Evidence for Fast Lithium-Ion Diffusion and Charge-Transfer Reactions in Amorphous TiO _x Nanotubes: Insights for...

The charge-storage kinetics of amorphous TiO _x nanotube electrodes formed by anodizing three-dimensional porous Ti scaffolds are reported. The resultant electrodes demonstrated not only superior storage capacities and rate capability to anatase TiO _x nanotube electrodes but also improved areal capacities (324 μAh cm^-2 at 50 μA cm^-2 and 182 μAh cm^-2 at 5 mA cm^-2) and cycling stability (over 2000 cycles) over previously reported TiO _x nanotube electrodes using planar current collectors. Amorphous TiO _x exhibits very different electrochemical storage behavior from its anatase counterpart as the majority of its storage capacity can be attributed to capacitive-like processes with more than 74 and 95% relative contributions being attained at 0.05 and 1 mV s^-1, respectively. The kinetic analysis revealed that the insertion/extraction process of Li⁺ in amorphous TiO _x is significantly faster than in anatase structure and controlled by both solid-state diffusion and interfacial charge-transfer kinetics. It is concluded that the large capacitive contribution in amorphous TiO _x originates from its highly defective and loosely packed structure and lack of long-range ordering, which facilitate not only a significantly faster Li⁺ diffusion process (diffusion coefficients of 2 × 10^-14 to 3 × 10^-13 cm² s^-1) but also more facile interfacial charge-transfer kinetics than anatase TiO _x.