Facile Solid-State Synthesis to In Situ Generate a Composite Coating Layer Composed of Spinel-Structural Compounds and Li₃PO₄ for Stable Cycling of LiCoO₂ at 4.6 V

Due to its high energy density, high-voltage LiCoO 2 is the preferred cathode material for consumer electronic products. However, its commercial viability is hindered by rapid capacity decay resulting from structural degradation and surface passivation during cycling at 4.6 V. The key to achieving stable cycling of LiCoO 2 at high voltages lies in constructing a highly stable interface to mitigate surface side reactions. In this study, we present a facile in situ coating strategy that is amenable to mass production through a simple wet-mixing process, followed by high-temperature calcination. By capitalizing on the facile dispersion characteristics of nano-TiO 2 in ethanol and the ethanol dissolubility of LiPO 2 F 2, we construct a uniform precoating layer on LiCoO 2 with nano-TiO 2 and LiPO 2 F 2 . The subsequent thermal treatment triggers an in situ reaction between the coating reagents and LiCoO 2, yielding a uniform composite coating layer. This composite layer comprises spinel-structured compounds (e.g., LiCoTiO 4 ) and Li 3 PO 4, which exhibit excellent chemical and structural stability under high-voltage conditions. The uniform and stable coating layer effectively prevents direct contact between LiCoO 2 and the electrolyte, thereby reducing side reactions and suppressing the surface passivation of LiCoO 2 particles. As a result, coated LiCoO 2 maintains favorable electronic and ionic conductivity even after prolonged cycling. The synergistic effects of spinel-structured compounds and Li 3 PO 4 contribute to the superior performance of LiCoO 2, demonstrating a high capacity of 202.1 mA h g –1 (3.0–4.6 V, 0.5 C, 1 C = 274 mA g –1 ), with a capacity retention rate of 96.7% after 100 cycles.