Design strategies and research progress for Water-in-Salt electrolytes

Electrolyte plays an essential role in ion transport among all electrochemical energy storage systems (EESs). Water-in-Salt (WIS) electrolyte as a novel aqueous electrolyte has attracted wide attention in recent years because it maintains the advantages of aqueous electrolytes and the wide electrochemical stable voltage window of nonaqueous electrolytes. In this review, we first summarized the structure, properties, and applications of the original LiTFSI-based WIS electrolyte. Then, we special emphasized the effects of ion and additional phase regulation on the structure and performance of WIS electrolytes based on the characteristic of different ions and additional phases in WIS electrolytes. Furthermore, the future research directions and opportunities of different WIS electrolytes are prospected. This review aims to provide guidance and reference for designing and manufacturing the next generation of aqueous electrolytes used in different EESs.