Aqueous MnO2/Mn2+ electrochemistry in batteries: progress, challenges, and perspectives

Abstract

Rechargeable aqueous MnO₂-based batteries have attracted extensive attention and development in recent years owing to their inherent low cost, high safety, and competitive specific capacity. Although the emerging MnO₂ dissolution/deposition chemistry directs the MnO₂-based batteries towards high energy density, it results in undesirable complexity in the reaction mechanism and capacity evolution in the batteries with traditional ion intercalation reactions. Batteries driven by the exclusive MnO₂ dissolution/deposition reaction provide promising practicability in high-energy-density systems with a high theoretical specific capacity (616 mA h g⁻¹) and desirable theoretical potential (1.991 V vs. Zn/Zn²⁺), along with the availability of a cathode-free design in the absence of ion intercalation, but are faced by several key challenges on the way to commercial applications and large-scale energy storage. Herein, we provide an in-depth review of the basic mechanisms and advanced development of MnO₂ dissolution/deposition chemistry and critically analyze the main issues in the two distinguishing systems for MnO₂-based batteries: partial MnO₂ dissolution/deposition with ion insertion/conversion and exclusive MnO₂ dissolution/deposition. Finally, the future development directions of MnO₂-based batteries are outlined to guide the research for fundamental science and next-generation aqueous batteries.