How do lithium-ion batteries explode in real-time?
The research tracks in real-time how internal structural damage to batteries evolves and can spread to neighbouring batteries.
The results have been released in the same week that international air-safety experts are meeting in Montreal, Canada, under the auspices of the United Nations, to consider proposals to restrict lithium batteries carried as cargo by commercial jets.
Lithium batteries, packed tightly together, can overheat or catch fire if they are damaged or experience short circuits and have been implicated in fires that have brought down two cargo jets in the past decade. Although battery failure is rare, three airlines have already declared plans to no longer carry bulk shipments of lithium-ion batteries in their cargo planes following US Federal Aviation Administration tests found overheating batteries could cause major fires.
The research results have been published in Nature Communications and the first author of the study, UCL PhD student Donal Finegan (UCL Chemical Engineering), said: “We combined high energy synchrotron X-rays and thermal imaging to map changes to the internal structure and external temperature of two types of Li-ion batteries as we exposed them to extreme levels of heat. We needed exceptionally high speed imaging to capture ‘thermal runaway’ – where the battery overheats and can ignite. This was achieved at the ESRF beamline ID15A where 3D images can be captured in fractions of a second thanks to the very high photon flux and high speed imaging detector.”
Previously, X-ray computed tomography (CT) had only been used to analyze battery failure mechanisms post-mortem with static images and to monitor changes to batteries under normal operating conditions.
The team looked at the effects of gas pockets forming, venting and increasing temperatures on the layers inside two distinct commercial Li-ion batteries as they exposed the battery shells to temperatures in excess of 250 degrees C.
The battery with an internal support remained largely intact up until the initiation of thermal runaway, at which point the copper material inside the cell melted indicating temperatures up to ~1000 degrees C. The heat spread from the inside to the outside of the battery causing thermal runaway.
In contrast, the battery without an internal support exploded causing the entire cap of the battery to detach and its contents to eject. Prior to thermal runaway, the tightly packed core collapsed, increasing the risk of severe internal short circuits and damage to neighbouring objects.
Corresponding author, Dr Paul Shearing (UCL Chemical Engineering), said: “Although we only studied two commercial batteries, our results show how useful our method is in tracking battery damage in 3D and in real-time. The destruction we saw is very unlikely to happen under normal conditions as we pushed the batteries a long way to make them fail by exposing them to conditions well outside the recommended safe operating window. This was crucial for us to better understand how battery failure initiates and spreads. Hopefully from using our method, the design of safety features of batteries can be evaluated and improved.”
The team now plan to study what happens with a larger sample size of batteries and in particular, they will investigate what changes at a microscopic level cause widespread battery failure.
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