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Univ/Corp Team Extends Lithium-Ion Capacity with Silicon

Treated silicon wafer showing nanopores (Biswal Lab/Rice University)

Treated silicon wafer showing nanopores (Biswal Lab/Rice University); click on image for full-size view

A team of scientists from Rice University and Lockheed Martin in Houston, Texas has discovered a way to use nanotechnology with silicon to increase the capacity of lithium-ion material in batteries. Research by Sibani Lisa Biswal and Michael Wong, on the chemical and biomolecular engineering faculties at Rice, and Steven Sinsabaugh, a Lockheed Martin Fellow, is investigating the ability of silicon to absorb lithium ions, which is a key ingredient in electric car batteries and large-capacity energy storage.

Increasing the battery’s capacity begins with the anode or negative side. The anode is made of graphite, but graphite can hold only so much lithium. While raw silicon can increase the anode’s capacity, silicon is brittle and will crack after a brief period of use.

To give the silicon more flexibility, the researchers found that putting micron-sized pores — a micron is one-millionth of a meter — into the surface of a silicon wafer (pictured right) gives the material sufficient room to expand. While common lithium-ion batteries hold about 300 milliamp hours per gram of carbon-based anode material, the researchers found that treated silicon could theoretically store more than 10 times that amount.

The nanopores — a micron wide and 10-50 microns long — form vertically when positive and negative charge is applied to the sides of a silicon wafer, which is then bathed in a hydrofluoric solvent. This straightforward process makes it adaptable for manufacturing.

Putting pores in silicon, the researchers caution, requires a balancing act. As more space is dedicated to the holes, 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 project was carried out under the auspices of the Lockheed Martin Advanced Nanotechnology Center of Excellence at Rice and presented at Rice’s Buckyball Discovery Conference, celebrating the 25th anniversary of the Nobel Prize-winning discovery of the buckminsterfullerene, or carbon 60, molecule.

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