Enhanced interfacial and cycle stability of 4.6 V LiCoO2 cathode achieved by surface modification of KAlF4

Abstract

Lithium cobalt oxide is one of the most popular cathode materials for lithium-ion batteries. Due to its relatively low capacity, increasing the output voltage is often necessary to release more capacity. However, subjecting cathode materials to high charging potentials inevitably induces surface-side reactions, compromising the integrity of the crystal structure and accelerating capacity degradation. Therefore, the stability of cathode materials at high voltage becomes crucial for lithium-ion batteries. In this study, we employed the ternary compound KAlF4 (KAF) as a surface modification coating for LiCoO2 (LCO). With the optimized ratio (1 wt%) of KAF, coated LCO cathodes exhibited enhanced capacity retention, excellent cycling, and rate stability. In the high voltage range of 3.0–4.6 V, the specific capacity of the initial discharge at a rate of 0.5 C reached 198.30 mA h g−1. Remarkably, the coated LCO retained 91.56 % of its initial capacity after 200 cycles at 0.5 C. Even at a high rate of 5 C, the coated sample retained a discharge capacity of 120.87 mA h g−1, significantly higher than that of bare LCO (80.24 mA h g−1). After a thorough investigation into the structure changes before and after cycling, we elucidated the mechanism of KAF coating in improving the stability of high-voltage LCO cathodes. The results indicated that KAF forms a stable passivation layer on the surface of LCO, which suppresses cracking and corrosion during charging and discharging, eventually stabilizes the crystal structure, and enhances the cycling stability of LCO.