Li+-(diglyme)2 and LiClO4-diglyme complexes:: Barriers to lithium ion migration

The lithium ion migration mechanism in Li+-(diglyme)(2) and LiClO4-diglyme complexes with coordination of Li+ by 3 to 6 oxygens has been investigated using ab initio molecular orbital theory. Local minima corresponding to different coordination sites of the Li+ cation and transition states between them have been located. The Li+ binding energies of the Li+-(diglyme)2 and LiClO4-diglyme complexes range from 94 to 122 and 167 to 188 kcal/mol, respectively. The binding energies increase with increasing coordination of Lif by oxygen, although the binding per Li-O bond decreases, and structures with higher coordination of Li+ by oxygen exhibit longer Li-O bond lengths than the ones with lower coordination number. The barrier heights for n + 1 --> n coordination of the cation by oxygen decrease with increasing coordination number n, with the smallest Li+ migration barriers (7-11 kcal/mol) occurring for complexes with the highest coordination numbers. The reaction coordinate for lithium ion migration between coordination sites is the torsional motion of the diglyme backbone. The implications of these results for Li+ migration in lithium poly(ethylene oxide) melts are discussed.