To help understand how commercial lithium-ion (Li-ion) batteries fail and potentially explode, researchers at University College London (UCL), the European Synchrotron Radiation Facility (ESRF; Grenoble, France), Imperial College London, and the National Physical Laboratory (Teddington, England), are using a combination of high-speed thermal imaging and x-ray tomography to show how internal structural damage to batteries evolves in real time.1 The method also provides an indication of how this breakdown can spread to neighbouring batteries.
The x-ray tomogram sections of a sample Li-ion cell were over a 360° rotation of the cell using a monochromatic beam with a 10.5 x 7.6 mm field of view, with radiographs recorded at a at a rate of 1250 frames per second (fps). The resulting tomograms had a resolution of 10.87 μm. Thermal imaging was done using a camera made by FLIR Systems France (Croissy-Beaubourg, France) that could detect IR wavelengths between 2.5 and 7 μm.
"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," says UCL PhD student Donal Finegan. "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 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°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 about 1000°C. This 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 neighboring objects.
Batteries pushed to the limit
"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," says UCL researcher Paul Shearing. "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 researchers now plan to study what happens with a larger sample size of batteries. In particular, they will investigate what changes at a microscopic level cause widespread battery failure.
1. Donal P. Finegan et al., Nature Communications (2015); doi:10.1038/ncomms7924