Composite electrolytes of lithium salt/polymeric ionic liquid with bis(fluorosulfonyl)imide

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 (VyMIM-FSI). Its structure and composition have been characterized by 1H and 19F NMR, and viscosity-average molecular weight (Mv). For comparison, the corresponding PIL, poly(1-vinyl-3-methylimidazolium) bis(trifluoromethanesulfonyl)imide (PVyMIM-TFSI), 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 (tLi+). Both the FSI−- and TFSI−-based PILs are thermally stable up to 250 °C. While both types of the prepared PILs show a significant decrease in glass transition temperature (Tg) 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.