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.
P&IT: How did you test out the spacesuit batteries? What did you learn?
Paul Shearing: In our group at University College London (UCL), we do a lot of 3D imaging across many, many time and length scales — 3D scans that might have resolutions of millimeters all the way down to tens of nanometers. We also take 3D scans at different speeds, and recently [in February of 2016], we imaged lithiumion batteries (similar to those used in NASA spacesuits) at 20,000 frames a second to capture rapid failure mechanisms. You can collect very, very rapid X-ray data.
Donal Finegan: Researchers at NASA and the National Renewable Energy Laboratories (NREL) designed an internal short circuiting device that would very quickly discharge the battery and release a lot of heat and consequently cause thermal runaway.
Paul Shearing: This internal short circuiting device has been developed by the NREL, in collaboration with NASA, to try to understand better what happens during failure of devices. NASA, under very controlled thermal conditions, can very repeatedly short circuit the battery with an external heat source.
Eric Darcy: This new high-speed X-ray tomography capability provides a greater understanding of how Li-ion battery cells respond to internal short circuits and how effectively the NREL/NASA internal short circuit device triggers similar responses. The device has the advantage of triggering on demand without compromising the cell enclosure or putting the cell at an irrelevant state of charge.
Paul Shearing: We’ve used our technology to image exactly what goes on inside that cell as the internal short circuiting device is activated. In collaboration with Eric Darcy and [NREL senior engineer] Matthew Keyser, we’re trying to evaluate the exact step-by-step process of the internal short circuit activation and the consequential failure that it causes in the cell.
Eric Darcy: We require that the severity of the hazard of a single-cell thermal runaway in a battery be appreciably mitigated. That means no cell-to-cell thermal runaway propagation and no flames exiting the battery enclosure are allowed. Our implantable device enables us to trigger a precisely localized internal short on demand by gently heating the cell greater than 57 °C (the melting point of the dielectric wax of the device). Without the device, one must heat a Li-ion cell greater than 130 °C to melt its separator to trigger an uncontrolled internal short.
Donal Finegan: This is what we imaged at a very high frame rate [in February of 2016] at The European Synchrotron (ESRF), as well as some mechanical abuse and some external puncturing tests that up until recently were used to simulate internal short circuiting. By looking at all of these together, you can understand how the failure of the safety devices can mitigate the catastrophic consequences of thermal runaway.
Eric Darcy: The new high-speed X-ray tomography capability is and will be providing unprecedented insight into how the device initiates a rapid electrical/thermal response which rapidly progresses throughout its electrode jellyroll and can defeat cell safety features such as center mandrels, shutdown separators, and burst disc vents. This will lead to improvements in the device to increase its consistency to trigger hard shorts and cell thermal runaway, and lead to cell and battery design improvements.
P&IT: Where else do you see this technology being used?
Paul Shearing: Historically where it’s been used, apart from medicine, is nondestructive testing. Auto manufacturers — if they want to do quality control on engine casting — routinely scan components to see if they have voids. The companies scan relatively large components for aircraft engines.
We’re normally interested in dynamic X-ray tomography. We can track how things evolve over time, and response to various different stress conditions. We’re particularly interested in pushing the boundaries of either temporal or spatial resolution.
P&IT: How can this information be used to create better batteries?
Donal Finegan: By imaging at very high speeds, we can understand the dynamic mechanism of how the materials inside the battery break down and move. By releasing this data to the manufacturing companies, they can potentially come up with design improvements to make these batteries safer with the knowledge that we have from what happens inside.
Paul Shearing: We want to understand as much as we possibly can about degradation and ultimately failure of these cells, so that we can begin to understand what safety devices can either prevent those failures from occurring, or worstcase scenario: how we can mitigate against the worst possible effects of those failure events. That could be the shutting down of a single cell level, or preventing the propagation of failure from one cell to another if we’re looking at much larger packs.
In the rare event when they do fail in response to these very demanding applications — whether that’s because of high-rate discharge or high pressure, low pressure, or higher temperatures — we want to understand how these failure events happen so we can design next-generation batteries where we can mitigate against these failures.
P&IT: What is most exciting about this kind of technology?
Paul Shearing: This is completely new. Only relatively recently have people been able to image using video cameras at tens of thousands of frames per second. All of a sudden, now we can image using Xrays at tens of thousands of frames a second. This is a really exciting and enabling technology. The limits are your imagination in terms of what you want to look at. You’re now slowing down time into this wonderful capability to be able to see subsurface.
There’s a huge challenge in understanding not just the changes in cell architecture but also the change in electric microstructure, the changes in the electrochemistry, the changes in the materials chemistry. It’s a very, very rich problem to be able to tackle using a range of imaging and spectroscopy tools, and one that we expect to keep us busy for some years to come.
For more information about UCL’s 3D imaging and battery testing efforts, visit https://www.ucl.ac.uk/electrochemicalinnovation-lab/research/imagingdiagnostics/battery_safety .