The reasons lithium batteries catch fire include rapid cycling or charging and discharging, and high temperatures in the battery. These conditions can cause the cathode inside the battery - which in the case of most lithium batteries is a lithium-containing oxide, usually lithium cobalt oxide - to decompose and release oxygen. If the oxygen combines with other flammable products given off through decomposition of the electrolyte under high enough heat, spontaneous combustion can occur.
"We thought that if there was a way to prevent the oxygen from leaving the cathode and mixing with other flammable products in the battery, we could reduce the chances of a fire occurring," said Reza Shahbazian-Yassar, associate professor at UIC.
The team knew that graphene sheets are impermeable to oxygen atoms. Graphene is also strong, flexible and can be made to be electrically conductive. But, had so far only used graphene to help modulate lithium buildup on electrodes in lithium metal batteries.
Assoc Prof Shahbazian-Yassar and Soroosh Sharifi-Asl, a graduate student at UIC and lead author of the paper, thought that if they wrapped very small particles of the lithium cobalt oxide cathode of a lithium battery in graphene, it might prevent oxygen from escaping.
First, the researchers chemically altered the graphene to make it electrically conductive. Next, they wrapped the tiny particles of lithium cobalt oxide cathode electrode in the conductive graphene.
When they looked at the graphene-wrapped lithium cobalt oxide particles using electron microscopy, they saw that the release of oxygen under high heat was reduced significantly compared with unwrapped particles.
Next, they bound together the wrapped particles with a binding material to form a usable cathode, and incorporated it into a lithium metal battery. When they measured released oxygen during battery cycling, they saw almost no oxygen escaping from cathodes even at very high voltages. The lithium metal battery continued to perform well even after 200 cycles.
"The wrapped cathode battery lost only about 14% of its capacity after rapid cycling compared to a conventional lithium metal battery where performance was down about 45% under the same conditions," Sharifi-Asl said.
As graphene is only a few nanometers thick, the team explains it would add no extra mass to the battery either.