Light it up: Battery particles tell the true story of a battery’s charge
Lithium-ion batteries power our phones, cars and even homes; Ensuring their safe and efficient behavior has become incredibly important. Using a simple optical technique, researchers at Purdue University have observed that a battery’s individual particles light up as they charge, providing a more complete picture of the battery’s overall health and performance.
“Lithium-ion batteries — in fact, all batteries — function because of millions of chemical interactions that occur at the particle level,” says Kejie Zhao, professor of mechanical engineering. “Characterizing it becomes a mechanical and electrochemical problem.”
In Zhao’s lab, they use many tools to bridge the gap between mechanics and electrochemistry to make better batteries. One of these tools is a simple RGB camera.
“Only recently has it been discovered that individual particles in a battery’s electrode appear brighter as they are charged,” Zhao said. “Our breakthrough is that we look at hundreds of particles at a time and can use their brightness levels to determine how evenly the charge is distributed throughout the electrode.”
This research was published in Proceedings of the National Academy of Sciences.
The experiment started with a lithium-ion coin cell battery in a glove box filled with inert gas (lithium is volatile when exposed to open air). Zhao’s team focused a simple optical microscope on a group of 100 to 1,000 individual particles. They slowly charged the battery and recorded a time-lapse video of the same group of particles over several hours. By analyzing the brightness level of these individual particles, they were able to reconstruct a highly accurate spatial model of how evenly the battery charges.
“The amazing thing about this process is that you don’t need any powerful tools, just a simple optical microscope and camera,” Zhao said. “It doesn’t even have to be sharp; the brightness levels provide consistently accurate data anyway.”
Using image processing and data analysis, the team extracted valuable data about the construction and operation of these batteries. “At present, the only way to check the quality of the particles in a battery is to examine them in the factory,” Zhao said. “But by optically observing how they charge over time, we get a more accurate picture. We found that there is a direct mathematical correlation between the optical brightness of particles and the overall state of charge of the battery.”
And that is important, because the charging and discharging behavior of a battery depends on its heterogeneity, or how evenly the particles are distributed over the electrode. If the charge is concentrated in one spot, the battery is more likely to fail, malfunction, or even suffer catastrophic burns. “Even at the particle level, charge clusters can lead to local defects, which can lead to reduced performance and ultimately thermal shutdown,” Zhao said.
Although Zhao’s experiments focused specifically on lithium-nickel-manganese-cobalt oxides (NMC), he said this optical process has been proven to work for many electrode materials – lithium cobalt oxide, graphite and others – due to the change in electrical conductivity during charging and discharging. In other words, this characterization process can be used for any type of battery formulation in the future.
“Batteries have always been difficult to diagnose,” he said. “Seeing them behave like this, in an active charge or discharge state, provides so much more information than analyzing them in a static state. We’ve proven the theoretical basis and now we can confidently use optical microscopes to analyze today’s batteries and establish the science for future battery technologies.”
Research report:On the scale of heterogeneity in battery composite electrodes
