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., MgCo₂O₄, 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 MgCo₂O₄, Mg insertion occurs at a considerably high potential of about 2.9 V vs. Mg²⁺/Mg, and similarly it occurs around 2.3 V vs. Mg²⁺/Mg with the valence change from Mn(III) to Mn(II) in MgMn₂O₄, 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 Mg₂X₂O₄ or MgX₃O₄ (X = Co, Fe, Mn, and Cr). In addition, the normal spinel MgMn₂O₄ and MgCr₂O₄ can be demagnesiated to some extent owing to the robust host structure of Mg_1−xX₂O₄, where the Mg extraction/insertion potentials for MgMn₂O₄ and MgCr₂O₄ are both about 3.4 V vs. Mg²⁺/Mg. Especially, the former “intercalation and push-out” process would provide a safe and stable design of cathode materials for polyvalent cations.