In-operando high-speed tomography of lithium-ion batteries during thermal runaway

Abstract

Prevention and mitigation of thermal runaway presents one of the greatest challenges for the safe operation of lithium-ion batteries. Here, we demonstrate for the first time the application of high-speed synchrotron X-ray computed tomography and radiography, in conjunction with thermal imaging, to track the evolution of internal structural damage and thermal behaviour during initiation and propagation of thermal runaway in lithium-ion batteries. This diagnostic approach is applied to commercial lithium-ion batteries (LG 18650 NMC cells), yielding insights into key degradation modes including gas-induced delamination, electrode layer collapse and propagation of structural degradation. It is envisaged that the use of these techniques will lead to major improvements in the design of Li-ion batteries and their safety features.

Figure 1. Schematic illustration of experimental setup and 3D reconstructions of battery cells.

(a) Cut-away of battery containment design attached to the rotation stage for in-operando X-ray CT; (b) arrangement of apparatus for X-ray CT thermal runaway experiments; (c) 3D reconstruction with orthoslices in the XY, YZ and XZ planes of the 2.6 Ah battery (Cell 1) with isolated XY slice; (d) 3D reconstruction with orthoslices in the XY, YZ and XZ planes of the 2.2 Ah battery (Cell 2) with isolated XY slice.

Figure 2. Temperature measurement.

(a) Mean surface temperature profiles of the three regions (shown in the thermal image) on Cell 1 during the thermal abuse test showing thermal runaway after 168 s. The hotspots shown in the thermal image occurred on the surface of the shell after 97 s in Supplementary Movie 1. As judged from the thermal image, the spot size of the heat gun on the surface of the cells was ca. 10 mm in diameter. (b) Mean surface temperature profiles of the three regions on Cell 2 during the thermal abuse test showing thermal runaway after 217 s. The thermal image was extracted from Supplementary Movie 2 after 80 s of heating.

Figure 3. Grey-scale slices from 3D reconstructions in Supplementary Movie 3 during in-operando high-frequency tomography of Cell 1.

(a) Enlarged grey-scale image of the XY plane 15 s before thermal runaway; (b) enlarged grey-scale image of the YZ plane 15 s before thermal runaway; (c) enlarged grey-scale image of the XY plane 1 s before thermal runaway; (d) enlarged grey-scale image of the YZ plane 1 s before thermal runaway. The dotted red lines indicate the through-plane slice with which the neighbouring image is associated. Scale bar, 1 mm.

Figure 4. Post test images in the XY plane comparing the spiral layered structure.

(a) Cell 1 with and (b) Cell 2 without an internal support. Cell 2, without the internal support, showed a severe structural collapse after venting.

Figure 5. Grey-scale slices from 3D reconstructions from Supplementary Movie 4 during in-operando high-frequency tomography of Cell 2.

(a) Enlarged grey-scale image of the XY plane 15 s before thermal runaway; (b) enlarged grey-scale image of the YZ plane 15 s before thermal runaway; (c) enlarged grey-scale image of the XY plane 1 s before thermal runaway; (d) enlarged grey-scale image of the YZ plane 1 s before thermal runaway. The dotted red lines indicate the through-plane slice with which the neighbouring image is associated. Scale bar, 1 mm.

Figure 6. Radiographs from Supplementary Movie 5 showing the propagation of thermal runaway in Cell 1.

(a) Radiograph of the YZ plane before thermal runaway; (b,c,d) sequential images showing the propagation of thermal runaway through the cell. The thermal runaway initiates at the inner layers where the maximum temperature is apparent and spreads radially outwards. The formation of copper globules can be observed as highly attenuating white blots in images b, c and d. Heating is applied from the right of the images but continuous rotation at 180° every 0.4 s maintains an even circumferential temperature distribution. Scale bar, 1 mm.

Figure 7. Radiographs from Supplementary Movie 6 showing the propagation of thermal runaway in Cell 2.

(a) Radiograph of the YZ plane before thermal runaway; (b) radiograph during thermal runaway where the red arrow indicates the region in which structural breakdown is first observed; (c) radiograph during ejection of contents; (d) radiograph immediately after ejection of contents. The time-stamped radiographs show that the entire process of initiation and ejection lasted <0.1 s. Heating is applied from the right of the images, but continuous rotation at 180° every 0.2 s maintains an even circumferential temperature distribution. Scale bar, 1 mm.

Figure 8. Post-mortem tomography of Cell 1 after thermal runaway.

(a) External view of Cell 1 after thermal runaway where the black marks indicate the points at which the bottom slice of the corresponding tomogram begins; (b) 3D reconstruction showing isolated copper phase (yellow), other broken down material (semi-transparent dark grey), battery casing (teal) and central cylindrical support (teal); (c) grey-scale slice from the XY plane; (d) tomogram of the battery vent region showing grey-scale slices from the XY, YZ and XZ planes; (e) 3D reconstruction of the cap region showing the placement of the central cylindrical support near the vent.

Figure 9. Post-mortem tomography of Cell 2 after thermal runaway.

(a) External view of Cell 2 after thermal runaway showing the burst cap and protruding internal contents. The black marks indicate the points at which the bottom slice of the corresponding tomogram begins; (b) 3D reconstruction showing isolated copper phase (yellow), other broken down material (semi-transparent dark grey) and battery casing (blue) where the copper phase is mostly still intact; (c) grey-scale slice from the XY plane.