Activating the Mn ²⁺ /Mn ⁷⁺ redox for a neutral Zn||Mn-compound battery with 2.2-V discharge plateau

In 1952, the first commercially alkaline Zn||MnO 2 primary battery was developed, which is based on Mn 4+ /Mn 3+ redox reactions (MnO 2 ↔Mn 2 O 3 ). Subsequently, the single-electron (Mn 3+ /Mn 2+ and Mn 7+ /Mn 6+ ) and two-electron (Mn 4+ /Mn 2+ ) redox reactions of Mn compound (Mn-Comp) were achieved in aqueous Zn-based batteries. After that, however, the new Mn multivalent change redox reaction was no longer observed. In this study, we report a neutral superhydrophilic hydrogel electrolyte that activates a Mn 2+ /Mn 7+ conversion reaction (Mn 2+ ↔MnO 4 − ). This advance is attributed to an expanding electrochemical stability window and high Mn 2+ reaction activity. This enables a discharge plateau of ≥2.2 V in aqueous Zn metal batteries. The high reversibility of Mn 2+ /Mn 7+ conversion reactions during cycling was achieved by incorporating chelation groups in the designed hydrogel electrolyte, which effectively stabilizes the MnO 4 − . The aqueous Zn||Mn-Comp battery with a 2.2-V discharge plateau operates stably for ≥360 hours.