Rechargeable fluoride-based batteries could offer very high energy density. However, current fluoride batteries use molten salt electrolytes, and thus need to operate at high temperatures. Now, a team of researchers from Caltech, the Jet Propulsion Laboratory (managed by Caltech for NASA), the Honda Research Institute and Lawrence Berkeley National Laboratory report two advances that could lead the way toward room-temperature fluoride batteries. Their paper is published in Science.
Fluoride-ion batteries offer a promising new battery chemistry with up to ten times more energy density than currently available Lithium batteries. Unlike Li-ion batteries, FIBs do not pose a safety risk due to overheating, and obtaining the source materials for FIBs creates considerably less environmental impact than the extraction process for lithium and cobalt.—Dr. Christopher Brooks, Chief Scientist, Honda Research Institute, co-author
Schematic of external electron flow, electrolyte ion shuttling, and redox reactions occurring at fluoride-ion battery (FIB) electrodes during charge or discharge cycles. Davis et al.
The first advance is the development of a room-temperature liquid electrolyte based on a stable tetraalkylammonium salt–fluorinated ether combination. The second is a copper–lanthanum trifluoride core-shell cathode material that demonstrates reversible partial fluorination and defluorination reactions.
Fluoride ion batteries are potential “next-generation” electrochemical storage devices that offer high energy density. At present, such batteries are limited to operation at high temperatures because suitable fluoride ion–conducting electrolytes are known only 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. Pairing this liquid electrolyte with a copper–lanthanum trifluoride (Cu@LaF3) core-shell cathode, we demonstrate reversible fluorination and defluorination reactions in a fluoride ion electrochemical cell cycled at room temperature. Fluoride ion–mediated electrochemistry offers a pathway toward developing capacities beyond that of lithium ion technology.—Davis et al.
The researchers have secured two US patents.
Co-author Robert Grubbs, Caltech’s Victor and Elizabeth Atkins Professor of Chemistry and a winner of the 2005 Nobel Prize in Chemistry, said:
Fluoride batteries can have a higher energy density, which means that they may last longer—up to eight times longer than batteries in use today. But fluoride can be challenging to work with, in particular because it’s so corrosive and reactive.
While lithium ions are positive (called cations), the fluoride ions used in the new study bear a negative charge (and are called anions). There are both challenges and advantages to working with anions in batteries.
For a battery that lasts longer, you need to move a greater number of charges. Moving multiply charged metal cations is difficult, but a similar result can be achieved by moving several singly charged anions, which travel with comparative ease. The challenges with this scheme are making the system work at useable voltages. In this new study, we demonstrate that anions are indeed worthy of attention in battery science since we show that fluoride can work at high enough voltages.—Simon Jones, a chemist at JPL and corresponding author of the new study
The key to making the fluoride batteries work in a liquid rather than a solid state is an electrolyte liquid called bis(2,2,2-trifluoroethyl)ether, or BTFE. This solvent is what helps keep the fluoride ion stable so that it can shuttle electrons back and forth in the battery.
BTFE is made up of several chemical groups that are arranged to give the molecule two positively charged regions that strongly interact with fluoride, since opposites attract. Simulations showed how these charged regions lead BTFE molecules to surround fluoride and dissolve it at room temperature.
The next step in beefing up fluoride-based batteries is extending the lifetimes of the cathode and anode. The team has already made some headway with this by stabilizing the copper cathode so that it doesn’t dissolve into the electrolyte.
Battery testing is underway. 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.
Victoria K. Davis, Christopher M. Bates, Kaoru Omichi, Brett M. Savoie, Nebojša Momčilović, Qingmin Xu, William J. Wolf, Michael A. Webb, Keith J. Billings, Nam Hawn Chou, Selim Alayoglu, Ryan K. McKenney, Isabelle M. Darolles, Nanditha G. Nair, Adrian Hightower, Daniel Rosenberg, Musahid Ahmed, Christopher J. Brooks, Thomas F. Miller III, Robert H. Grubbs, Simon C. Jones (2018) “Room-temperature cycling of metal fluoride electrodes: Liquid electrolytes for high-energy fluoride ion cells” Science Vol. 362, Issue 6419, pp. 1144-1148 doi: 10.1126/science.aat7070