Potassium secondary batteries are contenders of next-generation energy storage devices owing to the much higher abundance of potassium than lithium. However, safety issues and poor cycle life of K metal battery have been key bottlenecks. Here we report an ionic liquid electrolyte comprising 1-ethyl-3-methylimidazolium chloride/AlCl₃/KCl/potassium bis(fluorosulfonyl) imide for safe and high-performance batteries. The electrolyte is nonflammable and exhibits a high ionic conductivity of 13.1 mS cm−1 at room temperature. A 3.6-V battery with K anode and Prussian blue/reduced graphene oxide cathode delivers a high energy and power density of 381 and 1,350 W kg−1, respectively. The battery shows an excellent cycling stability over 820 cycles, retaining ∼89% of the original capacity with high Coulombic efficiencies of ∼99.9%. High cyclability is also achieved at elevated temperatures up to 60 °C. Uniquely, robust K, Al, F, and Cl-containing passivating interphases are afforded with this electrolyte, which is key to superior battery cycling performances.