Non-layer-transformed Mn3O4 cathode unlocks optimal aqueous magnesium-ion storage via synergizing amorphous ion channels and grain refinement

Aqueous rechargeable magnesium ion batteries (ARMBs) have obtained more attention due to the two-electrons transfer nature, low cost and safety. However, the scarcity of cathode materials seriously hinders the development of ARMBs because of the unfavorable strong interaction between Mg 2+ and cathode material. Herein, we choose a pre-treated spinel Mn 3 O 4 cathode for aqueous Mg 2+ storage. The pre-treatment in Na 2 SO 4 solution induces the grain refinement decorated with tortuous amorphous ion diffusion channels, facilitating the production of electrochemical reaction active sites and the diffusion of Mg 2+ , respectively, which achieve a (sub-)surface pseudocapacitance reaction between Mn (II) and Mn (III). As a result, the pre-treated Mn 3 O 4 cathode exhibits a package of optimal performances, i.e., a capacity of 98.9 mAh g −1 and a high capacity retention rate of 99.4% after 2000 cycles. To the best of our knowledge, our work not only provides a new reaction mechanism of spinel Mn 3 O 4 in aqueous batteries system, but also affords a high cycle stability electrode material for rechargeable Mg 2+ energy storage.