Publications

Ionic aggregation and counterion dynamics in model ionomers

Hall, Lisa M.; Stevens, Mark J.; Frischknecht, Amalie F.

Ionomers, polymers containing a small fraction of covalently bound ionic groups, have potential as solid, single ion conducting electrolytes in future batteries. However, the ions tend to form aggregates, making counterion diffusion unacceptably slow. A key materials design question is how molecular properties affect ionic aggregation and counterion dynamics. Recent experimental advances have allowed synthesis and extensive characterization of ionomers with a precise, constant spacing of charged groups. Because the molecular architecture is controlled and these materials show increased ionic aggregate ordering versus their randomly spaced analogs, this set of experiments is ideal for direct comparisons with molecular simulations. We perform molecular dynamics simulations of coarse-grained ionomers with either periodically or randomly spaced charged beads. The charged beads are placed either in the polymer backbone (ionenes) or as pendants on the backbone. To understand the range of ionic aggregate morphologies possible in real materials, we vary the spacing of charges along the chain, degree of randomness (from periodic to random block to fully random), and dielectric constant. The well-known "ionomer peak" in the scattering is present in all cases. The peak is significantly more intense for pendant ions with a long periodic spacing of charged beads, which form roughly spherical aggregates. This morphology is in qualitative contrast to the extended aggregates of ionenes that show increased counterion diffusion. Depending on the degree of randomness in spacing of charged beads along the chain, counterion diffusion can increase or decrease versus that of the precisely spaced materials. Possible implications for ionomer electrolyte design will be discussed. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.