Novel Concept Based on Fluoride Ions May Boost Battery Lifetime

Imagine if it were possible to go for weeks without charging your laptop or phone. That is the dream of scientists looking into substitute batteries that surpass the popular present-day lithium-ion versions. Now, in a new study published in the journal Science, chemists at several institutions, including Caltech and the Jet Propulsion Laboratory, which is managed by Caltech for NASA, as well as the Honda Research Institute and Lawrence Berkeley National Laboratory, have discovered a new way of developing rechargeable batteries based on fluoride, the negatively charged form, or anion, of the element fluorine.

An illustration of the electrolyte solution used in the new study, on the atomic scale. The fluoride ion (pink) is surrounded by a liquid of BTFE molecules. (Image credit: Brett Savoie/Purdue University)

“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.”

In the 1970s, scientists tried to build rechargeable fluoride batteries using solid components, but solid-state batteries function only at high temperatures, making them unfeasible for daily use. In the new research, the authors report at last discovering how to make the fluoride batteries function using liquid components—and liquid batteries easily function at room temperature.

“We are still in the early stages of development, but this is the first rechargeable fluoride battery that works at room temperature,” says Simon Jones, a chemist at JPL and corresponding author of the new research.

Batteries stimulate electrical currents by ferrying charged atoms—or ions—between a positive and negative electrode. This ferrying process proceeds more effortlessly at room temperature when liquids are utilized. With lithium-ion batteries, lithium is ferried between the electrodes with the aid of a liquid solution, or electrolyte.

Recharging a battery is like pushing a ball up a hill and then letting it roll back again, over and over. You go back and forth between storing the energy and using it.

Thomas Miller, Study Co-Author and Chemistry Professor, Caltech

While lithium ions are positive (referred to as cations), the fluoride ions used in the new research hold a negative charge (and are referred to as anions). There are challenges as well as benefits 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,” says Jones, who does research at JPL on power sources necessary for spacecraft. “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.”

The vital factor in making the fluoride batteries function in a liquid instead of a solid state turned out to be an electrolyte liquid called bis(2,2,2-trifluoroethyl)ether, or BTFE. This solvent is what helps maintain the fluoride ion in a stable condition so that it can ferry electrons back and forth in the battery. Jones says his intern at the time, Victoria Davis, who currently studies at the University of North Carolina, Chapel Hill, was the first to suggest trying BTFE. While Jones did not have much hope it would be successful, the team decided to give it a try anyway and were astonished it worked very well.

At that point, Jones sought Miller for help in figuring out why the solution was effective. Miller and his group conducted computer simulations of the reaction and learned which characteristics of BTFE were stabilizing the fluoride. From there, the team was able to modify the BTFE solution, altering it with additives to enhance its performance and stability.

We’re unlocking a new way of making longer-lasting batteries. Fluoride is making a comeback in batteries.

Simon Jones, Study Co-Author and Chemist, JPL

The Science research, titled, “Room Temperature Cycling of Metal Fluoride Electrodes: Liquid Electrolytes for High Energy Fluoride–Ion Cells,” was sponsored by the Resnick Sustainability Institute and the Molecular Materials Research Center, both at Caltech, the National Science Foundation, the Department of Energy, and the Honda Research Institute. Other authors at Caltech during the research include Christopher Bates, Brett Savoie, Nebojša Momčilović, William Wolf, Michael Webb, Isabelle Darolles, and Nanditha Nair.

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