High voltage supercapacitor based on nonflammable high-concentration-ionic liquid electrolyte

Abstract Supercapacitors (SCs) based on ionic liquids (ILs) often have wide potential windows due to the stable electrochemical properties of ILs, but their poor rate capabilities lead to low energy densities in high power output. Herein, we added small amounts of acetonitrile (ACN) to optimize the physical properties of 1-Ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4). The excellent performance of SC was obtained by assembling symmetric SC with the high-concentration-IL (HCI) electrolyte. Organic solvent has improved the conductivity and viscosity, following good rate capability and cycling performance at high current density. In addition, the stable electrochemical property of IL is guaranteed, its operating potential window can reach as high as 3.2 V, leading to the energy density of 40.29 Wh kg−1 (at 1.6 kW kg−1). The energy density has increased 214.25 %–228.48 % than generally accepted 1 M EmimBF4/ACN electrolyte and 137 %-140 % than commercial electrolyte, while the rate capability is still excellent. Therefore, our electrolyte is suitable for high power output. More important, the prepared electrolyte is nonflammable, which greatly increases the safty of SCs in practical applications.