Force Field Development and MD Simulations of Poly(ethylene oxide)/LiBF4 Polymer Electrolytes

Quantum chemistry-based force fields with many-body polarizable interactions and two-body effective polarizability parameters have been developed for the interaction of poly(ethylene oxide) (PEO) with Li+ and BF4-. The Li+/ether repulsion parameters were found to be transferable to another polyether, such as poly(methylene oxide), that is interacting with a Li+ cation. Molecular dynamics (MD) simulations have been performed for PEO (Mw = 2380)/LiBF4 for EO:Li = 15:1 at three temperatures: 363, 393, and 423 K. The Li+ environment was found to be in reasonable agreement with that measured for other lithium salts that have been doped in PEO. MD simulations employing the many-body (MB) polarizable force field predicted ion conductivity, self-diffusion coefficients, and the slowing of the PEO dynamics upon the addition of LiBF4 salt that were in good agreement with experiments. MD simulations employing the two-body (TB) force field yielded polymer and ion dynamics that were slower than those from the simulations employing the MB force field. Analysis of the Li+ cation diffusion mechanism revealed that the Li+ cations with significant motion along PEO chains have a much higher self-diffusion coefficient than do the Li+ cations that do not undergo a noticeable motion along PEO chains, which suggests that the Li+ motion along PEO makes an important contribution to the cation diffusion mechanism.