The technique provides a more effective way of using silicon as the anode, or negative, side of a lithium-ion battery. Batteries now use graphite anodes, which work well. “But it’s maxed out,” said Michael Wong, a professor of chemical and biomolecular engineering and of chemistry. “You can’t stuff any more lithium into graphite than we already have.”

Nothing holds lithium quite like silicon, which has the highest theoretical capacity for storing the stuff. “It can sop up a lot of lithium, about 10 times more than carbon, which seems fantastic,” Wong said. “But after a couple of cycles of swelling and shrinking, it’s going to crack.”

Others have tried using silicon nanowires, which work a bit like a mop to sop up lithium. The Rice University researchers, joined by scientists from Lockheed Martin, thought a sponge might work better.

They found that micron-sized pores in the surface of the silicon wafer (shown above) gave it plenty of room to expand. While common li-ion batteries hold about 300 milliamp hours per gram of carbon-based anode material, the treated silicon could, in theory, hold 10 times that much.

The other advantage is nanopores are easier to make than nanowires, said Sibani Lisa Biswal, an assistant professor of chemical and biomolecular engineering. The pores, which are a micron wide and 10 to 50 microns long (shown above), form when positive and negative charge is applied to a silicon wafer. The wafer is then bathed in a hydrofluoric solvent. “The hydrogen and fluoride atoms separate,” she said. “The fluorine attacks one side of the silicon, forming the pores. They form vertically because of the positive and negative bias.”

The resulting wafer “looks like Swiss cheese.” The process is straightforward and easily adapted for manufacturing. “The other advantage is that we’ve seen fairly long lifetimes. Our current batteries have 200 to 250 cycles, much longer than nanowire batteries,” Biswal said.

Manufacturing the wafers requires carefully balancing the space dedicated to nanopores with the amount of lithium that must be stored — more pores means less lithium. And if the silicon expands enough for the pore walls to touch, the material could degrade, the researchers warn. Still, they’re confident the easy availability of silicon coupled with the ease of manufacturing the nanopores will push their idea into the mainstream.

“We are very excited about the potential of this work,” Sinsabaugh said. “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.”

Main photo: Jeff Fitlow / Rice University. The team, clockwise from left: Lockheed Martin fellow Steven Sinsabaugh with post-doctoral researcher Mahduri Thakur, professor Michael Wong, undergraduate Naoki Nitta and assistant professor Sibani Lisa Biswal of Rice University. Mark Isaacson of Lockheed Martin is not shown.

Other photos: Biswal Lab / Rice University

Authors: Chuck Squatriglia

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