Feasible transformation of MgCo ₂ O ₄ from spinel to defect rocksalt structure under electron irradiation

Spinel MgCo ₂ O ₄ oxide, expected as a cathode material candidate for magnesium rechargeable battery, undergoes spinel-to-rocksalt transition accompanied with Mg insertion. Upon scanning-transmission-electron-microscopy observation, cations located at the tetrahedral sites readily migrate to the adjacent octahedral sites due to electron irradiation (without any Mg insertion), so that the spinel structure changes to a defect rocksalt structure with cation disorder. This together with first-principles calculations demonstrates that there is little energetic preference for Mg cation between the tetrahedral and octahedral sites, accounting for the feasibility of the Mg insertion into the spinel MgCo ₂ O ₄ cathode via intercalation and push-out process of Mg cation.