When a cell is operated outside of the conditions for which it was designed there is the possibility of “thermal runaway” where the cell experiences a significant temperature rise, gases are emitted, which under extreme conditions can catch fire. Whilst these events are rare, this can happen under various conditions of mechanical, thermal, or electrical stress or abuse.
The primary objective will always be to avoid such conditions. However, should an event occur it is imperative that the battery pack is able to safely contain failures of this type. Researchers to date have not had a good understanding of what is happening within the cell when an event occurs: the types and amounts of gases evolved, temperature variations in the cell and the energy contained within the material being ejected.
One of the challenges for industrial researchers is how to collect a wide variety of valuable data simultaneously when a cell is under extreme failure conditions. This includes electrochemical performance, volume and nature of the gases produced, temperature rise, the energy contained in ejected material, and information about chemical and structural changes in the cell. The range of test equipment required to do so is not widely available within industry laboratories. The complexity of the problem is increased by the wide range of operating environments that batteries for aerospace applications will be exposed to, and the strict qualification processes for the industry.
Researchers at UCL’s Electrochemical Innovation Lab have used calorimetry combined with mass spectrometry to provide data to the industry partner on mechanism of failure, heat release and gas composition under a range of conditions. X-ray computed tomography (X-ray CT, a non-destructive technique, similar to that used in medical applications) has been used to show internal structural differences after failure. The use of specialist instrumentation at Diamond Light Source has allowed the capture of high-speed videos of the internal components of a cell during failure using X-ray radiography.
The video below shows how X-ray radiography can be used to analyse thermal runaway. With thanks to UCL, NASA and NREL.
Significance and Impact
In the next phase of the programme, collected experimental data will be used, along with the combustion and mechanical strength modelling capability, to construct computer models that predict the potential combustion behaviour of the collected gases. This in turn will be used to anticipate the behaviour of cells in aerospace and other applications in order to inform design approaches that mitigate or eliminate failure risk. The experimental data will also be used to design verification test procedures to confirm the performance of pack designs at an early phase.
The project will enable significant advances in modelling capability, allowing UK industry to refine battery pack designs computationally before choosing the most promising designs and building physical test articles to be destructively tested. This will lead to a faster, cheaper, more efficient battery pack development process for a potentially huge industry; the results of a 2018 UK government consultation indicate the hybrid-electric aviation market could generate up to £4 trillion in the period to 2050.
Longer term the success of this programme could lead to a joined-up approach to battery safety research and validation in the UK, strongly linking academia in the UK and overseas with industry and government bodies such as the Health and Safety Executive and The National Physical Laboratory. It has the potential of making the UK a leader in the development of standards for aviation battery safety.
Outputs from this “industry sprint” will be made openly available. The success of this programme has generated significant interest from companies in adjacent industries, including automotive. Together the Faraday Institution and its industrial partners are integrating the sprint into a larger research programme drawing on researchers across six UK universities expert in the “science of safety” with the aim of improving battery safety across all industry sectors.
Over the past decade, we have developed a suite of tools to understand how battery failure initiates, and how it can propagate from one cell to another in a battery pack. Working with aerospace company as part of the sprint project has provided the opportunity to apply these tools to address key questions relating to battery safety in challenging aerospace applications. We are excited by the opportunity to translate our fundamental understanding to inform design approaches for a range of applications including the electrification of flight.”
Prof Paul Shearing, UCL.
Success story published April 2021.