Evidence for Fast Lithium-Ion Diffusion and Charge-Transfer Reactions in Amorphous TiOx Nanotubes: Insights for High-Rate Electrochemical Energy Storage

The charge-storage kinetics of amorphous TiOx 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 TiOx nanotube electrodes but also improved areal capacities (324 mu Ah cm(-2) at 50 mu A cm(-2) and 182 mu Ah cm(-2) at 5 mA cm(-2)) and cycling stability (over 2000 cycles) over previously reported TiOx nanotube electrodes using planar current collectors. Amorphous TiOx 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 TiOx 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 TiOx 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 x 10(-14) to 3 x 10(-13) cm(-2) s(-1)) but also more facile interfacial charge-transfer kinetics than anatase TiOx.