Foreign-ion and self-ion diffusion in a crosslinked salt-in-polyether electrolyte

Abstract

We present an extensive study of ionic transport in PolyG₃₀LiPF₆, which is a crosslinked poly(ethylene oxide)-poly(propylene oxide(PEO-PPO) random copolymer complexed with LiPF₆ to an oxygen-to-cation ratio of 30 : 1. Self-diffusion coefficients of the constituent ions were measured by pulsed field gradient nuclear magnetic resonance (PFG-NMR) as a function of temperature using the signals of ⁷Li and ¹⁹F. These data were compared with the charge diffusivity as derived with the Nernst–Einstein equation from the ion conductivity obtained by impedance spectroscopy. In addition, the diffusion behaviour of a foreign cation (sodium) and that of a foreign anion (iodine) in PolyG₃₀LiPF₆ were investigated by means of the radiotracers ²²Na and ¹²⁵I. All different types of diffusivities were evaluated in a comprehensive ion transport model which allows for the occurrence of charged single ions and neutral ion pairs. Simultaneous fitting of all data within this model yields best values of the model parameters which include Vogel–Tammann–Fulcher parameters and enthalpies/entropies of pair formation. Two distinct variants of the same general model reproduce the experimental data equally well, i.e., with closely similar results for the pair contribution to the migration of each ionic species. In the first variant, this pair contribution is due to a small fraction of ion pairs with a high mobility. By contrast, the second variant results in a very large fraction of pairs characterized by a relatively low mobility. The assumptions and implications connected with both model variants are discussed in detail.