Computation of Thermodynamic Oxidation Potentials of Organic Solvents Using Density Functional Theory

Abstract

Ethers and organic carbonates are commonly used as solvents in lithium battery electrolyte. It is important to determine the oxidation potentials of these organic solvents due to the high cathode potential (∼5 V) in many of these batteries. There are significant variations in the reported oxidation potentials for electrolytes containing these solvents. The factors contributing to the variation include the type of salt used in the electrolyte, composition of the electrode, and a somewhat arbitrary determination of the oxidation potential from the anodic cutoff current. We report here the application of density functional theory (DFT) to calculate solvent oxidation potentials assuming oxidation occurs via one-electron transfer to form the radical cation. No specific ion-ion, ion-solvent, or ion-electrode interactions are included. These values are then compared to the experimental observations. Eleven solvent molecules are studied: 1,2-dimethoxyethane, tetrahydrofuran, 1,3-dioxolane, diethylcarbonate, dimethylcarbonate, ethylmethylcarbonate, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, and catechol carbonate. Optimized geometries of the radical cations correlate well with the fragmentation patterns observed in mass spectrometry. The oxidation potentials of saturated carbonates are calculated to be approximately 1 V higher than the organic ethers, which is consistent with reported literature values. Quantitative comparison with experiment will require more careful measurements to eliminate other oxidation reactions and a standardized procedure for determining the oxidation potential. © 2001 The Electrochemical Society. All rights reserved.