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.

Understanding the Role of Fluorination on the Interaction of Electrolytic Carbonates with Li+ through an Electronic Structure Approach

“…The calculated IR frequencies of C=O bonds in the pristine and fluorinated carbonates are listed in Table , which shows that the C=O stretching frequencies is affected due to fluorination of the carbonates. The most affected IR vibrational mode, which is associated with the C=O bond, is found to be in good agreement with earlier reports for PRC and FLC ,. The stretching frequency of C=O bond shows shifting towards higher frequency region (blue shift) due to fluorination, which leads to improvement in the C=O bond strength.…”

Revealing electrolyte oxidation via carbonate dehydrogenation on Ni-based oxides in Li-ion batteries by in situ Fourier transform infrared spectroscopy

“…2b), possibly because the transformation to VC is faster or greater in LiClO 4 due to the less dissociative LiClO 4 salt 87,88,99,100 and more free solvents in the electrolyte, giving rise to more dominant VC features. DFT calculations show that the solvent molecules are stabilized with Li + interaction, while free solvents have higher energy levels of highest occupied molecular orbital (HOMO) and decreased stability against oxidation, 88,102,103 where here we also show the higher tendency towards dehydrogenation with the less dissociative LiClO 4 salt and less Li + -coordinated solvents. Furthermore, the kinetic barrier for H transfer would also be different between free carbonate and…”

“…Li + coordinated carbonate, giving lower energetic barrier to free carbonate to be decomposed on LiMO 2 surface. 75,87 The observation of VC in both EMC/EC with 1 M LiClO 4 and LP57 confirmed the universal dehydrogenation of EC in 1 M electrolytes, which had higher carbonate activity or lower oxidative stability 103 than concentrated electrolytes (3.1 M).…”

Current Understanding of Nonaqueous Electrolytes for Calcium‐Based Batteries

“…Solvation free energies, determined by density functional quantum chemical calculations for each MD-simulated Ca 2+ -solvent complex, ranged from -385 to -472 kcal/mol, [57] substantially more negative than Na + , Li + , and even Mg 2+ (-215 to -329 kcal/mol). [58][59][60] While higher solvation energies do promote solubility and thus have been argued to be a desirable feature of Ca electrolytes previously, [57] they also represent significant energy barriers that need to be overcome during Ca 2+ reduction. Of note, in a separate study, [61] increases in ion solvation strength of Li + when using high donor-number solvents (e.g., DMSO) were found experimentally and computationally to lead to an almost -600 mV shift in the Li + /Li redox potential vs. a Me10Fc + /Me10Fc reference, compared to less-strongly solvating solvents such as acetonitrile.…”

“…4a) as determined via molecular dynamics, MD, simulation. [57][58][59][60] Thus, the solvating environment surpasses others in terms of chemical complexity, yet, optimistically, also in degrees of freedom when considering possible future design of improved Ca 2+ /Ca redox. In addition to solvation number, the total solvation strength of Ca 2+ in a fixed set of neat or blended carbonates has been calculated [57] to be substantially higher than its Li + , Na + , and Mg 2+ counterparts (Fig.…”