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).

To meet the increasing energy demands of portable devices and electric vehicles, high-nickel lithium-ion cathode materials with the general formula Li(NixMnyCoz)O2 (NMCXYZ) have been extensively researched. Currently, NMC811 is used commercially due to its high capacity and low cobalt content. However, capacity fade is still a prominent issue, with a variety of degradation mechanisms responsible. Of particular interest is secondary particle cracking, where the stress-strain effect of c-parameter expansion in the NMC crystal lattice leads to fractures. Boundaries between the primary particle grain that make up the secondary particle structure act as nucleation points for particle cracking. The exposed surface then reacts with the liquid electrolyte, leading to oxygen evolution and capacity loss. Single-crystal materials have shown promise as a method of improving cycle life, as the lack of agglomerated primary grains appear to make the monocrystalline particles more resilient to the mechanical strain of expansion during charging1. In parallel with the cycle life concerns, attention over battery safety continues to increase as instances of catastrophic battery failure are publicised across the world. While the specific steps that lead to battery thermal runaway have been extensively researched, the safety and thermal stability of degraded cells remains a scarcely investigated topic2. To that end, the goal of this work is to characterise the safety properties of different NMC morphologies, and study how charge-discharge cycling affects the thermal stability of these materials. In this work, we have combined accelerating rate calorimetry (ARC) with lab-based, post-mortem macro, micro and nano X-ray CT to observe the influence of cell structure on thermal runaway of NMC811 pouch cells with single-crystal and polycrystalline cathode materials in both the pristine and aged state. The results presented show that when pristine, the larger surface area of single-crystal materials provides more reaction sites and leads to a lower self-heating onset and thermal runaway initiation temperature compared to polycrystalline, suggesting a lower thermal stability. However, as polycrystalline particles break up during thermal runaway, the fragments create fresh surface to react with the electrolyte and continue the series of exothermic reactions, leading to a higher peak temperature. Macro-CT has revealed how the wound jellyroll structure of the cell focuses the force of the explosion through the flat sides of the cells, possibly increasing the likelihood of failure propagation. It has also been observed that the polycrystalline materials show a greater material ejection out from the centre of the cell, reinforcing the findings from the ARC investigations that although polycrystalline cells are more thermally stable initially, the peak temperatures produced during catastrophic failure are higher due to particle fracture. Micro and nano-CT are used to probe this idea further, revealing that polycrystalline particles show significant damage and fracturing during failure, whereas single-crystal materials remain largely intact. Both polycrystalline and single-crystal morphologies were cycled to 80% capacity retention, with EIS and diagnostic cycles used to identify prevalent degradation modes for both particle structures. As previously found, the single-crystal cells showed superior cycle life to the polycrystalline materials3. The same combination of ARC and X-ray CT was then applied to aged cells, where it was found that aged cells show lower thermal stability than their pristine counterparts. Overall, this work aims to build understanding of the interplay between material structure, safety and degradation of nickel-rich cathodes, with the goal of informing future material development to produce batteries that are safer throughout their lifetimes. G. Qian et al., Energy Storage Mater., 27, 140–149 (2020) https://doi.org/10.1016/j.ensm.2020.01.027. X. Feng et al., Energy Storage Mater., 10, 246–267 (2018) https://doi.org/10.1016/j.ensm.2017.05.013. D. Ren et al., eTransportation, 2, 100034 (2019) https://doi.org/10.1016/j.etran.2019.100034.

To meet the increasing energy demands of portable devices and electric vehicles, high-nickel lithium-ion cathode materials with the general formula Li(NixMnyCoz)O2 (NMC) have been extensively researched. Currently NMC811 is used commercially for high-energy applications. The energy density of NMC also comes with concerns over cycle life and safety1,2. To improve the cycle life of NMC-based cells, single-crystal materials have recently gained attention to tackle the particle cracking issues found in polycrystalline cathodes3. However, for successful introduction to the lithium-ion battery market, inherent safety over a material’s lifetime also needs to be proven. Failure and degradation mechanisms both need to be fully understood to improve the stability of future cathode materials. Abusive testing, such as overheating, overcharge and nail penetration, has been used in conjunction with in-situ and ex-situ X-ray computed tomography (CT) 3D imaging to perform post-mortem studies and understand the relationship between thermal failure and cathode microstructure4,5. However, the interplay between safety characteristics, microstructural properties and material degradation remains unclear. This work first aims to compare the safety performance of polycrystalline and single-crystal NMC811 in 200 mAh pouch cells. Accelerating rate calorimetry (ARC) with a heat-wait-search (HWS) technique is used to heat cells and determine the onset of self-heating, onset of thermal runaway and the peak thermal runaway temperature. Laboratory-based pre- and post-mortem in-situ and ex-situ X-ray CT is also used for non-destructive imaging at multiple length scales to determine how failure propagates through the cells and the impacts on the electrodes and microstructure. Pouch cells containing polycrystalline and single-crystal NMC811 cathode and graphite anode are electrochemically cycled to induce material degradation. EIS measurements and diagnostic cycles are performed to identify prevalent degradation modes in both types of cathode materials. Finally, the same ARC and X-ray CT characterisations are performed on the aged cells to determine how degradation and changes to the material structure affect the safety performance in high-nickel cathode materials. The results of this work will improve the current understanding of capacity fade in high-nickel cathodes and the safety behaviour over the lifetime of a battery cell. This information can then be used to inform future materials development and strategies for mitigating thermal runaway in batteries. References L. Ma, M. Nie, J. Xia, and J. R. Dahn, J. Power Sources, 327, 145–150 (2016). H. J. Noh, S. Youn, C. S. Yoon, and Y. K. Sun, J. Power Sources, 233, 121–130 (2013). J. Langdon and A. Manthiram, Energy Storage Mater., 37, 143–160 (2021). D. Patel, J. B. Robinson, S. Ball, and D. J. L. Brett, (2020). D. P. Finegan et al., Phys. Chem. Chem. Phys., 18, 30912–30919 (2016).