A Defect Spinel Cathode Material for Long-Life and High-Energy-Density Magnesium Rechargeable Batteries

Magnesium rechargeable batteries (MRBs) have been a promising candidate for next-generation batteries due to the following reasons: (i) Mg can deposit without dendritic formation, enabling us to safely use Mg metal itself as a high-capacity (2205 mAh/g) anode. (ii) The electrode potential of Mg (-2.4 V vs. SHE) is relatively low among metal anodes for multivalent batteries, which is attractive for achieving high cell voltage. (iii) Mg is an abundant and non-toxic element. The development of MRBs, however, has been hindered by the lack of high-performance cathode materials. The difficulty in the development of cathode materials is derived from the high charge density of divalent Mg 2+ ion, which is almost twice that of monovalent Li + ion since their ionic radii are similar. This leads to the high activation energies for Mg 2+ diffusion in cathode materials by the strong electrostatic interaction with anions. High-temperature operation is a way to prevent the problem of the sluggish Mg 2+ diffusion; thus, we have been developing high-potential spinel oxide cathode materials that can work at 150℃. We have found several promising cathode materials (e.g., MgCo 2 O 4 , ZnCo 2 O 4 ) [1,2], but they show rapid capacity fading during battery cycling. Such stoichiometric spinel oxides coherently transform to the rocksalt structure accompanied by Mg insertion in the two-phase reaction [1]. Therefore, the cycle degradation can be attributed to the formation of the rocksalt phase on the surface of active material particles, which would impede the diffusion of Mg 2+ ions and degrade the reversibility of structural change in discharge/charge. Very recently, we have proposed a strategy of utilizing defect spinel oxides, which contain cation deficiencies at the octahedral sites in the spinel structure, to suppress the structure transition into the densely packed rocksalt phase in the early stage of discharge (i.e., Mg insertion) [3]. We have chosen ZnMnO 3 as a promising candidate for defect spinel-type cathode material in that Zn stabilizes the spinel structure [2] and Mn allows the valence change from tetravalent to divalent [1]. By the theoretical and experimental approach, we have confirmed that ZnMnO 3 with cation deficiencies at the octahedral sites is stable, and Mg 2+ ions are preferentially inserted into the cation-deficient sites at the potential of 2–3 V vs. Mg 2+ /Mg with keeping spinel-based structures. As a consequence, it has been demonstrated that ZnMnO 3