In an advance that could help electric vehicles run longer between charges, researchers have shown that silicon nanotube electrodes can store 10 times more charge than the conventional graphite electrodes used in lithium-ion batteries.
Researchers at Stanford University and Hanyang University in Ansan, Korea, are developing the nanotube electrodes in collaboration with LG Chem, a Korean company that makes lithium-ion batteries, including those used in the Chevy Volt. When such a battery is charged, lithium ions move from the cathode to the anode. The new battery electrodes, described online in the journal Nano Letters, are anodes and can store much more energy than conventional graphite electrodes because they absorb much more lithium when the battery is charged.
“In a hybrid car, the battery lasts only 30 minutes using the current technology,” says Jaephil Cho, professor of energy engineering at the Ulsan National Institute of Science and Technology in Korea, who led the research on nanotube anodes. If the new silicon anode can be matched to a cathode with comparable storage capacity, the resulting battery should be able to run a car for three to four hours without recharging, says Cho.
Silicon anodes have a higher energy-storage capacity than conventional graphite because the material can take up 10 times more lithium by weight than graphitic carbon. In fact, silicon takes up so much lithium–increasing in volume by as many as four times–that it can be a disadvantage. The mechanical strain on the brittle material is so great that silicon anodes tend to crack after they’re charged and discharged only a few times. So researchers, including Cho and Stanford materials scientist Yi Cui, have been developing nanostructured silicon designed to better withstand these stresses. They’ve made silicon nanowire anodes and nanoporous silicon anodes. Now they’ve collaborated to develop silicon nanotube anodes, whose storage capacity is better than those of other nanostructured silicon materials, says Cho.
The silicon nanotube anode looks like a bunch of hollow straws. While silicon nanowires can interact with lithium only on their surface, the nanotubes have more exposed surface area inside. “The nanotube has a large surface area–much more space for reaction sites than other types of materials,” says Cho. The shape also helps relieve mechanical strain when the battery is charged and discharged, because there’s extra space for the silicon to expand and contract.
The silicon nanotubes are made by repeatedly immersing an aluminum template in a silicon solution, and then heating it and etching the structure in acid to remove the aluminum. “It’s very simple, and the template is commercially available,” says Cho. Along with LG Chem, Cho is working with the template manufacturer to make a template compatible with large-scale manufacturing. He believes batteries incorporating the nanotube electrodes could be on the market in three years.
It’s too early to determine whether silicon anodes would add to the cost of lithium batteries. However, “even if the cost is higher, because you can get high capacity [with silicon], there will be an advantage,” says Arumugam Manthiram, professor of engineering and energy studies at the University of Texas at Austin.
LG Chem isn’t the only battery company working on silicon anodes; 3M and Sanyo are also developing the technology. However, major challenges remain before these electrodes will be built into vehicle battery packs, cautions Stanley Whittingham, professor of materials science and chemistry at the State University of New York at Binghamton. One of the problems with silicon is getting back all the energy you put in–a property called coulombic efficiency. Over time, less and less of the energy that’s been put in will discharge from a battery using a silicon anode. Cui and Cho have demonstrated their anodes’ performance after 200 charges. But before an anode is usable in a vehicle, Whittingham says, its coulombic efficiency needs to be proven over hundreds or thousands of charges, and such long-term performance hasn’t yet been shown with silicon.
Another challenge is that these high-performance anodes must currently be paired with less-stellar cathodes. “To fully realize the benefit of a silicon anode, you need a cathode whose charge-storage capacity is also 10 times better,” says Cui. In order to match them up in a working battery for testing, silicon anodes are currently paired with large-volume cathodes made of conventional materials. Cui and Cho are also developing new cathode materials in collaboration with LG Chem.