Scientists X-ray Li-ion batteries for deeper insight

Modeling the reaction

LFP as a material has two preferences: to be full of lithium ions or empty. Known as phase separation, scientists have been working on mathematical modeling for distinctive patterns of lithium-ion flow.

Back in 2015, Martin Bazant, a professor at MIT, teamed up with William Chueh, an associate professor of materials science at Stanford, to work on this problem. The duo used scanning X-ray microscopy to obtain images of LFP particles. The work led to detailed images of the electrode that revealed the concentration of lithium ions.

By taking multiple images of electrodes at different times of charge and discharge cycle, the researchers were able to create movies of the lithium-ion flow. In 2017, the team furthered their research by taking X-ray images of 63 LFP particles as they charged and discharged. Using 180,000 pixels, the researchers were able to train computational models to produce equations to describe the nonequilibrium thermodynamics and reaction kinetics of the material.

Relation to the carbon coating

"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," said Bazant in the press release. "It was a real surprise to us that we could learn the variations in surface reaction rate simply by looking at the images. There are regions that seem to be fast and others that seem to be slow."

The team found that the differences in reaction rates could be correlated with the thickness of the carbon coating on the LFP particles. LFP by itself is slow to conduct, so carbon coating is applied to aid in conducting electricity faster.

"We discovered at the nanoscale 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 added.

These findings also tell us that the performance of the electrodes in an LFP battery depends on the coupled ion-electron transfer at the interface between solid particles and carbon coating and not the rate of ion diffusion into the solid. This will help future research to be directed toward optimizing the thickness of the carbon layer to make more efficient batteries.

The approach of imaging material to study it in depth could also be applied to patterns in chemical and biological systems.

The research findings were published in the journal Nature.