Radiotracer Diffusion and Ionic Conduction in a PEO-NaI Polymer Electrolyte

Abstract

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 22Na and 125I were measured for temperatures between 67 and 180 degrees C 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.

Understanding Correlation Effects for Ion Conduction in Polymer Electrolytes

“…and collective diffusivity (from dc conductivity experiment) in PEO/NaI with EO:Na + =30:1 from Ref. [16]. Note that in Ref.…”

“…Though the ion-pairs contribute to mass transport, they do not participate in charge transport due to charge neutrality, thereby, resulting in a diminished ionic conductivity. Their model predicts that, over a broad temperature range, the ion-pairs are orders of magnitude faster than either of the free ionic species [16]. This led them to interpret that the ion-pairs are not coordinated with the ether oxygen atoms.…”

“…This attempt also brings out the necessity of additional information apart from merely the tracer diffusivities and ionic conductivity to quantify the microscopic dynamics. Fourth, we apply the extended SO-model to the experimental data, reported in [16]. It turns out that the data can be explained without invoking significant dynamical differences between the free ions and ion-pairs.…”

“…Recently Stolwijk and Obeidi proposed a model (SO-model) that aims to explain the depression of ionic conductivity relative to the average ionic diffusivity in a representative electrolyte system of PEO and sodium iodide (NaI) [16]. This model is an attempt to determine the mobilities of free ions and ion-pairs from the experimental information comprising of the tracer diffusivities of the ions and the ionic conductivity of the polymer electrolyte system.…”

“…(1a) expresses the Nernst-Einstein equation. D σ can be found from conductivity experiments [16,17,18,19] whereas D s can be obtained from tracer diffusion measurements [16,17] or PFG-NMR [20,21]. Without ionic correlations the Haven ratio H is unity, yielding D σ = D avg .…”