The importance of thermal runaway testing

Despite recent global events disrupting the automotive industry, the number of electric vehicles (EVs) on road continues to grow. In 2021, almost 10% of global car sales were electric, boosting the worldwide total to 16.5 million EVs– triple the amount in 2018. Already, the number of EVs sold in the first quarter of 2022 was 75% higher than Q1 of 2021 [1]

Battery standards

With more electric powertrains on our roads than ever before, the safety of lithium-ion batteries has become even more important. Before a battery can be certified for use, a variety of safety tests and standards must be passed successfully, involving a plethora of mechanical, electrical and thermal tests. To help customers achieve these standards, Bold has developed a comprehensive testing facility along with rigorous test programmes, giving customers confidence in their battery packs developed at BOLD.

The rate of innovation within the battery sector means that safety standards are constantly evolving; demanding battery manufacturers to be agile and adapt to new standards and tests. For example, following a Tesla Model S crash in Texas last year, changes were made to standards relating to exporting lithium-ion batteries as well as the nail penetration test.

Testing thermal runaway

Another challenge for battery manufacturers is testing thermal runaway. Despite being the biggest threat to battery safety, currently there is no official standard for thermal runaway. This makes designing a battery to mitigate the initiation and propagation of thermal runaway extremely difficult.

A cell can enter thermal runaway if it is overheated, overcharged or has a defect. The goal of the battery manufacturer is to reduce the risk of thermal runaway and prevent it from propagating throughout the module

On average, one in 10 Million cells experience a failure which could lead to thermal runaway [2]. These failures can be due to overcharging, overheating or defects which causes the cells to generate more temperature than can be dissipated; triggering an exothermic chemical reaction. The heat released fuels these exothermic reactions further, leading to a chain reaction which can spread to neighbouring cells.

‘Thermal runaway safety is a hot topic at the moment,’ explains Oscar Crespo, Project Manager at Bold. ‘The high energy and power densities now possible with modern lithium-ion batteries means we are using more of them and with that comes an increased risk to safety. That’s why we’ve been investigating new methods of testing thermal runaway. We want to ensure that our batteries are designed to not only prevent and detect the onset of thermal runaway, but also contain it so that it doesn’t propagate to other cells within the pack, or to the vehicle itself.’

Triggering thermal runaway

To test and analyse the behaviour of a thermal runaway event, you first need a reliable method of triggering it and here lies the initial problem. Accurately replicating an intense and violent phenomenon such as thermal runaway can be a real challenge. Particularly as the precise behaviour of thermal runaway varies depending on the geometry of the cell (cylindrical, prismatic or pouch) as well as it’s chemistry. By understanding the behaviour of thermal runaway for different types of cells, Bold can customise the battery design according to the specific cell type.

‘Nail penetration is one of the most common tests within the industry to trigger thermal runaway,’ highlights Crespo. ‘This is where a cell is penetrated with a nail to simulate an internal short circuit, leading to thermal runaway. However, it’s unusual for this event to occur in service, it’s much more likely that thermal runaway will initiate due to overheating or overcharging. That’s why we have been experimenting with heating different points on a cell to determine what temperature triggers thermal runaway, and whether the location of that temperature makes a difference. In this way we can try to define the most reliable and repeatable method of triggering thermal runaway.’

The behaviour of thermal runaway depends on the chemistry and geometry of the cell. This makes accurately replicating it under test conditions extremely challenging

Heat resistant materials

Once thermal runaway has been triggered, the direction and rate at which the flame propagates throughout the battery pack and the path of any ejected particles needs to be analysed. This will help to develop test procedures that can be used to assess the suitability of materials to endure a thermal runaway event within a battery.

‘Firstly, you want a material that doesn’t catch and propagate the flame and just continues burning itself,’ says Crespo. ‘On the other hand, this means that the material degrades with the flame. So ideally you need materials that don’t catch fire, but also suffer the least damage when burning. We’ve set ourselves the target of using materials that can withstand 2,000 degC for a short period of time, but it’s a real challenge to find those types of materials.’

However, one group of materials that comes close and is suitable for use in batteries is composites with thermoset resin systems. Unlike thermoplastics, when subjected to heat thermosets don’t completely melt away. Instead, they retain their structure long enough to prevent the flame propagating to neighbouring cells or modules.

BOLD UL94 flammability test procedure
Although there are no standards for thermal runaway, there are standards for materials used within batteries, such as the UL94 flammability standard. This is where materials are subjected to a flame for specific durations of time. The more heat resistant the material, the better it’s UL94 rating

‘If a module suffers thermal runaway it will most likely never be used in service again, but the aim is to protect the passengers and therefore stop a thermal runaway fire spreading to other modules within the pack or to the rest of the vehicle,’ explains Crespo. ‘The thermal performance of composites helps to achieve this, whilst also minimising weight. Steels and other metals also have high thermal resistance, but their weight means they are unsuitable for high performance battery packs.’

To achieve the thermal performance necessary to withstand the heat, pressure and flames of a thermal runaway event, Bold have been subjecting different laminates to a variety of thermal tests. Unfortunately, current simulation techniques to model thermal runaway are unreliable, therefore these physical tests have allowed Bold to specify the composite laminates that meet the thermal performance and weight targets of their battery packs.

Battery enclosures house the battery cells and can be designed to help prevent and contain a thermal runaway event

Lithium-ion batteries can be extremely dangerous and at Bold we are very aware of this,’ concludes Crespo. ‘This is why we put significant resource into our test procedures, so that we can develop our batteries to be as thermal runaway safe as possible. Currently there are no alternatives to the thermal runaway test that truly replicates the severity of a cell going into thermal runaway. So we will continue investigating potential solutions to this problem so that our customers can have even greater confidence in the batteries we develop, regardless of the application.’

References

[1] Global EVOutlook 2022 Executive Summary [Online]. The International Energy Agency. Available from: https://www.iea.org/reports/global-ev-outlook-2022/executive-summary

[2] Innovate UK, 2019. Battery testing in the UK report [Online]. Available from:https://www.e4tech.com/resources/243-uk-battery-testing-facilities-needed-to-amp-up-ev-sector.php


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