May 30 2018
Knowing how lithium responds to pressure developed during a battery’s charging and discharging could result in safer, improved batteries.
Stephen Hackney, professor, and Erik Herbert, assistant professor, both of materials science and engineering, reseach the properties of lithium at the nanoscale to understand how the metal reacts under pressure with an eye toward improving solid-state batteries. (Credit: Sarah Bird/Michigan Tech)
There’s an ancient proverb, “You must learn to walk before you learn to run.” Despite such wisdom, many industries overlook the basics and take on marathons instead, including the battery sector.
Lithium ion batteries hold a lot of promise for better storage capacity, however, they are volatile. The news about lithium ion batteries — especially the Samsung Galaxy 7 — causing phones to burst into flames is well known.
Most of the problem comes from the use of flammable liquid electrolyte within the battery. One method is to use a non-flammable solid electrolyte along with a lithium metal electrode. This would increase the energy of the battery while at the same time reducing the possibility of a fire.
Fundamentally, the destination is designing next-generation solid-state batteries that do not explode. The journey is to essentially understand lithium.
Everybody is just looking at the energy storage components of the battery.
Erik Herbert, Assistant Professor of Materials Science and Engineering, Michigan Technological University
Herbert added, “Very few research groups are interested in understanding the mechanical elements. But low and behold, we’re discovering that the mechanical properties of lithium itself may be the key piece of the puzzle.”
Michigan Tech scientists contribute considerably to gaining a deep understanding of lithium with results reported on May 30th, 2018, in an invited three-paper series in the Journal of Materials Research, published in collaboration with the Materials Research Society and Cambridge University Press. The team includes Herbert and Stephen Hackney, professor of materials science and engineering, together with Violet Thole, a graduate student at Michigan Tech, Nancy Dudney at Oak Ridge National Laboratory and Sudharshan Phani at the International Advanced Research Centre for Powder Metallurgy and New Materials. They share results that highlight the importance of lithium’s mechanical behavior in manipulating the performance and safety of next-generation batteries.
Like a freeze-thaw cycle damaging concrete, lithium dendrites damage batteries
Lithium is a very reactive metal, which makes it susceptible to misbehavior. But it is also very good at storing energy. People want their phones (and tablets computers, and other electronic gadgets) to charge as fast as possible, and so battery manufacturers face two challenges: Make batteries that charge very fast, passing a charge between the cathode and anode as fast as possible, and make the batteries reliable in spite of being charged repeatedly.
Lithium is a very soft metal, but it doesn’t act as anticipated when used in battery operation. The growing pressure that inextricably occurs during charging and discharging a battery causes microscopic fingers of lithium known as dendrites to fill pre-existing and inevitable microscopic flaws — pores, grooves, and scratches — at the interface between the solid electrolyte separator and the lithium anode.
During continued cycling, these dendrites can push their way into, and ultimately through, the solid electrolyte layer that physically divides the anode and cathode. Once a dendrite enters the cathode, the device short circuits and fails, often disastrously. Herbert and Hackney’s study concentrates on how lithium alleviates the pressure that naturally forms during charging and discharging a solid-state battery.
Their research documents the extraordinary behavior of lithium at submicron length scales — drilling down into the lithium’s smallest and debatably most perplexing attributes. The team examined how the metal reacts to pressure by indenting lithium films with a diamond-tipped probe to deform the metal. Their results confirm the surprisingly high strength of lithium at small-length scales published earlier this year by scientists at Cal Tech.
Herbert and Hackney build on that study by providing the inaugural, mechanical explanation of lithium’s remarkably high strength.
Lithium’s ability to diffuse or rearrange its own ions or atoms in an attempt to lessen the pressure imposed by the indenter tip, showed the scientists the significance of the speed at which lithium is deformed (which is associated with how fast batteries are charged and discharged), as well as the effects of defects and deviations in the arrangement of lithium ions that include the anode.
Drilling down to understand the behavior of lithium
In the article “Nanoindentation of high-purity vapor deposited lithium films: The elastic modulus,” the scientists measure lithium’s elastic properties to reflect changes in the physical orientation of lithium ions. These results highlight the need to incorporate lithium’s orientation-dependent elastic properties into all simulation work going forward. Herbert and Hackney also provide experimental evidence that specifies lithium may have a heightened ability to convert mechanical energy into heat at length scales less than 500 nm.
In the article that follows, “Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow,” Herbert and Hackney record lithium’s extraordinarily high strength at length scales less than 500 nm, and they provide their original outline, which aims to describe how lithium’s ability to manage pressure is regulated by diffusion and the rate at which the material is deformed.
Lastly, in “Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of the transition from diffusion to dislocation-mediated flow,” the authors provide a statistical model that explains the conditions under which lithium experiences an abrupt transition that further enables its ability to lessen the pressure. They also provide a model that directly connects the lithium’s mechanical behavior to the battery’s performance.
“We’re trying to understand the mechanisms by which lithium alleviates pressure at length scales that are commensurate with interfacial defects,” Herbert says. Enhancing one’s understanding of this important issue will directly enable the development of a steady interface that encourages safe, long-term and high-rate cycling performance.
Herbert added, “I hope our work has a significant impact on the direction people take trying to develop next-gen storage devices.”