For the first time, researchers have observed how lithium ions flow across battery interfaces, which could help engineers optimize the design of materials. Researchers from MIT, Stanford University, SLAC National Accelerator and Toyota Research Institute have made breakthrough progress in understanding lithium iron phosphate, an important battery material. Using advanced X-ray image analysis, they found that changes in the material's efficiency are related to the thickness of the carbon coating. The discovery could improve battery performance.
A team of researchers from MIT, Stanford University, SLAC National Accelerator Laboratory, and Toyota Research Institute used machine learning to reanalyze X-ray images of lithium ions entering and exiting battery electrode nanoparticles (left) during battery cycling. The false colors in this image show the charge state of each particle and reveal the inhomogeneous processes inside individual particles. Image source: Cube3D
By mining X-ray image data, researchers at MIT, Stanford University, SLAC National Accelerator and Toyota Research Institute have made significant new discoveries about the reactivity of lithium iron phosphate, a material used in electric car batteries and other rechargeable batteries.
The new technology revealed some previously unseen phenomena, including changes in the rate of lithium intercalation reactions in different regions of lithium iron phosphate nanoparticles.
The most important practical finding of the paper is that changes in these reaction rates are related to differences in the thickness of the carbon coating on the surface of the particles, which may improve the charge and discharge efficiency of such batteries.
By mining X-ray images, MIT researchers have made a major new discovery about the reactivity of lithium iron phosphate, a material used in electric car batteries and other rechargeable batteries. In each pair of particles in the figure, the actual particle is on the left and the researchers' simulated particle is on the right. Image source: Provided by researchers
Interface engineering
"What we learned from this study is that it's the interface that really controls battery dynamics, especially in today's modern batteries made of nanoparticles of active materials." Martin Bazant, the study's senior author and the E.G. Roos Professor of Chemical Engineering and Professor of Mathematics at MIT.
This method of discovering the physics behind complex patterns in images could also be used to delve into many other materials, including not only other types of batteries but also biological systems such as the dividing cells of a developing embryo.
"I think the most exciting thing about this work is that we are able to take images of a system that is forming a pattern and learn the principles that govern that pattern," Bazant said.
collaborative research
Dr. Hongbo Zhao, first author of the new study, was a graduate student at MIT and is now a postdoc at Princeton University. Other authors include Richard Bratz, the Edwin R. Gilliland Professor of Chemical Engineering at MIT, William Chueh, associate professor of materials science and engineering at Stanford University and director of the SLAC-Stanford Battery Center, and Brian Storey, senior director of energy and materials at Toyota Research Institute.
"Until now, we were able to make beautiful X-ray movies of battery nanoparticles at work, but measuring and understanding the subtle details of how they function was difficult because the movies were so informative," Chueh said. "By image learning from these nanoscale movies, we can gain insights that were previously unavailable."
Reaction rate modeling
Lithium iron phosphate battery electrodes are composed of many tiny lithium iron phosphate particles surrounded by an electrolyte solution. Typical particles are about 1 micron in diameter and about 100 nanometers thick. As a battery discharges, lithium ions flow from the electrolyte solution into the material through an electrochemical reaction called ion intercalation. When the battery is charged, the intercalation reaction is reversed and the ions flow in the opposite direction.
"Lithium iron phosphate (LFP) is an important battery material because of its low cost, good safety properties, and use of abundant elements," Storey said. "We are seeing increasing use of lithium iron phosphate in the electric vehicle market, so the timing of this study couldn't be better."
Prior to this study, Bazant had conducted extensive theoretical modeling on the modes of lithium ion intercalation formation. Lithium iron phosphate prefers to exist in one of two stable phases: either full of lithium ions or empty. Since 2005, Bazant has been working on mathematical models of this phenomenon, known as phase separation, which is driven by intercalation reactions that produce unique lithium ion flow patterns. In 2015, while on sabbatical at Stanford, he began working with Chueh to try to interpret images of lithium iron phosphate particles through scanning tunneling X-ray microscopy.
Using this microscope, researchers can obtain images that show, pixel by pixel, the concentration of lithium ions at every point in the particle. They can scan the particle multiple times as it charges or discharges, creating a movie of how lithium ions move in and out of the particle.
In 2017, Bazant and his colleagues at SLAC received funding from the Toyota Research Institute to conduct further research using this approach, along with other battery-related research projects.
insights and findings
By analyzing X-ray images of 63 lithium iron phosphate particles as they were charging and discharging, the researchers found that the movement of lithium ions inside the material was almost identical to previous computer simulations created by Bazant. The researchers used all 180,000 pixels as measurement data to train computational models to generate equations that accurately describe the non-equilibrium thermodynamics and reaction kinetics of battery materials.
"Every little pixel inside is jumping from full to empty, full to empty. We're mapping the entire process, using our equations to understand how this happens," Bazant said.
The researchers also found that the lithium ion flow patterns they observed could reveal spatial variations in the speed at which lithium ions are absorbed at each location on the particle surface.
"It really surprised us that we could look at the images to understand heterogeneities in the system—in this case, changes in the surface's reaction rates. Some areas seemed to react very quickly, and some areas seemed to react very slowly," Bazant said.
In addition, the researchers found that these differences in reaction rates were related to the thickness of the carbon coating on the surface of the lithium iron phosphate particles. The carbon coating on lithium iron phosphate helps it conduct electricity -- otherwise, the material would conduct electricity too slowly to be useful as a battery.
At the nanoscale, changes in carbon coating thickness directly control conductivity, something that would never have been discovered without these modeling and image analyses. The findings also provide quantitative support for a hypothesis proposed by Bazant several years ago: that the performance of lithium iron phosphate electrodes is primarily limited by the rate of coupled ion-electron transfer at the interface between the solid particles and the carbon coating, rather than the rate of lithium ion diffusion in the solid.
Optimize materials
The results of this study show that optimizing the thickness of the carbon layer on the electrode surface can help researchers design batteries that work more efficiently, the researchers said.
This is the first study to be able to directly link the properties of the battery material to the physical properties of the coating. The focus of optimizing and designing batteries should be to control the reaction kinetics at the electrolyte and electrode interface.
"The publication of this paper is the culmination of six years of hard work and collaboration," Storey said. "This technology allows us to uncover the inner workings of batteries in a way that has never been possible before. Our next goal is to improve battery design by applying this new understanding."
In addition to using this analytical method on other battery materials, Bazant anticipates it can be used to study pattern formation in other chemical and biological systems.