Intercalation and Push-Out Process with Spinel-to-Rocksalt Transition on Mg Insertion into Spinel Oxides in Magnesium Batteries.

On the basis of the similarity between spinel and rocksalt structures, it is shown that some spinel oxides (e.g., MgCoO, etc) can be cathode materials for Mg rechargeable batteries around 150 °C. The Mg insertion into spinel lattices occurs via "intercalation and push-out" process to form a rocksalt phase in the spinel mother phase. For example, by utilizing the valence change from Co(III) to Co(II) in MgCoO, Mg insertion occurs at a considerably high potential of about 2.9 V vs. Mg/Mg, and similarly it occurs around 2.3 V vs. Mg/Mg with the valence change from Mn(III) to Mn(II) in MgMnO, being comparable to the ab initio calculation. The feasibility of Mg insertion would depend on the phase stability of the counterpart rocksalt XO of MgO in MgXO or MgXO (X = Co, Fe, Mn, and Cr). In addition, the normal spinel MgMnO and MgCrO can be demagnesiated to some extent owing to the robust host structure of MgXO, where the Mg extraction/insertion potentials for MgMnO and MgCrO are both about 3.4 V vs. Mg/Mg. Especially, the former "intercalation and push-out" process would provide a safe and stable design of cathode materials for polyvalent cations.