Solid Electrolyte Interface Regulated by Solvent‐in‐Water Electrolyte Enables High‐Voltage and Stable Aqueous Mg‐MnO2 Batteries

Abstract Mg batteries utilizing divalent Mg 2+ as charge carriers have been attracting significant attention for energy storage owing to their uniqueness in terms of low cost, high safety, and high energy density. However, the short cycling life arising from the accumulation of a passivation layer on the Mg anode prohibits their further development. Here, a new strategy to suppress the accumulation of the passivation layer is presented, thus stabilizing the Mg anode by constructing a robust interfacial layer using organic solvent‐in‐water electrolytes. The organic solvents decompose on the Mg anode to form a MgO, polyether Mg enriched organic‐inorganic hybrid interfacial layer, preventing free water attack to the Mg anode. In addition, the organic solvent forms an H‐bond with H 2 O to suppress the water activity and therefore protecting the Mg anode. As an illustration to benefits of the solvent‐in‐water electrolytes, a Mg‐MnO 2 battery that is coupled by a MnO 2 cathode in Mn 2+ /MnO 2 chemistry with the Mg anode is developed. The Mg‐MnO 2 battery exhibits a well‐maintained discharge plateau at ≈2.5 V and a discharging capacity of ≈500 mAh g −1 over 1000 cycles. This result highlights the feasibility to stabilize the Mg anode by interface engineering through the solvent‐in‐water electrolyte design.