Elementary Steps of Lithium Ion Transport in PEO via Quantum Mechanical Calculations

A first attempt to model lithium ion transport along a poly(ethylene oxide), PEO, chain by quantum mechanical calculations is reported here. The PEO oligomer diglyme (CH 3 O−(CH 2 CH 2 O) 2 −CH 3 ) has been used as a model system for a PEO polymer. Different transition states have been calculated with ab initio methods for conformational changes leading to changes in coordination number for lithium from three to two in the lithium−diglyme complexes. The imaginary frequencies for the transition states and the associated intrinsic reaction coordinate paths have been calculated. Energy barriers of ∼90 kJ mol -1 are found along all the paths. The use of a bidentate structure as an intermediate between the two different tridentate structures is suggested. All structures have been optimized at the HF/6-31G** level of theory, and the total energy calculations have been performed at different levels of sophistication (HF/6-31G** and MP2/6-311+G**//HF/6-31G**). Furthermore, two equivalent transition states for the next oligomer in size, triglyme, complexed with a lithium ion have been calculated to test and show the stability of the calculations of the present model when elongating the oligomer. The results are compared with NMR data for the activation energies of conformational transformations in complexed PEO. A transport path for Li + along a single PEO chain involving tri- to “tetra-” to tridentate coordination changes with small energy differences has been calculated.