A new polymeric ionic liquid (PIL), poly(1-vinyl-3-methylimidazolium) bis(fluorosulfonyl)imide (PVyMIM-FSI), has been prepared by polymerization of the monomer of ionic liquids, 1-vinyl-3-methylimidazolium bis(fluorosulfonyl)imide (VyM1M-FS1). Its structure and composition have been characterized by H-1 and F-19 NMR, and viscosity-average molecular weight (M-v). For comparison, the corresponding PIL, poly(1-viny1-3methylimidazolium) bis(trifluoromethanesulfonyl)imide (PVyM1M-TFS1), is also prepared and characterized. The physicochemical properties of the binary composite electrolytes of lithium bis(fluorosulfonyl)imide (LiFSI)/PVyMIM-FSI with various concentrations of LiFSI have been comparatively studied with those of the corresponding lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)/PVyMIM-TFSI system, in terms of DSC, TGA, ionic conductivity, and lithium ion transference number (a). Both the FSI-- and TFSI--based Pas are thermally stable up to 250 C. While both types of the prepared PILs show a significant decrease in glass transition temperature (TO upon addition of lithium salt, the values of Tg for the composite electrolytes of LiFSI/PVyMIM-FSI are all significantly lower by 20-60 C in magnitude than those of the corresponding LiTFSI/PVyMIM-TFSI composite ones, indicating much better plasticizing effect for FSI- vs. TFSI-. The ionic conductivities of LiFSI/PVyMIM-FSI are all higher by about 2 orders in magnitude than those of the corresponding LiTFSI/PVyMIM-TFSI with the same concentration of lithium ions. (C) 2014 Elsevier B.V. All rights reserved.