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Realizing Lithium-Battery Potential

Nanoporous silicon that soaks up ions without self-destructing can make better batteries.
December 3, 2008

Lithium batteries are driving a renaissance in electric-vehicle development, and what’s attractive is not just the charge capacity of current prototypes, which is twice that of the nickel metal hydride batteries in hybrid vehicles. According to an assessment of electric-vehicle batteries published by the University of California, Davis, in May, “more important” is the potential for further performance improvement. A high-energy lithium-battery electrode developed at Hanyang University, in Ansan, South Korea, could make good on some of that potential.

Electrodes in 3-D: These silicon particles can absorb over six times more lithium ions by weight than graphite can, making them a candidate for creating electrodes for supercharged lithium batteries. The nanoporous structure shown in the electron micrograph close-up (lower image) enables the silicon to absorb a lot of lithium without shattering.

The Hanyang team, led by chemist Jaephil Cho, developed a nanoporous silicon electrode that could at least double the charge capacity of a lithium battery–essentially doubling the range of an electric vehicle. And unlike previously reported silicon anodes, the one created by Cho’s team can charge and discharge rapidly.

“It’s very good, very impressive work,” says Stanford University materials scientist Yi Cui, who is developing his own nanostructured silicon electrodes for lithium batteries.

Charging a lithium battery involves moving lithium ions from the battery’s positive electrode (or cathode) into its negative electrode (or anode). Silicon’s electrochemical affinity for lithium ions makes it an excellent material for an anode. But silicon tends to overindulge: anodes made of the material absorb so much lithium upon charging that they swell to four times their previous volume. Upon discharging, they deflate to their original size, and just a few charging cycles are usually enough to pulverize the brittle material.

Nanostructuring gives silicon strain-relieving flexibility, allowing it to recharge without deteriorating so quickly. Cui demonstrated this in January, unveiling silicon nanowire anodes that can elongate during charging to release some of the strain. These results showed a work in progress, however: batteries incorporating the nanowire electrodes still lost more than half their storage capacity after just a few cycles of rapid charging.

Cho’s new nanoporous silicon, in contrast, seems to last much longer even under rapid charging, according to his group’s paper published in November in the German journal Angewandte Chemie. The nanoporous electrodes still retained a charge greater than 2,400 milliamp-hours per gram–over six times more than the graphite anodes used in existing lithium batteries–after 100 rapid charging cycles. “That’s definitely good enough for commercialization,” says Cho.

The nanoporous silicon anodes consist of solid silicon crystals riddled with Swiss-cheese-style pores. Cho explains that this structure accommodates the strain because the walls between its pores are extremely thin–approximately 40 nanometers. This is less than half the thickness of Cui’s silicon nanowires.

Cho thinks that further optimization of the silicon nanostructure will also improve its lithium capacity per unit of volume, which is already about three times better than that of graphite. He believes that it is possible to tighten the pores by about half–essentially squeezing out more of the air within–without sacrificing the material’s charging performance. The result would increase the silicon per unit of volume available, thereby also boosting the charge per volume to six times that of graphite.

The process for making the nanoporous silicon material also marks an improvement over methods for making previous silicon anodes, says Cho. To create the nanoporous anodes, the Korean researchers mix silica nanoparticles with a viscous gel of carbon-coated silicon (to keep the silicon and silica from reacting chemically), heat the mixture to 900 °C to fuse it into a solid mass, and then selectively etch away the silica with hydrofluoric acid to create the pores. In contrast to most silicon assembly methods, the process takes place at atmospheric pressure and thus should be easier to scale up to large volumes. “It’s a much more economical process for mass production,” says Cho.

Cho says that he hopes to sell the technology to Korean battery maker LG Chem, where he has worked for the past four years and which may have won the lithium-battery contract for GM’s forthcoming Chevy Volt. But he could face competition. Cui says that his lab has also dramatically improved its nanowire synthesis and battery design, and in September, GM scientists presented impressive results on lithium anodes created using silicon-coated carbon fibers.

But the real question, say observers, is whether any of these materials can be produced at the right price. Marc Obrovac, a research specialist at 3M working on lithium-battery materials, points to a sophisticated silicon anode design already made by Sanyo Electric that achieves energy densities exceeding Cho’s. “Despite this superior performance, Sanyo apparently never commercialized its silicon electrode,” says Obrovac. “Fabrication cost may have been a factor.”

Cui points to another factor that could limit the impact of silicon anodes: cathode performance. If new cathode materials could match the energy density of the silicon anodes, this would multiply the energy storage capacity of finished batteries four- or fivefold, he says. Using conventional cathodes, however, would require a sixfold increase in the cathode’s mass and volume to deliver a doubling of the total energy storage. “We are actually limited more by the cathode,” says Cui. “Improving the anode will have a very big impact. But improving the cathode can have an even larger impact.”

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