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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 9: 1304 - 1315

DOI: https://doi.org/10.7536/PC240203

Review

Effect of Synthesis Conditions on The Properties of Cathode Materials for Lithium-Ion Batteries

  • Hang Li 1, 2 ,
  • Li Wang , 2, * ,
  • Youzhi Song 2 ,
  • Zhiguo Zhang 2 ,
  • Aimin Du , 1, * ,
  • Xiangming He 2, *
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  • 1 School of Automotive Studies, Tongji University, Shanghai 201804, China
  • 2 Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
*e-mail: wang-l@tsinghua.edu.cn(Li Wang);
(Aimin Du);
(Xiangming He)

Received date: 2024-02-05

  Revised date: 2024-05-26

  Online published: 2024-07-01

Supported by

National Natural Science Foundation of China(22279070)

National Natural Science Foundation of China(U21A20170)

National Natural Science Foundation of China(22279071)

National Natural Science Foundation of China(52206263)

Ministry of Science and Technology of China(2019YFA0705703)

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Abstract

Layered transition metal oxides(LiTMO2)are candidate cathode materials for high-energy-density lithium-ion batteries,primarily owing to their high theoretical specific capacity.Nevertheless,the persistent challenge of chemical-mechanical failure during charge-discharge cycling has impeded its progressive development.In numerous prior investigations,researchers have diligently explored the cycling failure of this material family,presenting a spectrum of modification strategies aimed at addressing this issue including doping,coating,surface or grain boundary modification.Given the impact of lattice defects and heterogeneous structures introduced throughout the synthesis process cannot be overlooked,a comprehensive comprehension of the influence exerted by various controlling factors on the structural formation of materials is imperative.This review aims to elucidate the ramifications of control factors,including precursor,lithium salt,sintering temperature,holding time,and sintering atmosphere,on the material structure during the synthesis process.The objective is to provide the battery community with valuable insights on strategies to synthesize high-performance LiTMO2materials 。

Contents

1 Introduction

2 Structural characteristics of high-performance LiTMO2

3 Reduction of inherent defects formed in the synthesis process

3.1 Effect of precursors on the inherent defects in LiTMO2

3.2 Effect of lithium salt species on the structure of LiTMO2

3.3 Effect of sintering regime on the structure of LiTMO2

3.4 Effect of sintering atmosphere and oxygen partial pressure on the structure of LiTMO2

3.5 Water-washing process

4 Conclusion and outlook

Key words： lithium-ion batteries; layered transition metal oxides cathode materials; synthesis; structure; performances

Cite this article

Hang Li , Li Wang , Youzhi Song , Zhiguo Zhang , Aimin Du , Xiangming He . Effect of Synthesis Conditions on The Properties of Cathode Materials for Lithium-Ion Batteries[J]. Progress in Chemistry, 2024 , 36(9) : 1304 -1315 . DOI: 10.7536/PC240203

