Full Depth‐of‐Discharge Cycling in Zn||MnO 2 Batteries Enabled by Alkaline Salt‐Concentrated Hydrogel Electrolyte

Abstract Alkaline Zn||MnO 2 batteries hold great potential for grid energy storage due to their material abundance, low cost, and high safety. However, their large‐scale application is limited to primary cells because of poor cycle stability, which is caused by hydrogen evolution on Zn anode and irreversible phase transitions of MnO 2 cathode. Herein, it develops alkaline salt‐concentrated hydrogel electrolytes that combine the benefits of both high‐concentration and hydrogel electrolytes. These electrolytes with high ionic conductivity of 370 mS cm −1 selectively block Zn 2+ crossover, prevent MnO 2 phase transitions, and mitigate parasitic reactions by tightly binding and confining salt‐coordinated water within the hydrogel matrix. Consequently, Zn||MnO 2 cells with high mass loading of 82 mg cm −2 exhibit minimal capacity loss after 1200 cycles. Additionally, reversible 100% depth‐of‐discharge cycling is demonstrated, leading to a fivefold increase in cell energy density. This work provides a cost‐effective and highly rechargeable solution for large‐scale grid energy storage.