第1~100次循环的充放电曲线^[82]

Fig. 4 Cycling stability of pristine SC-NCM88 and 1% LYTP@SC-NCM88 against a lithium metal anode at 0.5 C under testing temperature of (a) 25 ℃ and (b) 55 ℃. Charge/discharge curves for (c) SC-NCM88 and (d) 1% LYTP@SC-NCM88 from 1st to 100th cycle at 55 ℃^[82]. Copyright 2021, Springer Nature

In the process of preparing single crystal LiNi_0.8Co_0.1Mn_0.1O₂,Du et al.Innovatively added AlNO₃·9H₂O and(NH₄)₂HPO₄on the surface when preparing single crystal LiNi_0.8Co_0.1Mn_0.1O₂.By using the residual lithium on the surface of high nickel cathode,they successfully constructed a high-quality AlPO₄-Li₃PO₄hybrid coating on the single crystal surface.This strategy not only enhanced the stability of the coating,but also effectively improved the electrochemical performance of the battery^[80]。 The results show that the discharge capacity of the coated sample is 201.6 mAh·g^-1and the coulombic efficiency is as high as 87.9%when cycled at 0.1 C（1 C=200 mA·g^-1）,which is close to that of the initial sample.At 3 C,the discharge capacity of the coated sample is still up to 160.8 mAh·g⁻¹,which is 11%higher than that of the uncoated sample,and its rate capability and cycle stability are higher than those of the initial sample.On the one hand,amorphous AlPO₄can protect the surface of particles from the corrosion of harmful substances generated by electrolyte decomposition,relieve the stress,stabilize the interface,and make the material have good crystal structure stability.On the other hand,crystalline Li₃PO₄acts as a selective channel,allowing only the transfer of Li⁺,improving the transport of lithium ions and growing the diffusion coefficient of lithium ions by 141%.This strategy converts residual lithium into a specific medium,which not only eliminates the negative effect of residual lithium,but also constructs a robust lithium-ion conductive coating on the single crystal cathode 。

4.3 Ion doping

Substitution of some elements with other trace elements is considered to be a very effective method to stabilize the lattice structure and improve the lithium intercalation/deintercalation ability,which can reduce the structural disorder,improve the structural stability of materials,prevent the formation of intragranular cracks and inhibit oxygen evolution on the surface^{[84⇓⇓~87]}。 in addition,some studies have realized The combination of ion doping and surface coating.the specific method is to use the metal oxide coating HF with poor conductivity as an inhibitor,and at the same time,the ion doping is used to enhance the charge transfer efficiency of the surface of the material.the synergistic effect of the two methods optimizes the electrochemical performance of the cathode material.the electrochemical performance pairs of single crystal high nickel ternary materials with different doping elements are shown In Table 2。

表2 Comparison of Electrochemical Properties of Single Crystal High Nickel Ternary Materials Doped with Different Elements

Table 2 Comparison of electrochemical performance of single-crystal Nickel-rich ternary samples with different doping elements

Cathode material	Doping element	First-cycle specific discharge capacity/(mAh·g^-1)	Voltage range/V (vs Li⁺/Li)	T./℃	C-rate /C	Capacity retention (cycles)	Ref
LiNi_0.8Co_0.1Mn_0.1O₂	B	194.7	2.8~4.3	25	0.1	-	84
LiNi_0.8Co_0.1Mn_0.1O₂	B	-	2.8~4.3	25	0.5	98.2% (100)	84
LiNi_0.8Co_0.1Mn_0.1O₂	Ti	201.5	2.8~4.3	25	1	-	85
LiNi_0.8Co_0.1Mn_0.1O₂	Ti	166.7	2.8~4.3	25	3	92.02% (150)	85
LiNi_0.8Co_0.1Mn_0.1O₂	F	202.7	2.8~4.3	25	0.1	-	88
LiNi_0.8Co_0.1Mn_0.1O₂	F	-	2.8~4.3	25	1	86.84% (100)	88
LiNi_0.7Co_0.1Mn_0.2O₂	Nb	204	3.0~4.5	25	0.1	-	89
LiNi_0.7Co_0.1Mn_0.2O₂	Nb	-	3.0~4.5	45	1	73.18% (100)	89
LiNi_0.6Mn_0.2Co_0.2O₂	Zr	188.9	2.8~4.5	25	1	98.5% (150)	90
LiNi_0.6Mn_0.2Co_0.2O₂	Zr	157.1	2.8~4.5	25	5	93.9% (250)	90
LiNi_0.8Co_0.1Mn_0.1O₂	P	195.53	2.8~4.3	25	0.1	-	87
LiNi_0.8Co_0.1Mn_0.1O₂	P	-	2.8~4.3	25	1	83.53% (100)	87

