Imidazolium-based ionic liquid assisted poly(vinyl) alcohol-based solid polymer electrolyte for high energy sodium-ion electric double layer capacitors

High-performance solid-state electrolytes are essential for next-generation safe and efficient energy storage systems. In this study, a novel solid polymer electrolyte (SPE) was developed using polyvinyl alcohol (PVA) as the host polymer, sodium hexafluorophosphate (NaPF 6 ) as the sodium salt, and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF 4 ) as the ionic liquid for electric double-layer capacitor (EDLC) applications. The addition of EMIMBF 4 reduced PVA crystallinity from 27 % to 19 %, enhancing the amorphous phase and ion transport. The optimized composition containing 33.33 wt% (0.5 g) EMIMBF 4 achieved the highest ionic conductivity of 2.37 × 10 −4 S/cm at room temperature. The corresponding EDLC delivered a specific capacitance of 31.7 F/g at 0.1 mA/cm 2 over 1000 cycles, with an energy density of 17.6 Wh/kg and a power density of 829.1 W/kg. These results demonstrate the strong potential of the PVA–NaPF 6 –EMIMBF 4 system for high-performance, solid-state supercapacitors.