Manganese dioxide as cathode for aqueous zinc-ion batteries: reaction mechanisms, optimization strategies and further prospects

Aqueous zinc-ion batteries (AZIBs), as one of the most promising energy storage devices, have attracted widespread attention owing to their abundant resources, environmental friendliness, and high safety. As a crucial component of AZIBs, the electrochemical performance of cathode materials plays a decisive role in battery performance, thus necessitating in-depth investigations into the structure and properties of cathode materials. Manganese dioxide (MnO2), as a cathode material for AZIBs, has garnered significant interest owing to advantages such as the low cost of manganese, stable structure, simple synthesis process, and abundant raw materials. Additionally, it exhibits high specific capacity and tunable cycling performance. However, MnO2 as a cathode in AZIBs is plagued by structural deformation, side reactions, and the Jahn-Teller effect during cycling. Therefore, it is essential to comprehensively review the research progress, reaction mechanisms, and optimization strategies of MnO2 in AZIBs. Herein, MnO2 is taken as the research focus. Firstly, we comprehensively summarize the development status and research progress of MnO2 materials as cathodes for AZIBs. Subsequently, we conduct an in-depth analysis of the structural evolution and Zn2+ storage mechanisms of MnO2 during cycling, including the conversion reaction mechanism, Zn2+ intercalation mechanism, dissolution-deposition mechanism, and H+/Zn2+ co-intercalation mechanism. Building on this, various optimization strategies such as structural control, morphological regulation, defect engineering, and electrolyte development are systematically reviewed. Finally, we outline future research directions for high-performance MnO2 cathodes, put forward a rational research roadmap to maximize the electrochemical properties of MnO2, and facilitate the construction of stable AZIBs.