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.