Batteries that outperform today’s lithium-ion variety, in terms of how much energy they can cram into a small lightweight package, could push electric vehicle usage into high gear. A new study detailing an electrochemistry advance may nudge one such high-energy-density type, the fluoride-ion battery (FIB), from the drawing board toward application.
Rechargeable FIBs, which in theory can hold about eight times as much energy per volume as current lithium ion batteries can, aren’t new, but they are uncommon. That’s because these devices generate electricity by shuttling fluoride ions from one electrode to the other through a fluoride-ion-conducting electrolyte. The electrolytes are solids, and to coax them to conduct substantial ion currents, they need to be heated above 150 °C, which severely limits applications.
Now, a large team of researchers, including Simon C. Jones of California Institute of Technology and Christopher J. Brooks of the Honda Research Institute, have come up with a liquid electrolyte that shuttles fluoride ions to and fro and demonstrated its use in a room-temperature, rechargeable FIB (Science 2018, DOI: 10.1126/science.aat7070).
To make the electrolyte solution, the scientists searched for a combination of a fluoride salt and solvent that provided ample solubility, high ionic conductivity, and electrochemical stability. The search led to a neopentyl alkylammonium fluoride and bis(2,2,2-trifluoroethyl)ether, or BTFE.
Then the researchers devised a novel cathode consisting of a copper core and a lanthanum trifluoride shell. The shell prevents copper dissolution and BTFE decomposition while allowing facile diffusion of fluoride ions between the liquid electrolyte and the copper core. The diffusion enabled reversible conversion of copper to copper fluoride during charging cycles. In a proof-of-concept demonstration, the team made test cells with those components and operated them at room temperature for seven charging cycles.
“These results open up new opportunities for the scientific community” in high-energy-density batteries, says Jun Liu, a specialist in energy storage materials at Pacific Northwest National Laboratory. The study demonstrates good progress, Liu notes, but it also underscores the need for “much more research in this area” to develop a long-lasting, viable fluoride-ion battery that can exceed the performance of Li-ion batteries in terms of charge capacity.