Solid state synthesis is a traditional and simple material synthesis method,which mainly includes two steps:raw material mixing and high temperature sintering.Therefore,the main factors affecting the physical and chemical properties and electrochemical properties of materials in this process include the mixing mode of raw materials,sintering temperature,time,sintering atmosphere,etc.The method has the advantages of simple process and low requirement on equipment,so that the method is widely applied to the industrialized preparation of battery materials.Zhang et al.Mechanically mixed Li₂CO₃,NiO,Co₃O₄and MnO₂metal compounds according to the material design ratio,and then sintered in oxygen atmosphere to prepare micron-sized single crystal LiNi_0.8Co_0.1Mn_0.1O₂materials^[17]。 Under the optimized conditions,the initial discharge capacity of the material at 0.05 C was 224.5 mAh/G,and the capacity retention rate was 71%after 100 cycles at 1 C.Although the process of this method is simple,it is difficult to achieve atomic-level uniform mixing of raw materials by mechanical mixing,which easily leads to differences In material components,thereby affecting the electrochemical performance.in addition,high temperature sintering alone can not effectively control the morphology of the material,so it is often combined with other precursor synthesis processes to prepare the corresponding cathode materials。

Sol-gel method is based on the formation of complexes between transition metals and organic acid anions(such as citric acid,malic acid,lactic acid and other anions)in solution,followed by heating,dehydration and polymerization to form gel precursors,and further calcination to prepare the corresponding cathode materials.Compared with the traditional solid phase method,the sol-gel method can realize the atomic-level uniform mixing of product components,and can obtain materials with uniform morphology and high purity.Tung et al.Dissolved the acetate of lithium,nickel,cobalt and manganese in water according to the designed material ratio and fully mixed it with acetic acid,then heated and evaporated it at 80℃to form a gel precursor,and further sintered it to remove the organic matter to obtain a layered high nickel LiNi_0.8Co_0.1Mn_0.1O₂material.The electrochemical test showed that the discharge capacity of the material at 0.1 C was 185.9 mAh/G,and the capacity retention rate was 81.4%after 50 cycles at 1 C^[18]。 The material prepared by the sol-gel method has the characteristics of small particle size and high specific surface area,so that the material has strong reaction activity,the rapid deintercalation capability of the Li⁺is improved,the side reaction between the material and an electrolyte is intensified,and the capacity of the material is rapidly attenuated^[19]。 in addition,the gel preparation process needs to consume a large amount of organic reagents and energy,resulting in high cost and low efficiency.at the same time,a large number of organic anion groups will be decomposed during the sintering process of gel precursor,resulting in air pollution and low tap density of materials.Therefore,this method has not been used in the industrial preparation of high nickel materials At present。

Hydrothermal method is a method of synthesizing materials in a special closed reactor with water as the medium and high temperature and high pressure environment.In the preparation of high nickel materials,the hydrothermal method usually involves three applications.The first method is to use nickel,cobalt and manganese metal raw materials and auxiliary reagents to synthesize precursors,and then prepare cathode materials by lithium sintering.For example,Shi et al.Used nickel cobalt manganese acetate and urea as raw materials to synthesize a stoichiometric Ni_0.5Co_0.2Mn_0.3CO₃precursor at 200°C for 24 H under a suitable urea/transition metal ratio,and then prepared a LiNi_0.5Co_0.2Mn_0.3O₂cathode material with excellent electrochemical performance by solid phase sintering with a lithium source^[20]。 the second method is to use lithium,nickel,cobalt and manganese metal salts and auxiliary reagents to synthesize cathode materials In one step.Because the one-step synthesis needs to complete the wet intercalation of lithium in the liquid phase to form a layered cathode material,the system needs a high enough concentration of lithium,and at the same time,it needs to increase the recovery process of lithium;the third method is to mix the coprecipitated nickel-cobalt-manganese precursor with lithium source,and use the strong permeability of hydrothermal reaction to complete the lithium intercalation process of the precursor.Due to the low efficiency of hydrothermal lithium intercalation,this process also requires a high concentration of lithium source,which increases the cost.in addition,the morphology and grain assembly of the material can be controlled by adjusting the hydrothermal conditions(temperature,template,auxiliary reagent,etc.)^[21]。 Ju et al.Used nickel cobalt manganese chloride salt as raw material,sodium dodecyl sulfonate(SDS)as surfactant,and urea as precipitator to synthesize plate-like crystal self-assembled three-dimensional flower-like nickel cobalt manganese precursor,and then prepared the corresponding LiNi_0.6Co_0.2Mn_0.2O₂materials by sintering with lithium^[22]。 The fully exposed active(010)plane on the surface of the material significantly improves the Li⁺transport properties,and the reversible capacity loss is only 23.4%when the current density increases from 0.1 C to 10 C.Although the hydrothermal method can prepare cathode materials with good crystal form and strong electrochemical activity,it has not yet formed industrial application because of its high equipment requirements and limited production capacity 。

the spray drying method is to atomize the mixed raw material solution of lithium,nickel,cobalt and manganese,and then spray it into the hot environment to quickly volatilize water and other impurities to prepare the precursor powder,and then sinter it to prepare the corresponding cathode material^[23]。 Li et al.Dissolved chloride salt of nickel,cobalt and manganese in deionized water according to the proportion(molar ratio of 6:2:2),atomized it with an atomizer and then sprayed it into a pyrolysis reactor,dried and pyrolyzed it at 850°C to obtain a precursor of nickel,cobalt and manganese oxide,and then mixed it with a lithium source and sintered it to prepare a LiNi_0.6Co_0.2Mn_0.2O₂material^[24]。 the initial discharge capacity of the material is as high as 181.1 mAh/G at 0.1 C,and the capacity retention rate is 90.8%after 100 cycles at 1 C,showing good electrochemical performance.the spray drying method has low dependence on chemical reagents and does not produce acid and alkali wastewater,so it is an efficient and environmentally friendly material preparation method.the precursor with spherical morphology can be prepared by adjusting the raw materials and drying conditions.However,due to the rapid drying of pyrolysis,it is easy to cause the precursor to be loose and porous,which eventually leads to the low tap density of the sintered material^[24,25]。 Therefore,the spray drying method has not been widely used in the industrial preparation of high energy density nickel materials。

