Radiotracer diffusion and ionic conduction in a PEO-NaI polymer electrolyte

We studied ion transport in amorphous PEO30NaI consisting of poly(ethylene oxide) and sodium iodide in a Na-to-O ratio of 30. Diffusion coefficients of the radiotracers Na-22 and I-125 were measured for temperatures between 67 and 180 degreesC and compared with the overall charge diffusivity deduced from dc conductivity data. To explain the observed discrepancy between the sum of the tracer diffusivities and the charge diffusivity we propose a detailed model which is based on the formation of neutral ion pairs. Evaluating simultaneously all experimental data within this model yields not only the true diffusion coefficient of all individual species but also the ion-pairing reaction constant as a function of temperature.