Scientists at ETH Zurich have drastically lowered the quantity of fluorine—which is bad for the environment—needed to stabilize batteries. By developing a new electrolyte design for lithium metal batteries, the range of electric vehicles could be greatly increased. The study has been published in the journal Energy & Environmental Science.
If the electrolyte in a lithium metal battery is not properly tuned, this leads to the formation of dendrites (“whiskers”). Image Credit: Nobelprize.org
A lithium metal battery is one of the most promising options for the next generation of high-energy batteries. Compared to today's widely used lithium-ion batteries, lithium-metal batteries can store at least twice as much energy per unit of volume. This will result in smartphones not needing to be recharged as frequently and electric cars going twice as far on a single charge.
One major issue with lithium metal batteries still exists today: the liquid electrolyte needs to be heavily fluorinated with salts and solvents, which increases the battery's environmental impact. However, without fluorine, lithium metal batteries would become unstable, prone to short circuits, overheating, and spontaneous combustion, and cease functioning after very few cycles of charging.
A research group headed by Maria Lukatskaya, a Professor of Electrochemical Energy Systems at ETH Zurich, has developed a new technique. This method significantly lowers the amount of fluorine needed in lithium metal batteries, making them more stable, cost-effective, and environmentally friendly.
How Does a Lithium Metal Battery Work?
A positively charged cathode and a negatively charged anode make up a battery. The anode in a lithium-ion battery is composed of graphite, whereas the anode in a lithium-metal battery is composed of lithium metal. A liquid electrolyte separates the anode and cathode. Positively charged lithium ions move from the cathode to the anode during battery charging. The lithium ions transform into metallic lithium when they reach the anode and lose their positive charge.
A Stable Protective Layer Increases Battery Safety and Efficiency
The fluorinated compounds in electrolytes help form a protective layer around the metallic lithium at the battery's negative electrode.
This protective layer can be compared to the enamel of a tooth. It protects the metallic lithium from continuous reaction with electrolyte components.
Maria Lukatskaya, Professor, Department of Electrochemical Energy Systems, ETH Zurich
Without it, the cell would fail during cycling, the electrolyte would quickly run out, and instead of a conformal flat layer forming during recharging, lithium metal whiskers, or "dendrites," would form due to the absence of a stable layer.
If these dendrites come into contact with the positive electrode, a short circuit would result, increasing the possibility that the battery may overheat and catch fire. Therefore, controlling this protective layer's properties is essential to battery performance. A steady protective layer increases battery longevity, safety, and efficiency.
Minimizing Fluorine Content
The question was how to reduce the amount of added fluorine without compromising the protective layer’s stability.
Nathan Hong, Doctoral Student, ETH Zurich
The group's novel approach uses electrostatic attraction to produce the intended result. Here, fluorine is transported to the protective layer by electrically charged fluorinated molecules. This implies that the amount of fluorine needed in the liquid electrolyte is only 0.1 % by weight, at least 20 times less than in previous research.
Optimized Method Makes Batteries Greener
The study outlines the new approach and its fundamental principles. A patent application has been submitted. Lukatskaya conducted this research with funding from an SNSF Starting Grant.
One of the most challenging tasks was identifying the appropriate molecule to which fluorine could be attached and which, once it reached the lithium metal, would also break down again under the correct circumstances. As the group notes, one major benefit of this approach is that it can be easily incorporated into the current battery manufacturing process without adding to the expenses associated with changing the production setup. Coin-sized batteries were used in the laboratory. The researchers' next step is to test the scalability of the approach and apply it to pouch cells used in smartphones.
Journal Reference:
Yan, M., et al. (2024) Robust battery interphases from dilute fluorinated cations. Energy & Environmental Science. doi.org/10.1039/d4ee00296b.