Ionic interactions and transport in a low-molecular-weight model polymer electrolyte

Abstract

AC impedance, FT-Raman and pulsed field gradient (pfg) NMR measurements have been conducted on solutions of poly(ethylene oxide) dimethyl ether (MW 400) complexed with LiCF3SO3 as a function of temperature and salt concentration. From an analysis of the νS(SO3) and δS(CF3) vibrational band envelopes of the CF3SO3 anion, respectively, the relative concentrations of anions in various chemical environments have been calculated. We find spectroscopic evidence for a redissociation of associated ionic species into spectroscopically “free” anions with increasing salt concentration in dilute solutions. The relative abundance of associated ionic species increases with increasing temperature. Pfg-NMR measurements show that D−(19F) and D+(7Li) are very similar for all concentrations (i.e., O:Li⩾53:1) and temperatures (25–80 °C) investigated. Most notably, the diffusivity of the oligomer solvent, D(1H), is significantly faster than the self-diffusion coefficients of the dissolved ions in all cases. Predicted values for the ionic conductivity were obtained from the NMR diffusivities, using the Nernst–Einstein relation, and compared with those from direct measurement. We find that the calculated values are higher for all concentrations; the discrepancy increases with decreasing salt concentration and increasing temperature. A good correlation is found between the concentration dependence of the ionic redissociation pattern, as determined from the νS(SO3) Raman band envelope, and an increase in equivalent ionic conductivity with increasing salt concentration in dilute solutions (i.e., O:Li⩾110:1). We suggest that fluctuating, salt-rich heterogeneities of dissolved ions and polymer segments form at low salt concentrations, and that this may be a general behavior of dilute polymer–salt complexes.