Activating Sodium Intercalation in Cation-Deficient Fe₃O₄ Through Mo Substitution

Magnetite (Fe₃O₄), a conversion-type anode material, possesses high capacity, cost-effectiveness and environmental friendliness, positioning it as a promising candidate for the large-scale energy storage applications. However, the multi-electron reactions in sodium-ion batteries face challenges originated from the electrochemical inactivity of Na⁺ intercalation in the conversion-type oxides. In this work, controllable Fe vacancies are tailored in Fe₃O₄ lattice through the gradient Mo doping. The pair distribution function local structure analysis reveals that the key to stabilizing more Fe vacancies lies in the uniform occupation of Mo dopants at both tetrahedral (8a) and octahedral (16d) sites. The vacancy-rich structure, featuring 7.3% Fe vacancies, achieves a significantly enhanced capacity of 127 mAh g⁻¹ after 150 cycles at 100 mA g⁻¹, in comparison with the 37 mAh g⁻¹ for defect-free Fe₃O₄. A comprehensive understanding of how the defective structure relates to electrochemical performance is presented, combining physical-electrochemical characterizations with theoretical calculations. The occurred Mo-O interactions enhances electronic conductivity and diminishes electrostatic interactions between intercalated Na+ and lattice O²⁻. Concurrently, Fe vacancies facilitate bulk Na⁺ migration with lower energy barrier. This study presents a prospect for modulating the defective structure in transition metal oxides to activate fast and reversible sodium intercalation toward high-performance sodium-ion batteries.