Cryogenic Laser Reveals Short-Circuit Mechanism In Lithium Metal Batteries


Researchers developed a technique to look inside an intact lithium battery for diagnostics.

Lithium metal battery(LMB) technology is one of the promising battery technologies that can charge faster and can hold significantly more charge than lithium-ion batteries. But the failure rates are high and there are safety problems like fires and explosions. Moreover, these batteries degrade rapidly in a few hundred charging and discharging cycles. These issues are limiting the commercialization of these batteries. Researchers across the globe have hypothesized why and how these batteries fail, but there is no direct evidence based on those theories.

Researchers from the Sandia National Laboratories have taken images inside intact, lithium-metal coin batteries to challenge the existing theories. According to the researchers, this significant step could help make future high-performance batteries, such as for electric vehicles, safer, more powerful and longer lasting. Sandia scientists, in collaboration with Thermo Fisher Scientific Inc., the University of Oregon and Lawrence Berkeley National Laboratory, published the images recently in ACS Energy Letters.

Internal byproducts fails the batteries

The team of researchers repeatedly charged and discharged the lithium coin cells with the same high-intensity electric current. They observed that some cells went through a few cycles, while some went through more than a hundred cycles. The researchers then sent these cells to Thermo Fisher Scientific in Hillsboro, Oregon, for visual analysis.

The team in Oregon took images inside the lithium batteries and were expecting to see needle-shaped deposits of lithium spanning the battery. These deposits are thought to form spikes after repetitive cycles, and punch through a plastic separator between the anode and the cathode, forming a bridge that causes a short. But lithium is a soft metal and so scientists were unable to figure out how it could get through the separator. 

The team found out that every time the battery is recharged, solid electrolyte interphase (SEI) is formed. It tore holes in the separator, creating openings for metal deposits to spread and form a short. This suggests that the byproducts formed while recharging are much more destructive than previously thought. 

“The separator is completely shredded,” said battery scientist Katie Harrison, who leads Sandia National Laboratories, adding that this mechanism has only been observed under fast charging rates needed for electric vehicle technologies, but not slower charging rates.

Cryogenic lasers to take images inside the batteries

The metal shells in the battery limits what can be observed regarding the battery failure. X-rays can look inside without ripping the metal shell but distorts whatever evidence might be inside. Katie Jungjohann, a Sandia nanoscale imaging scientist at the Center for Integrated Technologies, and her collaborators used a microscope that has a laser to mill through a battery’s outer casing. They combined this with a sample holder that keeps the cell’s liquid electrolyte frozen at temperatures between minus 148 and minus 184 degrees Fahrenheit. The hole milled is large enough to pass a narrow electron beam inside and take a high-resolution image of the battery’s internal cross section with enough detail to distinguish the different materials.

“This is what battery researchers have always wanted to see,” Jungjohann said.


 





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