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.