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Abstract Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions—25 wt.% LiCl and 62 wt.% H 3 PO 4 —cooled to −78 °C exhibit a significantly widened stability window. Integrated experimental and simulation results reveal that, upon cooling, Li + ions become less hydrated and pair up with Cl − , ice‐like water clusters form, and H⋅⋅⋅Cl − bonding strengthens. Surprisingly, this low‐temperature solvation structure does not strengthen water molecules’ O−H bond, bucking the conventional wisdom that increasing water's stability requires stiffening the O−H covalent bond. We propose a more general mechanism for water's low temperature inertness in the electrolyte: less favorable solvation of OH − and H + , the byproducts of hydrogen and oxygen evolution reactions. To showcase this stability, we demonstrate an aqueous Li‐ion battery using LiMn 2 O 4 cathode and CuSe anode with a high energy density of 109 Wh/kg. These results highlight the potential of aqueous batteries for polar and extraterrestrial missions. 

Using elemental selenium as an electrode, the redox-active Cu 2+ /Cu + ion is reversibly hosted via the sequential conversion reactions of Se → CuSe → Cu 3 Se 2 → Cu 2 Se. The four-electron redox process from Se to Cu 2 Se produces a high initial specific capacity of 1233 mA h g −1 based on the mass of selenium alone or 472 mA h g −1 based on the mass of Cu 2 Se, the fully discharged product.