(121a) Ion Transport in Block Copolymer Electrolytes: Understanding Ion Correlations from Molecular Dynamics Simulations

Ion-containing block copolymers with both ion conductive and mechanically robust microphases have significant potential in applications such as battery electrolytes. Numerous design parameters such as polymer dielectric strength, architecture, and ion types can be tuned to improve ion conductivity, which is depends on ion diffusion and cation-anion correlations. The diffusion of ions as a function of polymer segmental dynamics, solvation site connectivity, and other factors has been extensively studied. However, the degree of correlation of cation and anion motion is often neglected and is difficult to assess in simulations. To calculate ion correlation and to better understand how dielectric strength and other chemical variables impact conduction, we use coarse-grained molecular dynamics simulations and calculate conductivity from ion mobilities in an external electric field. This method improves accuracy versus the typical use of fluctuation dissipation relationships to calculate conductivity from equilibrium simulations, for a given total simulation time. We first compare conductivity as a function of ion concentration, and find that ion correlation is dependent on salt loading in both the homopolymer and block copolymer, showing its considerable contribution to the difference in conductivity across various systems. We further probe the effects of Coulombic strength, polymer-ion interactions, and polymer architecture. By elucidating how these key factors affect ion conductivity, we hope to suggest design rules to optimize conduction in future materials.