Advances in Aqueous Electrolyte Engineering: From Fundamental Limitations to Emerging Strategies

Aqueous rechargeable batteries are promising candidates for large-scale energy storage owing to their inherent safety, low cost, and environmental compatibility, yet the narrow 1.23 V electrochemical stability window (ESW) of water limits their energy density. Since the seminal 2015 report of water-in-salt electrolytes (WiSE), electrolyte engineering—manipulating the molecular environment of water, including its O–H bond strength, hydrogen-bond topology, and interfacial reactivity—has emerged as the central strategy for overcoming this thermodynamic constraint. This review synthesizes the representative studies across Li+, Na+, K+, and Zn2+ chemistries into a unified framework that links the physicochemical fundamentals of aqueous electrolytes (ion solvation, hydrogen-bond networks, the electrical double layer, and ion transport) with five classes of strategies—concentration, solvation-structure, and interface engineering; emerging electrolyte systems (deep eutectic, ionic-liquid–water, asymmetric decoupled, gel, and high-entropy); and practical challenges (wide-temperature operation, cost, and scalability). Examining unresolved controversies—the thermodynamic versus kinetic origin of ESW expansion in WiSE, the long-term chemical persistence of aqueous solid electrolyte interphases, and the definitional boundary of aqueous batteries—this review proposes a research roadmap spanning multi-strategy orthogonal integration and standardized evaluation protocols, providing both a tutorial for newcomers and a critical reference for researchers pursuing practical high-voltage aqueous batteries.