Engineers have developed a novel paper-like material for lithium-ion batteries. It 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.
This paper-like material is composed of sponge-like silicon nanofibres more than 100 times thinner than human hair. It could be used in batteries for electric vehicles and personal electronics.
The findings were just published in a paper, ‘Towards Scalable Binderless Electrodes: Carbon Coated Silicon Nanofiber Paper via Mg Reduction of Electrospun SiO2 Nanofibers’, in the journal Nature Scientific Reports. The authors were Mihri Ozkan, a professor of electrical and computer engineering and Cengiz Ozkan, a professor of mechanical engineering, along with six of their graduate students.
The nanofibres were produced using a technique known as electrospinning, whereby 20,000 to 40,000 volts are applied between a rotating drum and a nozzle, which emits a solution, composed mainly of tetraethyl orthosilicate (TEOS), a chemical compound frequently used in the semiconductor industry. The nanofibres are then exposed to magnesium vapour to produce the sponge-like silicon fibre structure.
Conventionally produced lithium-ion battery anodes are made using copper-foil coated with a mixture of graphite, a conductive additive, and a polymer binder. As the performance of graphite has been nearly tapped out, researchers are experimenting with other materials. One such material is silicon, which has a specific capacity, or electrical charge per unit weight of the battery, nearly ten times higher than graphite.
Circumventing battery degradation
The problem with silicon is that is suffers from significant volume expansion, which can quickly degrade the battery. The silicon nanofibre structure created in the Ozkan’s labs circumvents this issue and allows the battery to be cycled hundreds of times without significant degradation.
“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,” said Zach Favors, one of the graduate students working with the group.
This technology also solves a problem that has plagued free-standing, or binderless, electrodes for years: scalability. Free-standing materials grown using chemical vapour 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 nanofibres at a time, even at the lab scale.
The researchers’ future work involves implementing the silicon nanofibres into a pouch-cell format lithium-ion battery, which is a larger-scale battery format that can be used in electric vehicles and portable electronics.
The research is supported by Temiz Energy Technologies. The UC Riverside Office of Technology Commercialization has filed patents for inventions reported in the research paper.
Zachary Favors, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed, Robert Ionescu, Rachel Ye, Mihrimah Ozkan, Cengiz S. Ozkan. ‘Towards Scalable Binderless Electrodes: Carbon Coated Silicon Nanofiber Paper via Mg Reduction of Electrospun SiO2 Nanofibers.’ Scientific Reports, 2015; 5: 8246 DOI: 10.1038/srep08246