A high-voltage quasi-solid-state flexible supercapacitor with a wide operational temperature range based on a low-cost "water-in-salt" hydrogel electrolyte.

Recently, "water-in-salt" electrolytes have provided a huge boost to the realization of high energy density for water-based supercapacitors by broadening the electrochemical stability window. However, the high cost and low conductivity of high concentration LiTFSI greatly restrict the possibility of practical application. Herein, we adopt a new strategy to develop a low-cost and quasi-solid-state polyelectrolyte hydrogel accommodating a superhigh concentration of CHCOOK through in situ polymerization, avoiding the problem that many conventional polymers cannot accommodate ultra-high ion concentration. The polyelectrolyte hydrogel with 24 M CHCOOK exhibits a conductivity of up to 35.8 mS cm and a stretchability of 950%. With advanced N-doped graphene hydrogel electrodes, the assembled supercapacitor yields a voltage window of 2.1 V with an energy density of 33.0 W h kg and superior cyclability with 88.2% capacitance retention at 4 A g after 6000 cycles comparable to those supercapacitors using high-cost LiTFSI salts. Besides, the supercapacitor with excellent temperature stability in the range of -20 to 70 °C can light an LED for more than one minute. The assembled flexible device with the PAAK/CMC-24 M gel film sandwiched in between demonstrates excellent bendability from 0° to 180° and shows great potential for flexible/wearable electronic devices. Our feasible approach provides a new route for assembling quasi-solid-state flexible high-energy storage devices with "water-in-salt" electrolytes.