Unravelling the Surface Structure of MgMn 2 O 4 Cathode Materials for Rechargeable Magnesium-Ion Battery

The spinel MgMn 2 O 4, a cathode material with theoretical capacity of 272 mA h g –1, holds promise for future application in high volumetric magnesium-ion batteries. Atomic-resolution imaging of the structure of the spinel and its surface composition would advance our understanding on its electrochemical properties, mass, and charge transport behavior in electrodes. We observe directly, by aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of cubic spinel MgMn 2 O 4 for the first time. More importantly, we find that a thin stable surface layer of rocksalt MgMnO 2 was grown on a bulk cubic spinel phase. The formation of a rocksalt phase was induced by reconstruction of the spinel phase, i.e., the insertion of Mg into the spinel lattice together with Mg/Mn cation exchange and Frenkel-defect-mediated relocation of Mg cations. This new structural analysis provides a critical step toward understanding and tuning the electrochemical performance of spinel oxide in rechargeable Mg-ion batteries.