Indium-Doped Rutile Titanium Oxide with Reduced Particle Length and Its Sodium Storage Properties

We hydrothermally synthesized In-doped rutile TiO₂ particles in an anionic surfactant solution and investigated the influences of In doping and the particle morphology on the Na⁺ storage properties. The solid solubility limit was found to be 0.8 atom % in In-doped TiO₂. In the case where no surfactant was used, the best anode performance was obtained for 0.8 atom % In-doped TiO₂ electrode by the benefits of three doping effects: (i) expanded diffusion-path size, (ii) improved electronic conductivity, and (iii) reduced electron charge density in the path. Further enhancement in the performance was achieved for the In-doped TiO₂ with a reduced particle length by the synthesis in the surfactant solution. This electrode exhibited a better cycle stability and maintained a high discharge capacity of 240 mA h g^-1 for 200 cycles. The reason is probably that Na⁺ can be inserted in the inner part of TiO₂ particles because of its reduced particle length.