It could significantly boost the range of electric vehicles.
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Lithium metal batteries represent a promising option for the next generation of high-energy batteries due to their capability to store at least double the energy per unit volume compared to the currently prevalent lithium-ion batteries. For instance, this could enable an electric car to cover twice the distance on a single charge or reduce the frequency of recharging for a smartphone.
However, the current use of fluorinated solvents and salts in the liquid electrolyte poses significant environmental concerns. If fluorine wasn’t added, the batteries would be unstable, stop working after a few charging cycles, and be prone to short circuits, overheating, and igniting.
To address this, a research team led by Maria Lukatskaya at ETH Zurich has developed a new method to significantly reduce the amount of fluorine needed in lithium metal batteries, making them more eco-friendly, stable, and cost-effective.
The fluorinated compounds in the electrolyte are crucial for creating a protective layer around the metallic lithium at the negative electrode of the battery. This protective layer acts like enamel on a tooth, shielding the metallic lithium from reacting with the electrolyte components.
Without this protection, the electrolyte would deplete quickly during cycling, leading to cell failure and the formation of hazardous lithium metal whiskers known as “dendrites” during recharging.
These dendrites can cause short circuits and even pose a risk of igniting the battery if they touch the positive electrode. Therefore, being able to control the properties of this protective layer is paramount for maximizing battery performance. A stable, protective layer enhances battery efficiency, improves safety, and extends the battery’s service life.
“The question was how to reduce the amount of added fluorine without compromising the protective layer’s stability,” says doctoral student Nathan Hong.
The group’s new method utilizes electrostatic attraction to achieve the desired reaction. Electrically charged fluorinated molecules act as a vehicle to transport the fluorine to the protective layer. This requires only 0.1% by weight of fluorine in the liquid electrolyte, which is at least 20 times lower than in prior studies.
Finding the perfect molecule for attaching fluorine and allowing it to decompose under specific conditions near lithium posed a significant challenge. The research team highlights a major advantage of this approach: seamless integration into existing battery production processes without incurring additional costs to modify production setup. The lab-tested batteries were the size of a coin.
Next, the researchers aim to assess the method’s scalability and its application to pouch cells commonly found in smartphones.
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