Lithium Ion Battery Cathode Material Surface Coating Effect And Mechanism Study

It is well known that lithium-ion battery is the best energy storage method for electrical vehicle nowadays. Among many of the cathode material, LiFePO4 has been thoroughly studied because of its low price, good cycle performance and high thermal stability. Besides, another polyanion-typed cathode material Li3V2(PO4)3 exhibits high capacity, showing great potential in future application. Spinal cathode LiNi0.5Mn1.5O4 has high energy density and environmental friendly, providing another possible choice. But the mentioned three materials all show low electronic conductivity and low rate performance, so it is of great importance to improve the electrochemical property.In this work, based on these three cathode material, surface coating were used for improving cycle and rate property.Cathode material LiFePO4 with C, Li3PO4 and Li4P2O7 as coating layer can be synthetized, written as LiFePO4/Li3PO4, LiFeP04/Li4P207(LiFe0.9P0.95O4-?) and LiFePO4/C. LiFePO4/Li3PO4 and LiFePO4/Li4P2O7 shows excellent cycle property and dynamic property as well as high specific capacity. Density functional methods were used to study the mechanism of Li3PO4 coating, it reveals that diffusion energy barrier of Li-ion along a-axis is comparable to that in C (0.33 eV vs.0.3 eV), and much lower than that in LiFePO4 (-0.6 eV). Simulation result also indicates that the Li vacancy in Li3PO4 has two defect energy levels which located at 0.81 eV and 1.03 eV, respectively, thus its electronic conductivity is mainly attributed to lattice vacancy. This lattice vacancy improves the Li3PO4 electronic conductivity of, leading to the similar effect of carbon coating.By optimizing synthetize method of LiPO3 and Li3PO4 coating, single spinel phase LiNi0.5Mn1.5O4 with space group of Fd3m can be obtained, with coating layer of 0.5-1.2 nm and 5-6 nm, respectively. Coated cathode shows advanced cycle and dynamic property, the additional mechanism lies in:i) restraining Ni-ion reaction induced lattice distortion; ii) preventing Li-ion from inactivity which trapped the side reaction product in cathode-electrolyte interface; iii) decreasing reaction activity of Mn3+ and Mn2+ dissolution.Monoclinic phase Li3V2(PO4)3 with Li4P2O7 coating has been prepared, exhibiting good cycle, specific capacity and dynamic properties. The additional mechanism is coating layer's protection reduces surface reaction and lower the increment of impendence.