A Superconcentrated Water‐in‐Salt Hydrogel Electrolyte for High‐Voltage Aqueous Potassium‐Ion Batteries

Abstract We report a high‐voltage aqueous full potassium‐ion battery (AFKIB) with a superconcentrated water‐in‐salt hydrogel electrolyte (WISHE). This WISHE exhibits an expanded electrochemical stability window (ESW) of 3.0 V as well as a high ionic conductivity of 4.34 mS cm−1. The hydrogen bonds between the polymer network and the water phase help to further widen the ESW over a liquid‐type water‐in‐salt electrolyte, and prevent liquid leakage. The assembled AFKIB exhibits a considerable capacity of 135 mA h g−1 and a superior stability after 3000 cycles. This work provides a meaningful way of designing high‐voltage aqueous energy storage devices.