Density Functional Theory Calculations of Ion Pair Association in Aprotic Electrolytes, and Decomposition Reactions at a Carbon Electrode Surface
Perla B. Balbuena, University of South Carolina
Research Objectives
This project is part of a comprehensive study for first-principles prediction of materials for electrolytes and electrodes used in batteries and fuel cells. We are investigating the pair interactions of lithium perchlorate in an organic solvent, ethylene carbonate (EC), and the formation of ion pairs for different solvent conditions. The assessment of ion pairing in these systems is important because of its effect on the ionic conductivity. We are also studying the possible paths of decomposition of the solvent in the presence of an electron donor.
Computational Approach
We are using density functional theory as implemented in the program GAUSSIAN 94 to investigate ion-ion and ion-solvent pair interactions, and charge distributions for effective force fields to be used in molecular dynamics simulations. Solvent effects are calculated using a polarizable continuum model. Reaction paths for the solvent decomposition are proposed, via formation of a transition state, which is calculated by ab initio techniques. The calculations are being done on the Cray C90.
Accomplishments
Solvation structures and selective solvation effects are analyzed and used to interpret experimental results. Interactions between lithium and perchlorate ions, and the solvent effect on these interactions, have been investigated. Among the monodentate, bidentate, and tridentate complexes for LiClO4, bidentate is the most stable one in gas phase and under solvent. Vacuum and solvent ion-pair potential-energy surfaces are examined. In solution, the minimum corresponding to the ion-pair association is shifted toward larger values of the ion-ion separation. Yet a second minimum is found in the potential energy surface in solution that may be attributed to a solvent-separated ion pair complex. A mechanism for the decomposition of EC has been proposed. It is a ring opening reaction taking place in two steps, each one with the formation of an intermediate transition state. All transition states and activation energies have been calculated, and transition state theory has been used to calculate the rate of reaction.
Significance
Aprotic electrolyte systems are being used in many current lithium-ion batteries. However, most of the design is still based on expensive trial and error experimentation. These studies provide data (ionic conductivity, heats of reaction) to macroscopic models to predict battery performance and help in the interpretation of experimental data obtained at the Center for Electrochemical Engineering at the University of South Carolina.
Ab initio calculations were performed to study the surface chemistry taking place on graphite/lithium electrode surfaces in an organic solvent, such as ethylene carbonate (EC) and propylene carbonate (PC). The calculated reaction paths, transition state structures, and activation energies were used to establish the feasibility of the proposed reactions at the operation conditions. The proposed ethylene carbonate (EC) reduction mechanism is shown.
Publications
T. Li and P. B. Balbuena, "Density functional theory studies of ion-ion and ion-molecule interactions of lithium perchlorate and organic solvents," J. Phys. Chem B (submitted, 1998).
A. Marquez, A. Vargas, and P. B. Balbuena, "Computational studies of lithium intercalation in model graphite in the presence of tetrahydrofuran," J. Electrochem. Soc. 145, 3328-3334 (1998).