Molecular Dynamics Simulations and Spectroscopic Studies of Amorphous Tetraglyme (CH₃O(CH₂CH₂O)₄CH₃) 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 tetraglyme [CH 3 O(CH 2 CH 2 O) 4 CH 3 ], a model for amorphous PEO, and tetraglyme:LiCF 3 SO 3 (lithium triflate) with an ether oxygen:Li + ratio of 10:1 were studied by molecular dynamics (MD) simulations at 300 and 400 K. Conformational and structural analyses of Li + interactions with tetraglyme and triflate ion oxygens are consistent with decreased Li + coordination by tetraglyme and increased ionic aggregation at the higher temperature. Dihedral angle population density distributions for tetraglyme chains show that the trans conformation is favored for C−O bonds while the more compact gauche conformation is favored for C−C bonds and is enforced by coordination of adjacent oxygens to Li + . Calculated populations of tetraglyme conformational triads indicate 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 K for tetraglyme:LiCF 3 SO 3 . The calculated Li + coordination number remains the same with increasing temperature, 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 + -CF 3 SO 3 - associated species are compared with vibrational spectral data. The increase in populations of [Li 2 CF 3 SO 3 ] + and [Li 3 CF 3 SO 3 ] 2+ from both MD simulations and IR data implies that Li + -CF 3 SO 3 - association is increased at higher temperature. Monodentate and bidentate coordination geometries of Li + by CF 3 SO 3 - were found. The increase in monodentate coordination of Li + by CF 3 SO 3 - at the higher temperature frees Li + to bridge between different CF 3 SO 3 - ions.