Fluoride-Based Battery Chemistry Could Leapfrog, Replace Lithium Ion

Fluoride is better known for its positive correlation with improved dental health than with batteries, at least in the eyes of the general public. But there’s reason to be hopeful that it could one day do double duty, substantially increasing the energy density of batteries. Fluoride was first explored in battery chemistry in the 1970s, but could only be made to work in solid-state batteries, which operate at much higher temperatures than conventional devices.

A breakthrough in fluoride battery chemistry could be instrumental in speeding the development of electric vehicles and allowing for an increased adoption of renewable power. According to a new paper published in the journal science and consisting of work done by Caltech, the Jet Propulsion Laboratory, the Honda Research Institute, and Lawrence Berkeley National Laboratory, there’s a way to build batteries with an energy density up to 8x higher than lithium-ion, using a new approach that allows fluoride to work in a liquid battery at room temperature. This is a significant scientific achievement regardless of whether the product sees commercialization — previously, it wasn’t known if there was a battery chemistry that would allow a room-temperature fluoride battery.

“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,” says study co-author Robert Grubbs, Caltech’s Victor and Elizabeth Atkins Professor of Chemistry and a winner of the 2005 Nobel Prize in Chemistry. “But fluoride can be challenging to work with, in particular because it’s so corrosive and reactive.”

Using a negatively charged anion to carry charge as opposed to a positively charged cation carries its own set of challenges. In this case, one problem has been creating a battery chemistry that functioned at voltage levels we’d actually find useful in modern electronics. Making fluoride batteries work at room temperature required an electrolyte solution called bis(2,2,2-trifluoroethyl) ether, or BTFE. In its spectrum of testing, the authors demonstrated that it’s possible to build a rechargeable fluorine battery that operates at room temperature.

It’s not clear what the exact barriers to commercialization are, but there’s still substantial work to do before any kind of actual functioning battery can be assembled. The problem with all battery chemistries is the intrinsic difficulty in finding a solution that will store energy reliably, discharge it safely and in the desired manner while minimizing events like thermal runaway. The importance of a new battery chemistry with an 8x improvement in energy density compared with lithium-ion can’t be understated, but neither can the complexity of building batteries in the first place. Fluoride’s 8x improvement in energy density won’t be very useful, for example, if the battery lasts only a fraction as long as a lithium-ion cell (at the very least, this type of issue would sharply curtail actual adoption). Companies around the world have poured a great deal of effort into finding new alternative battery chemistries and designs, including solid-state batteries, redox flow batteries, lithium-air batteries, and of course, continuing to incrementally improve lithium-ion chemistry.

Top image credit: Brett Savoie/Purdue University

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