Since their introduction in the late 1990s, lithium ion batteries have greatly evolved. They find use in many day-to-day appliances, such as mobile phones, laptop computers, and medical devices, in addition to aerospace and automotive platforms, among others.
On the other hand, the performance of lithium ion batteries may decay eventually, and they may not charge fully after many charge and discharge cycles and may discharge rapidly even when not in use.
By applying a technique using 3D X-ray tomography of an electrode, researchers at the University of Illinois attempted to gain a better understanding of the mechanism that takes place within a lithium ion battery and eventually design batteries with longer life and more storage capacity.
In other words, when charging a lithium battery, lithium ions are embedded into host particles which are present in the battery anode electrode and stored there until needed to generate energy during the battery discharge. Graphite is a host particle material that is often used in commercial lithium ion batteries. During charging, the graphite particles expand as the lithium ions enter them, and during discharging, they contract as the ions exit them.
“Every time a battery is charged, the lithium ions enter the graphite, causing it to expand by about 10 percent in size, which puts a lot of stress on the graphite particles,” explained John Lambros, professor in the Department of Aerospace Engineering and director of the Advanced Materials Testing and Evaluation Laboratory (AMTEL) at U of I. “As this expansion-contraction process continues with each successive charge-discharge cycle of the battery, the host particles begin to fragment and lose their capacity to store the lithium and may also separate from the surrounding matrix leading to loss of conductivity.”
“If we can determine how the graphite particles fail in the interior of the electrode, we may be able to suppress these problems and learn how to extend the life of the battery. So we wanted to see in a working anode how the graphite particles expand when the lithium enters them. You can certainly let the process happen and then measure how much the electrode grows to see the global strain - but with the X-rays we can look inside the electrode and get internal local measurements of expansion as lithiation progresses.”
Initially, the researchers customized a rechargeable lithium cell that was transparent to X-rays. However, during the development of the functioning electrode, they also added zirconia particles along with graphite particles.
The zirconia particles are inert to lithiation; they don’t absorb or store any lithium ions. However, for our experiment, the zirconia particles are indispensable: they serve as markers that show up as little dots in the X-rays which we can then track in subsequent X-ray scans to measure how much the electrode deformed at each point in its interior.
In order to measure internal changes in the volume, a Digital Volume Correlation routine—an algorithm in a computer code used for comparing the X-ray images before and after lithiation—is used, added Lambros.
Nearly a decade ago, the software was developed by Mark Gates, a U of I computer science doctoral student co-advised by Michael Heath, who is in U of I’s Department of Computer Science, and by Lambros. Gates enhanced the prevailing DVC schemes by introducing certain vital changes to the algorithm. Instead of being able to solve very small-scale problems with reduced amounts of data, Gates’ version includes parallel computations that run various parts of the program simultaneously and can generate results quickly, across a great number of measurement points.
“Our code runs much faster and instead of just a few data points, it allows us to get about 150,000 data points, or measurement locations, inside the electrode,” Lambros remarked. “It also gives us an extremely high resolution and high fidelity.”
Lambros added that this technique may be used by only a few research groups in the world.
Digital Volume Correlation programs are now available commercially, so they may become more common. We’ve been using this technique for a decade now, but the novelty of this study is that we applied this technique that allows internal 3D measurement of strain to functioning battery electrodes to quantify their internal degradation.
The paper titled, “Three-Dimensional Study of Graphite-Composite Electrode Chemo-Mechanical Response using Digital Volume Correlation,” was co-authored by Joseph F. Gonzalez, Dimitrios A. Antartis, Manue Martinez, Shen J. Dillon, Ioannis Chasiotis, and John Lambros. The article is published in Experimental Mechanics.
The study was partly funded by the University of Illinois Interdisciplinary Innovation Initiative, a National Science Foundation Graduate Research Fellowship, and the Air Force Office for Scientific Research.