The coprecipitation method comprises the following steps of:mixing and dissolving metal salts of nickel,cobalt and manganese,then controlling the conditions to react with a precipitator so that Ni²⁺,Co²⁺and Mn²⁺are synchronously precipitated out of the solution,filtering and drying to form precursor powder,and then adding a lithium source to be sintered to prepare the cathode material.The products prepared by coprecipitation method usually have spherical morphology and can achieve high volume energy density,which is the mainstream preparation method of high-nickel materials for industrialization.According to different precipitants,the precursors prepared by coprecipitation method are divided into hydroxide precursor（Ni_xCo_yMn_1-x-y(OH)₂）,carbonate precursor（Ni_xCo_yMn_1-x-yCO₃）and oxalate precursor（Ni_xCo_yMn_1-x-yC₂O₄）.Compared with carbonate and oxalate precursors,the mass ratio of anionic groups in the hydroxide precursor is low,and the degree of damage to the internal morphology caused by the gas released during sintering is small,so the prepared product is more compact and has more excellent electrochemical performance^[21]。 In the preparation of hydroxide precursor,because the direct reaction of nickel,cobalt and manganese ions with alkali liquor is too fast,which is not conducive to crystal growth,ammonia is usually added as a complexing agent,so that Ni²⁺,Co²⁺and Mn²⁺ions in the system slowly precipitate and grow to agglomerate into spherical precursors under the competitive action of NH₃complexation and OH^-precipitation.The complexation and precipitation reactions involved are as shown in formulas(1)and(2),where TM represents Ni,Co or Mn,and n=1 to 6.In fact,the solubility product constant for the hydrolytic precipitation of Ni²⁺,Co²⁺ions(ca.10^-15）is about two orders of magnitude lower than that for Mn²⁺（ca.10^-13）,While the complexation constants of Ni²⁺,Co²⁺ions with NH₃are about two orders of magnitude higher than those of Mn²⁺ions,Therefore,Ni²⁺,Co²⁺and Mn²⁺ions can be uniformly coprecipitated in the whole complexation-precipitation competition reaction when the conditions are properly controlled^[26]。 In addition,the hydroxide precursor has a layered structure(P-3m1 space group),in which the(001)face of the crystal surface contains the most unsaturated hydroxyl groups and has the strongest negative charge,thus attracting the positively charged TM(NH₃)_n²⁺in the liquid phase to grow preferentially on this face^[27,28]。

Generally speaking,the structure,morphology,size,grain arrangement and other physical and chemical properties of cathode materials are mainly inherited from its precursor,so the controllable preparation of the precursor is very important to obtain cathode materials with excellent electrochemical performance.Therefore,in the coprecipitation reaction,the raw materials such as nickel,cobalt and manganese salt solution,ammonia water and alkali liquor are usually pumped into a continuous stirring reaction kettle respectively,and the controllable and orderly growth of the precursor is realized by controlling the reaction pH value,ammonia water concentration,temperature,feeding speed,stirring intensity,time,nitrogen flow and other parameters.If the conditions are not properly controlled,it is easy to cause deviations in the composition,structure and morphology of the precursor.For example,when the equilibrium relationship between complexation and precipitation is out of balance,it is easy to cause the content of nickel,cobalt and manganese in the product to deviate from the design ratio,and at the same time,it may lead to the mixing ofα-Ni(OH)₂in the precursor ofβ-Ni(OH)₂;When the nitrogen protection is insufficient,the Mn(OH)₂phase in the precursor is easily oxidized into MnOOH or MnO₂,resulting in phase separation of the material,while too high nitrogen flow rate easily takes away the NH₃in the system and destroys the equilibrium between complexation and precipitation^[29]; low reaction pH value or high ammonia concentration tends to make metal ions complex,which is beneficial to primary grain growth,but it is easy to cause incomplete metal precipitation and component segregation;However,too high pH value or too Low ammonia concentration tends to precipitate metal ions,resulting in continuous nucleation in the reaction process,which makes it difficult for metal ions to grow continuously on the original grains,resulting in the reduction of precursor particle size and the deterioration of morphology consistency^[30]。 in addition,the internal structure of the coprecipitation reactor,the baffle setting and the blade type have a significant impact on the morphology of the precursor and the grain assembly mode by affecting the fluid motion in the reactor.First of all,the reaction solution injected into the kettle needs to be dispersed quickly to prevent local nucleation too fast,so the blades in the kettle should ensure that the fluid is efficiently and fully mixed in the radial and axial directions.Secondly,the formation of dense spherical precursor requires strong collision of crystal agglomerates in the suspension,and the shear force of the fluid is used to separate the part with weak binding force of the agglomerates,so as to retain the part with strong binding force,so high-speed stirring or even the installation of reactor wall baffles is needed.to sum up,the reasonable selection of coprecipitation reaction equipment and the precise control of various parameters in the reaction process are the key factors to ensure the preparation of high-quality precursors。

Based on the above introduction,the advantages and disadvantages of each synthesis method of high nickel materials are summarized in Table 2.in fact,sol-gel method,hydrothermal method,spray drying method and co-precipitation method are usually used to prepare the precursor of high nickel materials,and then combined with solid phase sintering to prepare the corresponding cathode materials.Among them,sol-gel method,hydrothermal method and spray drying method are convenient to introduce lithium source in the preparation of precursor,while coprecipitation method usually completes the lithium intercalation of materials in the sintering process.Among the preparation methods,the co-precipitation method can control the preparation of micron-sized dense polycrystalline spherical precursors to meet the morphology requirements of high energy density and high nickel materials,and has stable continuous production capacity,so the co-precipitation method is the mainstream synthesis method of high nickel materials for power lithium-ion batteries。

