MnO2 Polymorphs as Cathode Materials for Rechargeable Ca‐Ion Batteries

Abstract Calcium‐ion batteries (CIBs) have attracted extensive interest for energy storage due to the high operating potentials and the abundant resources of calcium. However, the application of CIBs has been limited by the absence of high‐voltage and high‐performance cathode materials. Herein, for the first time, four kinds of MnO 2 polymorphs (α, β, γ, and δ‐phase) as cathode materials for CIBs are reported, and the δ‐MnO 2 exhibits the highest electrochemical performance, which is mainly due to the higher Ca‐ion diffusivity of δ‐MnO 2 . The excellent rate capability obtained herein outperforms those of previously reported for all oxides as cathode materials for CIBs. Furthermore, the Ca‐ion storage mechanism of δ‐MnO 2 based on the reversible order–disorder transformation is demonstrated by the in situ/ex situ characterizations. In addition, the capacity decay mechanism of δ‐MnO 2 during the cycling process is confirmed as the dissolution of Mn and the optimization strategy is proposed. These results demonstrate that the δ‐MnO 2 is a promising cathode material for CIBs, providing new opportunity for the development of high‐performance CIBs.