Enhanced 4.7 V Electrochemical Performance of the LiCoO2 Cathode via a Fluoride and LiCo1–xAlxO2 Composite Coating

Surface coating is an effective way to suppress the structural collapse and surface side reactions of LiCoO2(LCO) at high voltage. Herein, KAlF4 is used as the raw material for coating modification and combining the wet chemical method and high-temperature solid-phase method to form a dense LiF, KF, and LiCo1–xAlxO2 composite coating layer on the surface of LCO. The fluoride composite coating layer can stabilize the surface of the material, and the solid solution phase can accelerate the transport of Li+ while stabilizing the surface. The synergistic effect of the composite coating phase has a positive effect on mitigating the surface side reactions and structural collapse of LCO at high cutoff voltages above 4.5 V. The modified sample had a first discharge specific capacity of 216.3 mAh/g at 0.5 C in the high-voltage range of 3.0–4.7 V and still had capacity retention of up to 60.4% after 200 cycles, while only 5.8% of unmodified LCO samples remained after 160 cycles. The improved electrochemical performance is attributed to the stabilized surface and phase structure, improved lithium ion diffusion coefficient induced by composite coating as evidenced by electrochemical impedance spectroscopy, cyclic voltammetry, and scanning electron microscopy.