Phase Transition Behavior of MgMn₂O₄ Spinel Oxide Cathode during Magnesium Ion Insertion

The 3d transition metal oxides with a spinel structure are among the most promising cathode materials for magnesium batteries. In this study, we investigated the reaction mechanism of magnesium ion insertion for magnesium spinel oxides, MgMn2O4, by electrochemical measurements, X-ray absorption spectroscopy (XAS), and synchrotron X-ray diffraction (XRD) with Rietveld analysis. Open-circuit-potential and XAS measurements showed that Mg2+ insertion into MgMn2O4 does not proceed via a simple two-phase coexistence reaction between the spinel and rock-salt phases. Synchrotron XRD measurements showed that Mg2+ insertion into MgMn2O4 involves crystal structural changes in three stages. In the early stage of the Mg2+ insertion process (0 < x < 0.2), Mg2+ is inserted into the spinel (MgMn2O4) phase and rock-salt (Mg1.2Mn2O4) phases, which are included in the pristine samples, without significant volume changes. In the middle stage of the Mg2+ insertion process (0.2 < x < 0.4), Mg2+ is inserted into the Mg1+αMn2O4 spinel phase and the Mg2−βMn2O4 rock-salt phases with a large volume change. In the last stage of Mg2+ insertion process (0.4 < x < 0.56), Mg2+ insertion proceeds via a two-phase coexistence reaction between Mg1.4Mn2O4 spinel and Mg1.6Mn2O4 rock-salt phases without Mg content changes in either phase. The phase transition from the Mg1+αMn2O4 spinel phase to the Mg2−βMn2O4 rock-salt phase with a large volume change resulted in significant polarization during the Mg2+ insertion process. Suppressing the phase transition, accompanied by a large volume change, is important in designing a spinel oxide cathode with a high rate performance.