Ion size controls the conductivity of solid polymer electrolytes

Solid polymer electrolytes (SPEs) are ion-containing solid materials composed of a polymer matrix that enables ionic transport while maintaining mechanical stability. The correlations among ion clustering, microstructure, and conductivity of SPEs are of significant interest for the development of more efficient energy storage materials. In pursuit of this objective, here, we assess and elucidate the effects of cation-anion and ion-monomer size ratios on the conductivity of a model SPE. We show that high ion-monomer and cation-anion size asymmetries promote better mixing of the ions with the polymer matrix. Under these conditions, the ion-dipole moment interaction dominates over the ion-ion interaction and improves the ion dispersion in the polymer matrix. At the same time, these interactions are not sufficiently strong to induce persistent ion pair localization, thereby facilitating faster ion transport. These two size ratios are thus key to tailoring the properties of SPEs with immediate relevance to the development of SPEs for energy storage devices.