Vertically Aligned Binder-Free TiO₂ Nanotube Arrays Doped with Fe, S and Fe-S for Li-ion Batteries

Vertically aligned Fe, S, and Fe-S doped anatase TiO₂ nanotube arrays are prepared by an electrochemical anodization process using an organic electrolyte in which lactic acid is added as an additive. In the electrolyte, highly ordered TiO₂ nanotube layers with greater thickness of 12 μm, inner diameter of approx. 90 nm and outer diameter of approx. 170 nm are successfully obtained. Doping of Fe, S, and Fe-S via simple wet impregnation method substituted Ti and O sites with Fe and S, which leads to enhance the rate performance at high discharge C-rates. Discharge capacities of TiO₂ tubes increased from 0.13 mAh cm^-2(bare) to 0.28 mAh cm^-2 for Fe-S doped TiO₂ at 0.5 C after 100 cycles with exceptional capacity retention of 85 % after 100 cycles. Owing to the enhancement of thermodynamic and kinetic properties by doping of Fe-S, Li-diffusion increased resulting in remarkable discharge capacities of 0.27 mAh cm^-2 and 0.16 mAh cm^-2 at 10 C, and 30 C, respectively.