Redox-active ionic liquid electrolyte with multi energy storage mechanism for high energy density supercapacitor

A bimodal redox-active ionic liquid electrolyte for supercapacitors with high energy density was demonstrated. The suggested bimodal electrolyte, which consists of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) and 1-ethyl-3-methylimidazolium halide (EMI-X, X = Br, I) as a redox active couple, shows the three types of energy storage mechanism: a classical EDL capacitance; a pseudo-capacitance from the redox reaction of halide species, such as bromide and iodide; and an EDL capacitance strongly enhanced by ion size effects. When EMITFSI is mixed with small ions, the thickness of the ionic layer becomes thinner and even more ions are packed into the electrode due to the decrement of excluded-volume effects and the increment of electrostatic interactions. The supercapacitor containing a mixture of EMITFSI and EMI-I showed a considerably high performance with 175.6 W h kg^-1 and 4994.5 W kg^-1 at 1 A g^-1 and excellent cycling stability up to 5000 cycles.