(561e) Coarse-Grained Modeling of Ion Transport in Salt-Doped and Single-Ion Block Copolymers

Microphase separating copolymers are attractive electrolyte materials because one microphase can solvate and transport ions while another is mechanically strong enough to prevent lithium dendrite growth. However, the relatively low ion conductivity in these materials remains a challenge. We apply generic coarse-grained models to efficiently consider a wide range of systems and analyze their ion transport as a function of polymer architecture, dielectric properties, and cation and anion size. Specifically, we use bead-spring chains which also interact with ions via a potential of form r^-4 to capture size-dependent ion solvation effects as a function of dielectric strength, and we include long-range Coulomb interactions between ions. This model captures the experimentally observed trends in lamellar domain spacing and ion conductivity versus ion concentration. Here, we first compare possible strategies to improve cation conduction in salt-doped systems, such as using larger anions or a higher dielectric strength polymer (which more strongly solvates ions). While these changes can reduce ion agglomeration and correlated cation/anion motion, strong ion-polymer interactions can concomitantly slow ion motion. Furthermore, when the polymer more strongly solvates smaller cations versus larger anions, this can lower the transference number (fraction of the conductivity contributed by the cation). We also study single-ion conducting systems in which the transference number goes to unity, and compare these to analogous salt-doped systems and systems with only some of their anions tethered to the chains. Tethering anions has only a modest effect on cation conduction in many cases, but can help improve cation conductivity in situations where local ion agglomeration is an issue (lower polymer dielectric constant or low ion concentration).

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0014209.

This material is based upon work supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under Award DE-SC0014209.