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

Abstract

Ion conducting polymer gel electrolyte membranes based on polymer poly(vinylidene fluoride-co-hexafluoropropylene) 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 × 10⁻³ S cm⁻¹ at 30 °C and ∼3 × 10⁻² S cm⁻¹ at 130 °C. A high total ionic transference number >0.99 and the cationic transference number (t_Li⁺) ∼ 0.22 with a wider electrochemical window (ECW) ∼ 4.0–5.0 V for the polymer gel electrolyte membrane containing higher loading of IL (∼70 wt% of IL) have been obtained. Temperature dependent ionic conductivity obeys Arrhenius type thermally activated behaviour.