Argonne National Lab Creates Lithium

May 10, 2023

Researchers at Argonne National Lab believe they may have a solution for lithium ion batteries that perform poorly in the cold.

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People who live in cold climates and drive electric cars know the lithium-ion batteries in their car don't work as well in freezing temperatures. They don't charge as fast and they don't go as far. It's a problem, but Argonne National Laboratory says it may have the answer.

In a blog post, the scientists at Argonne say that in today's lithium-ion batteries, the liquid electrolyte that serves as a pathway for ions to travel between the cathode and the anode as the battery charges and discharges begins to freeze at sub-zero temperatures. This condition severely limits the effectiveness of charging electric vehicles in cold regions and seasons.

A team of scientists from Argonne and Lawrence Berkeley national laboratories collaborated to develop a fluorinated electrolyte that performs well even in sub-zero temperatures. "Our research thus demonstrated how to tailor the atomic structure of electrolyte solvents to design new electrolytes for sub-zero temperatures," says John Zhang, who lead the research group at Argonne National Lab.

"Our team not only found an antifreeze electrolyte whose charging performance does not decline at minus 4 degrees Fahrenheit, but we also discovered, at the atomic level, what makes it so effective," said Zhang, who is a senior chemist and group leader in Argonne's Chemical Sciences and Engineering division. This low temperature electrolyte shows promise in working for batteries in electric vehicles, as well as in energy storage for electric grids and consumer electronics like computers and phones.

You don't have to know how a battery works to drive an electric car, just as you don't need to know how a four-stroke engine works to drive a conventional car. Most of us probably have little more than a rudimentary understanding of how lithium-ion batteries work. Argonne Lab explains that the electrolyte used in most lithium-ion batteries today is a mixture of a widely available salt — lithium hexafluorophosphate — and carbonate solvents such as ethylene carbonate. The solvents dissolve the salt to form a liquid.

When a battery is charged, the liquid electrolyte shuttles lithium ions from the cathode, which is typically an oxide that contains lithium, to the anode, which is usually made of graphite. These ions migrate out of the cathode, then pass through the electrolyte on the way into the anode. While being transported through the electrolyte, they sit at the center of clusters of four or five solvent molecules.

During the initial few charges, these clusters strike the anode surface and form a protective layer called the solid-electrolyte interphase. Once formed, this layer acts like a filter. It allows only the lithium ions to pass through the layer while blocking the solvent molecules. That's what allows the anode to store lithium atoms in the structure of the graphite when the battery is charged. During the discharge phase, electrochemical reactions release electrons from the lithium to generate electricity that is then used to power electric vehicles.

When the temperature drops, the electrolyte with carbonate solvents begins to freeze. That in turn causes it to lose its ability to transport lithium ions to the anode during charging because the lithium ions are so tightly bound within the solvent clusters. Therefore those ions require much higher energy to evacuate their clusters and penetrate the interface layer than they do at room temperature. The scientists believed the solution to poor cold weather performance was to find a better solvent that wouldn't freeze.

The team investigated several solvents that were infused with fluorine and were able to identify the one that had the lowest energy barrier for releasing lithium ions from the clusters at sub-zero temperature. They also determined at the atomic scale why that particular composition worked so well — it was dependent on the position of the fluorine atoms within each solvent molecule and their number.

In testing with laboratory cells, the fluorinated electrolyte retained stable energy storage capacity for 400 charge/discharge cycles at minus 4 degrees Fahrenheit. Even at that temperature, the capacity of the battery was equivalent to that of a cell with a conventional carbonate-based electrolyte at room temperature. "Our research thus demonstrated how to tailor the atomic structure of electrolyte solvents to design new electrolytes for sub-zero temperatures," Zhang said.

The antifreeze electrolyte came with an important bonus as well. It is much safer than the carbonate-based electrolytes that are currently used, since it will not catch fire. "We are patenting our low temperature and safer electrolyte and are now searching for an industrial partner to adapt it to one of their designs for lithium ion batteries," Zhang said.

Zhang's fellow scientists at Argonne are Dong-Joo Yoo, Qian Liu and Minkyu Kim. Berkeley Lab authors are Orion Cohen and Kristin Persson. The work was funded by the DOE Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office.

Image credit: Argonne National Lab via Advanced Energy Materials

The research is explained in exquisite detail in the journal Advanced Energy Materials. I am not a scientist, nor have I ever played one on television. That's a good thing because the turgid prose of most scientific writing makes my eyes glaze over. If you are interested in learning more about this research, I encourage you to follow the link above and knock yourself out. The research paper has the catchy title of "Rational Design of Fluorinated Electrolytes for Low Temperature Lithium-Ion Batteries."

Lots of CleanTechnica readers are quite savvy when it comes to things powered by electricity and I know many of you want to know whether this new electrolyte has any negative drawbacks at room temperature or reduces battery life — both of which could prevent battery manufacturers from being interested int this new technology. Here is an excerpt from the research that may address some of those concerns.

"Long term cyclability at high C-rates and low temperatures is considered as one of the challenging aspects for lithium ion batteries. To prove the superiority of our electrolytes, we conducted long term cycling tests with various conditions.

"When a current of 2 C was applied at 25 °C, the ethyl acetate electrolyte with fluorine gradually decayed to a capacity retention of 73% after 400 cycles while the ethyl acetate electrolyte with LiDFOB additive showed the best capacity retention of 91% after 400 cycles. This trend continues at a further high current of 6 C.

"While Gen 2 rapidly degraded to 34% within 50 cycles, the electrolyte with LiDFOB additive showed the best capacity retention of 85% even after 500 cycles. When a current of C/3 was applied at −20 °C, Gen 2 and ethyl acetate electrolytes showed a severe capacity degradation, corresponding to 7.5% and 34% capacity retention after 300 cycles, respectively.

"In stark contrast, the ethyl acetate with fluorine electrolyte with LiDFOB additive showed a negligible capacity loss and retained 97% capacity even after 300 cycles. In addition, in all test conditions, the Coulombic efficiencies of the EA-f electrolyte with LiDFOB additive were higher than those of other electrolytes. This cycling test result reveals the superior stability of our electrolyte for fast charging and low temperature operations."

The internal combustion engines of today share few characteristics with the engines of 100 years ago, except for the basic premise of the four stroke engine that can be reduced to its basics by this phrase — Suck, Push, Bang, Blow. Battery technology is progressing rapidly today, thanks to thousands of researchers around the world like Dr. Zhang and his colleagues at Argonne National Lab.

Poor cold weather performance is an issue, one that needs to be solved before the EV revolution can be considered complete. There's much we don't know about lithium-ion batteries with fluorinated elecrtrolytes, starting with how the presence of fluorine will affect the manufacturing and recycling of lithium-ion batteries.

After all, fluorine is a powerful chemical that damages the ozone layer when it gets into the atmosphere. In addition, different battery chemistries such as lithium iron phosphate seem to be less affected by cold temperatures than the more common NMC batteries. Who knows how sodium or sulfur batteries may perform in the real world once they exit laboratories and enter commercial production,

The only thing we can be relatively certain of is that the batteries in use a decade from now will be as different from today's batteries as transistors are from vacuum tubes. The EV revolution is just getting started. We can't wait to see what's coming next.

Steve writes about the interface between technology and sustainability from his home in Florida or anywhere else The Force may lead him. He is proud to be "woke" and doesn't really give a damn why the glass broke. He believes passionately in what Socrates said 3000 years ago: "The secret to change is to focus all of your energy not on fighting the old but on building the new."

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