Controlled Growth and Application in Lithium and Sodium Batteries of High-Aspect-Ratio, Self-Organized Titania Nanotubes

Searching for optimized electrode materials in alkali ion batteries we have prepared a large number of titania nanotubes (nt-TiO 2 ) electrodes by using different voltages in the 42–100 V range and Ti-anodization times and studied their behavior in test batteries. As a result of the high current densities observed in the transient curves for a fixed electrolyte composition, high anodization voltages allow the rapid growth of self-organized and amorphous nanotube arrays with up to 200 μm in length. The growth of nt-TiO 2 follows the parabolic rate law (L 2 = kt). Titania nanotubes with different lengths were used as electrode materials in lithium and sodium test cells. In lithium cells, the areal capacities depend on the nanotube length independently of the anodization voltage used to obtain a particular length. The very high areal capacities that are observed in lithium (around 2–4 mAh/cm 2 ) and sodium (ca. 1 mAh/cm 2 ) cells are attributed to the high length of the nanotubes. However, the longer titania nanotubes exhibit lower gravimetric capacity values (mAh/g) due to the longer ion diffusion path. The capacity to react with sodium is lower than with lithium, probably due to the poor conductivity of nt-Na x TiO 2 .