American university creates a "flood discharge channel" for lithium metal batteries so that the battery will not catch fire or explode when it short-circuits

Nanotechnology engineers at the University of California San Diego have developed a safety feature that can prevent lithium metal batteries from rapidly heating up and catching fire when a short circuit occurs inside them. The team has cleverly adjusted a part of the battery called the "separator" (the barrier between the positive and negative electrodes of the battery) so that when the battery shorts, the flow of energy (i.e. heat) accumulated inside the battery can be slowed down.

(Image credit: University of California, San Diego)

"Rather than trying to prevent the battery from failing, we are trying to make it safer so that when it fails it doesn't catch fire or explode with catastrophic consequences," the researchers said.

(Image credit: University of California, San Diego)

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

(Image credit: University of California, San Diego)

The UC San Diego team has developed a separator that mitigates this. One side of the separator is covered with a thin, partially conductive mesh of carbon nanotubes that intercepts dendrites as they form. When a dendrite pierces the separator and hits the mesh, there is a channel for the electrons to drain out slowly, rather than flowing all at once to the cathode. The researchers liken the battery separator to a spillway on a dam.

"When the dam starts to fail, people open the spillway to let some of the water out in a controlled way, so that when the dam does break and spills water, there is not much water left to cause a flood," said the researchers. "This is similar to the separator we developed, where we slowly drain the charge to prevent the electrons from 'flooding' to the cathode. When the dendrites are intercepted by the conductive layer of the separator, the battery starts to self-discharge, so when the battery does short, there is not enough energy to cause a dangerous situation."

Other battery research efforts have focused on building separators out of materials that are strong enough to stop dendrites from penetrating them. The problem with this approach, however, is that it only prolongs the time until the inevitable occurs. Such separators still need to have pores to allow ions to pass through in order for the battery to work. So if the dendrites eventually break through the separator, the battery short circuit will only be worse. Rather than preventing dendrites from penetrating the separator, the UC San Diego team sought to mitigate their effects.

(Image credit: University of California, San Diego)

In tests, lithium metal batteries equipped with the new separator showed signs of gradual failure after 20 to 30 charge and discharge cycles. Meanwhile, batteries equipped with a conventional (and slightly thicker) separator suddenly failed after just one charge and discharge cycle.

"In real use cases, people don't get an early warning if a battery is about to fail," the researchers said. "It could be fine one second, and the next second the battery is on fire or shorts out completely. It's unpredictable. But with our separator, people can get an early warning that the battery is getting worse every time they charge it."

Although the research focused on lithium metal batteries, the researchers say the separator could also be used in lithium-ion batteries and other battery chemistries. The team will continue to optimize the separator for commercialization, and UC San Diego has applied for a provisional patent for the separator.