1 Introduction

Lithium-ion batteries(LIBs)have become indispensable energy storage devices for 3C electronics,electric vehicles,and new energy grids[1⇓⇓~4]。 With the rapid expansion of applications,the demand for improving the energy density of LIBs is increasingly urgent.Layered transition metal oxides（LiTMO2,TM=Ni,Co,Mn)cathode materials have become the focus of attention due to their better performance in terms of energy density[5⇓⇓⇓~9]。 At present,it is mainly divided into two categories:LiNixMnyCo1−x−yO2(NCM)and LiNixMnyAl1−x−yO2（NCA）,and the higher the nickel content,the higher the available energy density.However,the cycle performance and thermal safety of LiTMO2（Ni>0.6）materials with high nickel content are relatively poor,which restricts their commercial application process[10⇓⇓~13]。 A lot of research has been done on this,and it is generally believed that the chemo-mechanical synergistic effect of LiTMO2in the process of lithium deintercalation is the main reason for the failure of material properties.However,the failure rates of materials reported by different research teams are different,indicating that the antecedents of chemo-mechanical synergy still need to be further clarified,and the synthesis process often determines the genes of the multi-level structure of materials[14,15][16⇓~18]。 Therefore,it is important to deeply understand the influence of different controlling factors in the synthesis process on the structure of materials and the structural evolution of materials during electrochemical cycling,which is a necessary condition for obtaining high-performance materials through synthesis。
The synthesis of LiTMO2materials involves many control factors.Although there are many research works on the synthesis of LiTMO2materials,most of them obtain the materials with optimal properties through multi-factor orthogonal experiments,and the discussion on the formation mechanism of material structure in the synthesis process is very limited[19⇓⇓⇓⇓~24][25⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~37]。 Based on this,the effects of different control conditions in the synthesis process on the structure of LiTMO2and the possible mechanisms are reviewed in this paper,and the problems to be explored in the future are discussed,so as to establish a clear and in-depth understanding of the synthesis mechanism of LiTMO2and promote the preparation and development of high-performance materials 。
Combining with the current mainstream technology of LiTMO2industry,this paper mainly focuses on the preparation of LiTMO2by solid state sintering.As shown in Figure 1,the structure of the material was mainly controlled by the precursor,lithium salt,sintering temperature,holding time,sintering atmosphere and washing process in the LiTMO2synthesis process.In order to provide a reference for the synthesis of LiTMO2materials 。
图1 LiTMO2合成过程中涉及的不同控制因素对结构演变的影响

Fig. 1 The effect of different control factors involved in the LiTMO2 synthesis process on the structural evolution

2 Structural characteristics of high performance LiTMO2;

The structure is the key to determine the properties of LiTMO2.The LiTMO2belongs to the R-3m space group,and the oxygen atoms are stacked in the O3 type(ABC ABC )[37,38]。 The bulk structure and surface structure of LiTMO2need to undergo different physico-chemical changes and environments in the process of lithium extraction and insertion,so the requirements for their structural characteristics are different.As shown in fig.2,the bulk structure should have less stress and strain,lower Li/Ni mixed antisite defects,and more stable lattice oxygen during cycling;The surface structure should have smaller surface defects(lithium residues and impurities)and lower interfacial side reaction activity 。
图2 LiTMO2层状结构示意图以及理想体相和表面结构所具备的特征

Fig. 2 Schematic representation of the LiTMO2 layered structure and the conditions that the ideal bulk phase and surface structure should have

3 Reduce material inherent defects caused by synthesis

The poor cycling stability of LiTMO2can be attributed to many factors,among which the intrinsic behavior of crystal structure evolution during cycling is a key reason,and its consequences include cracks caused by phase transformation,the reduction of lithium active sites caused by Li/Ni mixing,and the slow migration of Li+,which will lead to the loss of reversible capacity of the material[39⇓⇓⇓~43]。 in addition,the heterogeneous phase and various lattice defects produced In solid phase synthesis also have a significant impact on the decline of material structure and reversible capacity,which is another important reason for the decline of material properties[44]。 This section focuses on the effects of various factors controlling the synthesis process on the production and evolution of LiTMO2structures 。

3.1 Influence of inherent defect of precursor on LiTMO2;

The inherent defects formed during the synthesis of LiTMO2are the key factors affecting its electrochemical performance,but the origin of the defects is not clear.There is a consensus that different control conditions in the solid phase sintering process can affect the evolution of the material structure,but this is an intermediate process in the synthesis of solid phase sintered materials.Before solid phase sintering,the defects introduced by the precursor material can not be ignored.In recent years,it has been found that there is a strong correlation between the inherent defects of the precursor and the inherent defects of the LiTMO2.In this paper,the hydroxide precursor prepared by hydroxide coprecipitation is mainly discussed[45,46]。
Pokle et al.Took a simple LNO material as an example to study the relationship between precursor defects and final material defects[45]。 As shown in Fig.3,although the authors did not directly observe the direct transfer of precursor defects to the final material,they found that the defects such as dislocations and grain boundaries introduced during the synthesis of Ni(OH)2precursors may lead to uneven lithiation during the synthesis of LNO.The effect produced by this inhomogeneous lithiation is not absolutely favorable or unfavorable,but both affect the purity of the LNO layered structure 。
图3 Ni(OH)2前驱体固有缺陷在经历锂化过程后对LiNiO2结构缺陷的影响[45]

