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.