Novel Insights into Energy Storage Mechanism of Aqueous Rechargeable Zn/MnO Batteries with Participation of Mn.

Aqueous rechargeable Zn/MnO zinc-ion batteries (ZIBs) are reviving recently due to their low cost, non-toxicity, and natural abundance. However, their energy storage mechanism remains controversial due to their complicated electrochemical reactions. Meanwhile, to achieve satisfactory cyclic stability and rate performance of the Zn/MnO ZIBs, Mn is introduced in the electrolyte (e.g., ZnSO solution), which leads to more complicated reactions inside the ZIBs systems. Herein, based on comprehensive analysis methods including electrochemical analysis and Pourbaix diagram, we provide novel insights into the energy storage mechanism of Zn/MnO batteries in the presence of Mn. A complex series of electrochemical reactions with the co-participation of Zn, H, Mn, SO, and OH were revealed. During the first discharge process, co-insertion of Zn and H promotes the transformation of MnO into ZnMnO, MnOOH, and MnO, accompanying with increased electrolyte pH and the formation of ZnSO·3Zn(OH)·5HO. During the subsequent charge process, ZnMnO, MnOOH, and MnO revert to α-MnO with the extraction of Zn and H, while ZnSO·3Zn(OH)·5HO reacts with Mn to form ZnMnO·3HO. In the following charge/discharge processes, besides aforementioned electrochemical reactions, Zn reversibly insert into/extract from α-MnO, ZnMnO, and ZnMnO·3HO hosts; ZnSO·3Zn(OH)·5HO, ZnMnO, and ZnMnO convert mutually with the participation of Mn. This work is believed to provide theoretical guidance for further research on high-performance ZIBs.