Activating the Mn²⁺/Mn⁷⁺ redox for a neutral Zn||Mn-compound battery with 2.2-V discharge plateau

In 1952, the first commercially alkaline Zn||MnO₂ primary battery was developed, which is based on Mn⁴⁺/Mn³⁺ redox reactions (MnO₂↔Mn₂O₃). Subsequently, the single-electron (Mn³⁺/Mn²⁺ and Mn⁷⁺/Mn⁶⁺) and two-electron (Mn⁴⁺/Mn²⁺) 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 Mn²⁺/Mn⁷⁺ conversion reaction (Mn²⁺↔MnO₄^-). This advance is attributed to an expanding electrochemical stability window and high Mn²⁺ reaction activity. This enables a discharge plateau of ≥2.2 V in aqueous Zn metal batteries. The high reversibility of Mn²⁺/Mn⁷⁺ conversion reactions during cycling was achieved by incorporating chelation groups in the designed hydrogel electrolyte, which effectively stabilizes the MnO₄^-. The aqueous Zn||Mn-Comp battery with a 2.2-V discharge plateau operates stably for ≥360 hours.