Mo-Incorporated Magnetite Fe₃ O₄ Featuring Cationic Vacancies Enabling Fast Lithium Intercalation for Batteries

Transition metal oxides (TMOs) as high-capacity electrodes have several drawbacks owing to their inherent poor electronic conductivity and structural instability during the multi-electron conversion reaction process. In this study, the authors use an intrinsic high-valent cation substitution approach to stabilize cation-deficient magnetite (Fe₃ O₄ ) and overcome the abovementioned issues. Herein, 5 at% of Mo⁴⁺ -ions are incorporated into the spinel structure to substitute octahedral Fe³⁺ -ions, featuring ≈1.7 at% cationic vacancies in the octahedral sites. This defective Fe_2.93 ▫_0.017 Mo_0.053 O₄ electrode shows significant improvements in the mitigation of capacity fade and the promotion of rate performance as compared to the pristine Fe₃ O₄ . Furthermore, physical-electrochemical analyses and theoretical calculations are performed to investigate the underlying mechanisms. In Fe_2.93 ▫_0.017 Mo_0.053 O₄ , the cationic vacancies provide active sites for storing Li⁺ and vacancy-mediated Li⁺ migration paths with lower energy barriers. The enlarged lattice and improved electronic conductivity induced by larger doped-Mo⁴⁺ yield this defective oxide capable of fast lithium intercalation. This is confirmed by a combined characterization including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT) and density functional theory (DFT) calculation. This study provides a valuable strategy of vacancy-mediated reaction to intrinsically modulate the defective structure in TMOs for high-performance lithium-ion batteries.