Poly(vinylidene fluoride-co-hexafluoropropylene) membranes filled with deep eutectic solvent as non-flammable and flexible quasi-solid state electrolytes for high-voltage supercapacitors

This work outlines a method for developing green quasi-solid-state electrolytes using poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) membranes filled with deep eutectic solvent (DES) for application in high-voltage flexible supercapacitors (SCs). The synthesis is carried out in two steps. Firstly, porous PVDF-HFP matrices are formed by blending PVDF-HFP with polyethylene glycol (PEG), followed by the removal of PEG. In the second stage, a DES is prepared using a molar ratio of 1.8 : 1 : 1 of methylsulfonylmethane, LiClO₄, and H₂O. This DES is then introduced into the porous matrices through soaking. The PVDF-HFP-DES electrolyte, prepared with 15wt% of PEG400, exhibits the most developed specific surface area and pore volume. Additionally, FTIR and Raman spectroscopy confirm the co-existence of α-PVDF-HFP and β-PVDF-HFP phases in the membrane which facilitate the incorporation of both polar and non-polar solvents within its pores. A symmetrical carbon-based SC containing PVDF-HFP-DES electrolyte is evaluated within a voltage range from 0 to 2.2 V. The device demonstrates excellent ionic conductivity, energy density of 45.3 Wh kg^‒1, and remarkable stability over 10 000 consecutive cycles. More importantly, the flammability and electrochemical under bending conditions tests also indicate that the investigated quasi-solid-state electrolytes are safe for high-voltage flexible applications.