title: Evaluating the Free Energies of Solvation and Electronic Structures of Lithium-Ion Battery Electrolytes.
abstract: Adaptive biasing force molecular dynamics simulations and density functional theory calculations were performed to understand the interaction of Li(+) with pure carbonates and ethylene carbonate (EC)-based binary mixtures. The most favorable Li carbonate cluster configurations obtained from molecular dynamics simulations were subjected to detailed structural and thermochemistry calculations on the basis of the M06-2X/6-311++G(d,p) level of theory. We report the ranking of these electrolytes on the basis of the free energies of Li-ion solvation in carbonates and EC-based mixtures. A strong local tetrahedral order involving four carbonates around the Li(+) was seen in the first solvation shell. Thermochemistry calculations revealed that the enthalpy of solvation and the Gibbs free energy of solvation of the Li(+) ion with carbonates are negative and suggested the ion-carbonate complexation process to be exothermic and spontaneous. Natural bond orbital analysis indicated that Li(+) interacts with the lone pairs of electrons on the carbonyl oxygen atom in the primary solvation sphere. These interactions lead to an increase in the carbonyl (C=O) bond lengths, as evidenced by a redshift in the vibrational frequencies [ν(C=O)] and a decrease in the electron density values at the C=O bond critical points in the primary solvation sphere. Quantum theory of atoms in molecules, localized molecular orbital energy decomposition analysis (LMO-EDA), and noncovalent interaction plots revealed the electrostatic nature of the Li(+) ion interactions with the carbonyl oxygen atoms in these complexes. On the basis of LMO-EDA, the strongest attractive interaction in these complexes was found to be the electrostatic interaction followed by polarization, dispersion, and exchange interactions. Overall, our calculations predicted EC and a binary mixture of EC/dimethyl carbonate to be appropriate electrolytes for Li-ion batteries, which complies with experiments and other theoretical results.
authors.affiliation: Department of Chemistry, University of Missouri-Columbia, Columbia, MO, 65211, USA. m_shakori1361@yahoo.com.
authors.forename: Mehdi
authors.initials: M
authors.lastname: Shakourian-Fard
authors.affiliation: Department of Chemistry, University of Missouri-Columbia, Columbia, MO, 65211, USA. gkamath9173@gmail.com.
authors.forename: Ganesh
authors.initials: G
authors.lastname: Kamath
authors.affiliation: Center for Nanoscale Materials, Argonne National Laboratory, 9700 S. Cass Ave, Argonne, IL, 60439, USA. skrssank@anl.gov.
authors.forename: Subramanian K R S
authors.initials: SK
authors.lastname: Sankaranarayanan
keywords: charge transfer; electrolytes; energy conversion; energy storage; lithium-ion batteries