Dong et al.Improved the performance of the ternary cathode material LiNi_0.8Co_0.1Mn_0.1O₂by introducing B ions^[84]。 It can be seen from the XRD diffraction pattern that with the increase of B content,the ratio of crystal plane peak intensity I(003)/I(004)increases gradually,which means that the mixing degree of Li/Ni in the material has been effectively reduced.It can be inferred that the prepared Li(Ni_0.8Co_0.1Mn_0.1)_0.99B_0.01O₂sample will exhibit better electrochemical performance.The XRD refinement results show that the radius of the B³⁺ion is close to that of the Co³⁺and Mn⁴⁺ions,so it can smoothly enter the transition layer and occupy the 3A site in the lattice.This occupancy leads to a significant change in the Ni ion at the 3 B site.With the increase of B content,the number of Ni ions occupying 3 B sites decreases gradually.This means that the doping of B ions helps to reduce the degree of Li/Ni mixing,thus maintaining a high degree of order in the crystal structure.The electrochemical test results show that the initial specific capacity of the B-doped single crystal material is 194.7 mAh·g^-1at 0.1 C,which is 11.3%higher than that of the initial sample.The capacity retention after 100 runs at 0.5 C was 98.2%,which was 20.86%higher than that of the initial sample.The sample also showed the most excellent rate capability with a specific discharge capacity of 143.1 mAh·g^-1at 10 C.At the same time,the sample also has excellent reversibility.After 5 cycles at 0.1 C,the sample is cycled to 10 C with increasing current,and then to 0.1 C,the specific discharge capacity still has a 193.2 mAh·g^-1.The ion diffusion coefficient of Li(Ni_0.8Co_0.1Mn_0.1)_0.99B_0.01O₂is 3.88×10^-10cm²·s^-1.The addition of B₂O₃promotes the formation of crystals,the decrease of cation mixing,the increase of lithium ion diffusion and the decrease of interface impedance 。

Zhang et al.Selected Ti as the doping element to prepare Li-Ti-O lithium ion conductive coating,and studied the synergistic effect of Ti ion doping on the structure and morphology of the material,as well as the effect on the electrochemical performance of LiNi_0.8Co_0.1Mn_0.1O₂materials^[85]。 The results show that the sample has an initial coulombic efficiency of 94.82%and a specific discharge capacity of 201.5 mAh·g^-1at 1 C.The average specific capacity decay rate of Ti-doped sample is still low,which is 31.39%in the temperature range of 1~20 C.After 150 cycles at different rates of 3,10,20 C,2.8~4.3 V and 25℃,the average capacity retention can still reach 82.84%.After 150 cycles at 60℃and 2.8~4.5 V,the discharge capacity still remained in the 173.5 mAh·g^-1.The lower specific capacity loss and higher coulombic efficiency of the materials are mainly due to the low cation incorporation and enhanced passivation layer.There are three main reasons for the significant improvement of electrochemical properties by the introduction of Ti:the obtained Li-Ti-O coating inhibits the precipitation of lattice oxygen to a certain extent;The effective doping of Ti can reduce the cation mixing rate of the internal structure of the material and the phase change reaction during the charge-discharge process;The synergistic effect of double Ti modification can successfully prevent the inconsistency of intergranular crack evolution and structural degradation of LNCM samples 。

Zhang et al.Selected F as the doping element and used a multi-step calcination method to prepare LiNi_0.8Co_0.1Mn_0.1O₂materials with single crystal structure,and then doped the layered materials with F and heat treated at low temperature^[88]。 The test results show that the comprehensive electrochemical performance of the single crystal NCM material and NH₄F with a ratio of 1∶0.01 is the best,and the specific discharge capacity of the sample at 0.1 C is 202.7 mAh·g^-1,while that of the undoped initial sample is only 186.2 mAh·g^-1.After 100 cycles at 1.0 C,the capacity retention was as high as 86.6%,while that of the undoped initial sample was only 81.02%.By controlling the doping concentration of F,possible particle breakage and cracks are avoided,and the prepared single crystal material has small particle size and good dispersibility.Compared with undoped materials,the Co³⁺and lattice oxygen content of fluorine-doped materials increase,which in turn affects the rate and cycle performance.Appropriate amount of F doping can increase the crystal spacing and maintain a low cation mixing rate.The increase of Co³⁺can also improve the high speed performance of the material,and the increase of lattice oxygen content leads to the formation of strong Me—O(Ni,Co,Mn)bonds,which improves the stability of the crystal structure.Therefore,appropriate fluorine doping improves the overall performance of lithium energy storage in terms of initial discharge capacity,coulombic efficiency,rate capability,and cycling performance 。

