Simulation of Ion Transport through Percolated Aggregates in Precise Sulfophenylated Polyethylene Ionomers

Ionomers present an attractive potential alternative to standard electrolytes in Li ion batteries due to their capacity to function as single-ion conductors. When the spacing between adjacent pendant groups is precisely controlled, ionic groups within ionomers have been shown to self-assemble to form aggregates with well-ordered morphologies. The structure of these aggregates can have a significant impact upon ion conductivity. In order to gain greater understanding at the microscopic level, we perform fully atomistic molecular simulations to probe the assembled ionic aggregates of sulfophenylated polyethylene ionomers with a spacing of five carbon atoms between pendant groups (p5PhS), neutralized with a series of cations (Li, Na, and Cs). We find that the ionic aggregates form percolating clusters provided the polymers are neutralized with a sufficiently high fraction of cations. Li and Na aggregates take ribbon-like configurations, while Cs aggregates are more disordered. In each case the ions are able to slowly diffuse from sulfonate group to sulfonate group through the percolated aggregates, with larger ions exhibiting larger mean-squared displacements. Finally, structure factors computed from simulation agree reasonably well with experimental X-ray scattering data.