Systematics of ionic transport and pair formation in amorphous PEO–NaI polymer electrolytes

Mass and charge transport in the prototype polymer electrolyte system PEOyNaI were investigated by radiotracer diffusion and conductivity measurements over wide ranges of composition (y = 20 to 1000) and temperature (T = ∼ 60 to 200 °C). All data were simultaneously fitted within a comprehensive ion-transport model which takes into account for each composition the glass transition temperature (Tg), the fraction of ion pairs, and the diffusivities of single ions and pairs. This approach yields the pertinent model parameters as a function of salt concentration (Cs), including the Vogel–Tammann–Fulcher parameters of individual species and the enthalpy ( Δ H p ) and entropy ( Δ S p ) of pair formation. We find that the mobility of cations and anions distinctly increase with increasing T and decreasing Cs, whereas the fraction of free ions shows an anomalous reverse T- and Cs-dependence. This special constellation explains the appearance of a maximum in plots of the isothermal charge diffusivity versus salt concentration. Quantitative results for transference numbers, Haven ratios, and relative pair contributions to the tracer diffusion coefficients are presented as well.