This project involves a fundamental study of lithium polymer electrolytes used in lithium battery systems. Ionically conducting polymers were first discovered about 20 years ago and were subsequently used as electrolytes in solid-state batteries. Ion-conducting polymers are solutions of salts in polymers in which a macroscopically solid state is achieved by entanglement or cross-linking. Microscopically they behave as liquids. The polymer electrolytes are generally composites of a polyethylene oxide and a salt such as LiClO₄, LiAsF₆, or LiCF₃SO₃. It is generally believed that ionic conduction is a property of the amorphous phase and that ion association, ion-polymer interactions, and local relaxations of the polymer strongly influence the ionic mobility. However, much remains unknown about the nature of the ion association processes, the ion-polymer interactions, and the role that they play in ionic conductivity of the electrolytes. In this effort we are investigating the effects of the polymer host on ion solvation and the attendant effects of ion pairing, which strongly affect the ionic transport in these systems. The experimental part involves neutron scattering measurements and x-ray scattering measurements. The theoretical part involves electronic structure calculations using ab initio molecular orbital theory to investigate energetic, structural, and dynamical properties of ion-ion and ion-polymer interactions at a molecular level in combination with molecular dynamics simulations being carried out at the University of Minnesota. Information gained from this study will be used to help improve the performance of lithium battery systems.

In the theoretical work the polymer is being modeled using small alkyl oxides such as diethyl ether and diglyme.¹ The interaction of a lithium cation with the oxygens is being investigated, and the potential energy surfaces are being calculated to obtain potentials that are being used in molecular dynamics simulations.² The calculations are used to examine the stability of different coordinations of lithium with the polymer model and barriers to migration of the lithium cation from one coordination site to another coordination site. The barriers for transfer of lithium cation are very important in understanding the transport mechanism in the polymer electrolyte. The results of the calculations will be used in helping to interpret new experimental measurements on lithium polymer electrolytes. We have investigated the potential energy surfaces of Li⁺-diglyme and Li⁺-triglyme complexes, which are models for polyethylene oxide electrolytes. 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 the 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.