Beyond conventional aqueous electrolytes: Recent developments in Li-free "water-in-salt" electrolytes for supercapacitors

Water-in-salt electrolyte (WiSE) is an extremely high-concentration aqueous solution where the salt outnumbers the solvent by weight and/or volume. Supercapacitors (SCs) incorporating lithium-free WiSE are promising energy storage systems due to their wide electrochemical stability window (ESW), high-ionic conductivity, low cost, and ecofriendliness. This review provides an overview of WiSE-based SCs, focusing on physicochemical properties, recent progress, and prospects of non-lithium WiSE-based SCs. The mechanisms of how WiSEs exhibit expanded ESW, high-ionic conductivity, and low-temperature tolerance are elaborated. Then, we summarized the current strategies for the lithium-free WiSE-based SCs categorized by cations (Na+, K+, other metallic cations, and non-metallic cations). In addition, the utilization of redox additives in WiSE-based SCs is presented. Finally, current challenges and perspectives for the application of Li-free WiSE into SCs are provided.