Using sophisticated 3D imaging, a team at University College London, The European Synchrotron (ESRF), University of Manchester, Harwell Oxford, Oregon State University, and the National Physical Laboratory visualized a battery’s performance loss and internal structural damage. The images of active commercial Li/MnO2 disposable batteries, captured using X-ray computed tomography, will help to improve cell designs.
In a Photonics & Imaging Technology Q&A, University College London (UCL) PhD student Donal Finegan, UCL Chemical Engineering professor Paul Shearing, and Battery Technical Discipline Lead at NASA’s Johnson Space Center Eric Darcy explain how the imaging technique provides realtime tracking of degradation within a variety of batteries, including those powering NASA’s spacesuits.
Photonics & Imaging Technology: What are the benefits of X-ray imaging?
Paul Shearing: The beauty of X-ray imaging is that it’s non-destructive. We’re very interested in tracking the evolution of the materials and devices over different time scales. Some of the battery failures happen so quickly; you have to go to some of the most powerful particle accelerators to leverage their extremely fast imaging capability.
P&IT: How do you use 3D imaging to monitor the degradation of batteries? How does it work?
Paul Shearing: We use 2D and 3D imaging. If you rotate your X-ray source and X-ray detector relative to the sample, you build up a sequence of angular projections. Using some backprojection mathematics, we can then reconstruct that into a 3D volume.
That’s exactly the same thing that happens when you go to the hospital and get a CAT scan. In a CAT scan, the human patient is normally stationary; the X-ray source and detector spin around the patient’s head. When applying the technique to materials science and engineering problems, we typically rotate the sample relative to the source. We can do that across multiple time and length scales.
P&IT: What is the accelerator’s role in achieving the data?
Donal Finegan: The accelerator keeps electrons moving at close to the speed of light. As the electrons lose energy in the form of light, the accelerator boosts the electron ring with more electrons and further accelerates already present electrons for more laps of the ring, providing a continuous flux of X-rays at each beamline.
Paul Shearing: With all of our experiments, we‘ve been using The European Synchrotron [in Grenoble, France] to generate very brilliant, high-brightness X-rays. It’s the very-high-brightness, high-flux X-rays that have enabled us to image these cells at such a high frame rate, at thousands of frames a second. By contrast, if you were going to go to a lab-based source, you would be looking at maybe one frame a second, or several seconds per frame. At synchrotrons, we can do high-speed imaging in a way that’s completely inconceivable in a lab source.
P&IT: What antagonistic elements is this 3D imaging designed to find?
Paul Shearing: In terms of a battery, you might just see a very small capacity fade in a battery, just over the many tens and hundreds of cycles of a battery. That’s why your iPhone battery will begin to die after about eighteen months. We can image this. We can cycle a battery for 100 cycles, image it, cycle it for another 100 cycles, and image it; that’s something we routinely do, particularly when we’re looking at higher spatial-resolution degradation effects, which may be linked to material microstructure changes at relatively microscopic levels.
P&IT: What about faster failure events?
Paul Shearing: In terms of the faster failure events, we’re normally interested in either high electrochemical rate; fast charging and discharging; high voltage; and high temperature.
Donal Finegan: Considering the lithium-ion batteries, there are usually three types of safety tests that are performed: electrical, mechanical, and thermal. With electrical: Batteries can be charged or discharged at very high rates, and over-charged and over-discharged, which can put the batteries into a dangerous, unstable region and increase the risk of a process known as thermal runaway, where the battery gets very hot very fast, ultimately catches fire, and (depending on the design) can sometimes explode. Prevention and mitigation of this occurrence is of great interest for mission-critical applications, such as NASA space exploration missions.