CaF_2-coated Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_2 as cathode materials for Li-ion batteries

摘要

Li-rich cathode material Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_2 is prepared by a sol-gel method and coated with CaF_2 layer via a wet chemical process. The pristine and CaF_2-coated samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). An amorphous nanolayer coating of CaF_2 is obtained on the surface of layered pristine material. The CaF_2-coated Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_2 material exhibits excellent electrochemical performance. The initial coulombic efficiency is enhanced to 89.6% with high initial discharge capacity of 277.3 mAh g~(-1) after CaF_2 coating. Galvanostatic charge-discharge tests at 0.2 C display faster activation of Li_2MnO_3 phase and higher capacity retention of 91.2% after 80 cycles for CaF_2-coated material. Meanwhile it also shows higher rate capability with the capacity of 141.5 mAhg~(-1) at the 3 C-rate and stable cyclic performance above 190 mAh g~(-1) after 100 cycles at the 1 C-rate. The analysis of dQ/dV plots and electrochemical impedance spectroscopy (EIS) indicates that the obvious improvement of CaF_2 coating is mainly attributed to the accelerated phase transformation from layered phase to spinel phase and stable electrolyte/electrode interfacial structure due to the suppression of the electrolyte decomposition.