Amorphous and Crystalline TiO₂ Nanotube Arrays for Enhanced Li-Ion Intercalation Properties

We have employed a simple process of anodizing Ti foils to prepare TiO₂ nanotube arrays which show enhanced electrochemical properties for applications as Li-ion battery electrode materials. The lengths and pore diameters of TiO₂ nanotubes can be finely tuned by varying voltage, electrolyte composition, or anodization time. The as-prepared nanotubes are amorphous and can be converted into anatase nanotubes with heat treatment at 480 °C. Rutile crystallites emerge in the anatase nanotube when the annealing temperature is increased to 580 °C, resulting in TiO₂ nanotubes of mixed phases. The morphological features of nanotubes remain unchanged after annealing. Liion insertion performance has been studied for amorphous and crystalline TiO₂ nanotube arrays. Amorphous nanotubes with a length of 3.0 m and an outer diameter of 125 nm deliver a capacity of 91.2 A h cm⁻² at a current density of 400 A cm⁻², while those with a length of 25 m and an outer diameter of 158 nm display a capacity of 533 A h cm⁻². When the 3-m long nanotubes become crystalline, they deliver lower capacities: the anatase nanotubes and nanotubes of mixed phases show capacities of 53.8 A h cm⁻²and 63.1 A h cm⁻², respectively at the same current density. The amorphous nanotubes show excellent capacity retention ability over 50 cycles. The cycled nanotubes show little change in morphology compared to the nanotubes before electrochemical cycling. All the TiO₂ nanotubes demonstrate higher capacities than amorphous TiO₂ compact layer reported in literature. The amorphous TiO₂ nanotubes with a length of 1.9 m exhibit a capacity five times higher than that of TiO₂ compact layer even when the nanotube array is cycled at a current density 80 times higher than that for the compact layer. These results suggest that anodic TiO₂ nanotube arrays are promising electrode materials for rechargeable Li-ion batteries.