The morphological transition of anodic TiO2 structures and its application as an anode material of Li-ion batteries

초록

It is well-known that anodization of titanium in limited conditions allows the formation of highly organized TiO2 nanostructures, which can be morphologically transformed to other types of nanostructures by changing anodizing conditions. However, detailed mechanism and electrochemical conditions were not clearly identified yet. We systematically investigated the morphological transition of anodic TiO2 nanostructures as a function of applied voltage and the concentration of electrolytes, showing the scattering maps of the conditions for producing TiO2 nanotubes and microcones. We found that generally, as the voltage went up and the concentration of HF went down in the mixture electrolyte, TiO2 nanostructures changed from the amorphous nanotubes to the anatase microcones. Furthermore, we found that the mean diameter of the TiO2 microcones produced in (COOH)2 linearly increased with the applied voltage, whereas in other electrolytes, the mean diameters converged to a constant value. Regardless of the type of electrolyte, the percentage of area occupied by the TiO2 microcones was estimated to 20 % of the entire surface when the microcones were fully-grown and well-dispersed. In our previous study, TiO2 microcones, which have a hollow core with a multilayered anatase structure, showed three times higher capacity for an anode of Li-ion batteries (LIBs) than that of TiO2 nanotubes. However, although microcones showed higher capacity and excellent cycle performance, they could not overcome the low theoretical capacity and conductivity of TiO2 itself. Therefore, Graphene oxide as a conducting material on TiO2 microcones was coated via electrophoresis and reduced subsequently by cyclic voltammetry. As an anode material of LIBs, the prepared RGO-microcones showed a capacity of 157 mAh g?1 at even a high-rate of 10 C and sustained 1000 cycles with only 0.02 % capacity fading per cycle.