Modelling amorphous lithium salt-PEO polymer electrolytes: ab initio calculations of lithium ion-tetra-, penta- and hexaglyme complexes

Several stable structures of the 1:1 complexes of a lithium ion with tetra-, penta- and hexaglyme [CH3O(CH2CH2O)(n)CH3, n = 4-6] have been obtained with ab initio calculations at the Hartree-Fock level of theory employing the 3-21G* basis set. Twenty-three different stable complexes were found with coordination numbers of lithium ranging from four to six; i.e., no stable heptacoordinated complexes emerged. The total energies and the binding energies were evaluated by using density functional theory (DFT) calculations (B3LYP/6-31G*//HF/3-21G*) and showed the total binding energy to increase with the glyme length. The average binding energy for the different glymes reaches a maximum of similar to 620 kJ mol(-1) for the hexaglyme complexes, with an absolute maximum of 631 kJ mol(-1) obtained for a hexacoordinated Li+-hexaglyme complex. The average binding energy per bond for a specific coordination number for lithium shows only minor changes when extending the oligomer (<5 kJ mol(-1) bond(-1)). The large number of complexes obtained with clearly different geometry within a small energy range - six different complexes within 15 kJ mol(-1) for lithium-tetraglyme - clearly reflects the flexibility of the oligomer chains. (C) 1999 Elsevier Science Ltd. All rights reserved.