Fig. 3 Effect of intrinsic defects in Ni(OH)2 precursor on structural defects in LiNiO2 after undergoing lithiation process[45]. Copyright 2022 Wiley-VCH

In addition,the competitive reaction between precursor decomposition and lithiation during the synthesis of LiTMO2is an important reason for the formation of impurity phases[21,47]。 When Kim et al.Studied the synthesis of Li[Ni0.92Co0.03Mn0.05]O2materials,they improved the heating mode of the synthesis process and stayed at 250℃for 6 H to enhance the topological lithiation of the materials at low temperature stage as much as possible[47]。 As shown in Fig.4,this heating method using low temperature aging can promote the lithiation reaction while inhibiting the decomposition of TM(OH)2.However,the traditional sintering method will lead to lithiation of the surface layer and decomposition of the precursor inside the particles,resulting in pores of considerable size.This kind of pore will affect the degree of subsequent lithiation reaction,which may cause the remaining spinel or salt rock phase impurities produced by the decomposition of the precursor,or its lithiated derivatives;At the same time,it will also lead to a large number of defects in the particles of the LiTMO2.However,the authors did not characterize the rate and depth of the low-temperature topological lithiation.Although the experiment proves that the topological lithiation reaction can be strengthened by low temperature aging,the direct lithiation reaction between TM(OH)2and LiOH is essentially an acid-base reaction,while the TM(OH)2precursor and LiOH·H2O are both medium-strong bases,and it is difficult for them to react directly(Fig.4 )。
图4 低温老化的升温方式和快速升温方式对材料结构演变影响示意图。低温老化的升温方式可以在促进锂化反应的同时抑制TM(OH)2的分解，而传统的烧结方式会导致表层发生锂化，同时内部发生前驱体分解，进而导致不同尺寸的空洞缺陷[47]

Fig. 4 Schematic diagram of the effect of low-temperature aging heating method and fast ramping method on the structural evolution of the material. The low-temperature aging heating method can promote the lithiation reaction while inhibiting the decomposition of TM(OH)2, while the traditional fast ramping method leads to the lithiation of the surface layer while the decomposition of the precursor occurs in the internal layer, which in turn leads to the defects of different sizes of voids[47]. Copyright 2023 Wiley-VCH

The above studies show that the precursor has an important contribution to the formation of structural defects in LiTMO2.Future research should focus on two parts:(1)The precursor preparation process should avoid impurities and other defects as far as possible.Considering that the preparation of precursor needs to go through reaction,agglomeration,recrystallization and other processes,and the control factors are complex,it is not easy to control the purity of the precursor structure,among which the key is to clarify the specific influence path of different control factors on the evolution of the precursor structure,and to focus on the thermodynamic and kinetic aspects.And(2)regulating the competitive reaction of precursor decomposition and lithiation so as to avoid defects such as impurity phases,holes and the like generated by precursor decomposition as much as possible.To some extent,the doping,coating and heterostructure design of layered LiTMO2cathode materials are all remedial measures to make up for the defects introduced in the process of material synthesis.If the defects in the synthesis process are directly avoided by regulating the precursor,it will be more meaningful for the production and recycling of LiTMO2cathode materials 。

