Mechanism of Ion Transport in Amorphous Poly(ethylene oxide)/LiTFSI from Molecular Dynamics Simulations

The mechanisms of lithium cation (Li+) and bis(trifluoromethane)sulfonamide anion (TFSI-) transport in poly(ethylene oxide) (PEO, Mw = 2380) melts were examined using molecular dynamics (MD) simulations over a wide range of salt concentrations and temperatures. MD simulations using a quantum-chemistry-based many-body polarizable force field yielded ion self-diffusion coefficients, electrolyte conductivity, ion aggregation, and the coordination environment of Li+ in good agreement with experiment. Lithium transport was found to arise from a combination of the subdiffusive Li+ motion along PEO chains, motion together with PEO segments and intersegmental Li+ hops from one PEO segment to another. The rate of intersegmental hops was found to correlate well with times at which Li+ motion crosses over from subdiffusive to diffusive behavior. The contribution of Li+ motion along PEO chains to the total Li+ transport was found to be approximately equal to the contribution from Li+ moving together with PEO segments. Diffusion of both Li+ and TFSI- was found to be strongly coupled to PEO ether oxygen atom displacements and PEO conformational dynamics.