Lithium ion has become the battery of choice for electric vehicles, driving researchers to improve the technology's performance, longevity, and reliability. A new type of nanowire electrode developed by materials science and engineering professor Yi Cui at Stanford is a step toward that goal.

The new electrodes, discussed in last week's Nano Letters, can store six times as much charge as the graphite electrodes in current lithium batteries--that means electric cars that give more mileage per charging session.

When a lithium battery is charged, lithium ions move from the positive electrode (cathode) to the negative anode. Silicon is a promising material for anodes because it can store over 10 times as many ions as graphite at the same weight. But when silicon absorbs charge, it swells to four times its original volume, cracking after a few charging cycles.

The new nanowires exploit the properties of silicon and graphite. Cui and his colleagues make the material by depositing amorphous silicon on carbon nanowires. The wires can store a charge of about 2,000 milliamp hours per gram, while graphite anodes store less than 360 milliamp hours per gram. Meanwhile, the carbon core makes them robust. "Lithium ions can also get absorbed into carbon," says Cui, "but the volume expansion of carbon is 10 percent or smaller, so it provides a stable backbone." In tests, the nanowires performed well for more than 50 charging cycles.

The researchers had previously made electrodes from pure crystalline silicon nanowire. Those had triple the storage capacity of graphite electrodes but only lasted through 20 cycles.

The carbon-silicon nanowires are also easier to make. They don't require the high temperatures that are needed to grow the silicon-only nanowires. "Carbon nanofiber is already commercially available and you can produce tons," Cui says. "The coating process could be made a lot faster and is easy for large-scale manufacturing."