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.