Hydrogenated TiO2 Nanotubes as High-Power Anode Materials for Lithium-Ion Batteries

Titanium oxide (TiO₂) has received much attention as the most promising alternative to the conventional graphite anode of Li-ion batteries for high energy and power. The performance of TiO₂ anode for Li-ion batteries depends strongly on the crystalline phase, the morphology, and the porosity of the structure. The nanostructured TiO₂ materials, such as nanoparticles, nanorods, nanowires, and nanotubes have been studied to improve the performance of TiO₂ anode. Recently, it was reported that the hydrogenated TiO₂, called a ‘black TiO₂’ nanostructures are more attractive for photovoltaics, photocatalysis, and supercapacitors owing to their narrower bandgap (less than typical 3 eV value) and relatively high electrical conductivity.

The smooth and well-ordered TiO₂ nanotubes were synthesized on a Ti disk successfully by anodization in a non-aqueous solution containing fluoride ions. The as-prepared TiO₂ nanotubes were annealed to obtain crystalline anatase (A-TiO₂ NTs) and hydrogenated TiO₂ nanotubes (H-TiO₂ NTs) at 450 °C for 2 h in air and hydrogen atmosphere, and then their electrochemical performances were investigated as alternative anode materials for Li-ion batteries.

The initial discharge capacity of the H-TiO₂ NTs (0.117 mAh cm^-2) was superior to that of the A-TiO₂ NTs (0.110 mAh cm^-2) and the discharge capacities of the H-TiO₂ NTs and A-TiO₂ NTs maintained nearly 72 and 44 % at a current density of 10 mA cm^-2. In addition, the H-TiO₂ NTs (89 %) exhibited much higher the capacity retention than the A-TiO₂ NTs (70 %) at the current density of 1 mA cm^-2 (~10 C-rate) after 300 cycles, as shown in Figure 1. The H-TiO₂ NTs presented smaller the crystallite size and charge transfer resistance (R_c) compared with the A-TiO₂ NTs and the oxygen vacancies were formed in the H-TiO₂ NTs during hydrogenation, which was proved by the presence of Ti³⁺ from the XPS analysis.

These results indicate that the insertion and extraction of Li⁺ through the H-TiO₂ NTs were preferable to those through the A-TiO₂ NTs, which were probably attributed to the short diffusion length for Li⁺, innumerable reaction sites, and relatively high electrical conductivity. Therefore, the H-TiO₂ NTs exhibited vastly superior the rate capability and capacity retention property during cycling to the A-TiO₂ NTs at high current density as anode materials for Li-ion batteries.