Semi-Generic Coarse-Grained Modeling of Ion Transport in Solid Polymer Electrolytes

Solid polymer electrolytes are of interest for safer, nonflammable batteries. Polyethylene oxide (PEO) doped with various lithium salts has been extensively studied, though it has a relatively low modulus and its propensity to crystallize reduces its ion conductivity. Many other systems have been considered, including with different polymer and ion chemical structures and different polymer architectures, such as side chain PEO-based systems (in which crystallinity is disrupted) or diblock systems that microphase segregate to create regions of high modulus that can prevent lithium dendrite growth. Given this large macromolecular design space, we aim to use simple coarse-grained molecular dynamics simulations to identify what are the most important molecular features of these systems and how they act together to control ion conductivity. Specifically, we build on bead-spring models of polymers, but set different bead properties for different polymer types, adjusting their glass transition temperatures by scaling like-like interactions and applyling stiff angles set to match the Kuhn density. We include additional ion solvation potentials with settings based on the component polymer dielectric strengths, overall creating a semi-generic coarse-grained model that uses simplified potentials parameterized mostly with literature values for pure component properties. While still being simple, this allows us to map more specifically to experimental polymers and copolymers of interest and to better show the effects of different chain architectures. In one example, we find that such a model reproduces the experimentally observed effects of increasing side chain length on ion conductivity in a system with a poly(methyl methacrylate) backbone and PEO side chains (with most parameters set from these two polymers' properties). These effects were seen to depend on setting backbone-side chain angle potentials to appropriately couple the dynamics, providing insight into their origin.