3.2 Effect of lithium salts on LiTMO2;

Different lithium salts are needed for the preparation of LiTMO2with different Ni contents.When Ni is less than or equal to 0.6,both the Li2CO3and the LiOH·H2O can meet the preparation requirements of high-quality materials;However,when Ni>0.6,it is necessary to use LiOH·H2O as lithium source[48⇓~50]。 One of the important reasons why the type of lithium salt affects the formation of layered structure of LiTMO2is that there is a competition between the decomposition reaction of TM precursor and the lithiation reaction,and different lithium salts participate in the lithiation reaction at different temperatures.That is to say,when different lithium salts are used,the state of transition metal is different when lithiation occurs,so lithiation may be topological lithiation with little change in crystal structure and occupancy of transition metal.It may also be accompanied by great changes in transition metal occupancy and crystal symmetry,which will affect the speed of lithiation reaction and produce different product purity,lattice defect types and concentrations 。
He et al.Selected Ni0.83Mn0.1Co0.07(OH)2precursor with three different lithium salts of Li2CO3,LiOH·H2O and Li2O for comparative experiments[51]。 It should be noted that the layered precursor（TM(OH)2or TMOOH,P-3m1)has a very similar layered structure with the final product（LiTMO2,R-3m）,so keeping the layered structure without significant change to undergo lithiation reaction is an ideal LiTMO2generation path.However,the experimental results in Fig.5 show that dehydration of the TM(OH)2precursor occurs at 150°C,and if the hydrogen vacancy of the precursor cannot be replenished,the precursor will evolve from a layered structure to a spinel structure TM3O4（Fd-3m,which is composed of 2/3 TMO6octahedra and 1/3 TMO4tetrahedra)at a temperature higher than 280°C.Due to the high thermodynamic stability of TM ions in TM3O4,it is difficult to spontaneously and reversibly transform into a thermodynamically relatively unstable layered structure 。
图5 TM(OH)2前驱体搭配不同锂盐在不同烧结温度下的结构演变方式示意图[51]

Fig. 5 The schematic depiction below illustrates the structural evolution of the TM(OH)2 precursor in conjunction with various lithium salts at varying sintering temperatures[51]. Copyright 2023 Elsevier

The melting point of Li2CO3is high(733℃),and the decomposition reaction of TM(OH)2is relatively thorough at that time,so the main species involved in the lithiation reaction is TM3O4.The melting point of LiOH·H2O is low(478℃),so the temperature of lithiation reaction is relatively low.At that time,the decomposition reaction of TM(OH)2is not complete,and the main species involved in lithiation reaction is layered TM(OH)2or TMOOH;However,once the LiTMO2is generated,it is difficult to transform to spinel,so the selection of low melting point lithium salt is helpful for the layered precursor to participate in the lithiation reaction,which is conducive to the generation of layered products and avoids the generation of impurities.It should be noted that although the melting point of Li2O is higher(>1000℃),it will react with the dehydration product of H2O（precursor to form LiOH,so for the hydroxide precursor,Li2O also has a lithiation path similar to LiOH.Alternatively,we speculate that the H2O produced by the dehydration reaction may be able to act as a co-solvent to form a quasi-solid/liquid reaction interface between the hydroxide precursor and the lithium salt particles,thereby accelerating the early topological lithiation reaction.Therefore,the different hydrophilic characteristics of Li2CO3and LiOH·H2O and Li2O may also be one of the potential factors affecting the lithiation process of TM precursor,but due to the inherent water absorption characteristics of LiOH·H2O,it is difficult to prepare LiOH without crystal water,which also brings new challenges to experimental verification 。
In addition,low melting point lithium salts are also helpful for the synthesis of single crystal materials.Li et al.Used a planetary centrifugal mixer to demonstrate that the melting point of the LiOH-LiNO3mixture with a molar ratio of eutectic components of 40∶60 was as low as 183°C by simple mechanochemical activation of lithium salt and precursor,which could promote homogeneous reactions at the nanoscale[52]。 This process,in turn,can generate a unique driving force,promote depolymerization and surface phase transformation,and homogenize the lithium salt distribution,so that the particles can be easily coarsened into single crystal morphology later and the electrochemical performance can be improved。
In conclusion,the selection of low melting point lithium salt is a necessary condition for the formation of good layered structure during the synthesis of LiTMO2.In future studies,it is necessary to strengthen the understanding of the competitive reaction between TM precursor decomposition and lithiation,which is essential for understanding the mechanism of layered structure evolution 。
In addition,the effect of lithium salt particle size on the structure is also worthy of attention.It is generally believed that small particle size of lithium salt can increase the contact area with the precursor,which is conducive to the lithiation reaction.However,there is no relevant research results to prove how much the particle size of lithium salt affects the structure.At present,the smallest particle size of commercial LiOH·H2O is about 6μm,and it is not easy to achieve smaller particle size or even nanometer level,which also brings new challenges to experimental verification 。