Ion shuffling mainly refers to the process in which low-valence metal ions in the material migrate to the lithium layer and occupy the Li⁺site,as shown in Figure 3A^[31]。 Because the radius of low-valence Ni²⁺(0.6969Å)in the transition metal layer is similar to that of Li⁺(0.7676Å)in the lithium layer,and the barrier for their position exchange is low,the ion mixing of high-nickel materials mainly refers to Li/Ni mixing(Fig.3B )^[32,33]。 Crystal field theory demonstrates that Ni prefers to exist in the FCC octahedral structure as a Ni²⁺rather than an unstable Ni³⁺（e_gorbital with an unpaired spin electron^[34]。 Therefore,Li/Ni mixing can occur in the whole life cycle of materials,including synthesis,storage and electrochemical processes.For the synthesis process,when the lithium-containing precursor is heated and sintered,a large amount of Li/Ni mixed arrangement will be produced in the material first,and with the further increase of temperature and time,sufficient energy supply will make the ions in the crystal migrate orderly,thus reducing the degree of mixed arrangement and forming a layered compound with relatively complete structure^{[14,35⇓~37]}。 However,even at high temperature in pure oxygen atmosphere,the Ni²⁺in the material is difficult to be completely oxidized to Ni³⁺,resulting in some Ni existing in the transition metal layer as Ni²⁺,and then exchanging with the Li⁺in the lithium layer to form Li/Ni mixed row.Therefore,the key parameters in the synthesis process,such as the amount of lithium,sintering temperature,time,and oxygen partial pressure,can significantly affect the Li/Ni mixing degree of the material^[38]; In addition to synthesis,the unstable Ni³⁺on the surface of the material is easy to be spontaneously reduced to Ni²⁺during storage,which leads to the intensification of Li/Ni mixing and the formation of inert NiO rock salt phase^[39]。 At the same time,the reduction of Ni valence state will trigger the loss of active Li⁺and lattice O^2-in the material,and the reaction is shown in formula(3).If the Li₂O escaping from the surface of the material is exposed to the air,it will react with H₂O and CO₂to form residual alkali substances such as LiOH and Li₂CO₃,which will further promote the reduction of Ni valence state and the mixing behavior of Li/Ni^[40]; In addition,during the cycling process,a large number of Li⁺vacancies will be generated in the deep de-Li⁺state of the high-nickel material,and the repulsion effect will be generated between two adjacent oxygen atomic layers,which will drive the Ni²⁺in the transition metal layer to migrate to the Li⁺vacancies through the adjacent tetrahedral interstice,resulting in the aggravation of Li/Ni mixing^{[41⇓⇓~44]}。 After many cycles,the continuous expansion of the mixed arrangement region leads to the transformation of the material from the layered structure(R-3m)to the spinel(Fd-3m)and rock salt phase(Fm-3m)(Fig.3C),accompanied by the release of O₂（formula(4),(5))from the oxidation of the material lattice O^2-,thus inducing the oxidative decomposition of the electrolyte^{[3,45⇓⇓~48]}。 the phenomenon of Li/Ni mixing will be aggravated with The increase of Ni content,temperature,cut-off voltage and state of charge^[49]。

Severe Li/Ni shuffling not only hinders the migration of Li⁺in the layered structure and increases the interfacial impedance and polarization effect,but also causes the reversible capacity loss of the material,which eventually leads to the significant deterioration of the cycling and rate performance of the material.In addition to optimizing the synthesis conditions of the material itself(including lithium content,sintering temperature,atmosphere,etc.)To reduce Li/Ni mixing,specific exogenous ion doping can reduce Li/Ni mixing by enhancing the structural stability,thereby inhibiting the irreversible phase transformation of the material during cycling 。

The formation of residual alkali substances(mainly LiOH,LiHCO₃and Li₂CO₃)on the surface of high nickel materials is the main reason for the deterioration of their interfacial chemical properties.The formation of residual alkali mainly comes from two aspects:①In the synthesis process,the transition metal precursor is usually combined with an excessive lithium source to compensate for the volatilization loss of lithium at high temperature.Although lithium excess can inhibit the material Li/Ni mixing,the unreacted lithium source(mainly lithium oxides such as Li₂O and Li₂O₂)is easy to remain on the surface of the material,and further combine with H₂O and CO₂in the air to form residual alkali substances,as shown in Figure 4A.With the increase of Ni content,the sintering temperature of high nickel materials decreases,which makes it easier to form residual alkali during synthesis^[49]; ②During storage,the unstable Ni³⁺on the surface of high-nickel materials will spontaneously reduce to the Ni²⁺.In order to compensate the charge,the lattice O^2-in the adjacent structure will be oxidized to break away from the lattice and form a surface-active O^2-,as shown in formulas(6)and(7 )^[51]。 When the active O^2-meets the H₂O and CO₂in the air,it is transformed into OH^-/CO₃^2-.Further combining with the Li⁺on the surface of the material to form LiOH/Li₂CO₃residual alkali,and the reaction is shown in formulas(8)and(9 )^[52,53]。 Residual alkali is a kind of compound with poor conductivity,which seriously hinders the diffusion behavior of Li⁺at the electrode/electrolyte interface and increases the internal resistance of the battery.At the same time,the formation of residual alkali during storage is accompanied by the loss of Li⁺in the surface structure,the reduction of Ni valence and the loss of lattice O^2-,which makes the surface layer of the material transform from layered structure(R-3m)to rock salt phase(Fm-3m)and diffuse into the bulk phase,resulting in the loss of reversible capacity and the increase of impedance^[54]。 When the battery is at high voltage(potential to Li/Li⁺>4.3 V),the Li₂CO₃in the residual alkali component is easily oxidized and decomposed into CO₂and O₂,and the reaction is as shown in formula(10),resulting in battery flatulence and potential chain side reactions^[31]。 At the same time,the unstable Ni⁴⁺（(low LUMO energy level)on the surface of the material in the deep Li⁺state can be spontaneously reduced to Ni²⁺,At the same time,the oxidative decomposition of electrolyte was induced to produce O₂²⁻,O²⁻,O⁻and O₂oxygen-containing substances,and the nanoscale CEI film was formed on the surface of cathode^[13]。 However,the main components of the CEI film are composed of LiF,MF_n,Li₂O,Li₂CO₃in the inner layer and ROCO₂Li,Li_xPO_yF in the outer layer with poor conductivity(Fig.4 B),which leads to the increase of interface impedance and the attenuation of capacity and voltage^[49,55]。

