In an earlier study, Rice University researchers’ made a breakthrough in silicon anodes for lithium-ion batteries. Now they’ve topped themselves by crushing their anodes to make a high-capacity, long life, low-cost anode material that has the potential for use in commercial rechargeable lithium batteries.
As reported in the open access journal Scientific Reports, the anode, (a negative electrode of a battery) easily achieves 600 charge-discharge cycles at 1,000 milliamp hours per gram (mAh/g). This is a noteworthy improvement over the 350 mAh/g capacity of modern graphite anodes. The team leaders, Sibani Lisa Biswal, Rice engineer and Madhuri Thakurbegan, research scientist started investigating batteries four years ago.
“We previously reported on making porous silicon films,” said Biswal. “We have been looking to move away from the film geometry to something that can be easily transferred into the current battery manufacturing process. Madhuri crushed the porous silicon film to form porous silicon particulates, a powder that can be easily adopted by battery manufacturers.”
Silicon is able to hold 10 times more lithium ions than the graphite commonly used in anodes today, but there is a quandary holding back the technology. Silicon has more than triple the volume when completely lithiated. When repeated, the swelling and shrinking causes silicon to quickly break down.
Nanostructured Silicon
Fifty milligrams of the treated powder in the right vial has much more surface area than an identical weight of crushed silicon in the left vial. Credit: Jeff Fitlow/Rice University
Strategies for making silicon more suitable for battery use are being worked on by many researchers. Scientists have created nanostructured silicon with a high surface-to-volume ratio, allowing 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.
Earlier this year, they had advanced to making sponge-like silicon films that showed even more promise. But even silicon films presented a problem for manufacturers, said Thakur.
“They’re not easy to handle and would be difficult to scale up.”
However, by crushing the sponges into porous grains, the material gains far more surface area to soak up lithium ions.
In a news release photo, Biswal holds up two vials. One vial holds 50 milligrams of crushed silicon, the other 50 milligrams of porous silicon powder. The difference between them needs no explanation.
“The surface area of our material is 46 square meters per gram,” said Biswal. “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 said. “The material is very simple to synthesize, cost-effective and gives high energy capacity over a large number of cycles.”
Recent experiments included one in which 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. Our preliminary results with cobalt oxide as the cathode appear very promising, and there are new cathode materials that we’d like to investigate,”
Biswal said.
References:
Thakur, M., Pernites, MR., Nitta, N., Isaacson, M., Sinsabaugh, S., Wong, M.S., Biswal, S.L. Freestanding macroporous silicon and pyrolyzed polyacrylonitrile as a composite anode for lithium ion batteries. Chemistry of Materials (2012), DOI: 10.1021/cm301376t.
Thakur, M., Isaacson, M., Sinsabaugh, S., Wong, M.S., & Biswal, S.L. Gold-Coated porous silicon films as anodes for lithium ion batteries. Journal of Power Sources, 205 pp 426-432 (2012). DOI: 10.1016/j/jpowsour.2012.01.058.
