Water-in-salt hydrogel electrolyte for dendrite-free Zn deposition

Abstract

Zinc metal batteries suffer from dendrite formation, hydrogen evolution, and interfacial instability. While water-in-salt electrolytes (WiSE) suppress side reactions and hydrogels enhance interfacial stability, WiSE systems are costly and viscous, and conventional hydrogels contain excess water, promoting hydrogen evolution. To overcome these limitations, we developed a polyethylene glycol-based water-in-salt hydrogel (WiSH) electrolyte, incorporating tunable concentrations of zinc triflate (Zn(OTf)2) from 1 to 4 mol/kg. The optimized 4 mol/kg formulation enabled dendrite free and corrosion-free zinc plating/stripping in symmetric Zn||Zn cells for over 2000 hours at 1 mA/cm2 (1 mAh/cm2 area capacity), demonstrating exceptional long-term stability. The WiSH electrolyte exhibited improved mechanical strength and toughness with increasing salt concentration, attributed to stronger ionic crosslinking within the hydrogel matrix. Rheological and spectroscopic analyses confirmed the formation of a robust, densely crosslinked polymer network critical for stable and uniform Zn electrodeposition. Furthermore, a Zn–lignin full cell using the WiSH electrolyte achieved an energy density of 25 Wh/kg and 506 W/kg of specific power, highlighting its potential for energy storage systems. These results establish WiSH as a promising electrolyte platform for next-generation zinc batteries.