residual alkali not only affects the interface properties and electrochemical properties of high nickel materials,but also is harmful to the machinability of the electrode.in the process of electrode pulping,the presence of Residual alkali increases the pH of the slurry,which induces the binder PVDF to defluorinate and dehydrogenate and deactivate(formula(11)),causing the slurry to gel,resulting in uneven coating thickness of the slurry and weakening the adhesion with the current collector,as shown in Fig.4C and d^[53]。

At present,the residual alkali of high-nickel materials in industry is mainly removed by water washing process(Fig.4E).Although water washing can effectively remove the residual alkali on the surface,the direct exposure of active substances to water will destroy the surface structure and cause the loss of Li⁺,resulting in the decrease of reversible capacity of materials.The results show that organic solvents such as ethanol and polyaniline can not only effectively wash the residual alkali on the surface of high-nickel materials,but also maintain the structural integrity of the materials,thus obtaining ideal electrochemical performance^[56,57]。 In addition,some modification methods can effectively reduce or inhibit the surface residual alkali of high nickel materials,such as coating specific materials to transform the surface residual alkali into substances with good conductivity and stability^[52]。 Some functional ion doping can effectively resist the surface residual alkali side reaction by enhancing the chemical stability of the material itself^[51,58]。 Therefore,on the basis of strengthening its chemical stability,high nickel materials should be stored in dry or vacuum environment as far as possible to prevent water absorption failure of materials。

in addition to the deterioration of structure and surface properties,high-nickel materials are also prone to mechanical failure,which induces a series of side reactions and adverse structural transformations,resulting In battery performance decline and even safety problems.Among them,microcracks are one of the main causes of mechanical failure of high nickel materials.For polycrystalline spherical high nickel materials,the microcracks can be divided into intergranular microcracks and intragranular microcracks^[60]。

(1)Intergranular microcrack

Intergranular microcracks are microcracks caused by mutual extrusion between grains in secondary particles at the contact grain boundaries due to the volume effect caused by lattice contraction/expansion in different directions.As the Ni content in the high nickel material increases,the microcracking phenomenon of the material becomes more significant,as shown in Figure 5A.The reason for this phenomenon is that the high nickel material will experience different phase transitions such as H1-M,M-H2,and H2-H3 during the charge-discharge process(corresponding to different phase transition peaks on the differential capacity curve,as shown in Figure 5B),and the lattice parameters(a,C)of the material will also change with the charge-discharge(deintercalation Li⁺）)during this process(Figure 5C).When the material undergoes the H2-H3 phase transition(at the end of charging),the repulsive force between the adjacent oxygen atomic layers in the c-axis direction suddenly dissipates due to a large amount of Li⁺extraction,resulting in a sharp decrease in the lattice parameter C,which causes an obvious contraction change of the whole unit cell in the c-axis direction(Figure 5D )^[61,62]。 with the increase of Ni content,the H2-H3 phase transition of the material shifts to a lower potential during the charge-discharge process,and the(003)diffraction peak shift and the change of lattice parameter C in the XRD pattern become more intense(Fig.5e,f),which eventually leads to more serious microcracks in the material With higher Ni content^[10]。 Although the increase of Ni content can improve the initial reversible capacity of the material(Fig.5g),the micro-cracks generated during the cycle will make the electrolyte penetrate into the material particles.A drastic side reaction occurs at the newly exposed material interface,inducing the surface structure to transform from lamellar to rock-salt phase,causing the loss of active Li⁺and lattice O^2-,thus triggering the rapid capacity decay,as shown in Figure 5H^[11,63,64]。 in addition,Park et al.Studied the effect of internal cycling In different intervals(3.76~4.3 V(upper 60%DOD),2.7~4.0 V(lower 60%DOD),and 2.7~4.3 V(100%DOD))on the properties of high nickel materials^[65]。 The results show that the material exhibits the best cyclability at the lower 60%DOD,while the cyclability at the upper 60%DOD is even worse than that at 100%DOD,and the microcracks of the material after cycling are closely related to its capacity decay,which further proves that the H2-H3 phase transformation in the deep Li⁺state is the main reason for inducing microcracks and leading to the rapid capacity decay.When the charge-discharge interval is set before the H2-H3 phase transition,the cyclicity of the material can be significantly improved,but this will lead to a shorter range of the battery.Therefore,how to solve the intergranular microcrack phenomenon is an unavoidable difficult problem for polycrystalline high nickel materials.At present,the main methods to alleviate intergranular microcracks in high nickel materials include anion and cation doping,gradient material design,single crystal material design and heterogeneous material design^[66]。

