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

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.