Preparation and performance of gel polymer electrolytes doped with ionic liquids and surface-modified inorganic fillers

Ionic liquids (ILs) represent a fascinating, and yet not fully understood, medium for a variety of chemical, physical and biological processes. In the present work, lithium conducting electrolytes based on 1-cyanomethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([CCNIm+][TFSI−]) and 1-cyanopropyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C3CNIm+][TFSI−]) with lithiumbis(trifluoromethysulfonyl)imide (LiTFSI) were investigated. An IL gel electrolyte membrane was obtained by immersing a composite PVdF-HFP membrane in the IL electrolyte. PVdF/HFP copolymer-based gel polymer electrolytes containing Al2O3 and TiO2 fillers (raw and surface-modified with acid groups) were synthesized and studied. The highest values of ionic conductivity and lithium ion transference numbers were observed for membranes based on pristine copolymer and doped with TiO2 in 0.1 or 0.5 M LiTFSI in [C3CNIm+][TFSI−]. The ionic conductivity was equal to 3 × 10−4 S cm−1, and t+ = 0.3. The addition of Al2O3 led to a threefold decrease in the electrolyte/electrode interface resistance, which stabilized at 1500 Ω. Thermal analysis confirmed the stability of the membranes up to 200 °C and a twofold decrease in the polymer's degree of crystallinity after the addition of inorganic fillers.