(2)intragranular microcrack

High-nickel materials are not only prone to intergranular microcracks during cycling,but also to several fine intragranular cracks within a single grain(Fig.6a).Intragranular microcracks are usually caused by local microstress generated by lattice mismatch,distortion,dislocation and other defects within a single grain,and gradually develop into microcracks in the subsequent electrochemical behavior.The causes of these lattice defects include Li/Ni intermingling,loss of lattice O^2-,structural phase transition,Li⁺deintercalation inhomogeneity,etc^[67]。 Qiu et al.Found that in the state of deep Li⁺,due to the specific anisotropic change of lattice parameters a and C(Fig.6B),the material generates shear stress in the a and B planes,while the material shows tensile stress along the c-axis direction(Fig.6C),which easily leads to tearing dislocation of the material along the(003)crystal plane^[27]。 Yan et al.Further confirmed that the intragranular microcracks were mainly along the direction parallel to the(003)crystal plane,and their development was accompanied by the release of lattice O^2-and the formation of rock salt phase,and the microcracking phenomenon was intensified with the deep delithiation and heat generation of the material(Fig.6 d~I )^[68]。 Lin et al.Revealed the formation mechanism of intragranular microcracks during cycling of high nickel materials from the atomic scale^[69]。 The results show that the stress difference between the lattice distortion caused by the mixed arrangement of Li/Ni and the normal layered structure leads to the fracture between the atomic layers in the grain,and with the continuous migration and arrangement of transition metals at the fracture site and the increase of cation repulsion during the cycling process,the fracture surface expands and evolves into microcracks.Compared with intergranular microcracks,intragranular microcracks are smaller in size but more in number,and the exposed fresh surface of intragranular microcracks will also react with the electrolyte to promote the structure near the fracture surface to transform to the inert rock salt phase,thus inhibiting the migration of Li⁺and accelerating the failure process of the material^[70]。 Intragranular microcracks can be alleviated by introducing specific doping ions into the material to strengthen the lattice structure,fixing the lattice O^2-of the material,and reducing Li/Ni mixing 。

Transition metal(TM)dissolution is one of the common side reactions occurring on the surface of high-nickel materials,which is closely related to the formation of acidic substances such as HF in the electrolyte.During the battery manufacturing process,the injected electrolyte is usually mixed with a trace amount of H₂O,which promotes the hydrolysis of LiPF₆in the electrolyte to produce a trace amount of HF acidic substances,as shown in Equation(12 )^[49]。 When the electrolyte is in contact with the cathode material,free HF will attack the lattice structure of the surface of the material,so that TM in it will be converted into soluble metal salts and dissolved.Compared with the high-valent TM in the structure,the low-valent TM is more likely to form soluble metal salts,so the TM dissolution reaction can be expressed as formula(13)^[31]。 Because low-valent TM in high-nickel materials mainly exists in the form of Ni²⁺,the NiO rock salt phase caused by the increase of Li/Ni mixing and the loss of lattice O^2-is the main reason for the increase of TM dissolution^[31]。 Through visualization study,Ko et al.Found that compared with the initial high-nickel material,the TM content of the cycled material(especially the broken particles caused by microcracks)decreased significantly on the surface and near the interface between the material and the electrolyte,and the dissolution of Ni was more obvious than that of Co and Mn^[72]。 At the same time,Ni was detected on the surface of the negative electrode and in the electrolyte after cycling,which confirmed that TM dissolved from the surface of the positive electrode into the electrolyte and deposited on the surfaces of the negative electrode during cycling.Wachs et al.Used voltammetric cycling method to test the TM dissolution behavior of high nickel LiNi_0.8Co_0.1Mn_0.1O₂materials in organic electrolyte during cycling in real time by ICP-MS^[73]。 At the same scan rate,Ni,Co,and Mn began to dissolve significantly only at high potentials(4.7,4.8,and 5 V versus Li⁺/Li,respectively)(Fig.7A),while the dissolution behavior of TM intensified with the increase of cycle number and current(Fig.7B,C).The dissolution of TM can cause the loss of active sites of the positive electrode Li⁺,resulting in the decrease of reversible capacity of the material,while the TM migrating to the negative side deposits on the surface of the negative electrode or participates in the formation of SEI film,increasing the internal resistance of the battery.At present,the main methods to alleviate TM dissolution are ion doping,surface coating,core-shell/gradient material design and so on 。

Ion doping is a common method to control the stability of the crystal structure of high nickel materials.The effects of ion doping are usually reflected in the following aspects:(1)strengthening the TM-O bond in the transition metal layer,thereby enhancing the structural and chemical stability of the crystal;(2)the valence state of Ni is stabilized,the migration of the Ni²⁺of the transition metal layer to the lithium layer is inhibited,and the mixed arrangement of Li/Ni is reduced;(3)widen that interlayer space of the LiO₆octahedron in the layered structure and promoting the electrochemical migration of the Li⁺in the structure;(4)By constructing a heterostructure or changing the crystal surface,a suitable material microstructure can be induced,thereby optimizing the interface properties and improving the electrochemical stability of the material^[74]。 In the process of doping modification,the type of doping ions,doping methods and doping amount will affect the final electrochemical performance of the material.Common ions suitable for doping high nickel materials include Al³⁺,Mg²⁺,Nd³⁺ 、Zr⁴⁺ 、Ti⁴⁺ 、Sb⁵⁺ 、Ta⁵⁺ 、Nb⁵⁺ 、Mo⁶⁺ 、Cations such as W⁶⁺and U F^-UN S^2-NKCl^{-[75][76][77][78][64][79][80][81][82][83][84][85][86]}。

