A new electrolyte design for lithium metal batteries could significantly extend the range of electric vehicles. Researchers at ETH Zurich have radically reduced the amount of environmentally harmful fluorine needed to stabilize these batteries.
Tesla Roadster battery pack (on the left) with an integrated power control module on top. Image Credit: Via Don Endico FlickrCC BY-SA 2.0
Lithium metal batteries are among the most promising candidates for next generation high-energy batteries. They can store at least twice as much energy per unit volume as the lithium-ion batteries widely in use today. For example, this would mean that an electric car could travel twice as far on a single charge, or that smartphones would not have to be recharged as often.
At present, lithium metal batteries still have a significant drawback: the liquid electrolyte requires the addition of significant amounts of fluorinated solvents and fluorinated salts, which increases its environmental footprint. However, without the addition of fluorine, lithium metal batteries would be unstable, they would stop working after too few charging cycles and would be prone to short circuits as well as overheating and igniting. A research group led by Maria Lukatskaya, Professor of Electrochemical Energy Systems at ETH Zurich, has now developed a new method that dramatically reduces the amount of fluorine required in lithium metal batteries, making them more environmentally friendly and more efficient. Along with being stable, it is also cost-effective.
A stable protective layer increases battery protection and efficiency
Fluorinated compounds from the electrolyte help form a protective layer around the metallic lithium on the negative electrode of the battery. “This protective layer can be compared to tooth enamel,” explains Lukatskaya. “This protects the metallic lithium from continued reaction with the electrolyte components.” Without it, the electrolyte will quickly deplete during cycling, the cell will fail, and the lack of a stable layer will result in the formation of lithium metal whiskers – ‘dendrites’ – rather than a conformal flat layer during the recharging process.
Should these dendrites touch the positive electrode, it will cause a short circuit and the battery will become so hot that it will catch fire. The ability to control the properties of this protective layer is therefore critical to battery performance. A stable protective layer increases the efficiency, safety and service life of the battery.
If the electrolyte in a lithium metal battery is not properly tuned, it leads to the formation of dendrites (“whiskers”). Image credit: nobelprize.org
Minimizing Fluorine Content
“The question was how to reduce the amount of added fluorine without compromising the stability of the protective layer,” says doctoral student Nathan Hong. The group’s new method uses electrostatic attraction to achieve the desired reaction. Here, electrically charged fluorinated molecules serve as a vehicle to deliver fluorine to the protective layer. This means that only 0.1 percent fluorine is needed in the liquid electrolyte, at least 20 times less than in previous studies.
The newly developed method uses fluorinated cations as a vehicle to carry fluorine to the protective layer. As a result, the protective layer remains stable, fluorine use is reduced, production costs are reduced, and the battery becomes more durable. Image credit: ETH Zurich / Chulgi Nathan Hong
Optimized method makes batteries greener
The ETH Zurich research group describes the new methodology and its underlying principles external pagepaper Recently published in the journal Energy and Environmental Science, Patent has been applied for. Lukatskaya conducted this research with the help of an SNSF Starting Grant.
One of the biggest challenges was finding the right molecule to which fluorine could be attached and which would then decompose under the right conditions once it reached the lithium metal. As the group explains, a major advantage of this method is that it can be seamlessly integrated into the existing battery production process without generating additional costs for changing the production setup. The batteries used in the lab were the size of a coin. In the next step, the researchers plan to test the scalability of the method and apply it to sac cells used in smartphones.
Source: ETH Zurich
