This is what a lithium ion battery looks like when it's purposely heated to the point of explosion—for science. Sorry to dash any young pyromaniacs' hopes, but it's not something to try at home: Aside from the obvious dangers of exploding white-hot metal and chemicals, you'd need a synchrotron particle accelerator and a heat gun to pull off a vid like this.
For a new paper in Nature Communications, a group of UK and European researchers pushed two different commercial lithium ion batteries—which are commonly used in everything from smartphones to electric cars—to their breaking point, so they could observe what happens on the inside when they fatally overheat.
"Increasingly we're pushing lithium ion batteries into a greater and greater range of applications," said Paul Shearing, a chemical engineer at University College London and one of the paper's authors, in a phone call. "But actually, the kind of underlying chemistry and the underlying engineering design of these things hasn't changed enormously from when they were commercialised in the late 80s and early 90s."
The second battery, which didn't have the internal support
In order to help advise on safer battery designs, the group wanted to push them to their limits. In rare occasions, lithium ion batteries can fail due to "thermal runaway," which is when an increase in temperature makes the battery get hotter and hotter until it can give up to catastrophic effect. "It's very difficult to actually mitigate the heat that's generated and to spread it, because it's generated in such a short time span," explained UCL PhD student and first author Donal Finegan.
This kind of failure is thought to have contributed to a fire that led to a fatal Boeing 747 crash in 2010, and it's out of concern for this kind of safety risk that some airlines now refuse to carry bulk shipments of the batteries.
To trigger the failure, the researchers aimed a concentrated heat gun at over 200 degrees Celsius on the rotating batteries. In the video, you can see the increased temperature initially where the beam is focused—red, and ultimately white, show the hottest areas. As you can see, it doesn't take long for the heat to spread across the batteries, and both end with a "boom" moment as the cells explode.
The results were captured using the high-speed imaging capabilities of the European Synchrotron Radiation Facility (ESRF) in France, which let the researchers see what was going on beneath the "skin."
"This is taking a very large particle accelerator and using all of the x-rays it can generate, essentially, to image a battery at thousands of frames a second," said Shearing. "It's only by being able to access this extremely fast imaging capability that we can possibly hope to be able to explain these subsurface, very dynamic processes that are occurring."
What they observed, aside from the fact that both batteries tested ultimately exploded, was that each designed failed a bit differently. One had an internal support, and it seemed to fare a bit better up to the point of thermal runaway, by which time it had gotten so hot that the copper inside had melted (which must have meant it had reached around 1,000 degrees Celsius). Finegan said a cylindrical support inside battery cells could help ward off structural collapse.
Helping to inform potential design improvements and safety testing processes is the main point of the work, and Shearing and Finegan next want to test a greater selection of batteries and image the failure at even higher speeds. The current paper saw imaging at thousands of frames per second; Shearing said they hope to do hundreds of thousands of frames per second.
While they're actively seeking to explode batteries in their experiments, the researchers emphasised that this kind of failure is very rare. "We are very pro-battery," said Shearing. "We're keen not to make everyone panic and worry about the safety of their lithium ion battery in their iPhone in their pocket."