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.”
For use in commercial electric vehicles, lithium battery electrodes need to last through at least 300 charge cycles. In this respect, the nanowires could face stiff competition. In December 2008, a team from Hanyang University in Ansan, South Korea, unveiled nanoporous silicon anodes that lasted for more than 100 charging cycles and could store more charge than the nanowires. Chemist Jaephil Cho, who led the work, says that the nanoporous material has more silicon-per-unit volume than nanowires, so it can hold more charge per unit volume. However, he says, “carbon fiber [manufacturing] is easy to scale up and therefore [Cui’s] method for making carbon-silicon nanowires is believed to be very practical.”
General Motors and Applied Sciences, meanwhile, are developing nanowire anodes that are very similar to those of the Stanford team. The companies coat carbon nanofibers with silicon particles, as opposed to amorphous silicon, resulting in anodes that can store charge of 1,000 to 1,500 milliamp hours per gram. Gholam-Abbas Nazri, who is leading the work at the GM Research and Development Center in Warren, MI, says that the anode capacity can be increased by making the silicon layer thicker, but right now it’s best to stabilize the capacity at 1,000 milliamp hours per gram. Anodes that store more charge need cathodes that can supply higher charge, Nazri says, and “at the moment, there is no cathode [material] with enough capacity to match carbon-silicon anode.”
Cui is confident in the success of silicon as an anode material for lithium batteries. “In the next five years or less, we’ll see a battery with silicon anodes,” he says. However, cost will be the deciding factor. In the end, he says, it all depends on “whoever can come up with a low-cost, large-scale manufacturing process, produce the best performance, and put out products.”