Coarse-Grained MD Simulation of String-like Aggregates in Single-Ion Conductors

Single-ion conductors, or ionomers, have been investigated as a potential polymer electrolyte for advanced batteries. Ionic aggregates are prevalent in ionomers, and their sizes and shapes are not well characterized by experiment. Atomistic molecular dynamics simulations have been used to explore these aggregates, but may not be fully equilibrated because the aggregrates break and join infrequently. We report on an ion-only coarse-grained molecular dynamics simulation of a well-equilibrated ionomer system that reproduces structural features of the parent united atom simulation. Results for radius of gyration, shape anisotropy, and average ion coordination number from our simulation show that ionic aggregates are string-like, with random-walk configurations. An analogy to worm-like micelle equilibrium predicts an exponential length distribution for aggregates, in agreement with simulations. The implications of the size and structure of aggregates on conduction are discussed.