Mg-ion storage materials based on MnO₂ frameworks

A wide variety of crystal structures for MnO₂, so-called polymorphs, is promising for basic frameworks of cathode materials in next-generation batteries such as Mg rechargeable batteries. In this short review, we summarize recent progress on MnO₂-based cathode materials for Mg rechargeable batteries, especially focusing on the structures of α-MnO₂ and λ-MnO₂ to discuss the criteria for achieving topotactic reactions during magnesiation. In contrast to adequately stable α-MnO₂, spinel-structured λ-MnO₂ is unstable at elevated temperatures around 150 °C; however, its stability can be significantly improved by materials design. Here, we introduce as a successful example that defect spinel ZnMnO₃ exhibits long-term cycle performance even at 150 °C with high potential and capacity.