Innovative battery design More energy and less environmental impact

A new approach to the electrolyte in lithium metal batteries could significantly increase the range of electric vehicles. To stabilize these batteries, researchers from ETH  Zurich, Switzerland, need much less environmentally harmful fluorine.

Among the promising high-performance batteries of the next generation, lithium-metal batteries are leading the pack. They can store at least twice as much energy per unit volume as the widely used lithium-ion batteries of today. This means: An electric car can travel twice as far on a single charge, or you need to charge your smartphone less often.

Currently, lithium metal batteries have a critical disadvantage: Large amounts of fluorine-containing solvents and salts must be added to the electrolyte liquid, which comes at the expense of their ecological footprint. Without this fluorine, lithium metal batteries would be unstable, they would fail after only a few charging cycles and there could be short circuits or they would overheat and ignite. Maria Lukatskaya, ETH professor for electrochemical energy systems, and her team have now developed a new method to drastically reduce the amount of fluorine in lithium metal batteries, making them more environmentally friendly and also more stable and cost-effective.

Efficiency and safety thanks to a stable protective layer

The fluorinated compounds from the electrolyte help form a protective layer around the lithium metal at the negative pole of the battery. "We can compare this protective layer to tooth enamel. It protects the lithium metal from constantly reacting with the electrolyte components," Lukatskaya explains. Without this protective layer, the electrolyte would quickly deplete during charging, the cell would fail, and the absence of a stable protective layer would cause lithium metal peaks—'dendrites'—to form during charging instead of a uniform flat layer.

If these dendrites reach the positive pole, a short circuit occurs and the battery could heat up so much that it ignites. Therefore, controlling the properties of the protective layer is crucial for a battery's performance. A stable protective layer increases the efficiency, safety, and lifespan of a battery.

Minimize the fluorine content

"We considered how we could reduce the amount of added fluorine without losing the protective layer's stability," says doctoral student Nathan Hong. Their newly developed method uses electrostatic attraction to achieve the desired reaction. The ETH researchers developed a concept in which electrically charged fluorine-containing molecules serve as vehicles to bring the fluorine to the protective layer. This way, they only need 0.1 weight percent fluorine relative to the electrolyte liquid, which is at least 20 times less than in previous studies.

Optimized method for more environmentally friendly batteries

In a recently published external page publication in the journal Energy & Environmental Science, the ETH researchers describe their newly developed method and its basic principles, for which they have also applied for a patent. Lukatskaya conducted this research as part of a SNF-Starting-Grant project.

One of the biggest challenges was to find the right molecule to which fluorine can be attached and which also decomposes under the right conditions once it has reached the lithium metal. A big advantage of the method is that it integrates seamlessly into the existing production process, without generating additional costs for modifying the production facility. In the lab, the batteries were the size of a coin. In the next step, the researchers want to test the scalability of the method and move on to pouch cells, as used in smartphones.

This article was first published on our sister brand  'AT - Aktuelle Technik' (Suisse Edition), Vogel Communications Group