Researchers at the University of California, Riverside’s Bourns College of Engineering have developed a new silicon based paper-like material for lithium-ion batteries. The development has the potential to boost by several times the specific energy, or amount of energy that can be delivered per unit weight of the battery.

The new paper-like material is composed of sponge-like silicon nanofibers more than 100 times thinner than a human hair. The sponge-like material could be used in batteries for electric vehicles and personal electronics.

The development results have been published in the paper, “Towards Scalable Binderless Electrodes: Carbon Coated Silicon Nanofiber Paper via Mg Reduction of Electrospun SiO2 Nanofibers,” in the journal Nature Scientific Reports.

Electrospun Silicon Examples.  Click image for more info.

Electrospun Silicon Examples. Click image for more info.

The nanofibers were produced using a technique known as electrospinning, where 20,000 to 40,000 volts are applied between a rotating drum and a nozzle, which emits a solution composed mainly of tetraethyl orthosilicate, a chemical compound frequently used in the semiconductor industry. The nanofibers are then exposed to magnesium vapor to produce the sponge-like silicon fiber structure.

Scanning Electron Microscope Images of SiO2 Nanofibers.  Click image for more info.

Scanning Electron Microscope Images of SiO2 Nanofibers. Click image for more info.

Conventionally produced lithium-ion battery anodes are made using copper foil coated with a mixture of graphite, a conductive additive, and a polymer binder. But, because the performance of graphite has been nearly tapped out, researchers are experimenting with other materials, such as silicon, which has a specific capacity, or electrical charge per unit weight of the battery, at nearly 10 times higher than graphite.

The problem with silicon as a battery component is that is suffers from significant volume expansion, which can quickly degrade the battery. The silicon nanofiber structure created in the lab circumvents this issue and allows the battery to be cycled hundreds of times without significant degradation.

Graduate student Zach Favors offered, “Eliminating the need for metal current collectors and inactive polymer binders while switching to an energy dense material such as silicon will significantly boost the range capabilities of electric vehicles.”

The technology also solves a problem that has plagued free-standing, or binderless, electrodes for years: scalability. Free-standing materials grown using chemical vapor deposition, such as carbon nanotubes or silicon nanowires, can only be produced in very small quantities (micrograms). However, Favors was able to produce several grams of silicon nanofibers at a time even at the lab scale.

The UC Riverside Office of Technology Commercialization has already filed for patents on the inventions reported in the research paper. The research is being supported by Temiz Energy Technologies.

The researchers’ plan future work involving implementing the silicon nanofibers into a pouch cell format lithium-ion battery, which is a larger scale battery format that can be used in EVs and portable electronics.

The full team includes Mihri Ozkan, a professor of electrical and computer engineering, Cengiz S. Ozkan, a professor of mechanical engineering, and six of their graduate students: Favors, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed, Robert Ionescu and Rachel Ye.

The press release has lit up some bloggers that are making big claims. The team is offering a capacity of 802 mAh g−1 after 659 cycles with a Coulombic efficiency of 99.9%, which outperforms the conventionally used slurry-prepared graphite anodes by over two times on an active material basis.

Bloggers astonishment and back of the envelope multipliers aside, 802 mAh g−1 after 659 cycles is very impressive, indeed.


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