UC San Diego develops current diversion method to prevent lithium batteries from catching fire

Nanoengineers at the University of California San Diego have developed a safety feature that prevents lithium metal batteries from rapidly heating up and catching fire if they experience an internal short circuit. The researchers, led by UC San Diego nanoengineering professor Ping Liu and his doctoral student Matthew Gonzalez, published a paper detailing their work in the journal Advanced Materials.

The team made a clever tweak to a part of the battery called the separator, the barrier between the positive and negative electrodes, so that when the battery shorts, the flow of energy (and therefore heat) that builds up inside the battery is slowed down.

"We're not trying to prevent the battery from failing," said Gonzalez, the paper's first author. "We're just making the battery safer so that when it fails, the battery doesn't catastrophically catch fire or explode."

After repeated charging of lithium metal batteries, needle-like structures called dendrites appear on the anode. Over time, the dendrites grow long enough to penetrate the separator and create a bridge between the anode and cathode, causing an internal short circuit. When this happens, the flow of electrons between the two electrodes gets out of control, causing the battery to immediately overheat and stop working.

The separator invented by the UC San Diego team essentially mitigates this phenomenon. One side is covered with a thin, partially conductive carbon nanotube mesh that intercepts any dendrites that form. When a dendrite pierces the separator and hits the carbon nanotube mesh, the electrons have a pathway so they can drain out slowly, rather than rushing straight to the cathode.

Gonzalez likens the new battery separator to a spillway on a dam. "When the dam starts to fail, the spillway opens to allow some of the water to flow out in a controlled way," he said. "That way, when the dam really breaks and overflows, there won't be much water left to cause a flood. That's the idea of ​​our separator, to drastically slow down the rate at which charge is drained away, preventing a 'flood' of electrons to the cathode. When the dendrites are intercepted by the separator's conductive layer, the battery starts to self-discharge so that when the battery shorts, there won't be enough energy left to be dangerous."

Other battery research efforts have focused on making separators out of materials strong enough to block dendrites from penetrating. But one problem with that approach, Gonzalez said, is that it simply prolongs the inevitable. These separators still need to have holes to let ions pass through in order for the battery to work. So when a dendrite eventually gets through, the short circuit will be made worse.

In tests, lithium metal batteries equipped with the new separator showed signs of gradual failure over 20 to 30 cycles. Meanwhile, batteries with a normal (and slightly thicker) separator experienced sudden failure in just one cycle.

“In a real use case scenario, you wouldn’t have any advance warning that a battery is going to fail. It could be fine one second and catch fire or completely short out the next. It’s unpredictable,” Gonzalez said. “But with our separator, you get advance warning that the battery is getting worse and worse and worse, with every charge.”

While this study focused on lithium metal batteries, the researchers say the separator could also be used for lithium-ion and other battery chemistries. The team will work to optimize the separator for commercial use. UC San Diego has applied for a provisional patent for the research.