Li4Ti5O12 (LTO) is regarded as the most promising oxide negative electrode material for lithium ion batteries (LIB), with excellent charge-discharge cyclability and a flat plateau in potential-charge properties. However, the insulating nature of LTO hinders high-speed charge-discharge processes. Generally, the strategies to improve the conductance of a material include the enhancement of the conductivity by doping, and the mixing with a conducting material. The approach for the commercial oxide active materials is to use a carbon-based conductor, whereas studies on substitution of the tetravalent (4+) Ti ions with trivalent (3+) or pentavalent (5+) metal ions are ongoing in the laboratory level.
In the present study, we attempted to introduce oxygen deficiency to LTO thick films, expecting partial valence change of Ti ions. Amorphous LTO films that were deposited by an electrostatic spray deposition method in air at 250°C were post-annealed at 700°C in various ambient gases. A film annealed in Ar still showed sharp charge and discharge peaks in its current-potential properties, suggesting the possibility of reduction in an inert gas without the collapse of its crystal structure. Coating an LTO thick film with a glass-like carbon ultrathin layer, that also requires a post-treatment in the inert ambient gas, has been also attempted. Comparison of these films in conductivity is presented.