The new anode is made of germanium nanotubes. It charges and discharges five times faster than previously reported silicon anodes, lasts through twice as many charging cycles, and is easier to fabricate. Its 400-cycle life matches that of graphite and is long enough for portable-electronics batteries, says Jaephil Cho, a researcher at South Korea’s Ulsan National Institute of Science and Technology, who led the new work. "These anodes meet the practical requirements of lithium-ion cells," Cho says.
Cho collaborated with researchers at LG Chem, the Korean company that makes the lithium-ion batteries used in the Chevy Volt. Their results will soon be published online in the journal Angewandte Chemie. The researchers are also working on silicon nanotube anodes.
These advances are part of a broader push by LG Chem to develop better anode materials for higher-capacity batteries. "The company is looking for a breakthrough technology using both silicon and germanium materials for lithium-ion battery anodes," Cho says.
Charging and discharging a lithium-ion battery involves moving lithium ions into and out of the anode. The more lithium the electrode can pack, the more energy the battery can store. Silicon and germanium can, in theory, hold about 10 and four times as much charge as the same amount of graphite by weight. So far, silicon has been the main contender for anodes because it’s cheaper, but crystalline silicon breaks down from repeated swelling and shrinking.
Nanostructured materials better withstand stresses from changes in volume, so researchers and a handful of startups are making anodes from silicon nanowires, nanotubes, and porous nanoparticles. Of these, nanotubes have the best charge capacity, Cho says.
The drawback to silicon nanotube anodes, though, has been their low cycle life: they typically maintain their capacity for just 200 cycles. Not only do germanium nanotubes last longer, but they also charge and discharge faster, because lithium ions diffuse through germanium more rapidly.
"Cycling life is one of the key parameters for making practical anodes," says Stanford University materials science professor Yi Cui, whose startup Amprius is commercializing batteries with silicon nanowire anodes. "As an initial demonstration, this is very impressive," Cui says. But, he cautions, germanium’s higher cost could be a limitation.
Cho believes that increased interest in germanium anodes could bring about a decrease in the material’s cost. "Germanium is an abundant element, and the current price is maintained by the lack of demand," he says. "A hurdle for using germanium in real batteries is cost, but once big battery makers want to use it as an alternative candidate for [anodes], I believe its cost will drop."
The researchers make the nanotubes by heating antimony-coated germanium nanowires at 700 °C for five hours. Germanium atoms diffuse outward and form hollow nanotubes with walls 40 nanometers thick. The process should be easy to scale up to large volumes and could be used for silicon as well, Cho says. What’s more, unlike methods commonly used to synthesize silicon and other nanotubes, this method has a high yield and produces uniform nanotubes.
Cho continues to collaborate with LG Chem and other Korean companies on porous silicon nanoparticle anodes. Meanwhile, Cui and others are exploring various new materials for cathodes, which now have much lower energy densities than anodes and can limit a battery’s overall charge capacity.