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.”