High thermal safety and conductivity gel polymer electrolyte composed of ionic liquid [EMIM][BF4] and PVDF-HFP for EDLCs

Gel-polymer electrolytes (GPEs) are of great interest to build high-performing rechargeable electrical double-layer capacitors (EDLCs) owing to the combination of good electrochemical properties and improved safety. Herein, we report a quasi-solid-state GPE for EDLCs from a mixture of the poly (vinylidene fluoride-co-hexafluoro propylene) (PVDF-HFP) and ionic liquid 1-ethyl-3-methylimidazole tetrafluoroborate ([EMIM][BF4]), which avoids the leakage of liquid electrolyte and the low conductivity of common solid electrolyte. The intermolecular forces of GPE was characterized by in-situ infrared (IR) spectroscopy, and the interactions between ionic liquid and polymer were also analyzed. The prepared GPE was observed to be easily procurable, excellent transparency, high thermal safety (thermal stability up to 320 °C) and outstanding electrochemical properties at room temperature (wide electrochemical stability window of 3.91 V versus SS (SS being stainless steel foils) and high ionic conductivity of 6.9 × 10−3 S cm−1). The specific capacitance, the maximum specific energy, and the maximum specific power of the fabricated symmetrical supercapacitor had been determined to be 161.8 F g−1, 89.89 Wh kg−1, and 8 kW kg−1, respectively. Moreover, the EDLC shows extraordinary electrochemical behaviors and cyclic stability over up to 10 000 cycles at 25 °C with 86% capacitance retention under a current load of 2 A g−1. The experimental results indicate the excellent potential of the [EMIM][BF4] ionic-liquid-based gel polymer electrolyte for EDLCs.