The investigation of lithium-ion battery failures is a major challenge for personnel and equipment due to the associated hazards (thermal reaction, toxic gases and explosions). To perform such experiments safely, a battery abuse-test chamber has been developed and installed at the microtomography beamline ID19 of the European Synchrotron Radiation Facility (ESRF). The chamber provides the capability to robustly perform in situ abuse tests through the heat-resistant and gas-tight design for flexible battery geometries and configurations, including single-cell and multi-cell assemblies. High-speed X-ray imaging can be complemented by supplementary equipment, including additional probes (voltage, pressure and temperature) and thermal imaging. Together with the test chamber, a synchronization graphical user interface was developed, which allows an initial interpretation by time-synchronous visualization of the acquired data. Enabled by this setup, new meaningful insights can be gained into the internal processes of a thermal runaway of current and future energy-storage devices such as lithium-ion cells.

The safety of concerns of lithium-ion batteries continues to be a prevalent obstacle toward their widespread application from vehicle electrification to space exploration. Aside from the highly oxidising and reducing electrode materials, their safety is compounded by an inherent drawback of poor heat dissipation [1]. High-speed imaging with in-situ/operando X-ray CT has been used extensively to study various lithium-ion battery safety features and failure mechanisms [2][3], including thermal failure [4]. However, these are exclusively using synchrotron X-ray sources which are limited in terms of both access and data recording capabilities: high frame rates require the data collection window to be restricted to a few seconds. During lithium-ion battery failure, there are several changes to a cell structure leading up to thermal runaway (TR) which can take minutes, and as a result are often missed. Here, we present an instrument that simulates thermal failure for lab-based radiography at slower imaging speeds and longer recording lengths, which has been validated by correlative synchrotron measurements. The failure mechanisms within a fully charged (100 % SOC, 4.2 V) commercially available LiCoO2 cathode and graphite anode pouch cell (651628-2C, AA Portable Power Corp) rated at 210 mAh are investigated. Three samples are studied using lab-based radiography at a frame rate of 3.75 fps with a 16.1 µm pixel resolution and, for comparison, an additional three samples are studied using synchrotron X-ray sources at a higher speed of 20,000 fps with a 13.3 µm pixel resolution. For the six samples investigated, the total time taken from a start temperature of 80 °C to TR is approximately 20 minutes and the onset temperatures for TR are recorded within the range of 196 °C to 210 °C. The beginning of the TR event (defined as a sample temperature increase greater than 15 °C s-1), where the effects to the electrode structure are the most catastrophic, lasts for approximately 1 s. Operando radiographic images during this event reveal that the structural displacement of electrode layers begins at the centre of the cell and propagates outwards in a wave-like motion. The electrode displacement, as a result, is quantified by cross-correlating Gabor signals and spatiotemporal mapping [5] in both types of datasets. For the lab-based radiography, data is recorded from the start temperature to TR (lasting approximately 20 minutes), and reactions such as the electrolyte decomposition, ca. 105 °C, and separator melting, ca.130 °C are characterised in the context of electrode deformation and gas evolution. Investigations of pre- and post-failure 3D X-ray CT images further verify the uniformity of the pristine (or pre-failure) cell assembly as well as the estimated post-failure behaviour between samples. Finally, by comparison with correlative synchrotron measurements, the instrument for inducing thermal failure for lab-based X-ray CT is proven to be a viable and more accessible method to investigate thermal failure within a 210 mAh pouch cell. While synchrotron data has a higher-speed imaging advantage, it is limited to only recording the short TR event at a high temporal resolution. Whereas continuous imaging in lab-based radiography has the benefit of measuring the slower architectural changes taking place up to TR, albeit at a marginally lower spatial resolution. References [1] D. H. Doughty and E. P. Roth, Interface Mag., 21, 37–44 (2012). [2] D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, M. Di Michiel, G. Hinds, D. J. L. Brett, and P. R. Shearing, Phys. Chem. Chem. Phys., 18, 30912–30919 (2016). [3] D. P. Finegan, M. Scheel, J. B. Robinson, B. Tjaden, I. Hunt, T. J. Mason, J. Millichamp, M. Di Michiel, G. J. Offer, G. Hinds, D. J. L. Brett, and P. R. Shearing, Nat. Commun., 6, 6924 (2015). [4] M. T. M. Pham, J. J. Darst, D. P. Finegan, J. B. Robinson, T. M. M. Heenan, M. D. R. Kok, F. Iacoviello, R. Owen, W. Q. Walker, O. V. Magdysyuk, T. Connolley, E. Darcy, G. Hinds, D. J. L. Brett, and P. R. Shearing, J. Power Sources, 470, 228039 (2020). [5] A. N. P. Radhakrishnan, M. Buckwell, M. Pham, D. P. Finegan, A. Rack, G. Hinds, D. J. L. Brett, and P. R. Shearing, ChemRxiv (2021).

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