Poly(vinylidene fluoride-co-hexafluoropropylene) based in-situ silica composite polymer gel electrolyte for wide-temperature-range supercapacitor applications

Solid-state electrochemical devices with high energy density and enhanced safety were in growing demand for sustainable energy storage. Electrolytes played a vital role in device safety and stable operating voltage. Here, a flexible, free-standing poly(vinylidene fluoride-hexafluoropropylene) (PVdF-co-HFP)-based composite polymer electrolyte (CPE) operating over a wide potential window (2.5 V) was fabricated via phase inversion. The effects of ceramic nanofillers such as silica (SiO₂), bismuth oxide (Bi₂O₃), and zinc oxide (ZnO) on crystallinity, morphology, and electrochemical behavior were investigated. Among them, the in-situ silica-incorporated CPE (CPE-S) exhibited superior performance, showing high porosity (85%), ionic conductivity (0.520 mS cm⁻¹), electrolyte uptake (303%), and retention (0.81). A flexible supercapacitor assembled using CPE-S and graphite-coated paper delivered a specific capacitance of 5.13 F g⁻¹ and an energy density of 4.67 Wh kg⁻¹ at 2 mV s⁻¹, retaining 72% capacitance and ∼99% Coulombic efficiency over 10,000 cycles. The device demonstrated excellent thermal stability at 10, 30, and 80 °C and mechanical flexibility, maintaining comparable capacitance before and after bending (12.36 and 12.84 mF g⁻¹). These results confirmed the suitability of PVdF-co-HFP-based CPEs for high-temperature and flexible electric double-layer capacitor applications in extreme operating environments.