Why do batteries sometimes catch fire and explode?

We keep hearing now and then about battery explosions after charging. 

How likely would an electric vehicle battery self-combust and explode? The chances of that happening are actually pretty slim: Some analysts say that gasoline vehicles are nearly 30 times more likely to catch fire than electric vehicles. But recent news of EVs catching fire while parked have left many consumers—and researchers—wondering over how these rare events could possibly happen.

Researchers have long known that high electric currents can lead to "thermal runaway"—a chain reaction that can cause a battery to overheat, catch fire, and explode. But without a reliable method to measure currents inside a resting battery, it has not been clear why some batteries go into thermal runaway, even when an EV is parked.

Now, by using an imaging technique called "operando X-ray microtomography," scientists have shown that the presence of large local currents inside batteries at rest after fast charging could be one of the causes behind thermal runaway.

These scientists are the first to capture real-time 3D images that measure changes in the state of charge at the particle level inside a lithium-ion battery after it's been charged.

In a lithium-ion battery, the anode component of the electrode is mostly made of graphite. When a healthy battery is charged slowly, lithium ions weave themselves between the layers of graphite sheets in the electrode. In contrast, when the battery is charged rapidly, the lithium ions have a tendency to deposit on the surface of the graphite particles in the form of lithium metal.

What happens after fast charging when the battery is at rest is a little mysterious. But the method used for the new study revealed important clues.

Experiments  show that when graphite is "fully lithiated" or fully charged, it expands a tiny bit, about a 10% change in volume—and that current in the battery at the particle level could be determined by tracking the local lithiation in the electrode.

A conventional voltmeter would tell you that when a battery is turned off, and disconnected from both the charging station and the electric motor, the overall current in the battery is zero.

But in the new study, the research team found that after charging the battery in 10 minutes, the local currents in a battery at rest (or currents inside the battery at the particle level) were surprisingly large.

Parkinson's 3D microtomography instrument at the ALS enabled the researchers to pinpoint which particles inside the battery were the "outliers" generating alarming current densities as high as 25 milliamps per centimeter squared. In comparison, the current density required to charge the battery in 10 minutes was 18 milliamps per centimeter squared.

The researchers also learned that the measured internal currents decreased substantially in about 20 minutes. Much more work is needed before their approach can be used to develop improved safety protocols.

Alec S. Ho et al, Large Local Currents in a Lithium-Ion Battery during Rest after Fast Charging, ACS Nano (2023). DOI: 10.1021/acsnano.3c05470