PVDF-HFP in electrolytes for supercapacitors

Designing flexible electrochemical double-layer capacitors (EDLCs) that can operate efficiently at high and low temperatures remains a challenge specifi- cally related to packaging and electrolyte. The electrochemical performance of EDLCs is highly temperature dependent, and improved performance at high and low temperatures can be achieved if all components are designed for this purpose. Therefore, all solid-state symmetric EDLCs fabricated with solid-like electrolytes operating at high and low temperatures. A ionogel of poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and N-butyl-N-methyl pyrroli- dinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) ionic liquid was used as the solid electrolyte and simultaneously the separator in these all-solid EDLCs. The synthesized solid ionogel with the highest ion mobility was made with a polymer to IL ratio of 1:4 in weight, which exhibits high ionic conductivity over a wide temperature range, from 0,229 mS/cm at 5°C up to 0,435 mS/cm at 70°C. Inorganic additives were used in the ionogel formulation to increase the conductivity at low temperatures or to increase the IL concentration while maintaining the robustness. Silica nanoparticles of 20-50 nm in diameter and GO flakes of 300-800 nm in length were employed as such additives, demonstrating that the polymer to IL ratio could be increased to 1:5 in weight without compromising the conductivity. These membranes were observed by FESEM to appreciate the homogeneity and the ionic conductivity was measured between 5°C and 70°C, demonstrating that the GO ionogel performed better at high temperatures while the SiO2 one was better at low temperatures. All solid-state EDLCs were then prepared with both modified ionogels and tested to withstand operating temperatures from 5°C to 70°C, under a cell voltage of 2 V with long-term galvanostatic charge-discharge cycling over 10,000 cycles. This approach paves the way for flexible leakage-less devices.