Control of Electrolyte Decomposition by Mixing Transition Metal Ions in Spinel Oxides as Positive Electrode Active Materials for Mg Rechargeable Batteries

The development of Mg rechargeable batteries is hindered by both oxidative and reductive electrolyte decomposition on the positive electrode, which results in poor cyclability. Although improving the stability of the electrolyte is one solution, we found that the oxidative decomposition of the electrolyte can be suppressed by introducing Fe ions to spinel oxides. Furthermore, the mechanism was clarified from the viewpoint of the electronic state of the spinel oxides, with MgFe2O4 exhibiting the lowest valence band maximum in our previous study. Here, by developing an interpretation of this mechanism, we demonstrated that the type of transition metal ions in spinel oxides has an effect on the reductive decomposition of the electrolyte. Based on the above knowledge, we synthesized mixed Co-Fe spinel oxides that exhibited a suppressive effect on both oxidative and reductive electrolyte decomposition and successfully improved the cyclability. This study provides guidelines for developing positive electrode active materials for Mg rechargeable batteries.