The team tweaked the separator, which serves as a barrier between the anode and cathode, so that it slows down the flow of energy and the build up of heat during a short circuit.
"We're not trying to stop battery failure from happening. We're making it much safer so that when it does fail, the battery doesn't catastrophically catch on fire or explode," said researcher Matthew Gonzalez along with UC San Diego nanoengineering professor Ping Liu .
The separator stops the formation of the dendrites that cause the short circuits. One side is covered by a thin, partially conductive web of carbon nanotubes that intercepts any dendrites that form. When a dendrite punctures the separator and hits this web, electrons now have a pathway through which they can slowly drain out rather than rush straight towards the cathode all at once.
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 abrupt failure in a single cycle.
"In a real use case scenario, you wouldn't have any advance warning that the battery is going to fail. It could be fine one second, then catch on fire or short out completely the next. It's unpredictable," said Gonzalez. "But with our separator, you would get advance warning that the battery is getting a little bit worse, a little bit worse, a little bit worse, each time you charge it."
The researchers say the separator can also work in lithium ion and other battery chemistries and they are working on optimizing it for commercial use. A provisional patent has been filed by UC San Diego.