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Oct 31, 2023

Researchers Combat Corrosion To Reveal True Shape of Lithium for the First Time

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Lithium atoms on a surface naturally form a rhombic dodecahedron – a 12-sided shape similar to d12 dice – when they are protected from corrosion. The discovery could have important consequences for the development of safe lithium metal batteries, which have so far been hampered by unpredictable lithium growths that pose fire risks. The research is published in Nature.

Rechargeable lithium-ion batteries can be found in almost all households, powering everything from smartphones to electric vehicles. They are also used extensively for industrial energy storage applications, such as storing excess energy produced from solar and wind power at peak times.

Lithium-ion batteries hold energy by storing positively charged lithium atoms in a cage-like carbon structure that coats an electrode. But before there were lithium-ion batteries, there were lithium-metal batteries. Rather than carbon, a lithium-metal battery coats the electrode with a thin sheet of pure lithium metal. This small change can pack up to 10 times more lithium into the same space compared to lithium-ion batteries, resulting in a significantly increased performance.

But rechargeable lithium-metal batteries are also far more dangerous. Metallic lithium is highly reactive, to the point where corrosion begins to set in almost immediately during the deposition of the metal onto an electrode. This can cause the lithium atoms to form microscopic spikes and branches which, if they come into contact, can lead to a short circuit.

The inclusion of electrolyte in a lithium-metal battery will also affect the shapes that this surface lithium takes on, and this is partly why lithium-metal batteries are still deemed safe for use in some industries. Until now, the prevailing view had been that electrolyte choice was a major factor in determining whether lithium would form chunky, spiky or columnal shapes on the electrode surface.

“We wanted to see if we could deposit lithium so quickly that we outpace the reaction that causes the corrosion film,” said University of California Los Angeles (UCLA) doctoral student Xintong Yuan, the study’s first author. “That way, we could potentially see how the lithium wants to grow in the absence of that film.”

The researchers developed a new technique that deposits lithium more quickly than traditional methods. By running current through a much smaller electrode, the researchers were able to effectively push the electricity through faster and speed up the deposition process – similarly to how water pushed through a partially blocked hose will shoot out more forcefully.

This technique was used to lay down lithium metal using four different electrolytes, with the shape of the lithium deposits being analyzed using cryo-electron microscopy (cryo-EM) imaging. The shapes observed were compared against additional samples made under similar conditions, but using the more traditional deposition technique.

The new ultrafast deposition technique was found to be so fast that it outpaced lithium’s natural corrosion.

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With corrosion, the lithium formed four distinct microscopic shapes. But using the ultrafast corrosion-free process, the lithium always formed microscale dodecahedron shapes on the electrode surface. The researchers say that this likely reflects the “true” shape of lithium, in the instances where it is not allowed to corrode.

“There are thousands of papers on lithium metal, and most descriptions of the structure is qualitative, such as ‘chunky’ or ‘column-like,’” said Yuzhang Li, the study’s corresponding author and an assistant professor of chemical and biomolecular engineering at the UCLA Samueli School of Engineering.

“It was surprising for us to discover that when we prevented surface corrosion, instead of these ill-defined shapes, we saw a singular polyhedron that matches theoretical predictions based on the metal’s crystal structure. Ultimately, this study allows us to revise how we understand lithium-metal batteries,” Li added.

The discovery of this “true” shape of lithium could mean big things for battery science. If this knowledge can be used to prepare safer lithium-metal batteries that do not have the same structural short circuit risk, then they could represent a significant improvement over the current suite of lithium-ion batteries.

“Scientists and engineers have produced over two decades’ worth of research into synthesizing metals including gold, platinum and silver into shapes such as nanocubes, nanospheres and nanorods,” Li said. “Now that we know the shape of lithium, the question is, can we tune it so that it forms cubes, which can be packed in densely to increase both the safety and performance of batteries?”

Reference: Yuan X, Liu B, Mecklenburg M, Li Y. Ultrafast deposition of faceted lithium polyhedra by outpacing SEI formation. Nature. 2023;620(7972):86-91. doi: 10.1038/s41586-023-06235-w

This article is a rework of a press release issued by the California NanoSystems Institute. Material has been edited for length and content.