Molecular dynamics simulation study of LiI-doped diglyme and poly(ethylene oxide) solutions

Molecular dynamics simulations have been performed on solutions of diglyme/LiI at 363 and 450 K for compositions of ether oxygen:li (EO:Li) = 15:1 and 5:1 and have been compared with the results of the previous simulations of 12-repeat-unit PEO/LiI. In agreement with experiments on similar systems, the number of free ions was found to be smaller and the degree of ion aggregation greater in the diglyme/LiI solutions compared to PEO/LiI. The number of EOs coordinating each Li+ cation was found to be significantly fewer in the diglyme solutions, due to the lower solution density and short chain length, leading to the observed increase in ion aggregation. In contrast to PEO/LiI solutions, the number of free ions, ion pairs, and higher aggregates was nearly independent of temperature in the diglyme/LiI solutions for the temperature range investigated. Dynamically, the anion and cation self-diffusion coefficients were found to be greater in diglyme/LiI when compared to PEO/LiI for EO:LI = 15.1. However, the collective charge-diffusion coefficient and the conductivity were comparable or even lower in the diglyme/LiI solutions, reflecting the much greater extent of ion aggregation in these solutions. For EO:Li = 5:1, both the ion self-diffusion coefficients and the conductivity were greater for the diglyme/LiI solutions. The EO-Li+ bond lifetime was shown to correlate well with the torsional autocorrelation time for complexed -O-C-C-O- dihedrals. An average lifetime of an EO-Lif bond was estimated to be of the order 0.1 ns in diglyme/LiI solutions at 450 K, while the average diglyme molecule-Li+ bond lifetime was on the order of tens of nanoseconds, reflecting a slow rate of intermolecular cation hopping.