Strengthening Aqueous Electrolytes without Strengthening Water

Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions—25 wt.% LiCl and 62 wt.% H₃PO₄—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 LiMn₂O₄ 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.