Last summer, Mihri and Cengiz Ozkan, both professors at the University of California Riverside, put a small twist on all the attempts to use nanostructured silicon on the anodes of lithium-ion (Li-ion) batteries. They dispersed silicon particles onto nanostructures rather than making nanostructures on silicon.
Now the Ozkans are at it again. This time they and their colleagues at UC Riverside have created a paper-like nanofiber material that can be applied to the anodes of Li-ion batteries, boosting by several times a battery’s specific energy—the amount of energy that can be delivered per unit weight.
In research published in the journal Nature, the UC Riverside team produced the nanofibers using a technique known as electrospinning. In electrospinning 20,000 to 40,000 volts are applied between a rotating drum and a nozzle while the nozzle emits a solution composed mainly of tetraethyl orthosilicate (TEOS), a chemical compound commonly used in the semiconductor industry. The special ingredient in this case is that resulting nanofibers are then exposed to magnesium vapor to produce the sponge-like fiber structure.
The structure the researchers came up with is an evolution in the use of silicon in the anodes of Li-ion batteries to replace graphite. Years ago researchers determined that swapping silicon for graphene at the anode could improve a Li-ion battery’s charge life by a factor of ten.
Unfortunately, after a few charge-discharge cycles the silicon cracks from the expansion and contraction of the material and the battery becomes inoperable. This has led to numerous attempts to structure the silicon at the nanoscale to eliminate this cracking.
Other attempts at producing free-standing nanostructured silicon using chemical vapor deposition with carbon nanotubes or silicon nanowires can only be produced in very small quantities (micrograms). This new method is capable of producing several grams of silicon nanofibers at a time even at the lab scale.
In future work, the researchers will be using the silicon nanofibers in a pouch-cell format Li-ion battery, which is a larger scale battery format that can be used in electric vehicles and portable electronics.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.
