Restructuring of aqueous electrolytes using a soft-acidic/hard-basic zwitterion for low-temperature anode-free Zn batteries

Despite the growing interest in aqueous Zn batteries as a safe and low-cost alternative to commercial Li-ion batteries, the use of aqueous electrolytes has limited their application at sub-zero temperatures. Here, we propose a new electrolyte design using zwitterions based on the hard and soft acids and bases principle to restructure aqueous electrolytes. Incorporation of a soft-acidic/hard-basic zwitterion into an aqueous electrolyte results in the disruption of the hydrogen bonds of water molecules, weakening of Zn2+-OTf- interactions, and destabilization of the Zn2+ solvation sheath. The resulting electrolyte (ZT-electrolyte) enhances the anti-freezing phenomena with a solid-liquid transition temperature of -95 degrees C and Zn2+ desolvation kinetics. Consequently, the anode-free full cell (Cu||Znxa-V2O5@graphene) with the ZT-electrolyte exhibits high energy and power densities (142 W h kg-1 at 50 mA g-1 and 230 W kg-1 at 2 A g-1) with stable cyclability at -40 degrees C, which exceeded those of previously reported Zn batteries and are comparable to those of low-temperature Li-metal batteries. Restructuring of aqueous electrolytes using a soft-acidic/hard-basic zwitterion enabled low-temperature anode-free Zn batteries, with a focus on enhancing anti-freezing phenomena and Zn2+ desolvation kinetics at electrolyte-electrode interfaces.