Lithium batteries allow electric vehicles to travel several hundred miles on one charge. Their capacity for energy storage is well known — so is their tendency to occasionally catch on fire. This “thermal runaway” occurs most frequently when the batteries overheat or cycle rapidly.
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.
By preventing the oxygen from leaving the cathode and mixing with other flammable products in the battery, the chances of a fire occurring can be reduced. Graphene sheets — super-thin layers of carbon atoms with unique properties — are impermeable to oxygen atoms. Graphene is also strong, flexible, and can be made to be electrically conductive. Wrapping very small particles of the lithium cobalt oxide cathode of a lithium battery in graphene prevents oxygen from escaping.
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.
The wrapped particles were bound together with a binding material to form a usable cathode that was incorporated into a lithium metal battery. When released oxygen was measured during battery cycling, almost no oxygen escaped from cathodes, even at very high voltages. The lithium metal battery continued to perform well even after 200 cycles. The graphene is only a few nanometers thick so there is no extra mass added to the battery.