Emerging aqueous manganese-based batteries: Fundamental understanding, challenges, and opportunities

Aqueous manganese (Mn)-based batteries are promising candidates for grid-scale energy storage due to their low-cost, high reversibility, and intrinsic safety. However, their further development is impeded by controversial reaction mechanisms and low energy density with unsatisfactory cycling stability. Here, we summarized various types of emerging aqueous Mn-based batteries based on the active redox couples, including liquid-solid deposition/dissolution reactions of Mn-0/Mn2+ and Mn2+/MnO2, liquid-liquid conversion reactions of Mn2+/Mn3+ and MnO42-/MnO4-, and solid-solid intercalation reaction of XMnOy/MnOy (X: cations) with manganese oxide as the host materials. A critical review of the fundamental understanding of their physicochemical properties in each reaction, scientific challenges, and improvement strategies is presented. Finally, perspectives on aqueous Mn-based batteries design for future commercialization are highlighted.