[Image above] Silicon nanowires grown on a patterned substrate. Credit: Oak Ridge National Laboratory; Flickr CC BY-NC-ND 2.0
Today, there’s no denying it—we rely on our batteries.
Whether its your laptop, smartphone, or another device, they all require a robust battery to keep you connected and informed (or misinformed).
Advanced materials are the key to boosting the efficiency and capacity of tomorrow’s batteries. We need new materials that can do more, last longer, and fail less.
And when it comes to better lithium-ion batteries, it’s the anode that’s the problem. The capabilities of anodes, which are usually made of graphite, haven’t kept pace with the batteries’ cathodes.
One material that holds a lot of promise for drastically improving battery anodes is silicon—the same stuff that has revolutionized the computing world.
Because of its material properties, silicon has the potential to significantly increase battery capacity. But silicon also has one teensy, tiny, annoying little problem—silicon anodes massively expand when lithium ions migrate in from the battery’s cathode.
While expansion isn’t necessarily a bad thing—to some extent, it’s a really good thing—too much expansion definitely is. During lithiation, silicon expands up to 400%, which causes cracking and catastrophic failure within the battery.
Researchers at North Dakota State University (Fargo, N.D.) in collaboration with Pacific Northwest National Lab (Richland, Wash.) have made important progress in an alternative strategy to incorporate silicon into the anodes of lithium-ion batteries, however: silicon nanowires.
Nanowires can help mitigate the problems that have plagued silicon’s expansion problem, with the additional bonus that nanowires provide enhanced surface area within the battery as well.
The researchers electrospun amorphous silicon nanowires from a liquid precursor containing cyclohexasilane (Si6H12) and polymer carriers. And, added bonus: Using a liquid silicon precursor can help reduce fabrication costs.
That’s because silicon is normally incorporated into electronics from a gas (such as silane), but transporting such silicon-containing gases comes with their own slate of safety issues, which increases cost.
But liquid cyclohexasilane doesn’t transition into a gas until ~400ºF, meaning that, like natural gas, it can be transported as a liquid—which is safer and thus cheaper, according to Doug Freitag, VP of Technology and Business Development at 3DIcon, the company that is commercially advancing the cyclohexasilane technology.
The liquid precursor also contains much more silicon that its alternative gases, so liquid cyclohexasilane increases fabrication rate—again equating to time and thus cost savings.
After electrospinning the silicon nanowires from liquid cyclohexasilane, the scientists heated the nanowires to 350ºC to give them a stabilizing carbon coating. They then tested how well the silicon nanowires could perform in prototype lithium-ion batteries.
Although the authors report that the electrospun silicon nanowire anodes didn’t have as high of a discharge capacity as expected, they note several solvable factors that could be to blame. The important point, they add, is the proof of concept that such a battery didn’t significantly lose capacity during cycling—a pertinent milestone for a silicon anode.
In fact, the work shows that battery capacity decreased only 9.2% from cycle 2 to 30, indicating that the nanowires didn’t catastrophically expand upon lithiation.
So far, the research is a proof of concept, but according to Freitag, the team is currently negotiating with a pilot scale manufacturer to make the liquid material commercially available within the next year.
And its potential reach extends beyond batteries, too—Freitag says that the liquid silicon source is promising for the solar industry as well. Directly printing liquid silicon onto the siding of a house, for example, would afford the ability to integrate solar energy harvesting onto existing building materials. Yet another potential market for the liquid silicon is printable electronics, which would allow advances such as smart packaging, Freitag adds.
The paper, published in Electrochemical and Solid-State Letters, is “Si6H12/polymer inks for electrospinning a-Si nanowire lithium ion battery anodes” (DOI: 10.1149/1.3466994).