“Bullet-proofing” for Li-ion batteries prevents thermal runaway

While most researchers are chasing cheaper, faster charging and/or higher energy density battery chemistries, a group of University of Michigan researchers are looking to make existing lithium-ion technology safer and more reliable.

Although widely adopted, Li-ion does have its drawbacks. Beside the flammable electrolyte contained under pressure, common battery chemistries, like lithium cobalt oxide, can suffer from thermal runaway. Safety recalls over the past decade have spanned numerous cell phone and laptop batteries to the fires that grounded Boeing 787 Dreamliners in 2013.

To stifle runaway, the U of M research team have developed a new high-tensile material for the battery’s separator — the semi-permeable membrane between the electrodes — made with nanofibers extracted from Kevlar.

“Unlike other ultra strong materials such as carbon nanotubes, Kevlar is an insulator,” said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering. “This property is perfect for separators that need to prevent shorting between two electrodes.”

In a Li-ion battery, the separator allows lithium ions to pass back and forth between the anode and cathode. It also serves to block lithium dendrites — fern-shaped growths caused by the reaction of the anode material to the electrolyte — to span from one electrode to the other. Such growths short the battery internally and create dangerously high temperatures.

“The fern shape is particularly difficult to stop because of its nanoscale tip,” said Siu On Tung, a graduate student in Kotov’s lab, as well as chief technology officer at Elegus. “It was very important that the fibers formed smaller pores than the tip size.”

The pores in the membrane developed at U-M are 15-to-20 nanometers across, large enough to let individual lithium ions pass, but small enough to block the 20-to-50-nanometer 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.

“The special feature of this material is we can make it very thin, so we can get more energy into the same battery cell size, or we can shrink the cell size,” said Dan VanderLey, an engineer who helped found Elegus through U-M’s Master of Entrepreneurship program. “We’ve seen a lot of interest from people looking to make thinner products.”

While the team is satisfied with the membrane’s ability to block the lithium dendrites, 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.

To commercialize its research, the team founded start-up firm Ann Arbor-based Elegus Technologies and anticipates mass production will begin by the end of 2016.
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