Strengthening Aqueous Electrolytes without Strengthening Water

Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions-25 wt.% LiCl and 62 wt.% H₃ PO₄ -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 LiMn₂ O₄ 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.