(1)cation doping

Low valence Mg²⁺is a good dopant ion.Different from the doping of high-valence cations at the transition metal site,because the radius of Mg²⁺(0.7272Å)is equivalent to the radius of Li⁺(0.7676Å),it is generally believed that Mg²⁺doping occupies the Li⁺site of high-nickel materials,and plays a"pillar effect"on the layered structure when a large number of Li⁺are de-intercalated,thus inhibiting the intensification of Li/Ni mixing and the expansion of irreversible phase transformation during cycling,and effectively improving the cycling stability of materials^[87]。 Al³⁺doping is more prevalent.Since the hybrid orbital of Al³⁺is sp³d²,Resulting in coordination of Al³⁺with lattice O^2-in high nickel materials to form AlO₆octahedra similar to TMO₆,Therefore,it is generally believed that Al³⁺doping occupies the transition metal sites of high nickel materials^[88,89]。 At the same time,the valence state of Al³⁺in the material is constant,that is,it does not participate in the electrochemical reaction,so with the increase of the doping amount of Al³⁺,the material will have a slight capacity loss.However,the stronger Al—O bond(bond energy 512 kJ/mol)introduced by Al³⁺doping significantly enhances the structural and chemical stability of high nickel materials.Manthiram et Al.Showed that the effect of Al³⁺doping on inhibiting the dissolution and irreversible phase transformation of active materials was better than that of Mn doping,which made Al-doped materials show slower capacity fading and polarization development^[90]。 At the same time,a small amount of Al³⁺doping can improve the air storage performance of high nickel materials,inhibit the formation of residual alkali on the surface,and enhance the Li⁺migration ability and cycle stability of the materials^[51,58,91]。 Previous studies have generally considered that the mixed arrangement of Li/Ni in high nickel materials hinders the migration of Li⁺in the layered structure,resulting in the increase of polarization and the deterioration of cycle performance.However,some studies have shown that appropriate Li/Ni mixing in high nickel materials can significantly improve the structure and thermal stability of the materials,thereby improving the cyclicity^[92]。 For this reason,Sun et al.Studied cations with different valence States（Mg²⁺,Al³⁺,Ti⁴⁺,Ffect of Ta⁵⁺and Mo⁶⁺）doping on physicochemical property and electrochemical performance of high nickel Li[Ni_0.91Co_0.09]O₂（NC90）materials^[93]。 Compared with low-valence Mg²⁺,Al³⁺,and Ti⁴⁺doping,high-valence Ta⁵⁺and Mo⁶⁺can not only induce the ordered growth of primary grains along the radial direction during sintering(Fig.8 a)and inhibit the generation of microcracks during cycling,but also induce the generation of ordered Li/Ni mixed superlattice through the reduction of Ni valence state in the transition metal layer through the charge compensation effect of high-valence ion doping(Fig.8 B,C).This superlattice shuffling effectively suppresses the collapse of the layered structure during cycling(Fig.8d),and significantly enhances the mechanical stability and cycling performance of the material(Fig.8e )。

The above ion doping is generally introduced in the solid state sintering process,in addition,the in-situ introduction of doping ions in the precursor coprecipitation reaction is also an effective doping method.In view of this,Zhu et al.Introduced Nb⁵⁺doping in solid state sintering and coprecipitation reaction,respectively,and compared the two doping methods^[94]。 Although the two kinds of Nb⁵⁺doping can promote the radial growth of primary grain refinement of high nickel materials,the high energy barrier of ion diffusion in solid phase doping results in the aggregation distribution of Nb⁵⁺doping among grains(Fig.9a).Compared with the solid phase doping of Nb⁵⁺,the in-situ introduction of Nb⁵⁺in the coprecipitation reaction makes the doping of Nb⁵⁺in the material more uniform,avoids the generation of doping gradient and intergranular impurity phase(Fig.9b),and is more prominent in inhibiting Li/Ni mixing and irreversible phase transformation of high-nickel materials(Fig.9c and d),thus further improving the long cycle stability of the material(Fig.9e).However,the introduction of doping ions in the coprecipitation reaction may interfere with the nucleation and growth process of the precursor,so it is necessary to regulate the precipitation rate of doping ions to make it as consistent as possible with the precipitation rate of Ni²⁺,Co²⁺and Mn²⁺ 。

(2)anion doping

Compared with high-valence cation doping,halide anion doping is also a good means to control the mixing of Li/Ni in high-nickel materials,while enhancing the structural stability of doping sites^[95]。 Among several halide anions,F^-doping exhibits the lowest Li/Ni mixed-row formation energy,followed by Cl^-,I^-,and Br^-,respectively,as shown in Fig.10a^[95]。 During the F^-doping process,F^-forms bonds with metal ions by replacing the material lattice O^2-(Fig.10 B).Because the Li—F and TM—F bonds are stronger than the Li—O and TM—O bonds,respectively,the structural stability of the material is enhanced by F^-doping^[84]。 At the same time,in order to maintain the electrical neutrality of the material,the high-valence Ni³⁺in the F^-doped proximity structure is reduced to Ni²⁺,resulting in an increase in the number of Ni²⁺and an increase in Li/Ni mixing^[96,97]。 Therefore,the degree of Li/Ni mixing induced by F^-doping is closely related to the doping amount(Fig.10c–e).When the degree of shuffling is properly controlled,the Ni²⁺migrating into the lithium layer can stabilize and support the layered structure,preventing the Ni²⁺in the transition metal layer from continuously migrating into the lithium layer,thus further enhancing the structure and cycle stability of the material(Figure 10F )^[43,98]。 However,excessive F^-doping will induce severe Li/Ni mixing and reduce the electronic conductivity of the material(Figure 10g),resulting in reversible capacity loss and rate performance deterioration^[96,99]。 Therefore,halide anion doping needs to be regulated to an appropriate amount to promote the material。

(3)Multi-ion co-doping

Single ion doping can improve the electrochemical properties of high-nickel materials In some aspects,but the modification effect is limited,and it is difficult to resist the multiple failure behavior of high-nickel materials.in view of this,researchers have gradually focused on the co-doping of multiple ions or anions and cations to comprehensively improve the physical and chemical stability of high nickel materials,and achieved ideal results^[100]。 Guo et Al.Co-doped Al and B into high nickel LiNi_0.88Co_0.09Mn_0.03O₂materials^[58]。 Crystal field theory and simulation calculations show that Al tends to form AlO₆octahedra into the bulk structure of the material,while B tends to form BO₃triangular configuration or BO₄tetrahedra on the surface of the material(Fig.11a-d).The Li（Ni_0.88Co_0.09Mn_0.03）_0.985Al_0.005B_0.01O₂material with Al-rich bulk phase and B-rich surface was prepared(Fig.11e),which effectively alleviated the lattice contraction in the cycle process,inhibited the irreversible phase transformation,and improved the cycle stability of the material;Zhang et al.Introduced four cations of Ti,Mg,Mo and Nb to prepare high-entropy doped high-nickel materials,and the synergistic doping of multiple ions effectively alleviated the anisotropic shrinkage of the material lattice and stress accumulation during cycling,significantly improved the structure and thermal stability of the material,so that the capacity retention rate was as high as 85%during 1000 long cycles,and the thermal stability was close to that of LiNi_0.5Mn_0.3Co_0.2O₂materials^[101]。 However,at present,although multi-ion doping has achieved good results,there is still a lack of research on the screening,identification and reasonable collocation of doping ions,and the synergistic mechanism between doping ions needs to be further clarified。

