Cation-deficient Mg-Mn spinel as a near-zero-strain cathode material for aqueous Mg-ion batteries

Aqueous magnesium-ion batteries (AMIBs) are promising energy storage systems due to their high safety, abundant resources, and environmental friendliness. Manganese-based spinel cathode materials for AMIBs offer low cost and high theoretical capacity, but tetragonal MgMn 2 O 4 undergoes a tetragonal-to-cubic phase transition during charging and discharging, leading to large volume changes and limiting long-cycle stability. Herein, cation-deficient Mg-Mn spinel (MgMnO 3 ) is designed as the cathode material for AMIBs. During Mg-ion intercalation/de-intercalation, MgMnO 3 maintains the cubic spinel structure, showing a near-zero-strain characteristic with only 1.47% lattice volume variation during cycling. As a result, the optimized MgMnO 3 cathode exhibits a discharge capacity of 65 mAh g −1 with 98.5% capacity retention after 2000 cycles at 500 mA g −1 . In addition, the aqueous full cell comprising the MgMnO 3 cathode and Mg metal anode shows a high specific capacity of 131 mAh g −1 and a median voltage of 1.75 V. The results demonstrate that MgMnO 3 is a promising cathode material for AMIBs and provide insights into the development of high-performance AMIBs.