Structural Evolution and Stability of RF Sputter Deposited Li _xMn_2-yO₄ Thin Film Cathodes

Thin films of Li_xMn_2-yO₄ compounds were deposited by radio frequency (rf) magnetron sputtering from a sintered LiMn ₂O₄ target on Si substrate under working pressures of 0.4 Pa and 2 Pa (pure Ar gas). In situ substrate heating was applied to modify the film crystallography. The films deposited at a pressure of 2 Pa with substrate heating developed a (111) texture with increase of substrate temperature. To compare the structural stability of deposited films, rapid thermal anneal (RTA) of 300 - 700°C was performed. Films deposited under different conditions exhibited different trends of structural evolution, as treated by RTA. The crystal structure evolution of films deposited under different conditions was studied by X-ray diffraction and micro-Raman spectroscopy. The films deposited at elevated temperature retained the cubic spinel structure after RTA treatment of 700°C, while the films deposited without substrate heating showed significant cationic disorder after RTA. The discharge curves of the half-cells made of Li_xMn_2-yO₄ thin films were measured. It was demonstrated that the films deposited with in situ substrate heating exhibited larger capacity compared with other films. The more stable spinel phase on the films deposited with in situ heating may be responsible for the enhanced capacity.