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

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.