Ion Correlations and Transference Number in Model Polymer Electrolytes: Effects of Ion Size and Dielectric Strength

Salt-doped polymers have potential as safe electrolytes for batteries but suffer from low ion conductivity. Using bulky anions with delocalized charge may reduce ion agglomeration and increase conduction. However, size asymmetry between ions may increase preferential solvation of cations versus the larger anions, lowering the transference number t₊ (fraction of conductivity contributed by the cation). Here, we use coarse-grained molecular dynamics simulations, including a 1/r⁴ potential to capture size-dependent solvation effects, to relate polymer and ion chemistry to t₊ and overall conductivity. We calculate conductivity from ion mobilities in an external electric field, which improves accuracy versus the typical use of fluctuation dissipation relationships. We find that there is a discrepancy in t₊ estimated from ion diffusion constants and t₊ calculated from ion mobilities, especially at large ion size asymmetry or when ion-polymer interactions are strong. By understanding the impact of ion size, polarizability, and polymer dielectric strength on ion correlations, diffusion, and transference number, we aim to help guide design of future materials with improved conduction.