Fe-doping accelerated magnesium storage kinetics in rutile TiO₂ cathode materials

Rutile titanium dioxide (TiO₂) is theoretically favored as the efficient cathode materials for magnesium secondary batteries, yet rarely reported due to sluggish kinetics, low capacity, and inferior reversibility. Herein, a defect engineering strategy is developed via cationic Fe-doping to enhance the electrochemical magnesium storage kinetics of the rutile TiO₂ cathode materials and achieve the surface Mg²⁺ adsorption/diffusion mechanism. Abundant oxygen vacancies can be generated by Fe cations in rutile TiO₂ via a molten salt flux method. The optimized rutile TiO₂ cathode materials show large specific capacity of 204.8 mAh/g at current density of 100 mA g⁻¹, higher than that of the TiO₂-based counterparts reported previously. Additionally, the Mg²⁺ diffusion coefficient of Fe-doped rutile TiO₂ cathode materials are significantly improved to enable rapid magnesium storage. Fe cations can activate the cathode surface and weaken the Coulombic interactions between Mg²⁺ and anions in the host material. Oxygen vacancies can provide active adsorption sites for Mg²⁺ and MgCl⁺ to avoid slow solid-state diffusion and thus accelerate magnesium storage kinetics. Ex-situ XRD and XPS indicate that Fe-doped rutile TiO₂ cathode materials undergo the energy storage mechanism of Mg²⁺ adsorption/diffusion without phase transition or significant lattice expansion.