Aqueous Electrolytes: From Salt in Water to Water in Salt and Beyond

Traditional aqueous electrolytes have a limited electrochemical stability window due to the decomposition voltage of water (≈1.23 V). "Water-in-salt" (WIS) electrolytes, which expand the stability window of aqueous electrolytes from 1.23 to 3 V and spark a global surge of research in aqueous batteries, are developed. This breakthrough reveals novel aspects of solvation structure, ion transport mechanisms, and interfacial properties in WIS electrolytes, marking the start of a new era in solution chemistry that extends beyond traditional dilute electrolytes and has implications across electrolyte research. This review presents the current mechanistic understanding of WIS electrolytes and their derivative designs, focusing on the construction of solvation structures. The insights gained and limitations encountered in bulk solvation structure engineering are further discussed. Finally, future directions beyond WIS for advancing aqueous electrolyte design are proposed.