Larry A. Curtiss,
Argonne National Laboratory
Research Objectives
This project involves a fundamental study of lithium polymer electrolytes used in lithium battery systems. 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.
Computational Approach
Ab initio molecular orbital theory is being used 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. The polymer is being modeled using alkyl oxides chains. The interaction of a lithium cation with the oxygens from one and two chains is being investigated 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 calculations are being done on both the Cray C90 and J90 computers.
Accomplishments
Structures were located corresponding to coordination of a lithium cation with one to six oxygens of either one or two alkoxide chains. The binding energies of the complexes were found to increase with coordination of the cation by oxygen, although the binding per Li-O bond decreases. The barriers for lithium cation migration between coordination sites and transition states were located. While the barriers were found to be small for lithium cation migration from lower to higher coordination of lithium with oxygen, the barriers were large for higher to lower coordination.
Significance
Much is unknown about the nature of the ion association processes, the ion-polymer interactions, and the role that they play in ionic conductivity of lithium polymer electrolytes used in batteries. These electron structure computations in combination with molecular dynamics simulations at the University of Minnesota and neutron scattering measurements at Argonne National Laboratory will provide fundamental insight into the ionic conductivity mechanism in these materials.
Publications
Boinske, P. T., L. A. Curtiss, J. W. Halley, B. Lin, and A. Sutjianto. 1996. Lithium ion transport in a model of amorphous polyethylene oxide. Journal of Computer-Aided Materials Design 3:385-402.
Sujianto, A., and L. A. Curtiss. 1997. Theoretical study of the potential energy surface of diglyme. Chemical Physics Letters 264:127-133.
Coordination of lithium cation (green) with six oxygens (red) from two ethylene oxide
chains.