“In a hybrid car, the battery lasts only 30 minutes using the current technology,” says Jaephil Cho, associate professor of applied chemistry at Hanyang University, 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.
We present Si nanotubes prepared by reductive decomposition of a silicon precursor in an alumina template and etching. These nanotubes show impressive results, which shows very high reversible charge capacity of 3247 mA h/g with Coulombic efficiency of 89%, and also demonstrate superior capacity retention even at 5C rate (=15 A/g). Furthermore, the capacity in a Li-ion full cell consisting of a cathode of LiCoO2 and anode of Si nanotubes demonstrates a 10 times higher capacity than commercially available graphite even after 200 cycles.
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.
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.