3.3 Influence of sintering system on the structure of LiTMO2;

High temperature solid state synthesis is widely used in the synthesis of LiTMO2cathode materials,and the sintering temperature and holding time are the key parameters in the sintering process of LiTMO2.The decomposition temperature of different raw materials in the solid phase sintering process is different,and the solid phase reaction path and phase transformation path are also different.In addition to this,the materials sintered at different temperatures and atmospheres have different morphologies,electrochemical properties,and crystal structures[53]。
As shown in fig.6a,there is a significant difference in the synthesis temperature not only for LiTMO2materials with different Ni contents,even for single crystal(SC)and polycrystalline(PC)particles with the same Ni content.It should be emphasized that for large-scale solid phase sintering synthesis equipment,the heating rate is often relatively slow,and the spatial distribution of temperature and atmosphere is also biased,which makes it difficult for the controllability of large-scale preparation of solid phase chemical reactions with poor tolerance to the fluctuation window of control conditions to reach the level of laboratory research.However,the actual situation is that there is little characterization and research on the reaction field in the scale preparation equipment,so the understanding and control accuracy of the internal chemical reaction and phase change path are not enough 。
图6 （a）不同Ni含量LiTMO2材料单晶和多晶颗粒烧结温度和气氛；（b）不同温度下LiTMO2一次颗粒尺寸和内部应力变化

Fig. 6 (a) Sintering temperature and atmosphere of single and polycrystalline particles of LiTMO2 material with different Ni contents. (b) Variation of primary particle size and internal stress of LiTMO2 at different temperatures

In the current mainstream LiTMO2cathode material preparation technology,two processes,chemical reaction and crystal growth,mainly occur in the solid phase sintering,as shown in Fig.6 B.The size of the primary particle of the crystal is different at different temperatures,which correspondingly causes different stresses inside the secondary particle.Therefore,only by deeply understanding the temperature range and speed of chemical reaction and crystal growth,can we clarify the main problems to be solved in different temperature stages,and then rationally optimize the process parameters of solid phase sintering,and even develop new sintering methods[54]。

3.3.1 Effect of gradient heating on the structure of LiTMO2;

As shown in Fig.7,Zhou et al.Compared the samples prepared by different temperature rise processes and optimized a three-stage sintering(TSS)strategy to prepare single crystal NCM90 particles[54]。 the TSS includes three parts:phase formation(750℃),grain growth(985℃)and phase retention(750℃).In the crystal growth part,the particle growth is accelerated by increasing the temperature to promote the production of micron-sized single crystal particles.At the same time,combined with the doping of Mo element,the principle that the adsorption of Mo element on different crystal planes of particles can significantly change the crystal plane energy is used to further promote the formation of particles with characteristic crystal plane orientation through thermodynamic approach.Comparing Fig.7 a(traditional heating mode)and C(two-stage sintering mode),it can be found that the single crystal particle size prepared by TSS sintering mode is more uniform。
图7 掺杂1% Mo元素，（a）传统烧结方式，（b）两段烧结方式以及（c）三段烧结方式制备单晶NCM90颗粒升温示意图和晶体形貌[54]

