Novel Insights into Energy Storage Mechanism of Aqueous Rechargeable Zn/MnO2 Batteries with Participation of Mn2+

HighlightsPourbaix diagram of Mn-Zn-H2O system was used to analyze the charge-discharge processes of Zn/MnO2 batteries.Electrochemical reactions with the participation of various ions inside Zn/MnO2 batteries were revealed.A detailed explanation of phase evolution inside Zn/MnO2 batteries was provided. AbstractAqueous rechargeable Zn/MnO2 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/MnO2 ZIBs, Mn2+ is introduced in the electrolyte (e.g., ZnSO4 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/MnO2 batteries in the presence of Mn2+. A complex series of electrochemical reactions with the co-participation of Zn2+, H+, Mn2+, SO42-, and OH- were revealed. During the first discharge process, co-insertion of Zn2+ and H+ promotes the transformation of MnO2 into ZnxMnO4, MnOOH, and Mn2O3, accompanying with increased electrolyte pH and the formation of ZnSO(4)3Zn(OH)(2)5H(2)O. During the subsequent charge process, ZnxMnO4, MnOOH, and Mn2O3 revert to -MnO2 with the extraction of Zn2+ and H+, while ZnSO(4)3Zn(OH)(2)5H(2)O reacts with Mn2+ to form ZnMn(3)O(7)3H(2)O. In the following charge/discharge processes, besides aforementioned electrochemical reactions, Zn2+ reversibly insert into/extract from -MnO2, ZnxMnO4, and ZnMn(3)O(7)3H(2)O hosts; ZnSO(4)3Zn(OH)(2)5H(2)O, Zn2Mn3O8, and ZnMn2O4 convert mutually with the participation of Mn2+. This work is believed to provide theoretical guidance for further research on high-performance ZIBs.