Molecular dynamics simulations and spectroscopic studies of amorphous tetraglyme (CH3O(CH2CH2O)(4)CH3) and tetraglyme : LiCF3SO3 structures

Systems of poly(ethylene oxide) with dissolved inorganic salts are used as solid polymer electrolytes in high energy density batteries. Amorphous tetr aglyme [CH3O(CH2CH2O)(4)CH3], a model for amorphous PEO, and tetraglyme:LiC F3SO3 (lithium triflate) with an ether oxygen:li(+) ratio of 10:1 were stud ied by molecular dynamics (MD) simulations at 300 and 400 K. Conformational and structural analyses of Li+ interactions with tetraglyme and triflate i on oxygens are consistent with decreased Li+ coordination by tetraglyme and increased ionic aggregation at the higher temperature. Dihedral angle popu lation density distributions for tetraglyme chains show that the trans conf ormation is favored for C-O bonds while the more compact gauche conformatio n is favored for C-C bonds and is enforced by coordination of adjacent oxyg ens to Li+. Calculated populations of tetraglyme conformational triads indi cate that the most stable conformation around Li+-tetraglyme oxygens is tgt . Mean-square radius of gyration and end-to-end distance of pure tetraglyme chains decrease with increasing temperature and upon Li+-tetraglyme oxygen complexation, but increase at 400 vs 300 1( for tetraglyme:LiCF3SO3. The c alculated Li+ coordination number remains the same with increasing temperat ure, but triflate ions contribute more oxygens to Li+ coordination at 400 K (4.8) than at 300 K (4.6). The MD-derived populations of Li+-CF3SO3- assoc iated species are compared with vibrational spectral data. The increase in populations of [Li2CF3SO3](+) and [Li3CF3SO3](2+) from both MD simulations and IR data implies that Li+-CF3SO3- association is increased at higher tem perature. Monodentate and bidentate coordination geometries of Li+ by CF3SO 3- were found. The increase in monodentate coordination of Li+ by CF3SO3- a t the higher temperature frees Li+ to bridge between different CF3SO3- ions .