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Fall 2019

Five things you need to know about Deep Eutectic Solvents in Batteries

A crash course in the next big thing in battery technology

Ryan Greer

Scientists are working to understand how to improve energy storage with deep eutectic solvents. Image courtesy Nathan Johnson, Pacific Northwest National Laboratory.

Energy storage is crucial to the long-term viability of renewable energies, like wind and solar power. When the wind isn’t blowing, and the sun isn’t shining, we need to have stored energy from when they were. In Energy Frontier Research Centers (EFRCs), researchers believe that ionic liquids and deep eutectic solvents, two classes of materials with unique properties like unusually low melting points, hold the key to the future of energy storage technology.

As research into this rapidly progressing field continues, you may find yourself hearing more and more about them in the news. This article will help you better understand the science of these liquids and ongoing research at two EFRCs: Breakthrough Electrolytes for Energy Storage (BEES); and Fluid Interface Reactions, Structures, and Transport (FIRST).

Ionic liquids and deep eutectic solvents improve energy storage

Electrical energy can be very simply described as the energy of moving electrons. We can harness power from electrons moving through a circuit depending on the number of electrons moving (the current) and the “pressure” of that flow (the voltage). We have several ways to store the energy to make electrons move, including as chemicals which undergo electrochemical reactions, or in an electric field. Deep eutectic solvents and ionic liquids can find use in both methods.

At BEES, researchers are studying new electrolytes for redox flow batteries, where reactive chemicals are dissolved in a liquid called an electrolyte and pumped past a membrane, where they can react with one another and electrical energy can be harvested. Scientists at BEES believe that deep eutectic solvents could function as electrolytes in new and better redox flow batteries.

At FIRST, energy storage in supercapacitors is being studied. Supercapacitors can charge and discharge much more quickly than conventional batteries and tolerate more charge cycles, making them ideal for applications like regenerative braking. They store energy as the separation of charge between an electrode and charged particles, or ions, held in solution. These charges are held separate by a layer of liquid molecules adhered to the electrode surface. Scientists at FIRST are investigating the behavior of ionic liquids at electrode surfaces in situations just like this.

Deep eutectic solvents and ionic liquids are liquid at low temperatures - and that’s unusual

Both deep eutectic solvents and ionic liquids are materials that you might expect to be solid at relatively cool temperatures—lower than that of boiling water, 212 degrees Fahrenheit (100 degrees Celsius), for instance—but for a variety of reasons are not.

Ionic liquids are salts or mixtures of salts, compounds made of positively and negatively charged ions. Most salts are solid up to very high temperatures because the oppositely charged ions attract one another very strongly. These attractions are stronger than the “jiggling” of molecules due to temperature, and so the ions stick together as a solid.

Ionic liquids avoid this sticking in a couple of ways. They can have bulky, asymmetrical ions, which are usually organic, meaning they contain carbon. The awkward shapes of the organic ions prevent the charges from ordering neatly enough to stick in large groups. They also spread the charges of their ions across large areas, preventing the interaction from being too strong at any single point, so the movement of temperature can shake them back apart.

Deep eutectic solvents are mixtures of solid compounds that are not necessarily salts. A eutectic mixture is when a mixture has a lower melting point than the pure components, and a solvent is a liquid that is good at dissolving other substances. The eutectic effect in deep eutectic solvents occurs because of acid-base interactions, hydrogen bond interactions, or a combination of the two. These interactions spread out the strong forces between molecules, again preventing neat ordering and sticking together.

Deep eutectic solvents bring unique, useful electrical properties to the world of liquid electrolytes. Image courtesy Nathan Johnson, Pacific Northwest National Laboratory.

Most typical liquids, like water, are liquid at room temperature because they lack any strong interactions at all. Deep eutectic solvents and ionic liquids, on the other hand, have strong interactions which are stymied or altered, which gives them some unique and advantageous properties for the design of powerful, durable energy storage.

Deep eutectic solvents and ionic liquids can make energy storage stronger, safer, and longer-lasting

An important factor in the total power of an energy storage device is the amount of your electrochemically active components that can be stored in a given space. For devices where the components are in a liquid, such as the redox flow batteries being studied at BEES, how well those components dissolve into that liquid is very important. As it so happens, deep eutectic solvents and ionic liquids can usually dissolve and hold more of the types of electrochemical components used in these batteries than standard solvents.

According to Burcu Gurkan of BEES, “This translates to high energy density compared to traditional electrolytes.” Basically, if you have more of the chemicals that perform an electrochemical reaction, you are going to store more electrons, meaning a higher energy density.

