Development of deformable aqueous Li-ion batteries enabled by water-in-salt-based hydrogel electrolytes

The growing demand for safe, deformable, and high-performance energy storage systems, driven by advances in wearable electronics, soft robotics, and biomedical devices, has brought deformable aqueous lithium-ion batteries (ALIBs) into focus. Water as the electrolyte solvent imparts ALIBs with intrinsic safety and higher moisture tolerance than conventional organic-electrolyte LIBs, enabling the use of stretchable and deformable packaging that is generally non-hermetic. However, their electrochemical performance is fundamentally constrained by the narrow electrochemical stability window (ESW) of water, which limits the achievable cell voltage and energy density. Recent progress in water-in-salt electrolytes (WiSEs) and their hydrogel-based derivatives (WiS-HGEs) presents a promising strategy to overcome these challenges. Due to the high salt concentrations involved, WiSEs suppress the reactivity of water, thereby broadening the ESW. WiS-HGE offers enhanced mechanical robustness to support the deformability of ALIBs, where the hydrogel serves as both the electrolyte and a stretchable separator, preventing electrolyte leakage and maintaining ionic conductivity under mechanical deformation. This article reviews the latest advancements in deformable ALIBs enabled by WiS-HGE systems, with a focus on how salt selection, electrolyte composition, and polymer matrix collectively influence electrochemical performance. Perspectives on current challenges and future directions are discussed in the last section, highlighting material design strategies, interfacial engineering approaches, and component-level innovations. Our contributions alongside other breakthrough advancements in flexible and stretchable aqueous lithium-ion batteries based on water-in-salt gel electrolytes • Unlike conventional LIBs, ALIBs offer intrinsic safety, environmental friendliness, and cost-effectiveness. • High salt concentrations suppress water reactivity, expanding ESW, enabling high-voltage ALIBs. • Gel-based WiSEs prevent leakage, add flexibility/stretchability, and maintain ionic conductivity under deformation, critical for wearable applications. • Innovations in materials and structure design strategies are required to maintain electrochemical performance under mechanical deformation for flexible/stretchable batteries.