In news that could greatly extend the range of electric cars, researchers have shown that replacing the conventional graphite electrodes in lithium-ion batteries with silicon nanotubes can produce a battery that can store ten times more charge. The researchers developed a silicon anode that, aside from extending the range of electric cars, could also make gasoline-electric hybrid vehicles more efficient by allowing them to run in electric mode for longer periods.
The researchers say that, if the new silicon anode can be matched to a cathode with similar storage capacity, the resulting battery should be able to power a car for three or four hours without recharging. This is a marked improvement of six to eight times on today’s technology, which sees the battery in a current, typical hybrid car lasting only 30 minutes.
The silicon anode developed by researchers at Stanford University and Hanyang University in Ansan, Korea, in collaboration with LG Chem, a Korean company responsible for producing the lithium-ion battery used in the Chevy Volt, can store much more energy than graphite electrodes because they absorb higher levels of lithium when the battery is charged. In fact, the silicon can take up to ten times more lithium by weight than graphitic carbon.
But the ability of the silicon to absorb more lithium has a downside. Since it takes up so much lithium, it can increase in volume by as much as four times. This places so much mechanical strain on the brittle material that the silicon anodes tend to crack after only a few charge/discharge cycles. To combat this the researchers turned to nanostructured silicon.
Jaephil Cho, professor of energy engineering at the Ulsan National Institute of Science and Technology in Korea, and Stanford materials scientist Yi Cui, had made silicon nanowire anodes and nanoporous silicon anodes before teaming up to develop the silicon nanotube anodes that boast better storage capacity than either of those previous nanostructured materials.
The performance of the silicon nanotube anode lies in its shape, which looks like a bunch of hollow straws. This provides more surface area exposed inside and therefore, much more area for the lithium to interact with. Also, because the shape provides extra space for the silicon to expand and contract, there is a reduction in the mechanical strain caused when the battery is charged and discharged.
Cho believes that batteries incorporating the silicon electrodes could be on the market in as little as three years because the process to produce them is simple and the template used is already available commercially. It involves repeatedly immersing an aluminum template in a silicon solution, and then heating it and etching the structure in acid to remove the aluminum. Along with LG Chem, Cho is also working with the template manufacturer to make a template compatible with large-scale manufacturing.
There are, however, other challenges that will need to be overcome before silicon anodes find their way into electric vehicles. Although Cui and Cho have demonstrated their anode’s performance after 200 charges, the technology needs to be proven over hundreds of thousands of charges to become viable for use in vehicles. The problem lies in getting back from silicon all the energy that is put into it – a condition that worsens over time.
Additionally, to receive the full benefits of silicon anodes, they need to be paired with cathodes whose storage capacity is also ten times greater. To match the capacity of the silicon anodes in a working battery for testing their technology the researchers have been using large-volume cathodes made of conventional materials. However, Cui and Cho are working on developing new cathode materials in collaboration with LG Chem.
The team’s research is detailed in the study, Silicon Nanotube Battery Anodes, which appears in the journal Nano Letters.