Zhang et al.First prepared Ni_0.7Co_0.1Mn_0.2(OH)₂precursor by coprecipitation method,doped and prepared single crystal by high temperature sintering method.The doping effects of Nb⁵⁺,Sr²⁺and Y³⁺were compared,and finally the Nb⁵⁺doping was taken as the key research target,and the single crystal LiNi_0.7Co_0.1Mn_0.2O₂doped with Nb ions was prepared^[89]。 The specific discharge capacity of the Nb⁵⁺doped sample can reach 204.0 mAh·g^-1at 25℃and 0.1 C,and the capacity retention rate is 85.51%after 150 cycles at 1 C,while the capacity retention rate of the undoped sample is 70.60%under the same conditions;At 45℃and 0.1 C,it has a 217 mAh·g⁻¹discharge capacity with a capacity retention rate of 73.18%,while the capacity retention rate of the undoped sample is only 31.89%under the same conditions(the voltage range is 3.0~4.5 V),and its performance has been significantly improved compared with the undoped initial sample.The doping of Nb⁵⁺enlarges the transmission path of lithium ions and improves the intercalation/deintercalation efficiency of lithium ions.And that Nb⁵⁺can oxidize the Ni²⁺into Ni³⁺,so that the cation mix arrangement degree is reduced,the formation of microcracks is reduced due to the monocrystalline structure,and the structural stability of the ternary material is improve 。

Bao et al.Used ALD technology,ZrO₂coating and heat treatment to modify single crystal LiNi_0.6Mn_0.2Co_0.2O₂,and realized the combination of surface coating and doping^[90]。 The coating itself usually exhibits poor Li⁺conductivity and hinders surface charge transfer,whereas Zr⁴⁺surface doping can accelerate the charge transfer of Ni-rich layered oxides while providing sufficient protection.The thickness of the ZrO₂coating was controlled by adjusting the number of ALD deposition cycles and the way of heat treatment to promote the mutual diffusion of ions and obtain a surface layer with high Li⁺conductivity.The electrochemical performance test results of the uncoated single crystal NCM sample(SC-NCM)and the ALD ZrO₂coated single crystal NCM sample at different heat treatment temperatures are shown in Fig.5,and it is found that the sample（5-ZrO₂-500℃-NCM）with a deposition cycle of 5 and heat treatment at 500°C for 3 H has the best electrochemical performance,with a first-cycle discharge specific capacity of 188.9 mAh·g^-1at 2.8–4.5 V,1 C（180 mA·g^-1）,and room temperature,and the capacity retention rate can reach 98.5%after 150 charge-discharge cycles.The first cycle discharge specific capacity at 5 C is 157.1 mAh·g^-1,and the capacity retention rate is still 93.9%after 250 cycles.The Zr⁴⁺doping and ZrO₂coating can improve the cycle performance and rate capability at high pressure,and the synergistic effect between the surface doping and coating can achieve a sufficiently high Li⁺conductivity without affecting the surface protection.When the doping amount on the surface of the single crystal reaches the maximum,the electrochemical performance is optimized,and the surface ZrO₂is not lost and is temperature-dependent 。

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图5 未涂层单晶NCM样品和不同热处理温度下ALD ZrO₂涂层单晶NCM样品在2.8~4.5 V电压范围内的(a) 循环性能和(b) 倍率性能；(c) 未涂层单晶NCM样品和5-ZrO₂-500 ℃-NCM在5 C (1 C = 180 mA/g)、2.8~4.5 V电压范围内的循环性能^[90]

Fig. 5 (a) Cycle performance and (b) rate performance of SC-NCM and ALD ZrO₂ coated SC-NCM annealed at different temperatures between 2.8 and 4.5 V; (c) cycle performance of SC-NCM and at 5-ZrO2-500 ℃-NCM at 5 C (1 C = 180 mA/g) between 2.8 and 4.5 V^[90]. Copyright 2020, American Chemical Society

5 Conclusion and prospect

Compared with polycrystalline materials,monocrystalline materials have no grain boundaries,and the mechanical strength of particles is higher,which reduces the particle breakage during compaction and effectively increases the compaction density.the specific area of the monocrystalline material is smaller than that of the polycrystalline material,so that the contact with the electrolyte is reduced,the surface side reaction is reduced,and the oxygen evolution is reduced.the single crystal grain structure avoids the mutual extrusion between the polycrystalline structure grains,effectively inhibits the occurrence of microcracks,and improves the cycle stability of the material.Therefore,single crystal materials have significant advantages in terms of compaction density,gas generation,thermal stability,high temperature cycling performance,and thus have better thermal safety.This indicates that morphology monocrystallization is one of the most promising routes to reform high-nickel ternary cathode materials.At present,the energy density of the battery cell is increasing,and the safety problem is becoming increasingly prominent.the monocrystallization of the cathode material is very important to suppress the thermal runaway of the battery and improve the safety。

