Ab initio quantum chemistry and molecular dynamics simulations studies of LiPF6/poly(ethylene oxide) interactions

Ab initio and molecular mechanics studies of LiPF6 and the interaction of the salt with the poly(ethylene oxide) (PEO) oligomer dimethylether have been performed. Optimized geometries and energies of Li+/PF6- complexes obtained from quantum chemistry revealed a preference for C-3V symmetry structures for Li+-P separations under 2.8 Angstrom, C-2V symmetry for Li+-P in the range of 2.8-3.3 Angstrom and C-4V symmetry for Li+-P separations larger than 3.3 Angstrom. Electron correlation effects were found to make an insignificant contribution to binding in the Li+/PF6- complex. By contrast, analogous studies of PF6-/PF6- and PF6-/dimethyl ether complexes revealed important contributions of electron correlation to the complex interaction energy. A molecular mechanics force field for simulations of PEO/LiPF6 melts was parameterized to reproduce the geometries and energies of Li+/PF6-, PF6-/PF6-, PF6-/dimethylether complexes. Molecular dynamics simulations of PEO/LiPF6 melts were performed to validate this quantum chemistry-based force field. Accurate reproduction of the increase in solution density with addition of salt was found while the electrical conductivity of PEO/LiPF6 solutions was found to be within an order of magnitude of the experimental values. (C) 2001 John Wiley & Sons, Inc.