Phase Transition Mechanism of ZnMn₂O₄ Spinel Oxide with Electrochemical Magnesium-Ion Insertion

Spinel oxides with 3d transition metals are expected to be cathode materials with high energy density for magnesium rechargeable batteries. Although it is important to control their phase transitions in order to reduce the polarization during charge/discharge processes for practical use, the relationship between the electrochemical properties and the phase transition mechanism of spinel oxides is not well understood. In this study, we examined the electrochemical properties and the phase transition mechanism of Mg²⁺ insertion into ZnMn₂O₄ spinel oxide by using the galvanostatic intermittent titration technique (GITT), X-ray absorption spectroscopy (XAS), and synchrotron X-ray diffraction (XRD) measurements and compared them to those of MgMn₂O₄ spinel oxide. Compared to MgMn₂O₄, the polarization was relatively small in ZnMn₂O₄ in the early stage of the Mg²⁺ insertion process (0 ≤ x ≤ 0.3) because the ZnMn₂O₄ spinel phase has a larger solid-solution limit for Mg²⁺ insertion. On the other hand, in the late stage of the Mg²⁺ insertion process (0.3 < x ≤ 0.58), the polarization of ZnMn₂O₄ was larger than that of MgMn₂O₄ due to the larger volume change between the spinel and rocksalt phases. The finding that the use of zinc stable at the tetrahedral configuration in spinel oxides can expand the solid-solution limit for Mg²⁺ insertion into the spinel phase and reduce the polarization is significant for the development of cathode materials in magnesium rechargeable batteries.