Silicon ‘can radically increase capacity of lithium-ion batteries’

WASHINGTON - Rice University and Lockheed Martin scientists have found that silicon can radically increase the capacity of lithium-ion batteries.

“The anode, or negative, side of today’s batteries is made of graphite, which works. It’s everywhere. But it’s maxed out. You can’t stuff any more lithium into graphite than we already have,” said Michael Wong, a professor of chemical and biomolecular engineering and of chemistry.

Silicon has the highest theoretical capacity of any material for storing lithium but after a couple of cycles of swelling and shrinking, it cracks.

With Mahduri Thakur, a post-doctoral researcher in Rice’s Chemical and Biomolecular Engineering Department, and Mark Isaacson of Lockheed Martin, Sibani Lisa Biswal, an assistant professor of chemical and biomolecular engineering, Wong and Steven Sinsabaugh, a Lockheed Martin Fellow, found that putting micron-sized pores into the surface of a silicon wafer gives the material sufficient room to expand.

While common lithium-ion batteries hold about 300 milliamp hours per gram of carbon-based anode material, they determined the treated silicon could theoretically store more than 10 times that amount.

Nanopores are simpler to create than silicon nanowires, Biswal said. The pores, a micron wide and from 10 to 50 microns long, form when positive and negative charge is applied to the sides of a silicon wafer, which is then bathed in a hydrofluoric solvent.

“The hydrogen and fluoride atoms separate. The fluorine attacks one side of the silicon, forming the pores. They form vertically because of the positive and negative bias,” said Thakur.

The straightforward process makes it highly adaptable for manufacturing, she said.

“The other advantage is that we’ve seen fairly long lifetimes. Our current batteries have 200-250 cycles, much longer than nanowire batteries,” said Biswal.

The researchers are confident that cheap, plentiful silicon combined with ease of manufacture could help push their idea into the mainstream.

“This material has the potential to significantly increase the performance of lithium-ion batteries, which are used in a wide range of commercial, military and aerospace applications,” Sinsabaugh said.

“Our goal is to develop a model of the strain that silicon undergoes in cycling lithium. Once we understand that, we’ll have a much better idea of how to maximize its potential,” Wong said. (ANI)