Li-ion conducting polymer gel electrolyte membranes (PGEMs) based on polymer poly(vinylidene fluoride-co-hexafluoropropylene) PVdF-HFP, ionic liquid, 1-butyl-3-methylimidazolium tetrafluroborate BMIMBF₄ and Lithium bis(trifluoromethanesulfonyl)imide) LiTFSI salt (having different anion i.e. BF₄^- and TFSI^-)  have been synthesized and  characterized  by  scanning  electron microscopy  (SEM),  X-ray diffraction  (XRD), Differential  scanning  calorimetry  (DSC), Thermogravimetric  analysis  (TGA),  Fourier  transform infrared  (FTIR),  complex impedance spectroscopy and  cyclic voltammetry.  After incorporation of IL in polymer electrolyte, (i.e. PVdF-HFP+20wt.% LiTFSI) different physicochemical properties such as melting temperature (T_m), glass transition temperature (T_g), thermal stability, degree of crystallinity (X_c), and ionic transport behaviour of these materials are found to change. The pristine PVdF-HFP possessed a semi-crystalline structure and its amorphicity increased with the addition of LiTFSI salt and BMIMBF₄ ionic liquid. An impedance measurement indicated that the room-temperature ionic conductivity of the membranes increased with increasing IL content and attains a maximum value of 3.2 *10^-3 S/cm at 30 °C and ~ 8.7*10^-3 S/cm at 130 °C. An schematic representation exhibiting increase  in ionic conductivity with increasing     amount of dopant ionic liquid has been shown in the given figure. In addition, temperature dependent ionic conductivity seems to obey Arrhenius type thermally activated behaviour. A high total ionic transference number > 0.99 and the cationic transference number (t_Li⁺) ~0.33 with a wider electrochemical window (ECW) ~4.0–5.0 V for the PGEMs containing higher loading of IL have been obtained.

Fig. Schematic representation exhibiting increase  in ionic conductivity with increasing     amount of dopant Ionic Liquid.