The basic function of surface coating is to provide a physical protective layer for high-nickel materials,effectively prevent the interface side reaction between electrode materials and electrolyte,and inhibit the diffusion and escape of lattice O^2-[102]。 On this basis,the coating material can also promote the migration of ions and electrons at the interface and reduce the interface impedance.Some coating substances can also phagocytose impurities on the surface of materials and convert residual alkali compounds.When the charge and ionic radius are appropriate,the ions in the coating substance can diffuse into the interior of the material to form near-surface doping,thus resisting the structural instability of the material.The effect of surface coating depends on the type of coating material,coating method(mechanical mixing,wet chemical coating,atomic deposition technology,etc.)And the amount of coating.According to the coating function,the commonly used coating materials can be divided into electrochemical inert materials,Li⁺/electronic conductor materials and residual lithium conversion materials 。

(1)electrochemically inert substances

Electrochemical inert material coating is to protect the internal material and reduce the occurrence of interfacial side reactions,thereby prolonging the service life of the material.Such coatings mainly include metal oxides（Al₂O₃,MgO,SiO₂,ZrO₂,TiO₂ 、Nb₂O₅ 、TiNb₂O₇ 、V₂O₅,MoO₃,WO₃,etc.)Metal fluoride（AlF₃,U MgF₂U N CaF₂N K CoF₂metal phosphate（AlF₃（AlPO₄MnPO₄ 、UNi₃(PO₄)₂UNCo₃(PO₄)₂NKMn₃(PO₄)₂K1ZrP₂O₇）Kim et al.Uniformly deposited Al₂O₃on the surface of LiNi_0.8Mn_0.1Co_0.1O₂materials to form a nano-ultrathin coating layer by non-destructive and water-free targeted sputtering method,which effectively inhibited the side reaction of interfacial Li⁺/H₂O and the corrosion of HF under the condition of ensuring the diffusion efficiency of Li⁺,enhanced the stability of CEI film,and improved the cycle performance of materials^{[103][104][105][106][107][108][109][110][111][112][113,114][115,116][117]}。 Zaker et al.Characterized the modification characteristics of WO₃infiltration coating high nickel materials,and the results showed that the WO₃diffused inward along the primary grain boundary during sintering,and finally distributed in the secondary particles and the surface of internal primary grains as a thin layer of W-containing compounds such as Li₄Ni_1-xWO₆and Li_xW_yO_z,which inhibited the side reaction between the material and electrolyte and slowed down the stress accumulation between grains^[118]。

（2）Li⁺/electronic conductor material

The coating of the Li⁺/electron conductor substance can promote the transfer of the Li⁺/electron at the interface on the basis of inhibiting the side reaction at the interface,and ensure the cycle stability while achieving excellent rate performance.Such coatings are mainly lithium metal oxysalt（LiAlO₂,Li₃PO₄,Li₂TiO₃,Li₂ZrO₃,Li₂SiO₃,LiTaO₃,etc.)Fast ion conductors(NASION-type solid electrolyte,lithium phosphorus oxynitride,perovskite-type electrolyte,etc.),conductive carbon,conductive polymers(such as PEO),etc^{[119][120][121][122][123][124][125,126][127]}。 Yu et al.Uniformly deposited Al³⁺onto the surface of spherical Ni_0.9Co_0.1(OH)₂precursor with Al(OH)₃by using oxalic acid as a complexing agent^[128]。 Al³⁺is gradiently doped inward during the lithium-containing sintering process,and a LiAlO₂（Li⁺conductor)coating layer is generated in situ on the surface at the same time,as shown in Fig.12A.The Al³⁺doping effectively hinders the migration of Ni²⁺to the lithium layer and slows down the internal lattice stress and volume change(Fig.12 B–d),while the LiAlO₂coating layer prevents the electrolyte from penetrating into the material and inhibits the interfacial side reaction,so that the prepared Al-doped-LiAlO₂coated LiNi_0.9Co_0.1O₂material achieves ultra-long cycle stability(Fig.12e,f);Yan et al.Used the atomic layer deposition technique to uniformly attach the solid-state electrolyte Li₃PO₄to the surface of the spherical high-nickel material,and then the Li₃PO₄diffused inward along the primary grain boundary by sintering,and a coating layer was also formed on the surface of the internal grains^[129]。 The results show that the implanted Li₃PO₄coating not only promotes the intergranular Li⁺transfer,but also inhibits the electrolyte penetration along the grain boundary,which effectively alleviates the interfacial side reaction,intergranular microcracking and irreversible phase transformation,thus improving the cycle and voltage stability of the material;In addition,Cao et al.Coated polyaniline(PANI)and polyethylene glycol(PEG)on the surface of LiNi_0.8Mn_0.1Co_0.1O₂materials^[130]。 the polymer coating enhances the electronic and ionic conductivity of the material on the basis of inhibiting the interfacial side reaction,and effectively alleviates the stress-induced microcracks in the cycling process of the material by virtue of its good elasticity and toughness,so that the rate performance and cyclicity of the material are significantly enhanced。

