Aqueous rechargeable batteries have great prospects in the field of large-scale energy storage due to their high safety, low cost, and environmental friendliness. As a key component of batteries, electrode materials play important roles on their electrochemical performance. Vanadium oxides, manganese oxides, Prussian blue analogues, and organic materials are often used as active materials in aqueous batteries. Among these materials, vanadium oxides possess a variety of compounds, high theoretical specific capacity, and superior cycling performance. However, their redox potential is relatively low, restricting the operating voltage of aqueous batteries. Moreover, these materials are toxic, which are not conducive in the large-scale applications. Compared with vanadium oxides, Prussian blue analogues have a higher redox potential and a stable structure, but they have some disadvantage of low theoretical specific capacity, resulting in the low energy density of batteries. In contrast, organic materials possess abundant sources, facile structure regulation, and superior sustainability. However, their poor conductivity and low compaction density make it difficult to prepare high-loading electrodes. Compared with the materials above, manganese oxides have the advantages of diverse crystal structures, high theoretical specific capacity, high redox potential, non-toxicity, and low cost, which are beneficial for constructing high-performance aqueous batteries. Therefore, manganese oxides are considered as a promising electrode material in aqueous batteries. Recent efforts are made in the design of manganese oxides-based aqueous batteries, but the corresponding comprehensive review on this topic is still sparse.

This review firstly analyzed the crystal structure types and characteristics of manganese oxides. According to the connection mode between MnO₆ units, the crystal structure of manganese oxides can be divided into one-dimensional tunneled structure (i.e., α-MnO₂, β-MnO₂, γ-MnO₂, R-MnO₂, Todorokite-MnO₂), two-dimensional layered structure (i.e., δ-MnO₂), and three-dimensional spinel structure (i.e., λ-MnO₂, Mn₃O₄, LiMn₂O₄, ZnMn₂O₄). The characteristics of corresponding crystal structure were summarized. Manganese oxides exhibited unique physical and chemical properties, endowing their wide application as electrode materials in aqueous batteries.

The reaction mechanisms of manganese oxides are rather complex in aqueous batteries, especially for aqueous zinc-ion batteries, which were summarized according to the acidity of electrolytes. In alkaline Zn-MnO₂ batteries, MnO₂ is firstly converted into MnOOH, and then Mn(OH)₂ is formed. As the acidity of the electrolyte decreases, manganese oxides exhibit different electrochemical reactions, mainly including ion insertion-extraction, conversion, and dissolution-deposition (Mn²⁺/MnO₂). The different electrochemical reaction mechanisms of manganese oxides provide plentiful energy storage chemistry for the design of aqueous battery systems. However, there are also irreversible side reactions and structural distortions in manganese oxides during the cycling process, which hinder their further development.

The application of manganese oxides in aqueous batteries is briefly elaborated, including alkaline-metal-ions (such as Li⁺, Na⁺), multivalent-metal-ions (such as Zn²⁺, Mg²⁺, Al³⁺), and non-metallic-ions (such as H⁺, NH₄⁺) batteries. To address the poor conductivity, unstable structure, as well as manganese dissolution of manganese oxides, nanostructure design, hetero-element doping, defect engineering, and composite construction with other conductive materials are adopted to regulate the electronic structure and alleviate the Jahn-Teller distortion. As a result, the rate capability and cycling stability of manganese oxides-based aqueous batteries are significantly improved.