FT-IR investigation of ion association in plasticized solid polymer electrolytes

FT-IR spectroscopy has been utilized to monitor ion association in plasticized solid polymer electrolytes (SPEs). The SPEs were prepared from a random copolymer of ethylene oxide (EO) and propylene oxide (PO) and the salt lithium trifluoromethanesulfonate (lithium triflate, LiTf). Tetraethylene glycol dimethyl ether (tetraglyme) and N,N'-dimethylformamide (DMF) were chosen as model-plasticizers. Despite having a similar dielectric constant to that of the polymer host, epsilon similar to 5, the incorporation of tetraglyme into the SPEs resulted in increased ion association. The addition of a higher dielectric constant solvent, DMF, epsilon = 36.7, resulted in decreased ion association iu the SPE. The effects of salt concentration (0.05-1.25 mol dm(-3)) and temperature (25-100 degrees C) upon ion association in SPEs were also investigated. At low salt concentrations, ion association was found to increase with temperature, however, at 1.25 mol dm(-3) the temperature dependence of ion association was dominated by concentration effects. There appears to be a maximum in the fraction of ''free'' ions at a LiCF3SO3 concentration of 0.4 mol dm(-3), preceded by a minimum at approximately 0.2 mol dm-(3), consistent with the molar conductivity behavior previously observed in these electrolytes.