Mn-based oxides for aqueous rechargeable metal ion batteries

Aqueous rechargeable metal ion batteries (ARMBs), featuring safety, facile manufacturing and environmental benignity, have recently attracted extensive attention as promising energy storage systems. Particularly, the pursuit of electrode materials with abundance, low-cost and high capacity has directed the focus on Mn-based oxides for ARMBs. However, some barriers stand in the way of the development of Mn-based oxides for ARMBs, such as inherent poor electrical conductivity and rapid capacity degradation due to Jahn-Teller distortion and Mn2+ dissolution. Besides, the electrochemical window of aqueous electrolytes is too narrow to maximize the full potential of Mn-based oxides. In this review, we summarize recent developments of Mn-based oxides in aqueous batteries based on univalent ions (e.g., Li+ and Na+) and multivalent ions (e.g., Mg2+, Zn2+, and Al3+) as charge carriers. To be specific, we start with the introduction of crystal structures of Mn-based oxides reported so far, and outline the main shortcomings and the electrochemical reaction mechanisms (e.g., chemical conversion or intercalation) in combination with analysis of the Pourbaix diagram. Then research progress of Mn-based oxides in different battery systems is interpreted in detail indexed by the cation charge carrier. We highlight the prevalent optimization methods based on the electronic structure, morphology, additive, electrode-electrolyte interface, etc. for superb electrochemical performances. Finally, we systematically compare the applications in different battery systems with particular emphasis on battery energy density and discuss the reason behind the differences in terms of electrochemistry. And the research trends including electrode materials, electrode-electrolyte interfaces and high-concentration electrolytes are delineated for on-going studies.