Understanding Battery Failure: A Multi-Scale and High-Speed X-Ray CT Approach
Authors: Finegan Donal P , Scheel Mario , Tudisco Erika , Tjaden Bernhard , Robinson James , Taiwo Oluwadamilola O. , Eastwood David S , Hunt Ian , Di Michiel Marco , Rack Alexander , Hall Stephen A , Bay Brian , Offer Gregory James , Lee Peter D , Hinds Gareth , Brett Daniel J.L. , Shearing Paul R. 
Published: February 2020

The safety of Li-ion batteries is of upmost importance in particular for demanding applications such as electric vehicles and other mission critical systems. The thermal response of a cell is one of the most important characteristics to understand when assessing the safety of a cell design, as witnessed by recent high profile failures1, 2. However, there is limited understanding of the dynamic mechanisms associated with thermal runaway.
X-ray tomography has become a widely used technique for 3D imaging of materials and devices for electrochemical energy storage and conversion. Two of the major advances in tomography techniques in recent years are the reduction in tomogram acquisition time and the increased spatial resolution. In this study, high speed synchrotron X-ray CT of commercial Li-ion batteries during operation and failure was performed in beam-lines ID15A and ID19 at The European Synchrotron (ESRF). Tomograms were captured at a rate of up to 2.5 Hz allowing us to study some of the most rapid failure mechanisms including those associated with thermal runaway in 3D3. Simultaneous thermal imaging and X-ray tomography allowed structural and thermal evolution processes to be tracked dynamically. We have demonstrated the capability to track electrode layer delamination 4, cracking, and gas evolution processes occurring at multiple time and length scales.
Combined with multi-scale post-mortem tomography analysis our recent work has provided insight into the failure of battery materials from the cell scale down to the particle scale. Features which may be indicative of temperatures reached and reaction pathways during failure are identified via inspection of electrode microstructures. For example, surface layers of transition metals on electrode particles are seen; suggesting numerous exothermic reduction steps in the presence of electrolyte. Significant changes in particle morphology are also observed which can affect the rate of heat generation and onset temperatures of thermal runaway.
This combined high-speed and multi-scale tomography approach uses cutting-edge imaging techniques to explore failure mechanisms which were previously not understood. New insights into the structural and thermal dynamics leading up to and during thermal runaway and failure are achieved. The thermal response and catastrophic outcomes observed on the macro scale are linked to the structural properties of active materials on the micro-scale, revealing scope for further investigations on the impact of particle morphology on failure mechanisms of Li-ion cells.

References

1. GCAA Air Accident Investigation Report 13/2010; General Civil Aviation Authority: UAE; 2010.
2. AAIB Report on the serious incident to Boeing B787-8, Air accident report: 2/2015; 2015.
3. Finegan, D. P, et al. In-operando high-speed tomography of lithium-ion batteries during thermal runaway. Nature Communications 2015, 6.
4. Finegan, D. P., et al. Quantifying Bulk Electrode Strain and Material Displacement within Lithium Batteries via High-Speed Operando Tomography and Digital Volume Correlation. Advanced Science 2015, 3, 1.

Figure 1