A concentrated water-in-salt electrolyte (WiSE) effectively binds free water molecules, suppressing hydrogen evolution and the dissolution of potassiated perylene-3,4,9,10-tetracarboxylic dianhydride, resulting in improved cycling stability. In addition, easier breakup of the K⁺ solvation sheath in this WiSE also leads to improved kinetics for faster K⁺ storage.