Development of ion conducting polymer gel electrolyte membranes based on polymer PVdF-HFP, BMIMTFSI ionic liquid and the Li-salt with improved electrical, thermal and structural properties

Ion conducting polymer gel electrolyte membranes based on polymer poly(vinylidene fluoride-co-hexa-fluoropropylene) PVdF-HFP, ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide BMIMTFSI with and without the Li-salt (having the same anion i.e. the TFSI anion) have been synthesized. Prepared membranes have been characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared (FTIR), differential scanning calorimetry, thermogravimetric analysis (TGA) and complex impedance spectroscopic techniques. Incorporation of IL in the polymer PVdF-HFP/polymer electrolyte (i.e. PVdF-HFP + 20 wt% LiTFSI) changes 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. The ionic conductivity of polymeric gel electrolyte membranes has been found to increase with increasing concentration of IL and attains a maximum value of 2 X 10(-3) S cm(-1) at 30 degrees C and similar to 3 X 10(-2) S cm(-1) at 130 degrees C. A high total ionic transference number 40.99 and the cationic transference number (t(Li+)) similar to 0.22 with a wider electrochemical window (ECW) similar to 4.0-5.0 V for the polymer gel electrolyte membrane containing higher loading of IL (similar to 70 wt% of IL) have been obtained. Temperature dependent ionic conductivity obeys Arrhenius type thermally activated behaviour.