KEY CONCEPTS
• Glycerol carbonate is one of a series of cyclic compounds that may have potential as a renewable solvent in applications such as an electrolyte in lithium-metal batteries.
• Ab initio molecular dynamics simulations were done to better understand the solvent characteristics of glycerol carbonate. 
• Glycerol carbonate exhibits a higher viscosity than two other members of this series (ethylene carbonate and propylene carbonate) due to localized hydrogen bonding. 

Lubricants perform a number of critical functions, including friction reduction, minimization of wear and corrosion protection. To fulfill these functions, lubricants are formulated with a base stock and a series of additives that are designed to impart specific characteristics to boost performance.

One issue that can be overlooked is solvency (see Figure 4), which is critical because the additives used in lubricants must be miscible with the base stock. In some cases, additives that exhibit polar properties are not that compatible with a hydrocarbon base stock. For this reason, lubricant formulators will use a second base stock at a lower treat rate to assist with solvency.


Figure 4. Ab initio molecular dynamics simulations were run on glycerol carbonate to better understand this molecule’s solvent properties and potential suitability as an electrolyte in lithium-metal batteries. Figure courtesy of Colleen Kelley/University of Cincinnati Creative.

Solvents also are providing an important function in lithium-based batteries being used in electric vehicles. One issue that has been challenging to overcome is the concern about flammability due to the use of organic solvents as electrolytes in the battery.

In a previous TLT article,¹ a new approach was taken to reduce the formation of dendrites in lithium-metal batteries. Dendrites can grow from the battery’s lithium anode and, if left unchecked, will reduce battery performance and, ultimately, produce a short circuit at the cathode potentially leading to a fire. 

Dendrite formation can be accelerated due to the presence of a solid electrolyte interphase (SEI) that can form on the electrode surface and interact with both the electrode and the electrolyte. In this work, the researchers modified the commonly used ethylene carbonate, dimethyl carbonate electrolyte with a potassium-based salt. Nuclear magnetic resonance (NMR) analysis showed that the SEI formation was minimized in part because the potassium ions minimized the decomposition of ethylene carbonate and dimethyl carbonate at the lithium-metal anode.

Ethylene carbonate is a widely used solvent that exhibits a large dipole moment, high polarizabilities and large dielectric constants. Andrew Eisenhart, graduate student in the department of chemistry at the University of Cincinnati in Cincinnati, Ohio, says, “We have been working with ethylene carbonate and its analogue, propylene carbonate in our laboratory for some time. Both carbonates are cyclic in structure and are prepared by reacting carbon dioxide with the appropriate epoxide (ethylene oxide and propylene oxide).”

A third member of the cyclic solvent series is glycerol carbonate, which is structurally similar to propylene carbonate except for the substitution of a hydroxyl group for one of the hydrogens in the methyl group. Glycerol carbonate also exhibits a large dipole moment and is used as a solvent. To better understand its potential as a renewable solvent (derived from glycerin, a byproduct of biodiesel production), new research has now provided further insight on its solvation properties.

Hydrogen bonding
Eisenhart and his advisor, Thomas Beck, professor of chemistry and physics at the University of Cincinnati, conducted ab initio molecular dynamics simulations that were so complex that a supercomputer was required to do the calculations. The researchers evaluated pure glycerol carbonate liquid and electrolyte solutions of this solvent with potassium chloride and potassium fluoride ion pairs.

Eisenhart says, “Most solvents can be classified as compatible with organic or aqueous media based on how they interact with solutes. Glycerol carbonate is unique because this solvent can be placed in the middle between the two classes. The reason for glycerol carbonate’s positioning is that hydrogen bonding is present between the carbonyl group and the hydroxyl group functionalities in the molecule.”

In past experimental work, glycerol carbonate was found to exhibit a glass transition temperature when heated from a glassy state to a liquid state. Eisenhart says, “The presence of certain ions in glycerol carbonate will change the glass transition temperature. The difference between potassium fluoride and potassium chloride, when it comes to glassy phase transition, is that the potassium fluoride increases the transition temperature significantly while potassium chloride does not (relative to the pure liquid). Eventually, both solutions will form a glass phase.”

One of the objectives of the simulations was to find out why there is a difference in using the two salts. The researchers designed one unit cell containing 32 glycerol carbonate molecules to evaluate the solvent. For the two salts, unit cells containing one ion pair of either potassium chloride or potassium fluoride solvated with 27 glycerol carbonate molecules were analyzed.

As part of the evaluation of glycerol carbonate, the researchers also examined ethylene carbonate and propylene carbonate. Eisenhart says, “All three carbonates are difficult to model. The big difference is that ethylene carbonate and propylene carbonate had one dipole moment from the carbonyl, while glycerol carbonate contained a second dipole moment due to the presence of the hydroxyl group. The change in solvency for glycerol carbonate is all about its hydrogen bonding interactions.”

The simulations showed that glycerol carbonate’s high viscosity is due to localized hydrogen bonding between two molecules that pair off in a tail–tail orientation or as a dimer in a head–tail arrangement according to Eisenhart. The hydrogen bonding prevented the very organized molecular stacking that the researchers observed with ethylene carbonate and propylene carbonate but not with glycerol carbonate.

Glycerol carbonate interacts differently with potassium chloride and potassium fluoride. In the case of the former, contact ion pairs were observed where the potassium cation and the chloride anion are in contact with each other surrounded by a shell of glycerol carbonate molecules. For potassium fluoride, a shared solvent orientation was observed where a glycerol carbonate molecule is placed between the potassium cation and the fluoride anion.

Eisenhart says, “The higher electronegativity of the fluoride anion creates the enhanced hydrogen bonding situation leading to a shared solvent structure. The interaction also leads to the presence of a tetrahedral geometry between potassium fluoride and glycerol carbonate. This geometry is less defined with potassium chloride.”

Future work will involve modeling of glycerol carbonate with a series of lithium halogen salts including lithium fluoride, chloride, bromide and iodide. Hopefully, this will provide an indication of how glycerol carbonate might perform as an electrolyte in lithium-metal batteries.

Additional information can be found in a recent reference² or by contacting Beck at becktl@ucmail.uc.edu.

REFERENCES
1. Canter, N. (2021), “Improving the performance of lithium-metal batteries,” TLT, 77 (92), pp. 14-15.
2. Eisenhart, A. and Beck, T. (2021), “Quantum simulations of hydrogen bonding effects in glycerol carbonate electrolyte solutions, “ Journal of Physical Chemistry B, 125 (8), pp. 2157-2166.