Study On Coating Modification Of Lithium Cobalt Oxide And High Voltage Performance Of Lithium Battery

Li Co O₂（LCO）has the advantages of high compaction density,large discharge specific capacity and simple synthesis process,becoming the earliest commercial cathode material.Specifically,the LCO has a high theoretical capacity of 274 m Ah/g,but the practical discharge capacity is only 140 m Ah/g.More lithium ions can be extracted from LCO to increase its capacity by charging to higher voltage.However,LCO materials face irreversible changes in structure,rapid decline in capacity,and decline in safety performance at high voltage,which limit their practical application.Based on the above problems,Al₂O₃-coated Li Co O₂（AO-LCO）and Co O-coated Li Co O₂（CO-LCO）cathode materials were prepared by a simple wet chemical method.The influence regularity of calcination temperature and calcination time on material properties was explored by XRD,SEM and Electrochemical characterization,and the results show that AO-LCO has the best electrochemical performance under the conditions of calcination temperature of 600°C and calcination time of 6 h.The discharge capacity is 168.5 m Ah/g after 200 cycles,and the capacity retention rate is 89.5%.Comparing the CO-LCO samples with the AO-LCO samples under the same preparation conditions,it is found that the cycle stability of the CO-LCO samples is poorer than that of the AO-LCO samples,but the initial discharge capacity significantly increases.Finally,the two samples were tested at high temperatures,showing that AO-LCO had better electrochemical performance.In order to further optimize the electrochemical performance of Li Co O₂materials,combined with the cycle stability of AO-LCO and the high initial discharge capacity of CO-LCO,Al₂O₃/Co O co-coating were achieved on Li Co O₂（C-LCO）.The study shows that the capacity retention rate of C-LCO material is still as high as 86.5%after 500 cycles,and the initial discharge capacity is 218.6 m Ah/g.Meanwhile,C-LCO electrode can provide capacities of 215.1,209.6,200.8,187.6,161.9 and 131.1 m Ah/g at 0.1,0.2,0.5,1,2 and 5C.Through materials characterization of XRD,HRTEM,SEM and XPS,it is found that the coating layer on the surface of the material is a spinel structure Co Al₂O₄ that can provide a fast transport channel for lithium ions.Combined with in-situ XRD and kinetic properties,it is confirmed that the modified layer not only inhibits the detrimental phase transition of the material at high voltage,but also improves the Li⁺diffusion coefficient and effectively preventing the side reaction between the electrode and the electrolyte.At high temperature of 55°C,the capacity of C-LCO can still reach more than 100 m Ah/g after 700 cycles.Besides,the performance of lithium-ion batteries was studied at low temperature-20°C,and electrolytes with better compatibility with cathode material LCO were identified.