First principles modelling of amorphous polymer electrolytes: Li+–PEO, Li+–PEI, and Li+–PES complexes

Several structures of the 1:1 (cation:molecule) complexes of a lithium ion with linear oligomers, CH3X(CH2CH2X)nCH3, (n=0–5; X=O, NH or S) have been shown to be stable with ab initio calculations at the Hartree–Fock level of theory employing the 6-31G∗ basis set. Total energies and binding energies were further evaluated using single-point density functional theory calculations (B3LYP/6-311++G∗∗//HF/6-31G∗). The total binding energy is found to increase with increasing glyme length and follows: NH>O>S. The binding energy for the different glymes reaches a maximum of ∼582 kJ mol−1 for the Li+–PEI pentaglyme (n=5, X=NH) complex. From the present results we conclude the donicity of the heteroatom to be the dominant factor for the relative coordination strength, further supported by the suitability of the coordination geometry provided. Furthermore, transition states and associated energy barriers have been calculated for the conformational changes leading to changes in coordination number for lithium from three to two. The large energy barrier decreases as: O>NH>S. Implications for lithium ion transport and lithium salt solvation are discussed.