Effect of amorphous FePO_4 coating on structure and electrochemical performance of Li_(1.2)Ni_(0.13)Co_(0.13)Mn_(0.54)O_2 as cathode material for Li-ion batteries

摘要

Lithium-rich layered cathode Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_2 is synthesized by a co-precipitation method followed by high-temperature treatment and surface coated with different amount of amorphous FePO_4. The microstructure and electrochemical performance of the as-prepared cathode materials are investigated systematically. It is demonstrated that the Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_2 particles are uniformly coated with amorphous FePO_4. With proper amount of amorphous FePO_4 coating layer, significant improvements in discharge capacity, initial Coulombic efficiency, rate capability, cycle performance, and thermal stability are achieved at room temperature. Specifically, the 3 wt.% FePO_4-coated cathode exhibits the highest discharge specific capacities (271.7 mAh g-(~1) at C/20), improved initial Coulombic efficiency (85.1%), and best cyclability (discharge capacity of 202.6 mAh g~(-1) at C/2 after 100 cycles), while the 1 wt.% FePO_4-coated cathode displays the best rate capability (194.3 mAh g~(-1) at 1 C rate and 167.9 mAh g ~(-1) at 2 C rate). The charge-discharge curves and electrochemical impedance spectra reveal that the improved electrochemical performances are due to the suppression of both the oxygen vacancy elimination at the end of the first charge and side reactions of the cathode with the electrolyte, as well as the decrease in charge transfer polarization by the FePO_4 coating layer.