Iron-Doped Cauliflower-Like Rutile TiO₂ with Superior Sodium Storage Properties

Developing advanced anodes for sodium ion batteries is still challenging. In this work, Fe-doped three-dimensional (3D) cauliflower-like rutile TiO₂ was successfully synthesized by a facile hydrolysis method followed by a low-temperature annealing process. The influence of Fe content on the structure, morphology, and electrochemical performance was systematically investigated. When utilized as a sodium ion battery anode, 6.99%-Fe-doped TiO₂ exhibited the best electrochemical performance. This sample delivered a very high reversible capacity (327.1 mAh g^-1 at 16.8 mA g^-1) and superior rate performance (160.5 mAh g^-1 at 840 mA g^-1), as well as long-term cycling stability (no capacity fading at 1680 mA g^-1 over 3000 cycles). Density functional theory (DFT) calculations combined with experimental results indicated that the significantly improved sodium storage ability of the Fe-doped sample should be mainly due to the increased oxygen vacancies, narrowed band gap, and lowered sodiation energy barrier, which enabled much higher electronic/ionic conductivities and more favorable sodium ion intercalation into rutile TiO₂.