Molecular dynamics simulation study of LiI-doped diglyme and poly(ethyleneoxide) 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 aggre gation 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 l ength, leading to the observed increase in ion aggregation. In contrast to PEO/LiI solutions, the number of free ions, ion pairs, and higher aggregate s was nearly independent of temperature in the diglyme/LiI solutions for th e temperature range investigated. Dynamically, the anion and cation self-di ffusion coefficients were found to be greater in diglyme/LiI when compared to PEO/LiI for EO:LI = 15.1. However, the collective charge-diffusion coeff icient and the conductivity were comparable or even lower in the diglyme/Li I solutions, reflecting the much greater extent of ion aggregation in these solutions. For EO:Li = 5:1, both the ion self-diffusion coefficients and t he conductivity were greater for the diglyme/LiI solutions. The EO-Li+ bond lifetime was shown to correlate well with the torsional autocorrelation ti me 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 t ens of nanoseconds, reflecting a slow rate of intermolecular cation hopping .