Sodium-Ion Conducting Gel Polymer Electrolytes for Quasi-Solid-State Supercapacitors

Poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) as a host polymer, 1-ethyl-3-methylimadizilium tetrafluoroborate (EMIMBF4) as an ionic liquid, succinonitrile (SN) plastic crystal as a plasticizer, and sodium tetrafluoroborate (NaBF4) as a salt are used to synthesize highly conducting gel polymer electrolytes (GPEs). The GPEs have been synthesized by the standard solution cast method and applied as a self-separator in quasi-solid-state supercapacitors. The AC conductivity spectra at different temperatures are analyzed using the universal power law model to obtain the dc conductivity and power law exponent. The inverse temperature dependence of the dc conductivity follows the Arrhenius behavior for all compositions. A significant enhancement of the dc conductivity (8.72 mS cm–1) at 30 °C of the GPE containing NaBF4 salt is achieved compared to that (0.17 mS cm–1) of the electrolytes without NaBF4 salt. Supercapacitors have been designed using activated carbon (AC) as the electrode and GPEs of different compositions as electrolytes. The electrochemical impedance of the supercapacitors has been analyzed using an equivalent circuit model. The supercapacitor fabricated with the gel polymer electrolyte containing NaBF4 salt exhibits a higher specific capacitance (Csp) of ∼257.0 F g–1 and an energy density (Esp) of ∼9.0 W h kg–1 than those (specific capacitance ∼135.0 F g–1 and energy density ∼4.5 W h kg–1) of the GPE without salt. The supercapacitor fabricated with sodium salt-added GPEs also exhibits higher cycling stability and capacity retention ability (∼79%) after 8000 GCD cycles than those (∼64%) of the supercapacitor fabricated without NaBF4 salt.