From Salt in Water, Water in Salt to Beyond

Traditional aqueous electrolytes have a limited electrochemical stability window due to the decomposition voltage of water (approximate to 1.23 V). "Water-in-Salt" (WIS) electrolytes are developed, which expand the stability window of aqueous electrolytes from 1.23 to 3 V and sparked a global surge of research in aqueous batteries. This breakthrough revealed 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. In this review, the current mechanistic understanding of WIS electrolytes and their derivative designs, focusing on the construction of solvation structures is presented. The insights gained and limitations encountered in bulk solvation structure engineering is further discussed. Finally, future directions beyond WIS for advancing aqueous electrolyte design is proposed.