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Microscopic pores dot a silicon wafer prepared for use in a lithium-ion battery. Silicon has great potential to increase the storage capacity of batteries, and the pores help it expand and contract as lithium is stored and released. (Credit: Biswal Lab/Rice University)
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A team of Rice University
and Lockheed Martin scientists has discovered a way to use simple silicon to
radically increase the capacity of lithium-ion batteries.
Sibani Lisa Biswal, an assistant professor of
chemical and biomolecular engineering, revealed how she, colleague Michael
Wong, a professor of chemical and biomolecular engineering and of chemistry,
and Steven Sinsabaugh, a Lockheed Martin Fellow, are enhancing the inherent
ability of silicon to absorb lithium ions.
Their work was introduced today at Rice's
Buckyball Discovery Conference, part of a yearlong celebration of the 25th
anniversary of the Nobel Prize-winning discovery of the buckminsterfullerene,
or carbon 60, molecule. It could become a key component for electric car
batteries and large-capacity energy storage, they said.
"The anode, or negative, side of today's
batteries is made of graphite, which works. It's everywhere," Wong said.
"But it's maxed out. You can't stuff any more lithium into graphite than
we already have."
Silicon has the highest theoretical capacity of
any material for storing lithium, but there's a serious drawback to its use.
"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."
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A side view of microscopic pores in silicon. (Credit: Biswal Lab/Rice University)
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Other labs have tried to solve the problem with
carpets of silicon nanowires that absorb lithium like a mop soaks up water, but
the Rice team took a different tack.
With Mahduri Thakur, a post-doctoral researcher in
Rice's Chemical and Biomolecular Engineering Department, and Mark Isaacson of
Lockheed Martin, Biswal, Wong and Sinsabaugh 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.
Sinsabaugh described the breakthrough as one of
the first fruits of the Lockheed Martin Advanced Nanotechnology Center of
Excellence at Rice (LANCER). He said the project began three years ago when he
met Biswal at Rice and compared notes. "She was working on porous silicon,
and I knew silicon nanostructures were being looked at for battery anodes. We
put two and two together," he said.
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," she said. "The fluorine attacks one side of
the silicon, forming the pores. They form vertically because of the positive
and negative bias."
The treated silicon, she said, "looks like
Swiss cheese."
The straightforward process makes it highly
adaptable for manufacturing, she said. "We don't require some of the
difficult processing steps they do—the high vacuums and having to wash the
nanotubes. Bulk etching is much simpler to process.
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Rice University and Lockheed Martin researchers joined forces through LANCER to create silicon anodes for lithium ion batteries. Clockwise from left: Lockheed Martin Fellow Steven Sinsabaugh and post-doctoral researcher Mahduri Thakur, Professor Michael Wong, undergraduate Naoki Nitta and Assistant Professor Sibani Lisa Biswal, all of Rice. Lockheed Martin's Mark Isaacson is missing from the photo. (Credit: Jeff Fitlow/Rice University)
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"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.
They said putting pores in silicon requires a real
balancing act, as the more space is dedicated to the holes, the less material
is available to store lithium. And if the silicon expands to the point where
the pore walls touch, the material could degrade.
The researchers are confident that cheap,
plentiful silicon combined with ease of manufacture could help 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
Biswal and Wong plan to study the mechanism by which silicon absorbs lithium
and how and why it breaks down. "Our goal is to develop a model of the
strain that silicon undergoes in cycling lithium," Wong said. "Once
we understand that, we'll have a much better idea of how to maximize its
potential."
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