The storage or recovery rate of electrons through high current is important, but high voltages are too. Even a large amount of current at a low voltage doesn’t produce much power. Luckily, deep eutectic solvents and ionic liquids can help with this. They can show excellent electrochemical stability, meaning their chemical composition doesn’t break down at high voltages. This enables those higher voltages to be achieved in energy storage applications, and thus higher power.

These liquids also stand to make energy storage substantially safer. Traditional liquid electrolytes are organic or aqueous in flow batteries. This has the unfortunate side-effect that they often vaporize easily. Not only does this mean they can exert substantial pressure on the energy storage device, potentially causing failure, but it also makes it easier for them to form explosive mixtures with air, as organic solvents are also frequently flammable.

Deep eutectic solvents and ionic liquids, on the other hand, vaporize much less easily, and thus are much less likely to strain or burst energy storage devices and are usually non-flammable. They are also more chemically durable than traditional organic electrolytes, and less likely to chemically degrade over time into different chemicals that reduce efficiency and lower capacity. Both their lower tendency to vaporize and their better chemical stability also mean that energy storage devices with these alternative electrolytes could endure harsher conditions of heat and wear.

Altogether, the potential promise of these liquids seems incredible: Replacing traditional electrolytes with deep eutectic solvents and ionic liquids could give us energy storage devices that suffer less physical stress, require less maintenance, and last longer in harsher conditions at higher capacity.

Deep eutectic solvents and ionic liquids have tunable properties

A great benefit to both deep eutectic solvents and ionic liquids is that their properties, such as conductivity, melting point, viscosity, and so on, are tunable. Not only does that mean that we are more able to match or exceed the capabilities of traditional electrolytes, but we can tune the liquids for different energy storage applications. The supercapacitors studied at FIRST, for instance, require very different properties in their electrolytes than the redox flow batteries at BEES, and so our choice and design of electrolytes must accommodate this.

There are two primary ways by which we can tune these materials. The first is by altering the molecules themselves. This might seem obvious—if you want different properties, just use a different chemical; what’s special about that? But a unique characteristic of these chemicals is that they often contain large, asymmetrical organic molecules.

We can alter a lot about these large organic molecules while retaining the basic backbone, and thus retaining the essential properties of a deep eutectic solvent or ionic liquid. We can append large carbon chains or add different functional groups, or even replace hydrogen atoms with fluorine atoms. Organic chemists are very good at exactly this sort of precision alteration, giving us a convenient route to a vast potential space of material design.

The second method of tuning is related to the fact that these special liquids do not have to be a single chemical compound, but can be a mixture. Small changes in the proportions of different components can have a big effect on the overall properties. For example, the deep eutectic solvent formed by choline chloride and urea melts at a low melting point of 53 degrees Fahrenheit (12 degrees Celsius) when mixed in a 1:2 ratio, but when urea is replaced by ethylene glycol, melting occurs at -66 degrees Celsius.

Scientists are getting answers

Using deep eutectic solvents and ionic liquids in energy storage is not without its challenges. The high viscosity of many formulations can be problematic for the speed of charge and discharge. There can be a tendency for solvent molecules to crowd electrodes and block their function, reducing speed and efficiency.

To fully leverage the potential of these materials, we need to not search only for fixes to the obvious problems, but to achieve a full understanding of their properties. After all, not all problems are always problems: While traditional batteries suffer from solvent molecules crowding their electrodes, double-layer supercapacitors depend on this effect.

Researchers at BEES and FIRST are working to understand the behavior of these materials in many different energy storage applications. At BEES, they hope deep eutectic solvents can enable totally new chemistries and varieties of redox flow batteries, and at FIRST, researchers are working to uncover what happens at the interface between the fluid and a solid surface.

Along the way, the better understanding cultivated by these researchers will not only improve energy storage, but will help enable the full potential of deep eutectic solvents in a myriad of disparate fields, such as the capture and separation of gases, novel chemical synthesis, metal processing, and more.

More Information

Smith EL, AP Abbott, and KS Ryder. 2014. Deep Eutectic Solvents (DESs) and Their Applications. Chem. Rev. 114:11060–11082.

Acknowledgments

The authors thank the EU (FP6 and 7), EPSRC, and TSB for funding numerous projects in this area.

About the author(s):

  • Ryan Greer is a graduate student in chemistry at Florida State University and is a member of the Center for Actinide Science and Technology Energy Frontier Research Center. His research explores the nature of covalent bonding in actinide complexes using charge density studies and the quantum theory of atoms in molecules.

More Information

Smith EL, AP Abbott, and KS Ryder. 2014. Deep Eutectic Solvents (DESs) and Their Applications. Chem. Rev. 114:11060–11082.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.