A Novel “Water-in-Ionic Liquid” Electrolyte for Zn Metal Batteries

Success in the development of aqueous batteries hinges on strict design principles for the aqueous electrolyte: utilize the high ionic conductivity but limit the redox activity of water. Here we develop a new type of electrolyte, a “water-in-ionic liquid” electrolyte, where the water molecule is sealed in a water trap composed of ions by short-range intermolecular interactions. In this way, the water molecule is protected by the surrounding anion-abundant framework and simultaneously plays its lubricant role for ionic conduction, realizing a high stability together with a high ionic conductivity (1.18 × 10^-2 S cm^-1) of the electrolyte. This water-confined motif promotes the formation of an explicit interphase (SEI) on a Zn metal anode, which is clearly observed for the first time and contributes to a substantially reversible Zn electrochemistry. Notably, due to the intermolecular constraint, the volatilization of this electrolyte is alleviated, which is helpful for batteries with an open system. As a proof of concept, we demonstrate zinc-air batteries (ZABs) with superior longevity compared to those using typical aqueous electrolyte under lean-electrolyte conditions (300 h vs 72 h, respectively).