Fig. 7 Schematic temperature rise and crystal morphology of single-crystal NCM90 particles prepared by doping 1% Mo element using (a) conventional sintering method, (b) two-section sintering method, and (c) three-section sintering method [54]. Copyright 2023 John Wiley and Sons Ltd

The design of gradient heating is based on the deep understanding of chemical reaction and crystal growth in the sintering process,and the growth rate and direction of the crystal are controlled by orderly regulating the sintering temperature in a thermodynamic or kinetic way.This demonstrates the importance of insight into the solid phase reaction mechanism to optimize the synthesis.However,there are still many unknowns about the thermodynamic and kinetic pathways of the effects of different controlling factors on the structure of materials in the synthesis of LiTMO2,which need to be explored and studied in the future 。

3.3.2 Enhanced low temperature equilibrium path

Much attention has been paid to the study of the synthesis mechanism of LiTMO2materials.Hua et al.Studied the chemical and structural evolution during the synthesis of LiTMO2,as shown in Fig.8,and proposed the growth model of LiTMO2crystal in air atmosphere,and it should be noted that the model is discussed from the starting point of spinel TM3O4（product of precursor decomposition )[55]。 First,lithium and oxygen will attach to the surface of TM3O4and nucleate,and gradually form a"lithium-rich"nucleus.With the growth of the"Li-rich"core,the Li+gradually diffuse into the inner layer of the particle,while the TM ions migrate to the near-surface region driven by the concentration gradient.This process is accompanied by the oxidation reaction between TM ions and oxygen atoms,and the intercalation of external oxygen anions into the particle lattice,which provides the impetus for the crystallization and growth of the particle 。
The chemical reaction and phase transformation of materials in the real sintering process are far more complex than currently recognized,which is mainly due to the lack of understanding of the metastable intermediates in the synthesis process,so the real phase transformation path of materials is not clear;Secondly,the temperature field and the distribution of reactants in sintering are not uniform,which may make the reaction deviate from the ideal path.In this regard,some studies have gone deep into the specific chemical reactions in the synthesis process,and through the external synthesis process parameters to achieve the control of the material structure.Kang et al.'s Research results show that strengthening the topological lithiation of LiTMO2at low temperature is conducive to reducing the internal structural defects of particles,so the improved sintering method can achieve less defective LiTMO2and enhance the cycle stability of materials[21]。 However,it should be noted that for the LiTMO2（Ni>0.6）of high nickel system,the currently used precursors are mainly prepared by hydroxide coprecipitation,while the topological lithiation temperature(~200℃)and thermal decomposition temperature(~220℃)of hydroxide precursors are very close,and there is a competitive reaction between them.Once the thermal decomposition of the hydroxide precursor occurs,a spinel phase TM3O4is formed,which is fundamentally different from the layered hydroxide precursor in structure and does not undergo spontaneous reversible transformation,resulting in a change in the subsequent lithiation path of the material.Therefore,how to avoid the occurrence of competing reactions is the key to achieve low temperature topological lithiation 。
图8 以尖晶石TM3O4为起点，空气氛围下LiTMO2晶体生长的潜在机理模型示意图[55]

Fig. 8 Schematic diagram of the potential mechanism modeling of LiTMO2 crystal growth under air atmosphere, starting with spinel TM3O4[55]. Copyright 2020 American Chemical Society

3.3.3 Rapid heating enhancement nonequilibrium path

High temperature shock(HTS)is a new method to prepare cathode materials,which has the advantages of high heating rate（≈104℃·min−1）,high sintering temperature and high cooling rate（≈103 to 104 C·min−1）,as shown in Fig.9.Chen et al.Verified the feasibility of preparing LiMn2O4materials by this sintering method[56]。 Compared with the traditional sintering method,HTS avoids the degree of harmful side reactions in the low temperature section and accelerates the non-equilibrium reaction through the ultra-high heating rate,which not only avoids the demand for energy consumption in the high temperature and long time of traditional sintering,but also reduces the harmful competitive reactions in the low temperature section。
图9 高温冲击（HTS）烧结方式升温示意图[56]

