Preparation And Research Of Ternary LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Material For Lithium Ion Battery

With the growth in per capita electric vehicles in China,the problems,such as unsatisfactory endurance of electric vehicles,short battery life span,and high battery replacement cost,are becoming more and more anxious.Therefore,the demand for electric vehicles is also restricted.Compared with lithium iron phosphate cathode material（Li Fe PO4）,because lithium nickel cobalt manganese oxide cathode materials（Li Nix Coy Mn1-x-y O2）have higher specific capacity,better cycle life and thermal stability,they have broadly been used in the new generation of electric vehicles.Among recent researches of cathode materials,high nickel layered cathode materials of polycrystalline secondary particles with spherical structure have prominent advantages of high energy density.However,the inter-granular fracture and even pulverization cannot be avoided in polycrystalline materials,due to the contraction and expansion of the lattice volume during the Lithium ions intercalation/deintercalation processes,which causes an unsatisfied cycling life of the materials.The uneven stress profile inside the single particle can be significantly inhibited by the single crystal structure of ternary oxide cathode materials,in which the microcracks can be retarded,the side reactions can also be eased.Ultimately,cycling stability of Lithium ternary materials will be maintained.In this paper,the influences of the morphologies and particle sizes of single crystal materials were explored by different lithium doping ratios and calcination temperatures through solid phase high temperature calcination method.On this basis,lithium metaborate was introduced to reduce the calcination temperature within high temperature stage,control the particle morphologies,and investigate the relationship between the single crystal structure and their electrochemical performances.1.Preparation of single crystal material（NCM523）by high temperature solid state methodNCM523 precursor was prepared by the traditional coprecipitation method.Subsequently,NCM523 precursor was prepared by the traditional coprecipitation method.Subsequently,the calcination temperature and the proportion of lithium doped inside NCM523 precursor were regulated for manipulating the sizes of single crystal granules within the high temperature calcination section.It was found that the size of the particle increasing with the increase of the proportion of lithium doped in the precursors,but excessive lithium amount would lead to excessive residual lithium amount after calcination,which will also affected the cycling stability.With the lithium doping ratio of 1:1.1 inside NCM523 precursor and the calcination temperature of930℃,the single crystal NCM523 has uniform particle size about 1-3μm grains,less residual lithium amount on the NCM523 surface,evident layered structure,and the lowest lithium-nickel mixing degree.The discharge specific capacity was 164.2 m Ah·g^-1 at the initial discharge rate of 0.1 C,the working voltage of 2.5-4.3 V at 25℃.After400 discharge cycles at the rate of 1 C,the discharge specific capacity was still 116.2m Ah·g^-1,and the capacity retention rate of 79.5%.2.The synthesization of single crystal NCM523 via assisted molten saltBased on the preparation of NCM523 precursor by co-precipitation method,lithium metaborate assisted molten salt with different molar ratios was added into NCM523 precursors to change the grain growth behavior and the formation of single crystal.Additionally,under the effects of reactive atoms and the migration of the crystal boundary during the high-temperature solid-phase reaction,because the gaps among large crystal or secondary crystal particles were filled by Lithium metaborate assisted molten salt,the contact area among the single crystal NCM523 can be enlarged.Ultimately,the smaller crystal particles are dissolved into the larger crystal particles with regular morphology by the solid melting reaction.The as-synthesized octagonal single crystals were observed by Scanning Electron Microscope（SEM）.As the molar ratio of lithium metaborate to precursor of 1:0.4,the octahedral single crystal NCM523with uniform particle size was synthesized.For this reason,the best electrochemical performancees were derived.Namely,first discharge capacity was 153.7 m Ah·g^-1 at the operating voltage of 2.8 V-4.3 V,0.1 C initial rate at 25°C.The discharge capacity was 117 m Ah g^-1 after 400cycles at 1 C rate,with the capacity retention rate of the battery of 80.5%.And compared with the single crystal NCM523 synthesized at high temperature without cosolvent,the electrochemical stability of NCM523 synthesized by assisted molten salt was improved,and the calcination temperature was reduced by30°C,which is beneficial to energy conservation.