(382i) Coarse-Grained Simulations of Polymer Electrolytes: Ion Correlations and Transference Number

Salt-doped polymers have potential as safer, nonflammable electrolytes for batteries, but improvements to their ion conductivity are needed. Using bulky anions to decrease the strength of Coulomb interactions at contact 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). We study these effects using coarse-grained molecular dynamics simulations which include a 1/r⁴ potential to capture size-dependent ion-monomer and ion-ion solvation effects. 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, at low dielectric constant, size asymmetry between ions (at fixed cation-anion contact distance) significantly improves cation conduction due to the mitigation of ion aggregation and correlated cation-anion motion. On the other hand, at high dielectric constant, size asymmetry slows cation mobility as t₊ reduces due to strong preferential solvation. 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.