An intrinsically non-flammable organic electrolyte for wide temperature range supercapacitors

Non-aqueous supercapacitors have received considerable attention owing to their durability, high energy and power densities. However, the traditional organic electrolytes used are a combination of ionic liquids (ILs) and highly flammable organic solvents, causing severe safety concerns due to the potential catastrophic fires or explosions of the electrolytes. Herein, we report a non-flammable triethyl phosphate (TEP)-based binary electrolyte. The interaction between TEP and 1-butyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) was investigated by the Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and Nuclear Magnetic Resonance (NMR). The result demonstrated that the hydrogen bonding in EMIMBF4 ion pairs were effectively broken/weakened by TEP, endowing improved ionic conductivity and reduced viscosity. By coupling carbon nanosheet electrodes, the symmetric supercapacitor fabricated with the binary electrolyte can operate well in a wide temperature range (from −20 °C to 80 °C), delivering a highest energy and power density of 52.1 Wh kg−1 and 19.7 kW kg−1, an outstanding capacitance retention of 86.5 % after 10,000 cycles and a self-discharge as low as 44 %. This work opens up new frontiers to develop safe and superior supercapacitors at wide temperature range.