Li⁺−Diglyme Complexes:  Barriers to Lithium Cation Migration

The potential energy surfaces of Li + −diglyme and Li + −triglyme complexes, which are models for poly(ethylene oxide) electrolytes, have been investigated at the HF/6-31G(d) and MP2/6-31+G(d) levels of theory. Eighteen local minima were located that correspond to coordination of Li + with one to four oxygens. The binding energies of the complexes increase with coordination of Li + by oxygen, although the binding per Li−O bond decreases. The potential energy surfaces for lithium cation migration between one- and two-coordination sites and two- and three-coordination sites in the Li + −diglyme complexes were investigated, and five transition states were located. While the barriers are small (less than 2 kcal/mol) for lithium cation migration from lower to higher coordination, the barriers are large (20−30 kcal/mol) for higher to lower coordination. The latter corresponds to the barrier for transfer of Li + from one end of diglyme to the other and is approximately the difference in binding energy of the higher and lower coordination structures. The implications for Li + migration along a single polymer chain in lithium−poly(ethylene oxide) are discussed.