Source: ScitechDaily

China Energy Storage Network: Lithium metal batteries can store twice as much energy as lithium-ion batteries, but they face environmental challenges because they require the use of fluorinated solvents and salts. A research team at ETH Zurich, led by Maria Lukatskaya, has developed a method to reduce the fluorine content, thereby improving the stability of the battery, making it more environmentally friendly and economical.

Lithium metal batteries are the main contender for the next wave of advanced high-energy batteries. Compared with commonly used lithium-ion batteries, metal lithium batteries store at least twice the energy per unit volume. As a result, this advance could double the driving distance of electric vehicles on a single charge or reduce the number of charging times for smartphones.

Currently, lithium metal batteries still have an important drawback: liquid electrolytes require the addition of large amounts of fluorinated solvents and fluorinated salts, which increases the impact on the environment. However, without the addition of fluorine, lithium metal batteries are unstable, stop working after a few charges, and are prone to short circuits, overheating, and fire. A research team led by Maria Lukatskaya, Professor of Electrochemical Energy Systems at ETH Zurich, has now developed a new method that drastically reduces the amount of fluorine required in lithium metal batteries, making them more environmentally friendly, more stable and more cost-effective.

Fluorine compounds in the electrolyte help form a protective layer around the lithium metal at the negative electrode of the battery. "This protective layer can be likened to the enamel of a tooth. It protects the lithium metal from continuous reactions with electrolyte components," Lukatskaya explains. "Without it, the electrolyte is quickly depleted during cycling and the battery fails, and the lack of a stable protective layer leads to the formation of dendrites during charging instead of a conformal, planar protective layer."

If these dendrites touch the positive electrode, they can cause a short circuit, which can potentially heat up the battery too quickly and ignite. The ability to control the properties of the protective layer is therefore crucial to battery performance. A stable protective layer can improve the efficiency, safety and lifetime of the battery.

"The question is how to reduce the amount of fluorine added without affecting the stability of the protective layer," says doctoral student Nathan Hong.

The team's new method uses electrostatic attraction to achieve the desired reaction. Here, charged fluorine-containing molecules are the carriers that deliver the fluorine to the protective layer. This means that only 0.1 weight percent of fluorine is needed in the liquid electrolyte, which is at least 20 times lower than in previous studies.

The ETH Zurich research team presents the new method and its basic principles in a paper recently published in the journal Energy & Environmental Science. A patent application has been completed.

One of the biggest challenges was to find suitable molecules that could attach fluorine and, once attached to lithium metal, also decompose again under the right conditions. The research team explains that a key advantage of this method is that it can be seamlessly integrated into existing battery production processes without incurring additional costs due to changes in production settings. The battery used in the laboratory is only the size of a coin.

Next, the researchers plan to test the scalability of the method and apply it to batteries used in smartphones.

[Editor: Gao Qian]