Mass and charge transport in the PEO-NaI polymer electrolyte system: effects of temperature and salt concentration

Ionic transport in amorphous complexes of poly(ethylene oxide) (PEO) and sodium iodide (NaI) was investigated by means of radiotracer diffusion and electrical conductivity measurements for three different compositions characterised by O-to-Na ratios of 20, 30, and 60. The diffusivity of Na-22 and I-125 as well as the charge diffusivity each show a systematic dependence on both temperature and salt concentration. The experimental data can be described within a transport model based on the occurrence of neutral ion pairs in addition to charged single cations and anions. It is found that both the enthalpy and entropy of ion pair formation decrease with increasing salt concentration. This indicates that the state of coordination between cations and polymer chain segments correlates with the number of available oxygen atoms per cation.