A new process may soon make lithium batteries, which can hold twice or more the charge of current lithium-ion batteries, more environmentally sound.
The stability of lithium batteries relies on the highly toxic chemical fluorine, the amount of which would be drastically cut by the new process. From the intermittent generation capabilities of solar and wind to the range of electric cars, battery capacity is one of the most significant limiting factors in the transition toward sustainable energy systems.
Fluorine in Batteries
Maria Lukatskaya, Professor of Electrochemical Energy Systems at ETH Zurich and leader of the research group that created the new process, compares fluorine to “the enamel on a tooth.” The fluorinated salts and solvents prevent the metallic lithium at the negative electrode from being in constant contact with the electrolyte liquid in the battery.
Beyond quickly depleting the battery to the point that it would become ineffective after a handful of uses, more dangerous and severe repercussions are possible. Lacking the stable layer provided by the fluorine compounds, long, thin dendrite “whiskers” can crystallize out of the lithium’s normal flat layer. If these reach the positive electrode, the battery can potentially ignite and explode.
A layer called solid electrolyte interphase, or SEI, provides this stability. SEI is where electrolyte chemicals decompose and form a coating on the electrode. The processes for creating this layer most commonly utilized today were developed for use with graphite in earlier battery technologies, rendering them suboptimal for lithium. These more primitive processes have the disadvantages of allowing for the growth of dendrites and a short battery life. Promise has been shown in replacing the carbon and lithium-ion of older processes with fluorine for lithium batteries.
Recent work on utilizing fluorine has shown that a high level of fluorine in SEI increases performance over earlier processes. However, this research used a more heavy-handed approach than Lukatskaya and her team. Researchers dumped high levels of fluorine into the electrolyte, counting on the statistical likelihood of fluorine being reduced on the electrode surface from its sheer preponderance in the solution. Lukatskaya and her team have developed a more nuanced method for stabilizing lithium in batteries.
Development of a new process
The new process relies on a much smaller amount of fluorine, nearing almost twenty times less. However, in this case, less becomes more through its electrostatic attraction to the negative anode, resulting in the fluorinated cations in the additive that the team developed reaching the electrode surface before anything else and, therefore, becoming heavily present in the layer formed there. Essentially, it’s a case of the early bird getting the worm, but on a chemical level.
Recent tests showed that Coulombic efficiency, the rate at which the battery charge can be passed between electrodes without loss, increased from 96.4% to 99.6% under this new process. 99% Coulombic efficiency is the barrier that rechargeable batteries need to achieve to be effective for long-term use. Without the additive, test batteries’ storage capacity severely degraded after just 30 cycles. Even after 275 cycles, batteries with the additive tested at 94% of their original capacity. For comparison, Apple states that the lithium-ion batteries of the iPhone 14 should hold 80% charge after 500 cycles, and the iPhone 15 should hold 80% charge for 1000 cycles.
To check their work, the team examined their work using electron microscopes and X-ray photoelectron spectroscopy to get the closest possible eye on exactly what was occurring in the SEI. These investigations showed a high level of fluorine in the SEI formed with their additive and steady, rigid formation process. SEI formed without their additive showed an uncontrolled mass gain.
The fluorine’s stabilizing properties also counteracted the corrosivity of chlorine impurities in the electrolyte, increasing long-term cycling. Additionally, tests showed no dendrite growth when using the additive, despite dendrites occurring in control tests not using it.
The impacts of fluorine
While fluorine is an important component of battery efficiency, it can also be dangerous. Overheated batteries have been studied to reveal their ability to produce highly toxic gases containing fluorine. These gases can have severe effects on the human body, including tissue destruction and systemic toxicity. In addition to the gas leak issue, Florine itself is highly toxic and corrosive.
The work here is not just theoretical science but a practical way forward for solving the energy storage issues plaguing the transition to renewables. The group has received a patent for their process, which they claim will be cost-effective. From a production point of view, one of the main selling points is that it can be utilized on current battery production setups, not requiring a significant overhaul of manufacturing facilities.
While lithium-ion batteries dominate the current market, Lukatskaya’s group has pushed one step closer to a lithium revolution, potentially doubling or more current storage capabilities.
Ryan Whalen is a writer based in New York. He has served in the Army National Guard and holds a BA in History and a Master of Library and Information Science with a certificate in Data Science. He is currently finishing an MA in Public History and working with the Harbor Defense Museum at Fort Hamilton, Brooklyn.