Battery researchers stare into the void

Using X-ray tomography, a research team was able to observe the internal evolution of the materials inside solid-state lithium batteries as they were charged and discharged. Detailed three-dimensional information from this study could help improve the reliability and performance of these batteries, which replace the flammable liquid electrolytes in existing lithium-ion batteries with solid materials.

The operando synchrotron X-ray computed microtomography imaging revealed how the dynamic changes of electrode materials at lithium/solid-electrolyte interfaces determine the behavior of solid-state batteries. The researchers found that battery operation caused voids to form at this interface, which created a loss of contact that was the primary cause of failure in the cells. They report their findings in a paper in Nature Materials.

"This work provides fundamental understanding of what is happening inside the battery, and that information should be important for guiding engineering efforts that will push these batteries closer to commercial reality in the next several years," said Matthew McDowell, an assistant professor in the George W. Woodruff School of Mechanical Engineering and the School of Materials Science and Engineering at the Georgia Institute of Technology. "We were able to understand exactly how and where voids form at the interface, and then relate that to battery performance."

The lithium-ion batteries now in widespread use for everything from mobile electronics to electric vehicles rely on a liquid electrolyte to carry lithium ions back and forth between electrodes within the battery during cycles of charging and discharging. The liquid electrolyte uniformly coats the electrodes, allowing free movement of the ions.

Rapidly evolving solid-state battery technology uses a solid electrolyte instead, helping to boost energy density and improve the safety of future batteries. But removing lithium from electrodes can create voids at the interface with the solid electrolyte; these voids cause reliability issues that limit how long the batteries can operate.

"To counter this, you could imagine creating structured interfaces through different deposition processes to try to maintain contact through the cycling process," McDowell said. "Careful control and engineering of these interface structures will be very important for future solid-state battery development, and what we learned here could help us design interfaces."

The Georgia Tech research team, led by first author and graduate student Jack Lewis, built special test cells about 2mm wide, designed to be studied at the Advanced Photon Source, a synchrotron facility at Argonne National Laboratory. Four members of the research team studied changes in battery structure during a five-day period of intensive experiments.

"The instrument takes images from different directions, and you reconstruct them using computer algorithms to provide 3D images of the batteries over time," McDowell explained. "We did this imaging while we were charging and discharging the batteries to visualize how things were changing inside the batteries as they operated."

Because lithium is so light, imaging it with X-rays can be challenging and required a special design of the test cells. The technology used at Argonne is similar to that used for medical computed tomography (CT) scans. "Instead of imaging people, we were imaging batteries," McDowell said.

Because of limitations in the testing, the researchers were only able to observe the structure of the batteries through a single cycle. In future work, McDowell would like to see what happens over additional cycles, and whether the structure somehow adapts to the creation and filling of voids. The researchers believe their results would likely apply to other electrolyte formulations, and that this characterization technique could be used to obtain information about other battery processes.

Battery packs for electric vehicles must withstand at least 1000 cycles during a projected 150,000-mile lifetime. While solid-state batteries with lithium metal electrodes can offer more energy for a given size battery, that advantage won't overcome existing battery technology unless they can provide comparable lifetimes.

"We are very excited about the technological prospects for solid-state batteries," McDowell said. "There is substantial commercial and scientific interest in this area, and information from this study should help advance this technology toward broad commercial applications."

This story is adapted from material from the Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.