We attribute this excellent stability to the internal porosity of the SiNFs, which allows for internal volume expansion of the small SiNPs. This internal expansion of Si within the SiNFs effectively preserves the crucial SEI layer that coats the outside of the SiNFs. The existence of the native oxide shell and C-coating also contribute to mitigating volume expansion related effects through creation of a buffer layer.

—Favors et al.

Conventionally produced lithium-ion battery anodes are made using copper foil coated with a mixture of graphite, a conductive additive, and a polymer binder. The absence of conductive powder additives, metallic current collectors, and polymer binders in addition to the high weight percent silicon in the nano-silicon paper electrodes all contribute to significantly increasing capacity at the cell level, the researchers said.

The nanofibers 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 nanofibers are then exposed to magnesium vapor to produce a sponge-like silicon fiber structure.

(a) Schematic representation of the electrospinning process and subsequent reduction process. Digital photographs of (b) as-spun SiO2 NF paper, (c) etched SiNF paper, and (d) C-coated SiNF paper as used in the Li-ion half-cell configuration. Favors et al. Click to enlarge.

This 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, the team was able to produce several grams of silicon nanofibers at a time even at the lab scale.

… we have successfully demonstrated the first synthesis of a scalable carbon-coated silicon nanofiber paper for next generation binderless free-standing electrodes for Li-ion batteries that will significantly increase total capacity at the cell level. The excellent electrochemical performance coupled with the high degree of scalability make this material an ideal candidate for next-generation anodes for electric vehicle applications. C-coated SiNF paper electrodes offer a highly feasible alternative to the traditional slurry-based approach to Li-ion battery electrodes through the elimination of carbon black, polymer binders, and metallic current collectors.

—Favors et al.

The researchers’ future work involves implementing the silicon nanofibers into a pouch cell format lithium-ion battery.

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.

Resources

• Zachary Favors, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed, Robert Ionescu, Rachel Ye, Mihrimah Ozkan & Cengiz S. Ozkan (2015) “Towards Scalable Binderless Electrodes: Carbon Coated Silicon Nanofiber Paper via Mg Reduction of Electrospun SiO₂ Nanofibers” Scientific Reports 5, Article number: 8246 doi:10.1038/srep08246