A model for diffusion and ionic conduction in polymer electrolytes

Abstract

We present a model which describes tracer diffusion and ion conduction in electrolytes consisting of a polymer matrix and a solvated inorganic salt. In particular, the model is capable of reproducing the experimentally observed discrepancy between the sum of the cation and anion tracer diffusivity, on one hand, and the overall charge diffusivity as deduced from the ionic conductivity with the aid of the Nernst-Einstein equation, on the other. A crucial feature of the model is the increasing formation of highly mobile, neutral ion pairs with increasing temperature. Another key property is that the motion of each individual species is dictated by the segmental motion of the polymer chains, which manifests itself by a uniform Vogel-Tamann-Fulcher-type temperature dependence. By contrast, the magnitude of the diffusivity reflects the dissimilar degree of coupling of cations, anions and ion pairs to the polymer host.