At present,single crystal materials still have the problem of low specific discharge capacity,which is mainly due to the large primary particle size of single crystal,resulting in a longer diffusion path of Li⁺from the inside of the particle to the outside of the particle,thus affecting the transmission efficiency of Li⁺.In addition,for commercialization,problems such as surface reconstruction,intragranular cracking and kinetic limitation of lithium ion diffusion need to be solved.Among them,H2/H3 phase transformation,oxygen vacancy and interface deformation are the main reasons for the formation of intragranular cracks^[20,77]^[76]^[46]。 The surface doping technique can alleviate the intragranular crack problem of single particle,and the M-O bond formed by heterogeneous elements can effectively stabilize the lattice structure.in addition,the coating method minimizes side reactions with the electrolyte,enabling long-term cyclic preservation of the surface.the combination of these methods makes it feasible for single crystal high nickel ternary materials to be used In high energy density lithium ion batteries。

At present,the problem of low discharge specific capacity caused by large primary particle size of single crystal has not been solved,and the modification scheme to improve the discharge specific capacity of single crystal materials needs to be explored in the future.the development trend of single crystal high nickel ternary materials is that the proportion of Ni content increases gradually,the proportion of Co content decreases gradually,and the direction of higher specific capacity and lower cost is developed.However,this trend will also lead to a decrease in the cycling stability of the positive electrode.Therefore,it is necessary to explore how to improve the stability of single crystal materials while reducing the production cost and improving the specific capacity of materials.At the same time,we should continue to develop controllable preparation and mass production processes of single crystal materials to meet the needs of large-scale production。

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模态框（Modal）标题

Abstract

Cite this article

1 Introduction

2 Property difference between monocrystalline and polycrystalline materials

图1 多晶颗粒和单晶颗粒界面变化[57]

图2 (a) 单晶NCM622，(b) 多晶NCM622，(c) 单晶NCM811，(d) 多晶NCM811材料储存一段时间后的SEM横截面图[62]

3 Ynthesis method of monocrystal high nickel ternary material

3.1 Coprecipitation method

3.2 Molten salt synthesis

3.3 Etching method

3.4 Spray pyrolysis

3.5 Hydrothermal method

4 Modification Strategy of Single Crystal High Nickel Ternary Material

4.1 Problems of single crystal materials

4.2 Surface coating

表1 Comparison of electrochemical properties of single crystal high nickel ternary materials with different coatings

图3 LYTP修饰SC-NCM88正极的合成方法示意图[82]

图4 原始SC-NCM88和1% LYTP@SC-NCM88在0.5 C下以锂金属为负极且测试温度为(a) 25 ℃和(b) 55 ℃下的循环稳定性。(c) SC-NCM88和(d) 1% LYTP@SC-NCM88在55 ℃下第1~100次循环的充放电曲线[82]

4.3 Ion doping

表2 Comparison of Electrochemical Properties of Single Crystal High Nickel Ternary Materials Doped with Different Elements

图5 未涂层单晶NCM样品和不同热处理温度下ALD ZrO2涂层单晶NCM样品在2.8~4.5 V电压范围内的(a) 循环性能和(b) 倍率性能；(c) 未涂层单晶NCM样品和5-ZrO2-500 ℃-NCM在5 C (1 C = 180 mA/g)、2.8~4.5 V电压范围内的循环性能[90]

5 Conclusion and prospect

References

图1 多晶颗粒和单晶颗粒界面变化^[57]

图2 (a) 单晶NCM622，(b) 多晶NCM622，(c) 单晶NCM811，(d) 多晶NCM811材料储存一段时间后的SEM横截面图^[62]

图3 LYTP修饰SC-NCM88正极的合成方法示意图^[82]

图4 原始SC-NCM88和1% LYTP@SC-NCM88在0.5 C下以锂金属为负极且测试温度为(a) 25 ℃和(b) 55 ℃下的循环稳定性。(c) SC-NCM88和(d) 1% LYTP@SC-NCM88在55 ℃下第1~100次循环的充放电曲线^[82]

图5 未涂层单晶NCM样品和不同热处理温度下ALD ZrO₂涂层单晶NCM样品在2.8~4.5 V电压范围内的(a) 循环性能和(b) 倍率性能；(c) 未涂层单晶NCM样品和5-ZrO₂-500 ℃-NCM在5 C (1 C = 180 mA/g)、2.8~4.5 V电压范围内的循环性能^[90]