Silicon Anodes Promise Next-Generation Lithium Battery
ture with millions of fossil-fuel powered cars displaced from American roadways by plug-in electric vehicles presents significant economic and environmental benefits. It also poses a challenge to develop a next-generation battery technology that offers far greater energy density. Silicon anodes for Li-ion batteries present a promising solution.
Today's electric vehicles rely on nickel-metal hydride batteries. They are heavy, bulky and have a specific energy that is too low, about 80 watt hours per kilogram (Wh/kg), for long-distance travel. Li-ion batteries, commonly used in handheld electronics, offer greater capacity. Composed of three main components—a graphite anode, a cathode and electrolyte (lithium salt dissolved in organic solvent)—the graphite anode has specific capacity of about 350 mAh/g. Li-ion batteries using graphite anodes exhibit a specific energy of more than 160 Wh/kg, double that of nickel-metal hydride batteries.
"If we want to increase driving distance of electric vehicles, we need to have much better capacity -- at least double the capacity of graphite anodes and cathodes used in the Li-ion battery," said Dr. Jason Zhang, a PNNL scientist.
One of the limiting factors of the Li-ion battery is its anode—the graphite. Lithium is added to graphite when charging and removed as the battery is used. Graphite anodes are used in nearly all Li-ion batteries, but recent research has sought to capitalize on a better anode solution—silicon. With a theoretical capacity of more than 10 times that of graphite, silicon anodes can at least double the capacity of graphite-anode batteries. However, it is this very ability to absorb lithium and expand during charging that is the problem: The silicon breaks down quickly.
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