Science & Technology - Posted by Mike Williams-Rice on Thursday, November 1, 2012 13:26 - 1 Comment
Crushed porous silicon could boost battery life
RICE (US) — Scientists have created a porous silicon powder that could make rechargeable lithium batteries more powerful and cheaper to produce.
“We previously reported on making porous silicon films,” says lead researcher Sibani Lisa Biswal, an assistant professor of chemical and biomolecular engineering at Rice University. “We have been looking to move away from the film geometry to something that can be easily transferred into the current battery manufacturing process.”
Research scientist Madhuri Thakur crushed the porous silicon film “to form porous silicon particulates, a powder that can be easily adopted by battery manufacturers,” Biswal explains, who reports the results in the journal Scientific Reports.
Porous silicon powder mixed with pyrolyzed polyacrylonitrile is the basis for a robust anode for lithium-ion batteries. Anodes developed with the powder at Rice University have achieved more than 600 charge-discharge cycles in the lab. View larger. (Credit: Madhuri Thakur/Rice University)
Their silicon-based anode, the negative electrode of a battery, easily achieves 600 charge-discharge cycles at 1,000 milliamp hours per gram (mAh/g). This is a significant improvement over the 350 mAh/g capacity of current graphite anodes.
That puts it squarely in the realm of next-generation battery technology competing to lower the cost and extend the range of electric vehicles.
Pros and cons of silicon
Silicon can hold 10 times more lithium ions than the graphite commonly used in anodes today. But there’s a problem: silicon more than triples its volume when completely lithiated. When repeated, this swelling and shrinking causes silicon to quickly break down.
Many researchers have been working on strategies to make silicon more suitable for battery use. Scientists have created nanostructured silicon with a high surface-to-volume ratio, which allows the silicon to accommodate a larger volume expansion.
Biswal, lead author Thakur, and co-author Michael Wong, a professor of chemical and biomolecular engineering and of chemistry, tried the opposite approach: they etched pores into silicon wafers to give the material room to expand.
By earlier this year, they had advanced to making sponge-like silicon films that showed even more promise.
Even those films presented a problem for manufacturers, Thakur notes. “They’re not easy to handle and would be difficult to scale up.”
But by crushing the sponges into porous grains, the material gains far more surface area to soak up lithium ions.
Biswal held up two vials, one holding 50 milligrams of crushed silicon, the other 50 milligrams of porous silicon powder. The difference between them was obvious. “The surface area of our material is 46 square meters per gram,” she continues.
“Crushed silicon is 0.71 square meters per gram. So our particles have more than 50 times the surface area, which gives us a larger surface area for lithiation, with plenty of void space to accommodate expansion.”
The porous silicon powder is mixed with a binder, pyrolyzed polyacrylonitrile (PAN), which offers conductive and structural support.
“As a powder, they can be used in large-scale roll-to-roll processing by industry,” Thakur says. “The material is very simple to synthesize, cost-effective, and gives high energy capacity over a large number of cycles.”
“This work shows just how important and useful it is to be able to control the internal pores and the external size of the silicon particles,” Wong says.
In recent experiments, Thakur designed a half-cell battery with lithium metal as the counter electrode and fixed the capacity of the anode to 1,000 mAh/g. That was only about a third of its theoretical capacity, but three times better than current batteries. The anodes lasted 600 charge-discharge cycles at a C/2 rate (two hours to charge and two hours to discharge).
Another anode continues to cycle at a C/5 rate (five-hour charge and five-hour discharge) and is expected to remain at 1,000 mAh/g for more than 700 cycles.
“The next step will be to test this porous silicon powder as an anode in a full battery,” Biswal says. “Our preliminary results with cobalt oxide as the cathode appear very promising, and there are new cathode materials that we’d like to investigate.”
Researchers at Lockheed Martin co-authored the study.
Source: Rice University