Photo-assisted low temperature crystallization of solution-derived LiCoO₂ thin film

LiCoO₂ is one of the most successfully commercialized cathode materials for battery applications, owing to its excellent capacity and cycle performance. LiCoO₂ exhibits two main crystallographic structures depending on the calcination temperature: low-temperature cubic spinel (Fd3m) structure, often called LT-LiCoO₂, and high-temperature hexagonal layered structure (R3¯m), referred to as HT-LiCoO₂. HT-LiCoO₂, which generally requires a high processing temperature, shows a better battery performance than LT-LiCoO₂. However, in the fabrication of thin film batteries, a low processing temperature is essential considering the thermal instability of the polymer substrates. Therefore, the low temperature fabrication of layered LiCoO₂ is a primary concern for the development of thin film batteries. In this study, we observe that the ultraviolet (UV) treatment of sol-gel derived LiCoO₂ precursors triggers the crosslinking of polyvinylpyrrolidone and impedes the phase crystallization of impurities, such as Co₃O₄. After the decomposition of the polymer during post-annealing above 350 °C, crystallites are found to exist in the metastable state, making them more efficiently oxidizable. Finally, with the UV treatment, the transition to the layered LiCoO₂ is mostly completed at a low temperature of 400 °C.