(3)Residual lithium conversion material

As mentioned before,high nickel materials have poor resistance in air and are easy to react with H₂O and CO₂to form residual alkali such as LiOH and Li₂CO₃,which deteriorates the surface properties.In order to remove the residual alkali,besides water washing,a coating substance can be introduced to the surface of the material to react with the residual alkali,and a new coating layer which is beneficial to the migration of Li⁺is generated while the residual alkali is consumed^[131,132]。 For example,Chen et al.Employed(NH₄)₂HPO₄to cooperatively coat LiNi_0.8Mn_0.1Co_0.1O₂materials with PPy^[133]。 Wherein the(NH₄)₂HPO₄forms a Li₃PO₄compound through the action of PO₄^3-and Li₂O and LiOH on the surface of the material,so as to achieve the purposes of removing residual alkali and improving the conductivity of the material.However,due to the uneven distribution of Li₃PO₄coating,PPy with good ductility was further introduced for coating,which not only inhibited the interfacial side reaction,but also enhanced the electronic conductivity and mechanical strength of the material 。

Core-shell and gradient material design are functionally similar to surface coating.The region near the core of the material is composed of components with high capacity and strong electrochemical activity,while the outer region(or near-surface region)is composed of similar materials with stable structure and chemical properties or strong Li⁺transport,thus preventing the internal region with high capacity but poor stability from directly contacting with the electrolyte to induce side reactions^[134]。 Su et al.Used a two-step co-precipitation method to synthesize a core-shell LiNi_0.8Mn_0.1Co_0.1O₂material with constant composition but different grain assembly(Fig.13 a),in which the inner core was formed by the agglomeration of primary grains with high packing density,and the outer shell was formed by the intercalation of plate-like grains with exposed(010)active facets,thus realizing short-range Li⁺transport and improving the rate capability of the material^[135]。 Park et al.Synthesized a high-nickel material with an average composition of Li[Ni_0.865Co_0.120Al_0.015]O₂and a gradient of Ni and Co concentrations along the radial direction of the particle,in which Ni is enriched in the core region to exert high capacity,while Co is enriched in the surface region to enhance structural and chemical stability.This design makes the capacity retention rate as high as 92%after 500 cycles at 1 C and 3.0–4.2 V^[136]。 However,during the sintering process,the original core-shell/gradient structure design is often destroyed by the diffusion between the internal and external components,so Rathore et Al.Used WO₃to coat the spherical core-shell precursor with Ni-rich center and Mn-rich or Al-rich surface.The Li_xW_yO_zspecies formed during the sintering process of Li addition diffuse inward along the grain boundary,which not only hinders the component exchange between the core and shell layers,but also adds additional mechanical strength to the secondary particles^[137]。 in addition,Liu et al.kept The concentration of Ni component(80%)in the high nickel material unchanged,and studied the influence of the gradient change of Co and Mn components from inside to outside on the structure and mechanical stability of the material.the element distribution is shown in Fig.13b^[138]。 It is found that the Co-rich phase exhibits lower Young's modulus and superior electronic conductivity than the Mn-rich phase when the Ni content is constant.Finally,in the three prepared materials of NC-NM82 with Co-rich center and Mn-rich surface,NCM811 with constant composition and NM-NC82 with Mn-rich center and Co-rich surface,the design of NM-NC82 effectively alleviated the micro-cracks caused by the internal expansion of the material,significantly improved the cycle reversibility of the material,and at the same time,the good conductivity of the surface enhanced the rate performance of the material and reduced the overpotential。

For polycrystalline spherical high nickel materials,the internal random grain orientation is easy to cause internal stress accumulation,which eventually evolves into microcracks and induces serious side reactions.In order to avoid intergranular microcracks,the most straightforward approach is to design high-nickel materials as single crystal particles^[139]。 the micron-sized single crystal particles improve the mechanical strength and pressure resistance of the high-nickel material,ensure the morphological integrity of the material during cycling,and have an excellent effect on inhibiting the side reactions of the new interface,as shown in Figure 14a^[140]。 at present,in order to reduce the process,single crystal materials are mainly prepared by sintering polycrystalline spherical precursors At high temperature,but the sintering temperature is generally higher than that of polycrystalline cathode materials with the same composition^[141⇓~143]。 Qiu et al.studied the formation mechanism of single crystal particles during the sintering process of polycrystalline precursor,and the results confirmed that when the temperature rises to a certain value,the primary grains promote the fusion growth of grains through the formation of Li/Ni mixed intermediate phase at the contact interface,as shown in Fig.14b^[144]。 As the grains grow,the adhesion between the grains weakens and eventually falls off from the polycrystalline agglomerate to form a single crystal material.In this process,the smaller the polycrystalline particles,the higher the lithium content and the sintering temperature,the more beneficial to the growth and separation of single crystal particles.However,too high sintering temperature easily leads to the volatilization of Li⁺and lattice O^2-in the layered structure,resulting in the intensification of Li/Ni mixing and the generation of local rock salt phase(Fig.14C),which makes it more difficult to prepare high-performance single crystal materials with stoichiometric ratio^[145]。 in order to reduce the high temperature requirement In the synthesis process,the researchers introduced a cosolvent to catalyze the low-temperature growth of single crystal particles during the sintering process of polycrystalline precursors to achieve the purpose of controlling the morphology,and the crystal melting growth mechanism is closely related to Ostwald ripening^[146]。 The commonly used cosolvents are KCl,NaCl,Na₂SO₄,LiNO₃,LiCl,Li₂SO₄,etc^[146]。 Wherein the molten Na⁺can promote the formation of octahedral grains,and the molten Li⁺can induce the formation of fine polyhedral grains^[147]。 In addition,single crystal materials with uniform morphology and particle size distribution can also be prepared by hydrothermal method and sol-gel method.Although single crystal materials can avoid intergranular microcracks,they also have design drawbacks.For example,studies have shown that there is a significant difference in the distribution of Li⁺concentration in the grain of single crystal materials during cycling,which will cause obvious lattice mismatch between the Li⁺rich region and the Li⁺poor region and lead to structural defects(Fig.14d),thus hindering the migration of Li⁺in the grain,resulting in capacity reduction and rate performance deterioration^[148⇓~150]。 Therefore,single crystal materials still need to be further optimized to effectively compensate for the inherent defects of polycrystalline materials。