Researchers have discovered potential pathways to making enhanced, solid-state batteries with crystalline materials called garnets.

A new study explores how a promising class of gemstone materials could be a key ingredient in next-generation batteries. The research team included the U.S. Department of Energy’s (DOE) Argonne National Laboratory, DOE’s Oak Ridge National Laboratory, Princeton University, and Purdue University. 

The study used newer x-ray techniques at Argonne’s Advanced Photon Source, a DOE Office of Science user facility.

Garnets as a Potential Battery Material

The ideal battery is one that can store a large amount of energy safely. Solid-state batteries, which contain all solid materials, offer great potential to do that. But these technologies face significant technical hurdles. A key challenge with using lithium metal as the anode (negative electrode) in solid-state batteries is the formation of needle-like growths called filaments. Filaments make batteries less safe and less durable.

Gemstone materials known as garnets are a potential electrolyte for solid-state batteries. Electrolytes are materials inside batteries that move ions from one electrode to the other. Garnets are promising because they can move ions through batteries quickly and are relatively stable in the presence of lithium. However, filaments can form with garnets. The research team sought to understand why that happens and how garnets can be engineered to eliminate filaments.

Combining Two Techniques

At the Advanced Photon Source (APS) facility, the team simultaneously applied two techniques to observe an operating battery with a garnet electrolyte and lithium anode. The battery was repeatedly charged and discharged until it failed due to filaments and other damage. The objective was to track filament growth and material degradation.

The first technique, called far-field high-energy diffraction microscopy, involves directing high-energy synchrotron x-ray beams into a sample as it is rotating. A detector records where the x-ray beams scattered.

“Compared to traditional laboratory-scale x-rays, synchrotron x-rays are a billion times brighter and can penetrate much deeper into materials,” says Jon Almer, an Argonne physicist and one of the study’s authors. ​“This capability allows us to evaluate changes occurring deep inside batteries as they operate. When using traditional x-ray techniques with batteries, researchers would need to cut the batteries open to characterize the changes inside. It would not be possible to track changes as a battery operates.”

Peter Kenesei, an Argonne physicist and one of the study’s authors, also adds: “Analysis of the x-ray scattering patterns from this advanced tool enables us to characterize much smaller features in materials than would be possible with laboratory x-rays. With the garnets, the analysis allowed us to determine the features of individual grains. In our study, a grain is a tiny crystal with roughly the same diameter as a single human hair.”

The diffraction technique showed that the garnet grains can have different shapes, structures, and orientations. While all grains have the same chemical formula, the elements may be arranged in different ways. This condition is known as polymorphism.

“Before this study, researchers didn’t know where and at what concentrations polymorphism occurs in garnet solid electrolytes,” says Kelsey Hatzell, a Princeton materials scientist and one of the study’s authors. ​“If you evaluate these materials with laboratory x-rays, they look uniform, with repeating, identical crystal arrangements. Our finding is important because tiny structural differences in grains can significantly impact how ions move through garnet materials when a battery is operating.”

The technique also revealed very small changes in the arrangement of atoms in the garnet material as the battery charged and discharged. This information enabled the team to quantify the mechanical stresses on grains—and how these stresses deformed the grains. A better understanding of grain deformation can inform how to build a high performance battery.

The second characterization technique, known as x-ray tomography, involved sending high-energy synchrotron x-ray beams into the battery and measuring how and where the intensity of the x-ray beams was reduced. The team used the measurements to generate 3D digital images of the garnet materials’ interior. This technique reveals much larger scale features in materials than the x-ray diffraction technique.

“By combining the two techniques, we could track changes in both small-scale and large-scale features in the garnets as the material damage progressed,” says Marm Dixit, a battery researcher at Oak Ridge National Laboratory and one of the study’s authors. ​“This allowed us to correlate tiny, grain-level structural features with much bigger damage features, like filaments and fractures. The analysis gave us clues about when, where, and how filaments began to grow.”

Making Garnets Uniform in Structure

The team found that the polymorphic (nonuniform) regions of the garnet materials tend to be where much of the filament formation and other large-scale structural damage occurred. “The correlation between polymorphism and filaments suggests that it may be beneficial to figure out how to make garnets more uniform in structure,” says Dixit.

The team theorized that polymorphic regions in garnets may form as a result of the use of dopants during materials processing. Dopants are chemicals added in tiny amounts to battery materials to optimize their electrical properties.

“It’s possible that the nonuniform distribution of the dopants is causing the nonuniform structure of garnets,” says Dixit. ​“A logical next research step would be to investigate new battery processing methods that apply dopants so that polymorphism doesn’t occur.”

Future Garnet Development

What made the diffraction technique in this study so significant is that it enabled the team to ​“see” individual grains in battery materials. With the upgrade of the APS facility underway, this kind of research is aiming to become more powerful. The upgrade will increase the brightness of the facility’s x-ray beams by up to 500 times.

“We will be able to zoom in on individual grains and observe how their internal structure changes,” says Argonne physicist Jun-Sang Park, one of the study’s authors. ​“This could help to identify the root causes of filaments.”

Featured image: The research team used a powerful X-ray diffraction technique to characterize grains in next-generation battery materials called garnets. Each colored blob represents a grain. Left image: the grains before battery operation; right image: after battery failure. Photo: Argonne National Laboratory.