Silicon-based anode research breakthrough
A group of researchers at Rice University decided to seek a different approach to their previous work in order to develop a high-capacity, durable and low-cost anode material with a great commercial potential for next-generation rechargeable lithium batteries. The newly devised method to use silicon as anode material can nearly triple the capacity of silicon-based anode in lithium-ion based batteries.
Unlike graphite, which is a commonly used material in anodes (negative electrodes of batteries), silicon can hold 10 times more lithium ions. However, silicon’s volume gets tripled once it is completely lithiated, and the repetition of this process when the anode increases and decreases its volume causes silicon to quickly break down.
Led by Sibani Lisa Biswal, an assistant professor of chemical and bio-molecular engineering, and research scientist Madhuri Thakur, the group managed to create a silicon anode able to achieve 600 charge-discharge cycles at 1,000 milliamp hours per gram (mAh/g). That’s nearly triple the capacity of currently used graphite anodes.
Two years ago, Biswal, Thakur and Michael Wong, a professor of chemical and biomolecular engineering and of chemistry, etched micron-sized pores into the silicon wafers surface to provide the material with room to expand. Earlier this year, they made progress by creating sponge-like silicon films that showed even more promise but were hard to be used in scaled up production.
“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.”
By crushing the sponges into porous grains, the material gains far more surface area to soak up lithium ions. Use of crushed silicon powder provides more than 50 times the surface area (46 opposed to 0.71 square meters per gram), thus providing a lot more space for lithiation, as well as plenty of void space to accommodate expansion. The porous silicon powder is mixed with pyrolyzed polyacrylonitrile (PAN) – a binder which offers conductive and structural support.
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 Rice researchers plan to conduct is to test this porous silicon powder as an anode in a full battery, where they plan to use cobalt oxide as the cathode because it appears to be very promising. Silicon powder anode material can be easily synthesized and cost effective, and it can be used in large-scale roll-to-roll processing in production.
For more information, you can read the paper published in Nature’s open access journal Scientific Reports: “Inexpensive method for producing macroporous silicon particulates (MPSPs) with pyrolyzed polyacrylonitrile for lithium ion batteries” [1.4MB PDF].