Spinning Electrons with Silicon Paper Electrodes

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Husband and wife team, Cenzig S. Ozkam and Mihri Ozkam and their graduate student, Zach Favors, have achieved another innovative approach to creating better batteries.  The blog has cited the Ozkan’s earlier effort that involving a new architecture for high-performance batteries capable of charging and discharging at much higher rates, and Favors’ discovery that beach sand in nano-sized form has some potential to increase battery performance considerably.  It’s silicon, after all.

Zach Favors will share his findings in his presentation, “Beach Sand for Long Cycle Life Li-Ion Batteries,” at the ninth annual Electric Aircraft Symposium.

The three, working in the University of California, Riverside’s Bourns College of Engineering “have developed a novel paper-like material for lithium-ion batteries,” with “the potential to boost by several times the specific energy, or amount of energy that can be delivered per unit weight of the battery.”

Schematic showing silicon spinning, chemical reduction which produces porous fiber

Schematic showing silicon spinning, chemical reduction which produces porous fiber.  (b) shows as-spun fiber, (c)  etched SiNF paper, and (d) C-coated SiNF paper as used in the half-cell test specimen.  Courtesy Zach Favors

etched SiNF paper, and (d) C-coated SiNF paper as used in the Li-ion half-cell configuration.

The material can be spun on a high-speed spinneret, much like a spider’s, but revolving quickly enough to leave even Spiderman dizzy.  More than 100 times thinner than a human hair, 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 the sponge-like silicon fiber structure,” according to the University.

Usually , lithium-ion battery anodes are composed of copper foil coated with a mixture of graphite, a conductive additive, and a polymer binder.   Graphite, though, has reached its limit of conductivity in lithium batteries, so researchers are looking at lighter, more conductive materials such as silicon.  As noted before in the blog, silicon expands and contracts when charging and discharging, leading to performance degradation and eventually material disintegration.

Silicon fiber cycling data (top blue line) compared to carbon electrodes (bottom black line)

Coated Silicon fiber cycling data (top blue line) compared to uncoated fiber (bottom black line).  Note high Coulombic efficiency

Countering graphite’s life-shortening limit, silicon nanofibers from the Ozkan’s lab allow the battery to be cycled hundreds of times without significant degradation.

Favors explains, “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.”

Another limiting factor, small-scale production of free-standing, or “binderless” electrodes – seems to be solved here.  Instead of the micrograms usually created in the lab, these nanofibers can be made in grams at the laboratory level, with scalability feasible for commercial production.  Even though it may be a few years out, the team hopes to mount 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 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, Cengiz S. Ozkan, a professor of mechanical engineering, and six of their graduate students: Zach Favors, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed, Robert Ionescu and Rachel Ye.

Part of that report puts numbers on the high performance of these electrodes: “The free-standing (i.e., binderless) carbon-coated Si nanofiber (C-SiNF) electrodes produce a capacity of 802 mAh g−1 after 659 cycles with a Coulombic efficiency of 99.9%, which outperforms conventionally used slurry-prepared graphite anodes by more than two times on an active material basis. The silicon nanofiber paper anodes offer a completely binder-free and Cu current collector-free approach to electrode fabrication with a silicon weight percent in excess of 80%.”

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.

 

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