Iron-Induced Partial Inversion Occupancy of MgMn2O4 for Magnesium-Ion Batteries with Enhanced Cycling Stability

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As a promising cathode material for magnesium-ion batteries, MgMn 2 O 4 has been increasingly studied due to its high theoretical capacity and low cost. However, the inherent Jahn–Teller distortion of Mn 3+ ions compromises its cycling stability. In this study, nanoscale single-phase MgMn 2 O 4 and MgMn 1.5 Fe 0.5 O 4 cathode materials were synthesized by Pechini method. The crystal structure of MgMn 1.5 Fe 0.5 O 4 has been systematically analyzed, along with its impact on electrochemical properties. Fe‑doping induces a phase transformation from tetragonal spinel to cubic spinel in MgMn 2 O 4 structure. X‑ray Rietveld refinement combined with first-principles calculations reveal that Fe occupies octahedral sites in the Fe-doped structure, accompanied by partial inversion occupancy. During this process, tetrahedral Mg 2+ and octahedral Mn 3+ sites undergo exchange, thereby stabilizing the cubic phase. This structural modification effectively mitigates Jahn–Teller distortion, enhancing electrochemical cycling stability and Mg 2+ diffusion kinetics. After 100 cycles at 200 mA g –1 , MgMn 1.5 Fe 0.5 O 4 exhibits an outstanding capacity retention of 90% with a specific discharge capacity of 126.6 mA h g –1 .