Progress and Challenges of Water-in-Salt Electrolytes: Exploring Physical Chemistry Properties and Solid Electrolyte Interphase Formation Mechanisms

“Water-in-salt” (WIS) electrolytes endow the possibility of commercial aqueous devices due to the extending electrochemical stability window (ESW). However, there is still a long way to address current issues until future practical applications, such as the high cost of salts, the cathodic limit, and the controversial mechanism of solid-electrolyte interphase (SEI). In this review, we first introduce cutting-edge WIS electrolytes and display their current issues. After, the reported tactics of solving issues and achievements in our group are listed, including four sections: 1) physical structure; 2) SEI formation analysis; 3) additives contributions; and 4) devices. In the end, we focus on the current challenges and perspectives of WIS electrolytes for aiming at the practical applications of aqueous energy storage devices.