Re-understanding the function mechanism of surface coating: Modified Li-rich layered Li1.2Mn0.54Ni0.13Co0.13O2 cathodes with YF3 for high performance lithium-ions batteries

As a promising positive electrode material for high energy lithium-ion batteries, Li-rich manganese based cathode materials (LNCM) have a capacity over 250 mA h g−1 but suffer from capacity fade and voltage decay during electrochemical cycling. Surface coating and bulk doping can improve the performance of LNCM from different aspects but both of them have shortcomings. Herein, we contrive to integrate the advantages of surface coating and bulk doping by YF3 modification. The phase transformation and gradient diffusion at the interface between the coating layer and bulk region were investigated by XRD, XPS, SEM, EDS and TEM. A LNCM material modified with 5 wt% YF3(0.05-YF3) shows an enhanced initial coulombic efficiency of 87.56% and an improved rate performance of 179.6 mA h g−1 at 5C compared to 76.94% and 148.3 mA h g−1 for the pristine, a discharge capacity of 217.9 mA h g−1 and a retention of 85.2% after 150 cycles at 0.5C compared to 161.2 mAhg−1,72.71% compared to the pristine. The improved performance can be attributed to the fact that part of Li2MnO3 had been activated during the YF3 modification and secondary calcination process, forming a spinel phase with three dimensional Li+ diffusion channels, which are beneficial to enhance rate performance. Besides, some F− ions diffused into the bulk region due to the gradient and bonded with TM, which are help to stabilize the structure. Further treatment at high temperature of the secondary calcination for the 0.05-YF3 modified materials show a lower voltage decay of 331.9 mV after 100 cycles at 0.5C compared to 608.6 mV for pristine, which further confirms the diffusion of F− and its effects on reducing the voltage decay and stabilizing the structure during cycling.