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 1 H and 19 F NMR, and viscosity-average molecular weight ( M v ). 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 ( t Li + ). 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 ( T g ) upon addition of lithium salt, the values of T g 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.