Evaluation of a 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid-Based Electrolyte and Its Performance in an Electrochemical Double-Layer Capacitor

An ionic liquid (IL)-based polymer electrolyte (PE) was prepared with the polymer polyvinylidene fluoride-co-hexafluoropropylene (PVdF-co-HFP), using 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (1B3MITFSI) and zinc trifluaromethenesulfonate (Zn(CF3SO3)2-ZnTf) as the IL and the salt, respectively. The composition 23.6% PVdF-co-HFP: 52.9% ZnTf: 23.6% 1B3MITFSI showed the highest room temperature ionic conductivity of 9.9×10−3 S cm−1 with good mechanical properties. The electrolyte was purely an ionic conductor with dominant contribution from cations. The electrochemical stability window of the electrolyte was found to be from 0.1 V to 2.25 V. An electrochemical double-layer capacitor (EDLC) with the configuration polyvinylidene fluoride (PVdF):natural graphite (NG)/1B3MITFSI-based PE/PVdF:NG was fabricated successfully. Impedance results showed that the single electrode specific capacitance (Csc) of the EDLC was 4.5 F g−1 and the relaxation time constant was 10.7 s. Continuous cyclic voltammetry was done within the potential window of 0.1 V to 2.0 V at a scan rate of 10 mV s−1. The initial Csc was 26.5 F g−1 and its retention was above 65.5% over 500 cycles. The single electrode-specific discharge capacitance (Csd) at the first cycle was found to be 5.0 F g−1 through galvanostatic charge discharge test. It remained steady at 4.0 F g−1 over 10,000 charge-discharge cycles. Results confirm that the EDLC has a good stability and its degradation is quite low with the efficiency above 70%. This further proves the viability of the system as a promising energy storage device.