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Because a silicon-based anode holds so much promise for lithium batteries, many other companies and entities are trying to find new methods to stabilize silicon as well, but Yang feels that the Calbattery method is the best so far. “To stabilize silicon, you want the silicon to be intermittently mixed with graphene,” Yang said. “The silicon must not agglomerate to produce a material with the best performance and longest cycle life. The way other researchers or companies incorporate silicon into graphite is different. They basically have silicon loosely sitting on the surface of the graphite, which does not help, because during cycling the silicon particles migrate and then agglomerate, causing rapid capacity fading. I tested many other silicon procedures. We know ours is the best, because we tested the materials with the same procedure, so we can make an apples-to-apples comparison.” Roberts claims that the result is a stable anode material with three times the specific capacity of any other, and that it’s closer to being commercial-ready than any other silicon-based solution. “It has triple the capacity to absorb lithium ions,” Roberts said, “and when you combine it with other high-energy-density cathode and high-
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Polycrystalline silicon rod
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Photo courtesy of Argonne National Lab (Flickr)
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You can’t just mix them up. We tried it; it didn’t work. To coat it doesn’t work. The only way it works is to put it in as a gas.
Photo courtesy of Warut Roonguthai
Yang found a composite production process that works around the limitation of silicon’s expansion. It’s a gas phase deposition process that uses organosilane heated up to a gas. The sub-micron-size particles form between crevices in the graphene layers and uniformly embed themselves throughout these platelet layers, creating a stable silicon-based anode composite material that maintains its structural integrity for long cycle life, for use not only in EVs but also in consumer portable device applications. “You can’t just mix them up,” Roberts said. “We tried it; it didn’t work. To coat it doesn’t work. The only way it works is to put it in as a gas.” “The silicon gas goes everywhere, and then it decomposes to form solid particles,” Yang added. “That’s how you form this embedded structure.” The resulting composite allows silicon to expand and contract, but not so much that it breaks the battery electrode contact.
Graphene’s hexagonal structure