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

Abstract

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.