Diffusive Flux as a New Metric for Ion-Conducting Soft Materials

7 Li NMR diffusion measurements for single-ion conducting ionomers with strong solvation for lithium reveal that diffusion is considerably faster than expected from ionic conductivity measurements, suggesting that neutral ion pairs dominate lithium transport in this class of materials. Ion aggregation is controlled by the overlap parameter for ion pair polarizability volumes; at ion contents higher than the polarizability volume overlap threshold, ion pairs aggregate strongly, making the dielectric constant saturate and the glass transition temperature increase more rapidly with ion content. Neutral ion pairs hop from one aggregate to a neighboring one by polymer segmental motion. Only when the ion pair has an extra lithium (i.e., a positive triple ion) do such hops contribute to ionic conductivity.