X-Ray Images Help Researchers Visualize Lithium-Ion Reaction Rates

By Battery Power Online Staff 

September 15, 2023 | In new research published in Nature (DOI: 10.1038/s41586-023-06393-x), researchers from MIT, Stanford University, SLAC National Accelerator, and the Toyota Research Institute have been able to visualize the reactivity of lithium iron phosphate, watching the patterns of lithium-ion flow via X-ray images, which revealed spatial variations in the rate at which lithium ions are absorbed at each location on the particle surface.

According to an MIT press release on the work, the paper’s most significant practical finding is that these variations in reaction rate are correlated with differences in the thickness of the carbon coating on the surface of the particles. The discovery could lead to improvements in the efficiency of charging and discharging such batteries, according to the press release.

“What we learned from this study is that it’s the interfaces that really control the dynamics of the battery, especially in today’s modern batteries made from nanoparticles of the active material. That means that our focus should really be on engineering that interface,” says Martin Bazant, the E.G. Roos Professor of Chemical Engineering and a professor of mathematics at MIT, who is the senior author of the study.

“What I find most exciting about this work is the ability to take images of a system that’s undergoing the formation of some pattern, and learning the principles that govern that,” Bazant says. He foresees that this approach to discovering the physics behind complex patterns in images could also be used to gain insights into other materials, including biological systems.

The imaging approach took scanning tunneling X-ray microscopy images of battery nanoparticles at work and then applied computer vision algorithms to extract new insights from the images. The researchers obtain images that reveal the concentration of lithium ions, pixel-by-pixel, at every point in the particle. They can scan the particles several times as the particles charge or discharge, allowing them to create movies of how lithium ions flow in and out of the particles, according to the press release. By analyzing X-ray images of 63 lithium iron phosphate particles, and using all 180,000 pixels as measurements, the researchers trained the computational model to produce equations that accurately describe the nonequilibrium thermodynamics and reaction kinetics of the battery material, the statement explained.

“Every little pixel in there is jumping from full to empty, full to empty. And we’re mapping that whole process, using our equations to understand how that’s happening,” Bazant says. The researchers also found that the patterns of lithium-ion flow that they observed could reveal spatial variations in the rate at which lithium ions are absorbed at each location on the particle surface.

“It was a real surprise to us that we could learn the heterogeneities in the system—in this case, the variations in surface reaction rate—simply by looking at the images,” Bazant says. “There are regions that seem to be fast and others that seem to be slow.”

Modeling Reaction Rates

Lithium iron phosphate battery electrodes are made of many tiny particles of lithium iron phosphate, surrounded by an electrolyte solution. A typical particle is about 1 micron in diameter and about 100 nanometers thick. When the battery discharges, lithium ions flow from the electrolyte solution into the material by an electrochemical reaction known as ion intercalation. When the battery charges, the intercalation reaction is reversed, and ions flow in the opposite direction.

The new images show that these reaction rates are the same in practice as previous in silico models. The researchers showed that differences in reaction rate were correlated with the thickness of the carbon coating on the surface of the lithium iron phosphate particles. That carbon coating is applied to lithium iron phosphate to help it conduct electricity — otherwise the material would conduct too slowly to be useful as a battery.

“We discovered at the nano scale that variation of the carbon coating thickness directly controls the rate, which is something you could never figure out if you didn’t have all of this modeling and image analysis,” Bazant says.

The results from this study suggest that optimizing the thickness of the carbon layer on the electrode surface could help researchers to design batteries that would work more efficiently, the researchers say.

“This is the first study that’s been able to directly attribute a property of the battery material with a physical property of the coating,” Bazant says. “The focus for optimizing and designing batteries should be on controlling reaction kinetics at the interface of the electrolyte and electrode.”