Promoting operating voltage to 2.3 V by a superconcentrated aqueous electrolyte in carbon-based supercapacitor

Aqueous supercapacitors(SCs)have attracted more and more attention for their safety,fast charge/discharge capability and ultra-long life.However,the application of aqueous SCs is limited by the low working voltage due to the narrow electrochemical stability window(ESW)of wate r.Herein,we report a new"water in salt"(WIS)electrolyte by dissolving potassium bis(fluorosulfonyl)amide(KFSI)in water with an ultra-high mass molar concentration of 37 mol/kg.The highly concentrated electrolyte can achieve a wide ESW of 2.8 V.The WIS electrolyte enables a safe carbon-based symmetrical supercapacitor to operate stably at 2.3 V with an ultra-long cycle life and excellent rate performance.The energy density reaches 20.5 Wh/kg at 2300 W/kg,and the capacity retention is 83.5%after 50,000 cycles at a current density of 5 A/g.This new electrolyte will be a promising candidate for future high-voltage aqueous supercapacitors.