A novel, paper-like material composed of sponge-like silicon nanofibers could boost by several times the amount of energy delivered per unit weight of a lithium-ion battery.
Researchers at the University of California, Riverside's Bourns College of Engineering, have developed a material which, if viable, could be used in everything from electric vehicles to personal electronics.
The nanofibers — which measure more than 100 times thinner than a strand of human hair — were produced via electrospinning, a technique wherein 20,000 to 40,000 volts are applied between a rotating drum and nozzle for the purpose of emitting a solution composed of tetraethyl orthosilicate, a chemical compound often used with semiconductors. These nanofibers are then exposed to magnesium vapor which, in turn, produces the sponge-like silicon fiber structure.
The obvious question is — why is this even necessary? Well, the problem with lithium-ion batteries is that the anodes are made using copper foil coated with a mix of graphite and polymer binder. Due to the fact that the performance of the graphite has been nearly tapped out (by consumer demand), researchers are actively looking for other materials to use.
This led to experimenting with silicon, which has an electrical charger per unit weight of the battery nearly 10 times higher than graphite. The problem with the material, though, is that it suffers from significant volume expansion. And when this occurs, it will (very quickly) degrade the battery.
The silicon-based nanofibers that the researchers created in their lab circumvents this issue, and allows the battery to be cycled hundreds of times without any display of 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 six graduate students that contributed to this project, while explaining the impact their solution could have on the mass market.
The group’s solution also solves a problem that has plagued free-standing electrodes for years: scalability. In the past, free-scaling materials grown by way of chemical vapor disposition (e.g. carbon nanotubes), could only be produced in small quantities (micrograms). The UC-Riverside team proved in their paper that they were actually able to produce several grams of silicon nanofibers at a time — even at the lab scale.
Looking ahead, the group will try and figure out how best to implement their silicon nanofibers into a pouch cell format lithium-ion battery, basically a larger scale battery format that can be used in electric vehicles and portable electronics.
To learn more, read the group’s paper, entitled “Towards Scalable Binderless Electrodes: Carbon Coated Silicon Nanofiber Paper via Mg Reduction of Electrospun SiO2 Nanofibers,” which was published in the journal Nature Scientific Reports.
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