An advanced barrier between the electrodes in lithium-ion batteries could help head off fires and overheating that occasionally arises in laptops or industrial applications. The barrier uses nanofibers extracted from Kevlar. The barrier helps minimize the growth of metal tendrils on battery electrodes that can otherwise lead to electrical shorts.

In normal battery operation, there are holes in the barrier that let lithium ions pass through but not electrons. If the holes in the membrane get too big, lithium atoms build up into fern-like structures called dendrites, which eventually poke through the membrane. If they reach the other electrode, the resulting structure gives a path for electrical current within the battery, shorting it out.

“The fern shape is particularly difficult to stop because of its nanoscale tip,” said University of Michigan graduate student Siu On Tung, who is also chief technology officer at Elegus Technologies, a spin-off company that will be commercializing the technology. “It was important that the fibers formed smaller pores than the tip size.”

Other membranes might have pores that are a few hundred nanometers wide. In contrast, the pores in the membrane developed at U-M are 15 to 20 nm across. That is large enough to let individual lithium ions pass, but small enough to block the 20 to 50-nm tips of the fern structures.

The researchers made the membrane by layering the fibers on top of each other in thin sheets. This method keeps the chain-like molecules in the plastic stretched out, which is important for good lithium-ion conductivity between the electrodes, Tung said.

Researchers say they can make the Kevlar material in super-thin sheets that allow squeezing more energy into a given battery size or reducing the size of a cell with a given energy capacity. They also say Kevlar’s heat resistance could lead to safer batteries as the membrane stands a better chance of surviving a fire than most membranes currently in use.

The team is satisfied with the membrane’s ability to block the lithium dendrites, but they are currently looking for ways to improve the flow of loose lithium ions so that batteries can charge and release their energy more quickly.

Elegus Technologies expects to begin shipping the material in the fourth quarter of 2016.