New Kevlar-based nanocomposite serves as dendrite-suppressing Li-ion battery separator with high ionic conductivity
Researchers at the University of Michigan, with colleagues at Ford and the Harbin Institute of Technology in China, have developed a dendrite-suppressing membrane exhibiting high modulus, ionic conductivity, flexibility, ion flux rates and thermal stability for Li-ion batteries by using a composite made from Kevlar-derived aramid nanofibres assembled in a layer-by-layer manner with poly(ethylene oxide).
In a paper published in Nature Communications, they report that the porosity of the ion-conducting membrane (ICM) is smaller than the growth area of the dendrites; the aramid nanofibers thus eliminate “weak links” where dendrites can pierce a membrane. The aramid nanofiber network also suppresses poly(ethylene oxide) crystallization detrimental for ion transport.
A U-M team of researchers also founded Ann Arbor-based Elegus Technologies to bring this research from the lab to market. Mass production is expected to begin in the fourth quarter 2016.
Dendrite growth threatens the safety of batteries by piercing the ion-transporting separators between the cathode and anode. Finding a dendrite-suppressing material that combines high modulus and high ionic conductance has been a major technological and materials science challenge.
Unlike other ultra strong materials such as carbon nanotubes, Kevlar is an insulator. This property is perfect for separators that need to prevent shorting between two electrodes.—Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering and corresponding author of the paper
While the widths of pores in other membranes are a few hundred nanometers, the pores in the membrane developed at U-M are 15-to-20 nanometers across. They are large enough to let individual lithium ions pass, but small enough to block the 20-to-50-nanometer tips of fern-like dendrite structures.
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. We’ve seen a lot of interest from people looking to make thinner products.—Dan VanderLey, an engineer who helped found Elegus through U-M’s Master of Entrepreneurship program
Thirty companies have requested samples of the material.
Kevlar’s heat resistance could also lead to safer batteries as the membrane stands a better chance of surviving a fire than most membranes currently in use.
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.
The research was funded primarily by the National Science Foundation under its Chemical, Bioengineering, Environmental and Transport Systems and its Innovation Corp. Partial funding also came from Office of Naval Research and Air Force Office Scientific Research. Kotov is a professor of chemical engineering, biomedical engineering, materials science and engineering and macromolecular science and engineering.
Siu-On Tung, Szushen Ho, Ming Yang, Ruilin Zhang & Nicholas A. Kotov (2015) “A dendrite-suppressing composite ion conductor from aramid nanofibres” Nature Communications 6, Article number: 6152 doi: 10.1038/ncomms7152