Fig. 9 Schematic diagram of heating up by high temperature shock (HTS) sintering method[56]. Copyright 2023 Wiley-VCH

This sintering method is also feasible in the synthesis of LiTMO2,and its advantage is that it can cross the competitive reaction temperature range of low temperature lithiation and thermal decomposition of hydroxide precursor.However,the reaction time of this method is too fast,which poses a new challenge to the analysis of the phase transformation process of materials 。
In addition,the cooling process of sintering also affects the structure of LiTMO2,which is mainly reflected in two aspects:the first is the atmosphere,the material is in a state of oxygen deficiency during high temperature sintering,and the cooling process can supplement the missing oxygen;The second is to maintain the phase transformation.The material has large defects at high temperature,and the cooling process can continue to maintain the phase transformation of the material,thereby reducing the generation of material defects.Therefore,the rate of the cooling process should not be too large,usually natural cooling to room temperature 。

3.4 Influence of sintering atmosphere and oxygen partial pressure on the structure of LiTMO2;

The selection of sintering atmosphere in the sintering process of LiTMO2materials also has an effect on the structural phase transition.For high nickel LiTMO2,the oxygen concentration must be ensured during sintering because of the need to oxidize Ni2+to Ni3+,otherwise it will lead to excessive Ni2+residues and aggravate the phenomenon of Li/Ni mixed row.Many reports have also shown that the concentration of O2plays a crucial role in the layered structure evolution of materials[16,57⇓⇓~60]。 in addition,some studies have shown that oxygen enriched on the particle surface can reduce the energy barrier for particle growth,resulting In smaller primary particles[61]。 In the synthesis process,the morphology of the material particles can be controlled by oxygen pressure[62]。
Ben et al.Studied the effect of different O2pressures on the morphology of LiNi0.90Co0.05Mn0.05O2(NCM90)during sintering[62]。 As shown in Fig.10,the primary particle size of the material(D5-NCM90)prepared at an oxygen pressure of 5 MPa is smaller than that of the material(Bare-NCM90)prepared at a normal oxygen atmosphere.the mechanical failure mechanical test of the two particles found that the maximum mechanical failure resistance of D5-NCM90 particles was 30.5 mN,which was greater than the maximum mechanical failure resistance of Bare-NCM90 particles(10.2 mN).Therefore,D5-NCM90 also showed more stable cycling performance。
图10 (a) Bare-NCM90 (b) D5-NCM90颗粒形貌和半电池电化学性能[62]

Fig. 10 Particle morphology and half-cell electrochemical performance of (a) Bare-NCM90 and (b) D5-NCM90[62]. Copyright 2023 American Chemical Society

This way of regulating the material structure by adjusting the oxygen pressure in the sintering process has great commercial application value,but the sintering device needs to be upgraded to ensure that the muffle furnace can operate under pressure,which also points out the direction of upgrading the commercial production line of LiTMO2in the future 。

