Image: Researchers discovered a new molecule, BTFE, that helps dissolve fluoride at room temperature to create higher-energy batteries.
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Credit: Purdue University Image / Brett Savoie
WEST LAFAYETTE, Ind. – The chemical element that makes up most of today’s batteries, lithium, could soon be challenged by its polar opposite on the periodic table: fluoride. Yeah, the same stuff in toothpaste.
The two elements would compete to make the electronics last longer while charging, e.g. B. Electric cars that have to cover more kilometers than is possible with lithium-ion batteries on the market.
Researchers are one step closer to equipping fluoride-based batteries for battle, with improvements that allow the technology to operate at room temperature. Up until that point, fluoride was limited to building high temperature batteries, which are impractical for our electronic devices.
A team of researchers from the Jet Propulsion Laboratory, California Institute of Technology (Caltech), Honda Research Institute, Inc., and Lawrence Berkeley National Laboratory – including a Caltech postdoctoral fellow who is now an assistant professor at Purdue University – has enlisted two U.S. Patents for the improvements and results published in Science on December 6th.
Fluoride has long been in the running to outperform lithium because of its potential for better energy storage in electrodes that ions move between to charge a battery.
“Fluoride-based battery electrodes can store more ions per location than typical lithium-ion electrodes, which means this technology has a much higher energy density,” said Brett Savoie, a Purdue assistant professor of chemical engineering.
Lithium and fluoride share a yin and yang relationship: lithium is the most electropositive element on the periodic table, which means that it likes to lose electrons, while fluoride is the most electronegative element and just wants to take in electrons. Giving away lithium electrons that it doesn’t want stores energy, while removing electrons from fluoride also stores energy.
To build a battery, the ions of elements like fluorine and lithium must dissolve in the battery’s electrolyte, a solution that helps them migrate between the electrodes. The problem is that fluoride ions have so far only been able to dissolve well in solid electrolytes, which limits their use to high-temperature batteries.
In order for fluoride-based batteries to be operated at room temperature, fluoride ions would have to dissolve better in a liquid electrolyte, as is the case with lithium ions.
The technology could then move in the direction of replacing lithium, a cation-based battery, as the first high-performance, anion-based rechargeable battery.
Researchers at the Jet Propulsion Laboratory discovered a liquid electrolyte, a synthesized molecule called BTFE, that allows fluoride to dissolve at room temperature. Savoie contributed to this discovery by simulating how BTFE and other related solvents successfully dissolve fluoride.
BTFE consists of several chemical groups arranged in such a way that they give the molecule two positively charged regions that interact strongly with fluoride as opposites attract. Simulations showed how these charged regions cause BTFE molecules to surround fluoride and dissolve it at room temperature.
Savoie’s simulations also provided a mechanism for testing other solvents for fluoride, such as “Glyme” molecules, which expand the voltage and stability window of BTFE. This means the battery is less likely to fail at higher voltages.
The next step in upgrading fluoride-based batteries is to extend the life of the positive and negative electrodes, known as the cathode and anode. The team has already made some progress in stabilizing the copper cathode so that it does not dissolve in the electrolyte.
Battery tests are in progress. The work was supported by the Resnick Sustainability Institute and the Molecular Materials Research Center, both at Caltech, the National Science Foundation, the Department of Energy Office of Science, and the Honda Research Institute.
This research is also in line with Purdue’s Giant Leaps celebration, which recognizes the university’s global advances toward a sustainable economy and planet as part of Purdue’s 150th anniversary. This is one of the four themes of the annual celebration’s ideas festival, which aims to present Purdue as an intellectual center for solving real-world problems.
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ABSTRACT
Room temperature cycling of metal fluoride electrodes: liquid electrolytes for high-energy fluoride ion cells
Victoria K. Davis1, Christopher M. Bates1, Kaoru Omichi2, Brett M. Savoie1 *,
Nebojsa Momcilovi? 1, Qingmin Xu2, William J. Wolf1, Michael A. Webb1,
Keith J. Billings1, Nam Hawn Chou2, Selim Alayoglu3, Ryan K. McKenney2,
Isabelle M. Darolles1, Nanditha G. Nair1, Adrian Hightower1, Daniel Rosenberg3,
Musahid Ahmed3, Christopher J. Brooks2, Thomas F. Miller III1,
Robert H. Grubbs1, Simon C. Jones1
1 California Institute of Technology, Pasadena, CA, USA
2 Honda Research Institute, Inc., Columbus, OH, USA
3Lawrence Berkeley National Laboratory, Berkeley, CA, USA
* Currently at Purdue University, West Lafayette, IN, USA
Fluoride-ion batteries are potential next-generation electrochemical storage devices that offer high energy density. Such batteries are currently limited to operation at high temperatures, since suitable fluoride ion-conducting electrolytes are only known in the solid state. We report a liquid fluoride ion conducting electrolyte with high ionic conductivity, wide operating voltage and robust chemical stability based on dry tetraalkylammonium fluoride salts in ether solvents. By combining this liquid electrolyte with a copper-lanthanum trifluoride (Cu @ LaF3) core-shell cathode, we demonstrate reversible fluorination and defluorination reactions in an electrochemical cell of a fluoride-ion battery that is cyclized at room temperature. Fluoride-ion-mediated electrochemistry offers a way to develop capabilities beyond that of lithium-ion technology.
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