UCL PhD student Donal Finegan 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 battery 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 European Synchrotron Radiation Facility's 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."
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 the battery shells were exposed to temperatures in excess of 250°C.
The battery with an internal support remained largely intact until the initiation of thermal runaway, at which point the copper material inside the cell melted indicating temperatures up to 1000°C.
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.
UCL's Dr Paul Shearing noted: "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, what changes at a microscopic level cause widespread battery failure.
Joining UCL in the work was the European Synchrotron Radiation Facility, Imperial College London and the National Physical Laboratory.
