Fluorine found as possible substitute for lithium in rechargeable batteries

With increased use of rechargeable batteries to power modern technology, particularly electric vehicles, researchers have been looking for alternative materials for lithium-ion in rechargeable batteries. Modern batteries use lithium and cobalt, but these have a very limited supply.

Materials scientists in the McKelvey School of Engineering at Washington University in St. Louis have found a potential alternative for lithium in fluorine, a relatively abundant and light element. Interestingly, fluoride ion is the mirror opposite of lithium ion, having the strongest attraction for electrons, which allows it to easily carry out electrochemical reactions. Researchers in Japan also are testing fluoride-ion batteries as possible replacements for lithium-ion batteries in vehicles. They say these batteries could allow electric vehicles to run 1,000 kilometers on a single charge. However, current fluoride-ion batteries have poor cyclability — that is they tend to degrade rapidly with charge-discharge cycles.

Washington University researchers Steven Hartman and Rohan Mishra have adopted a new approach to fluoride-ion battery design, identifying two materials which easily gain or lose fluoride ions while undergoing small structural changes to enable good cyclability. Mishra, assistant professor of mechanical engineering & materials science, said that the new battery materials are both layered electrides. Electrides are a relatively new class of materials that have been known in principle for about 50 years, but it wasn’t until the past 10 to 15 years that their properties were better understood, Mishra said. While these materials conduct electrons like ordinary metals, unlike the “sea of electrons” in metals where the electrons are delocalized throughout the crystal, in electrides, the electrons reside at specific interstitial sites within the crystal structure, similar to an ion.

“We predict that these interstitial electrons can be easily replaced with fluoride ions without significant deformations to the crystal structure, thus enabling cyclability,” Mishra said. “The fluoride ions can also move or diffuse fairly easily due to the relatively open structure of the layered electrides.”

Hartman, an Institute of Materials Science & Engineering alumnus who earned a doctorate in Mishra’s lab prior to accepting a postdoctoral fellowship at Los Alamos National Laboratory, used quantum-mechanical calculations to test dozens of potential battery candidates. The computerized tests introduced fluoride into the interstitial spaces of the layered electrides dicalcium nitride and yttrium hypocarbide, showing energy storage capabilities that were close to the performance of lithium-ion batteries. In the case of dicalcium nitride, it is made up of relatively abundant elements and can help overcome the supply shortage of elements for current lithium-ion batteries.

Hartman contrasted the battery study with some of the Mishra group’s other work, which uses machine learning “big data” techniques to sift through thousands of candidates. 

“This took more intuition and trial-and-error than other studies we’ve done,” Hartman said. “In principle, you can add a lot of fluoride ions to conventional electrodes to store a lot of charge, but in practice, these theoretical capacities are hard to manage. When we add in fluoride to conventional electrodes, they swell and shrink dramatically as it charges and discharges, and that can lead to cracking and loss of electrical contact.” 

Minimizing this volume and shape change is essential to create a durable fluoride battery.

“In these layered electride materials, we predict that adding and removing the fluoride ions would cause significantly smaller structural changes, thus helping achieve a longer cycling life,” Hartman said.

Mishra’s lab is looking to collaborate with researchers who can synthesize the promising electrides identified in this study and test them in prototype batteries.

McKelvey School of Engineering has strong group of interdisciplinary faculty members conducting battery research. Recent research by Peng Bai, assistant professor of energy, environmental & chemical engineering, resulted in the ability to approximate the battery’s current density threshold and accurately predict the short circuit time for any particular current density. Jason He, professor of energy, environmental & chemical engineering, recently conducted a feasibility study for electrochemical "refilling" of lithium-ion batteries into the spent electrodes to regenerate useful compounds, such as lithium cobalt oxide. In addition, Vijay Ramani, the Roma B. & Raymond H. Wittcoff Distinguished University Professor, recently received $2 million from the Department of Energy's Advanced Research Projects Agency-Energy (ARPA-E) to continue research into a battery concept that he pioneered for long-duration, grid-scale energy storage.

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Funding for this research was provided by the National Science Foundation (DMREF-1729787 and DMR-1806147). The research used computational resources of the Extreme Science and Engineering Discovery Environment (XSEDE), supported by the NSF (ACI-1548562).

Hartman S, Mishra R. Layered electrides as fluoride intercalation anodes. Journal of Materials Chemistry A. In print Dec. 7, 2020. Pp. 1-8. DOI: 10.1039/d0ta06162j.

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