LAUSR

Abstract Concentrated aqueous electrolytes have been widely explored for high-voltage aqueous batteries. To achieve higher voltage, even higher alkali-ion molalities are being pursuit up to 55.5 mol kg−1 (m) that corresponds to monohydrate, but the sacrificially lowered ionic conductivity is problematic. Here we report a 61.7 m K-ion aqueous electrolyte composed of KN(SO2F)2 (KFSI) and KSO3CF3 (KOTf). The pioneered beyond-monohydrate realm is unique to K-ion systems, in which [FSI]− anion is stable to hydrolysis. The K-ion electrolyte has two-order higher ionic conductivity (12 mS cm−1 at 30 °C) than a comparable 55.5 m Li-ion system (0.1 mS cm−1) while exhibiting a 2.7 V potential window on Pt being among the widest in all alkali-ion aqueous electrolytes. This work suggests the superior overall performance of aqueous K-ion batteries over other alkali-ion batteries.