Linking High-Throughput, Non-Destructive Quality Control Methods with Electric Vertical Take-Off and Landing Mission Cycles and Battery Safety
Authors: Reid Hamish Thomas , Jones Matthew , Singh Gaurav , Robinson James B 
Published: December 2024

Electric vehicles are the key to the migrating away from fossil fuels. However, the wider electrification of transport, in particular aircraft is attracting increasing attention, with the UK government putting eVTOLs at the centre of its ‘Future of Flight’ plan 1. Electric vertical take-off and landing (eVTOL) innovations offer opportunities for rapid, low operating cost, emission-free aviation for urban and inter-city transport. The utilisation of unused airspace opens the possibility of reducing congestion and demand for limited traffic networks. eVTOLs are also flexible, able to land of flat ground or small helipads, infrastructure that is straightforward to integrate into cities and urban environments 2.
For electric-powered flight to be feasible, currently battery technology must be pushed to its extreme limits. Rapid charging and discharging are necessary to make passenger aircraft logistically and economically practical 3. Changes in altitude and geography also mean batteries will need to operate over a wide range of temperatures. The demands placed on the cells used in eVTOLs requires robust quality-control (QC) to ensure that batteries are of the highest quality to meet the stringent safety standards and demanding power needs of aerospace applications. In this work, we collaborated with industrial partners to create a high-throughput X-ray computed tomography protocol to identify and quantify defects in 21700 cylindrical cells geared towards electric-powered flight. A virtual unrolling workflow was applied to quantify and compare the defects across cells. We then applied electrochemical impedance spectroscopy to link the defects found in imaging with electrochemical data, with the aim of devising a rapid, non-destructive QC method for aerospace cells. Both passing and defective cells were then cycled using fast charge and eVTOL mission profile protocols, to identify the prominent degradation mechanisms that cells are expected to suffer from during use. Finally, we used accelerating rate calorimetry to understand the effects of aerospace mission cycling on battery safety, with the aim of informing which second-life applications expired eVTOL packs may be appropriate for.
Initial X-ray screening has shown two major defects: jellyroll buckling and electrode delamination. The severity of defects links to the initial open-circuit voltage (OCV) of the cells, where a lower OCV cell is more likely to have problematic defects. As cells are cycled, particularly in low temperatures and under fast charging, swelling and relaxation of the electrode layers exacerbates the buckling and delamination observed at the centre of the jellyroll. Fast charging is also known to initiate Li metal plating in the graphite anode 4,5. The combination of electrode defects and plating is expected to impact the safety behaviour of the aged cells, reducing the temperature at which cells begin self-heating before going into thermal runaway.
Overall, this work aims to build understanding around eVTOL mission cycling and safety to develop reliable, low-cost QC methods that can be applied to the aerospace sector and beyond.

References

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Figure 1