Mixture of non-ionic and organic ionic plastic crystals immobilized in poly(vinylidene fluoride-co-hexafluoropropylene): A flexible gel polymer electrolyte composition for high performance carbon supercapacitors

Flexible and quasi-solid-state gel polymer electrolytes (GPEs) comprising plastic crystals are emerged as new class of materials for the energy storage applications. Here, a novel composition of a flexible matrix of GPEs comprising a mixture of a non-ionic plastic crystal (succinonitrile) and an organic-ionic plastic crystal 1-ethyl-1-methyl-pyrrolidinium bis(trifluoromethyl sulfonyl)imide, immobilized in a polymer poly(vinylidine fluoride-co-hexafluoropropylene) for application in carbon supercapacitors. Different structural, thermal and electrochemical properties including high ionic conductivity (σ ~3.3 × 10⁻³ S cm⁻¹ at room temperature) and a wide electrochemical stability window (~3.1 V versus Ag) make the GPE films attractive as potential electrolyte for supercapacitors. Effect of addition of Li-salt (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) in GPE is also studied and found improvement in supercapacitors' performance. Supercapacitors are fabricated using the mixed plastic crystals-based GPE films separated by activated carbon electrodes, optimized via different techniques namely cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge-discharge measurements. The optimized capacitor with GPE containing LiTFSI offers the specific capacitance of ~370.7 F g⁻¹ with specific energy and maximum power density of ~80.5 Wh kg⁻¹ and ~70.6 kW kg⁻¹, respectively. The supercapacitor offers stable cyclic performance for ~10,000 charge-discharge cycles with ~93% of retention and safe performance over a temperature range from −40 to 80 °C.