Spectroscopic studies of triflate ion association in polymer gel electrolytes and their constituents

Raman and NMR spectroscopy have been used to investigate the state of ion association in systems comprising salt, polymer and/or solvent. The dissolved salt in each case was lithium triflate (lithium trifluoromethane sulphonate). Five systems were studied, comprising polymer gel electrolytes or constituents of such electrolytes. These were: salted (a) N,N'-dimethyl formamide (DMF), (b) tetraethylene glycol dimethylether (tetraglyme), (c) polyethylene glycol (PEG), (d) end-esterified PEG and (e) poly(vinylidene fluoride) (PVDF) gels containing either DMF or tetraglyme. Raman results give a broad indication of the significance of end-group, solvent and polymer choice in triflate-salted systems. In all cases, anion association rises with temperature, often with a significant increase in ion aggregation. The reliability of these results is supported by an analysis of systematic errors incident in this technique for ion association measurements. A limited comparison performed on the system having the highest concentration of ion aggregates (end-esterified PEG) suggests that Raman and NMR spectroscopy provide different information concerning ion association. Reasons for the difference are discussed, concluding that complementary information is obtained owing to the different time constants relevant to the two techniques and to the dependence of the Raman results on molecular proximity.