Design Rules for Highly Conductive Polymeric Ionic Liquids from Molecular Dynamics Simulations

Polymeric ionic liquids (PILs) are of considerable interest as next-generation battery materials due to their potential to combine the solid-state stability of polymers with the high ion conductivities of ionic liquids. However, polymerization of ionic liquids to form a polymer generally leads to a suppression in ion transport rates that has proven to be a major barrier to the of commercially viable PIL solid electrolytes. Here we employ a combination of all atom and coarse-grained molecular dynamics simulations to identify strategies by which ion conductivity can be maximized by maximizing both PIL segmental relaxation rates and the extent of ion transport decoupling from chain dynamics. Results indicate that combined ion size correlates well with PIL glass transition temperatures and segmental dynamics but that ion/polymer decoupling is controlled primarily by the size of the free ion. We also find that ion aggregation promotes both reduced glass transition temperatures and enhanced ion/polymer decoupling. These results suggest that PIL ion mobility can be improved by combining ultralarge bound ions with very small free ions and with chemistries that promote ion aggregation.