A team at the University of Michigan (U-M) has used operando video microscopy to develop a comprehensive understanding of the voltage variations observed during Li metal cycling, which is directly correlated to dendrite growth. Specifically, they observed the evolution of the morphology of the Li electrode through operando high-resolution video capture, and directly correlated the morphology to time synchronized voltage traces.
They then developed a model to relate electrode morphology and competing electrochemical kinetics to cell voltage. This allowed for an in-depth understanding of the electrochemical processes occurring. This work, published in an open-access paper in ACS Central Science, provides a level of detailed understanding that can help researchers take the next steps toward bringing Li metal anodes to commercial reality.
Lithium-sulfur and lithium air batteries have the potential to store 10 times more energy in the same space as the current state-of-the-art lithium-ion batteries. However, the Li-metal electrodes in these next-generation batteries are especially prone to forming dendrites. Dendrites rapidly reduce a battery’s performance, raise safety concerns and cut short its lifetime.
|Dendrites growing in a lithium metal battery. Image courtesy: Neil Dasgupta. Click to enlarge.|
The most frequently used previous techniques to investigate dendrite growth focus on electrochemical measurements while the battery runs. Then, autopsies of the batteries after the experiment finishes reveal what physical changes occurred inside. With this approach, researchers can see the dendrites—but they can’t see how they’d grown.
To capture dendrites in action, the researchers cut a window in a battery and mounted it on a high-definition video microscope, wiring it so that they could monitor both the dendrite growth and the voltage between the two electrodes, which changes during charge and discharge cycles.
In this way, they tied their observations of the electrode—whether dendrites grew or shrunk, and the general state of degradation—with the voltage measurements. Then, they linked the voltage patterns to specific dendrite activity.
To avoid complicating the problem with a different electrode that would develop its own problems, they studied a battery with two lithium electrodes.
Our window battery is a simple platform that can be used by researchers worldwide. It can be reproduced in any lab with an optical microscope, simple electrochemical equipment, a machine shop and a $100 budget.—Neil Dasgupta, U-M assistant professor of mechanical engineering
The researchers were able to see dendrites grow as lithium accumulated on the surface of an electrode, and shrink when the cycle reversed, pulling lithium away from the surface. They saw pits form in the electrode when the lithium was removed, and they saw how these pits became nucleation sites for dendrites on the next cycle.
The lithium dendrites looked strangely organic, like plants growing and withering over the course of a battery cycle. Some dendrites broke off and became “dead lithium” floating around in the battery.
The team discovered that not all dendrites mean serious damage. If the dendrites are small and evenly carpet the surface of the electrode, more lithium remains in play. In this case, the battery performance remains stable, said Kevin Wood, a postdoctoral researcher in mechanical engineering who helped develop the battery window.
If you want to get to practical operating conditions, I don’t think there’s any way to truly prevent dendrite growth. But by controlling dendrite growth you can enable batteries that have long lifetimes and better safety.Kevin Wood
Using this insight, the team discovered a way to significantly extend the lifetime of lithium electrodes, to be revealed in a future publication.
The research was supported by the Department of Energy's Joint Center for Energy Storage Research, Pacific Northwest National Laboratory and the National Science Foundation.
Kevin N. Wood, Eric Kazyak, Alexander F. Chadwick, Kuan-Hung Chen, Ji-Guang Zhang, Katsuyo Thornton, and Neil P. Dasgupta (2016) “Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy” ACS Central Science doi: 10.1021/acscentsci.6b00260