Study On Preparation And Surface Coating Modification Of LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Material For Lithium Ion Batteries

The ternary cathode material LiNi_0.6Co_0.2Mn_0.2O₂ is considered to be a promising cathode material for lithium ion batteries because of its low preparation cost,good thermal stability and high discharge capacity during charging and discharging.However,LiNi_0.6Co_0.2Mn_0.2O₂ material is easy to be corroded by HF,the decomposition product of electrolyte,during charging and discharging.On the one hand,the material structure is destroyed,on the other han d,the transition metal elements in the material dissolve in the electrolyte,and combine with fluoride ions to form high resistance metal fluoride deposited on the electrode surface,wh ich hinders the transfer of Li⁺and reduces the electrochemical properties of the material.The research object of this paper is LiNi _0.6Co_0.2Mn_0.2O₂ cathode material.Firstly,materials are prepared by different sintering parameters,and the optimum sintering time and sintering temperature are determined.On this basis,LaPO₄ and La₂O₃ are coated by hydrothermal method and solvothermal method respectively to improve the electrochemical properties of the materials.（1）LiNi_0.6Co_0.2Mn_0.2（OH）₂ and Li₂CO₃ precursors are used as raw materials to prepare LiNi_0.6Co_0.2Mn_0.2O₂ at different temperatures（800℃,850℃,900℃,950℃）and sintering time（10,11,12 hours）.The effects of sintering time and temperature on the structure and properties of LiNi_0.6Co_0.2Mn_0.2O₂ are studied.The results show that different sintering parameters have n o obvious effect on the layered structure of the material itself,and the lattice parameters of the material have little change.When the sintering temperature is 900℃,the material has higher average coulomb efficiency and the highest average discharge capacity.The average discharge capacity of the material reaches 154.1 mAh g^-1,and shows good cycling performance.When the sintering time is 12 hours,the material exhibits the highest average discharge capacity and the highest average Coulomb efficiency,156.9 mAh g^-1 and 89.50%,respectively.At the same time,the material has good cycling performance.The optimum parameters for the preparation of LiNi _0.6Co_0.2Mn_0.2O₂ material in this experiment:sintering at 900°C for 12 hours.（2）Different content（1wt.%,2wt.%,3wt.%）of LaPO₄-coated LiNi_0.6Co_0.2Mn_0.2O₂ composites are prepared by hydrothermal method.The results show that LaPO₄ is coated on the surface of the material and does not significantly affect the structure and lattice parameters of the materi al.LiNi_0.6Co_0.2Mn_0.2O₂ coated with LaPO₄ exhibits higher cycling and rate performance.The material with 3wt%.The best cycle performance is obtained.After 100 cycles,the capacity retention rate of the material reaches 84.78%,while that of the uncoated material is only 74.21%.The profit from the strong P=O bond and corrosion resistance of LaPO ₄,the LaPO₄-coated materials have lower ion transfer impedance,and the corrosion degree of electrolyte is much lower after 100 cycles,which enhances the cyclin g and rate performance of the materials.（3）The LiNi_0.6Co_0.2Mn_0.2O₂ material is modified by La₂O₃ coating in an anhydrous ethanol solvent.The results show that La₂O₃ is coated on the surface of the material and does not significantly affect the structure and lattice parameters of the material.The cycle performance and rate performance of LiNi_0.6Co_0.2Mn_0.2O₂material after La₂O₃ coating were significantly improved under the high cut-off voltage of 4.5V.The capacity retention rate of uncoated materials is only 66.14%after 100 cycles,while that of uncoated materials is more than 80%.The La₂O₃-coated LiNi_0.6Co_0.2Mn_0.2O₂ material has a lower ion transfer resistance,and the degree of corrosion by the electrolyte is significantly lower after 100 cycles,thereby enhancing the cycle and rate performance of the material.The enhanced performance is attributed to the strong La-O bond of La₂O₃ itself,and La₂O₃ has good high temperature resistance and acid corrosion resistance,which slows down the corrosion of electrolyte on materials.