Modelling lithium ion transport in helical PEO by ab initio calculations

Several potential energy surfaces were calculated for a lithium ion inside a helix of poly (ethylene oxide) (PEO). The calculations were ah initio calculations at the Hartree-Fock level of theory employing the 3-21G* basis set. The model system used was a methyl terminated oligomer chain of PEG, CH3O(CH2CH2O)(12)CH3. A general chain geometry was used, approximate to that of several experimentally determined crystalline structures of MXPEO3. Preferred lithium ion positions at different levels of intrusion into the PEO helical structure and energy barriers for ion transport inside the helix were calculated. The binding energies were further evaluated using DFT methods, B3LYP/6-31 G*//HF/3-21G*, and compared to previously calculated values for coordination of lithium in low M-w PEO oligomer systems. The present results suggest that LiXPEO3 crystalline complexes are poor ion conductors not due to high coordination strengths, but rather due to the high energy barriers, > 43 W mol(-1), and the need for a co-operative motion of lithium ions between the preferred sites. (C) 2001 Elsevier Science Ltd. All rights reserved.