3.5 Water washing process

The production of high nickel LiTMO2generally has a water washing process to remove the residual alkali on the surface of the particles[63⇓⇓~66]。 After washing,the material needs to be sintered twice.However,the current problem is that the effect of different control conditions in the washing process on the structure and electrochemical performance of materials is not clear,and the effect of residual alkali on the surface of LiTMO2particles on the electrochemical performance is different in different studies,and there is no unified understanding by comparison.For example,the research results of Chen et al on LiNi0.83Mn0.1Co0.07O2materials show that the surface Li2CO3will aggravate the parasitic reaction between the particle surface and the electrolyte,and then accelerate the process of material failure[67]。 However,the research results of Guo et al on LiNi0.9Co0.06Mn0.04O2materials show that the formation of a Li2CO3coating on the surface of its particles can greatly improve the cycle performance of the material:a dense Li2CO3coating can be used as a physical protective layer to isolate the cathode from humid air[68]; In addition,amorphous Li2CO3can be converted into a fluorine-rich electrolyte interphase layer(CEI)during cycling,enhancing the interface stability.In addition,high-nickel materials are very sensitive to moisture,and there are many uncertainties in the impact of the washing process on the material structure due to the control accuracy,so the washing process of high-nickel materials still needs to strengthen the exploration of the mechanism.In the future research,on the one hand,it is necessary to reveal the influence of temperature,time,washing method,pH and other parameters on the structural evolution of materials and the specific action path,on the other hand,it is necessary to establish a complete washing evaluation system,including the necessity of washing process from the aspects of process cost and electrochemical performance of materials 。

4 Conclusion and prospect

As shown in Figure 11,the urgent demand for the specific energy of lithium-ion batteries for a variety of energy storage applications drives the research and development efforts of LiTMO2cathode materials.High-performance LiTMO2cathode materials have high requirements on the structure,so the research on the synthesis of materials needs to be paid attention to:(1)The inherent defects caused by the preparation of precursors can not be ignored.In the process of preparing precursors by coprecipitation,the influence path of different control parameters(temperature,concentration,feed rate,pH and stirring mode,etc.)On the structure evolution needs to be further clarified.(2)The mechanism of the synthesis process should be further explored to regulate the reaction process.Precursor,sintering temperature,holding time and sintering atmosphere affect the structural phase transformation of materials to a great extent,and the kinetic and thermodynamic paths involved in these complex controlling factors affecting the structural phase transformation should be deeply analyzed.(3)The study of material modification should pay more attention to the revelation of mechanism.A large number of experimental studies on material modification for electrochemical performance optimization accumulated in the early stage provide a sufficient basis for mechanism exploration,which is an important way to realize the scientific design of material design and preparation technology.(4)To carry out the analysis and mechanism exploration of the"synthesis-failure"linkage of materials.There is a strong causal relationship between material cycle failure and synthesis process,and only by clarifying this causal mechanism can the efficiency of material research and development be improved 。
图11 LiTMO2未来研究总结和展望

Fig. 11 LiTMO2 future research summary and prospects

The problems to be solved in the future development of LiTMO2can be briefly summarized as follows:(1)The cycle stability of high-nickel cobalt-free(low-cobalt)materials needs to be broken through.Cobalt salt has become an important reason for the instability of the current supply chain of lithium battery materials due to its small reserves,high toxicity,high price and special supply sources.Facing the increasing demand for energy storage in the future,it is necessary to further reduce the cobalt content and develop low-cobalt(content<0.1)or cobalt-free materials.However,the issue of material cyclic stability remains a challenge.(2)Research and development of more efficient preparation methods for single crystal materials.Single crystal materials have better cycle stability,which is a promising way for future material development.However,its commercial application is limited due to its complex preparation process and poor controllability of material morphology.These problems need to be solved in the future.(3)Develop modification methods with commercial application value.At present,most of the reported modification methods are still in the laboratory research stage,and some modification methods are limited by cost factors and complex preparation processes,and do not have the value of commercial application,so it is necessary to adjust the target orientation and strive to contribute to the commercial application of materials.(4)Artificial intelligence helps efficient scientific research.At present,the synthesis and optimization of LiTMO2mostly use trial and error method.In recent years,the rapid development of artificial intelligence is expected to break this time-consuming and labor-intensive research form.In addition,for layered LiTMO2materials,previous studies have hoped that the layered structure will become more and more standardized and the defects will be reduced as much as possible.However,more and more studies have shown that there are beneficial defects,and there are many ways to introduce beneficial defects.For the future research of layered LiTMO2materials,first of all,we need to have a clear understanding of the high-performance layered structure,and then how to introduce beneficial defects through synthesis is worth discussing 。

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