Pseudocapacitive Lithium-Ion Storage in Oriented Anatase TiO2 Nanotube Arrays

We report on the synthesis and electrochemical properties of oriented anatase TiO ₂ nanotube (NT) arrays as electrodes for Li-ion batteries. The TiO ₂ NT electrodes displayed both pseudocapacitive Li ⁺ storage associated with the NT surface and the Li ⁺ storage within the bulk material. The relative contribution of the pseudocapacitive and bulk storages depends strongly on the scan rate. While the charges are stored primarily in the bulk at low scan rates (>1 mV/s), the surface storage dominates the total storage capacity at higher scan rates (>1 mV/s). The storage capacity of the NT electrodes as a function of charge/discharge rates showed no dependence on the NT film thickness, suggesting that the Li ⁺ insertion/extraction processes occur homogeneously across the entire length of NT arrays. These results indicated that the electron conduction along the NT walls and the ion conduction within the electrolyte do not cause significant hindering of the charge/discharge kinetics for NT electrode architectures. As a result of the surface pseudocapacitive storage, the reversible Li ⁺ storage capacities for TiO ₂ NT electrodes were higher than the theoretical storage capacity for bulk anatase TiO ₂ materials.