Many owners of electric vehicles (EVs) worry about how their battery will perform in very cold weather. However, a new battery chemistry may have solved that problem.
In current Li-ion batteries, the main problem lies in the liquid electrolyte. This key battery component transfers charge-carrying particles — ions — between the battery’s two electrodes, causing the battery to charge and discharge. But the liquid begins to freeze at sub-zero temperatures. This condition, of course, severely limits the effectiveness of charging electric vehicles in cold regions and seasons.
To address this issue, a team from the U.S. Department of Energy’s (DOE) Argonne and Lawrence Berkeley national laboratories developed a fluorine-containing electrolyte that performs well — even when the temperature drops below zero.
“Our team not only found an antifreeze electrolyte whose charging performance does not decline at -4 °F, but we also discovered, at the atomic level, what makes it so effective,” said Argonne group leader Zhengcheng “John” Zhang.
This new electrolyte shows promise of working for batteries in EVs as well as in energy storage for electric grids and consumer electronics.
In today’s lithium-ion batteries, the electrolyte is a mixture of 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 Li-ions from the cathode to the anode. 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 Li-ions to pass through the layer while blocking the solvent molecules. In this way, the anode is able to store lithium atoms in the structure of the graphite on charge. Upon discharge, electrochemical reactions release electrons from the lithium that generate electricity that can power vehicles.
The problem is that in cold temperatures the electrolyte with carbonate solvents begins to freeze. As a result, it loses the ability to transport Li-ions into the anode on charge. This is because the Li-ions are so tightly bound within the solvent clusters. Hence, these ions require much higher energy to evacuate their clusters and penetrate the interface layer than at room temperature. For that reason, scientists have been searching for a better solvent.
The team investigated several fluorine-containing solvents. They were able to identify the composition that had the lowest energy barrier for releasing Li-ions from the clusters at sub-zero temperatures. They also determined at the atomic scale why that particular composition worked so well: It depended on the position of the fluorine atoms within each solvent molecule, and their number.
In lab tests, the team’s fluorinated electrolyte retained stable energy storage capacity for 400 charge-discharge cycles at -4 °F. Even at that sub-zero temperature, the capacity 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 has a bonus property: 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 Li-ion batteries,” Zhang said.
Here is a Tech Briefs interview with Zhang, edited for length and clarity.
Tech Briefs: What inspired your research?
Zhang: We realized that low temperature is a big challenge. Certain parts of the Earth suffer from low temperature, so in that case, batteries suffer huge capacity drops and low power capability. That’s the challenge we faced. We received a proposal to perform solicited research on this area. So, that’s why we have a low-temperature electrolyte.
Tech Briefs: Can you explain in simple terms how the technology works?
Zhang: At -4 °F, what happens to a battery? The first thing you think about, the electrolyte, which is a liquid, will be frozen. When it becomes frozen, the liquid ions will not move or not move fast enough. There’s very high resistance for transferring Li-ion and transferring Li-ion is a fundamental step to support the electrochemical reactions.
So, at room temperature, it’s in a liquid state, and the Li-ion can be transferred very fast. But once the temperature drops below zero, this process is significantly reduced. We call that overpotential. For example, if you charge to 4.2 volts, you can get full capacity. But at low temperatures, you only charge 50 percent to get to 4.2 volts. The actual capacity is only half. That’s why we tackled this. We realized that an ester-based solvent has a very low melting point. This means at an extremely low temperatures, they still remain in a liquid state instead of frozen.
But commercially available esters cannot be used because they have very high reactivity on the graphene. So, they can’t perform a very good SEI (solid electrolyte interface) — a layer of decomposed electrolyte product passing the surface of graphene to allow lithium to go through the SEI, to go to the graphene side, for regular charging and discharging. To solve this issue, we introduced fluorine to the ester solvent. We found, one of the specific solvent fluorine esters to be the best for low temperature applications.
This solvent has an extremely low melting point, and can dissolve lithium salt, which has a very high conductivity. So, that can enable low-temperature performance.
This was one major discovery, but later we realized that the fluid ester, this solvent itself, is not good enough. So, we had to develop tailored additives to combine with the fluid ester. We’ve developed what we call a Co-Additives Strategy. There are just two additives. It performs very well at a low temperature. We didn’t suffer from capacity drop, power failure, or low temperature. We also found that this electrolyte does not catch fire. So, it’s very safe and suitable.
Tech Briefs: You’ve said the team is patenting the electrolyte and is now searching for an industrial partner to adapt it to one of their designs for Li-ion batteries. How is that coming along?
Zhang: It is officially filed by the attorney. The lawyer who helped us has already submitted the paperwork to the patent office.
Tech Briefs: Aside from the patent and the partner searching, what are your next steps? Do you have any further research or work planned?
Zhang: We still have something going on in the low-temperature area, and we’re also trying to see if there’s anything we can further improve in this system. So, there’s research going on for low-temperature applications; pretty much focusing on some brand-new ideas.