Carnegie Mellon Engineering

Next-generation batteries will benefit from the discovery of lithium’s exotic mechanical properties

December 21, 2016

Contact: Lisa Kulick
Carnegie Mellon University

A team of researchers at Carnegie Mellon University and California Institute of Technology have discovered that the mechanical properties of lithium metal are stronger at the microscale and highly dependent on orientation. The findings represent a breakthrough in understanding dendrite formation and electrode/electrolyte interactions in lithium ion and next generation batteries.

Dendrites have remained a major hurdle in advancing battery technology that is based on lithium metal anodes. Lithium remains one of the most promising anode materials because it can lead to 200% improvement over current state of the art batteries.

Dendrites are microscopic fibers of lithium that can form during the charging cycle. Over time, they can grow long enough to connect the battery’s electrodes to one another, causing the battery to short-circuit or ignite. The team’s findings show that a promising solution is to suppress the dendrites mechanically with solid electrolytes by tuning the electrode/electrolyte interface properties.

This study was the first to report a significant enhancement in the mechanical strength of lithium at the microscale. Additionally, the authors found that the mechanical properties of lithium metal have a high sensitivity to the crystallographic orientation.

Extreme anisotropy of Lithium metalIt is challenging to examine lithium’s mechanical behavior due to its high reactivity. This has limited researchers’ abilities to prepare samples, characterize its microstructure, and perform mechanical testing.

For this study, the Caltech team conducted micro-mechanical experiments in a controlled environment with a scanning electron microscope that observed lithium micropillars. Viswanathan’s team then performed first-principles simulations to calculate the orientation and temperature dependence of the mechanical properties of lithium.

“We observed that the mechanical properties of lithium can vary by a factor of four, depending on the orientation,” said Ph.D. candidate Zeeshan Ahmad, a leading author of the study who performed the simulations.

“The emergence of such high strengths in small-scale lithium and sensitivity of this metal’s stiffness to orientation help to explain why the existing methods of dendrite suppression have been mainly unsuccessful,” said Assistant Professor of Mechanical Engineering Venkat Viswanathan, co-author of the paper. “This has significant implications for practical design and safety of future-generation batteries.”

Base on the results, the team presents rational guidelines for anode/electrolyte selection and operating conditions that will lead to better cycling performance for next generation batteries.

The study, performed in collaboration with researchers at California Institute of Technology's Greer Group, was published in Proceedings of the National Academy of Sciences this week. Other co-authors are Julia R. Greer and Chen Xu of California Institute of Technology, and Asghar Aryanfar of UCLA. The work was funded by the Department of Energy and the National Science Foundation.

More information: Enhanced strength and temperature dependence of mechanical properties of Li at small length scales and its implications for Li metal anodes, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1615733114

Photo caption: Extreme anisotropy of Li metal, where elastic properties vary up to a factor of four depending on crystal orientation. Li metal anodes with compliant surface orientation may alleviate dendrite growth, leading to extended cycle life in secondary batteries. Image courtesy of Zeeshan Ahmad and Chen Xu.

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