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 degrees 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 center dot center dot center dot 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.

Automated summary

Aqueous electrolytes usually have poor electrochemical stability, but eutectic aqueous solutions containing 25 wt.% LiCl and 62 wt.% H3PO4, cooled to 78 degrees C, show a significantly improved stability. Experimental and simulation results reveal that this improved stability is due to the reduced hydration of Li+ ions, the formation of ice-like water clusters, and the strengthening of H center dot center dot center dot Cl- bonding. Surprisingly, the low-temperature solvation structure does not strengthen water molecules' O-H bond, contrary to conventional wisdom. A proposed mechanism for water's low temperature inertness in the electrolyte is the less favorable solvation of OH and H+, the byproducts of hydrogen and oxygen evolution reactions. An aqueous Li-ion battery using LiMn2O4 cathode and CuSe anode is demonstrated to showcase this stability, with a high energy density of 109 Wh/kg. These findings highlight the potential of aqueous batteries for polar and extraterrestrial missions.