Novel Mg Rechargeable Battery Cathodes: Chevrel to Spinel

Abstract

In this chapter, it is shown that spinel oxides such as MgCo₂O₄ work as cathode materials for Mg rechargeable batteries with a high redox potential about 2–3 V versus Mg²⁺/Mg on the basis of the similarity between spinel and rocksalt structures (Okamoto et al., Adv. Sci., 1500072, 2015, [1]). The Mg insertion into spinel lattices occurs via “insertion 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 versus Mg²⁺/Mg, and similarly, it occurs at around 2.3 V versus Mg²⁺/Mg with the valence change from Mn(III) to Mn(II) in MgMn₂O₄. In addition, Mg²⁺ ions originally in MgMn₂O₄ and MgCr₂O₄ can be extracted to some extent because of the robust host structure. The “insertion and push-out” process proposed here provides a  new design of cathode materials for Mg rechargeable batteries, and various approaches are introduced to develop cathode materials based on this mechanism in the subsequent chapters.