Modeling the Effects of Ion Association on Alternating Current Impedance of Solid Polymer Electrolytes

This work presents a rigorous continuum model describing the transport of ions and associated ion pairs in solid polymer electrolytes subjected to small amplitude alternating current ~ac! excitation. The model treats ion association as a reversible reaction among ions and ion pairs. Dimensionless governing equations are developed from component mass balances, flux equations based on dilute solution theory, and the Poisson equation. Assuming reversible electrode reactions and electroneutrality, the model equations have an analytical solution. Further simplifications are possible in limiting cases ~weak and strong association, zero and infinite frequency excitation!, giving expressions consistent with previously published models. We use the model to explore the effect of association/dissociation reaction rates, ion pair diffusivity, and fractional dissociation on ac impedance behavior. We present a scheme for establishing component diffusivities and fractional dissociation from independent experimental data for lithium perchlorate in poly~ethylene oxide!. With no additional adjusted parameters, satisfactory agreement exists between calculated and experimental ac impedance data. © 2002 The Electrochemical Society. @DOI: 10.1149/1.1480018# All rights reserved.