Morphology dependence of the lithium storage capability and rate performance of amorphous TiO₂ electrodes

Amorphous TiO₂ film electrodes of controllable and reproducible nanostructure and porosity were grown via evaporation of titanium in an oxygen ambient (i.e., reactive ballistic deposition (RBD)). The cyclability, rate capability, and Coulombic capacity of the electrodes depended on their morphology and porosity, which varied with the angle of incidence of the evaporated titanium. When films are deposited via evaporation at a glancing angle of 80° with respect to surface normal, nanocolumnar arrays with high internal porosity, high surface area, and optimal pore size and connectivity can be prepared. The optimized films deposited at 80° exhibit a reversible lithium capacity of ∼285 mA h g^-1 at a low cycling rate (0.2 C) and maintain a reversible capacity near 200 mA h g^-1 at rates as high as 5 C. About 70% of the theoretical capacity (235 mA h g^-1) was retained with indiscernible capacity decay after 100 cycles at 1 C. The total charge stored in the TiO₂ RBD films involves both surface capacitive and diffusional processes.