Molecular dynamics simulations of poly(ethylene oxide)/LiI melts. 1. Structural and conformational properties

Molecular dynamics (MD) simulations of PEO/LiI melts at 363 and 450 K for the ether oxygen (EO)/LiI compositions 48:1, 15:1, and 5:1 have been performed. An explicit atom quantum-chemistry-based force field developed in our previous work was used. The Li+ cation environment from our MD simulations was found to be in good agreement with neutron diffraction isotopic substitution (NDIS) experiments performed on the EO/Li = 5:1 system. Cation complexation was found to be strongly temperature dependent. The Li+ cations have a tendency to be coordinated by six or more contiguous EO from a single chain at the lower temperature (363 K), but as temperature increases, coordination by shorter contiguous ether oxygen atom sequences becomes increasingly probable. The number of "free" ions was found to decrease with increasing temperature, consistent with results from Raman spectroscopy experiments on similar polymer electrolyte systems. The strong temperature dependence of the cation environment indicates the importance of entropic factors in cation complexation. In contrast to the strong temperature dependence, simulations revealed the local Li+ environment at 450 K to be nearly independent of salt concentration for systems more dilute than 15:1. In this dilute regime, the system forms salt-rich and pure PEG-like domains of size comparable to that of half of the simulation box. The Li+ cations strongly perturb the conformations of PEO compared to those of the pure melt but only locally. It was found that addition of salt leads to a decrease of the radius of gyration of PEO chains. This effect is more pronounced at lower temperature. At higher salt concentration, the Li+ cation environment becomes composition dependent, and the local heterogeneities begin to disappear.