Silicon is the most common element in the electronics industry, but a lot of it gets wasted when the devices are thrown away. Now scientists at Rice University and the Université Catholique de Louvain, Belgium, have found a way to recycle it as flexible parts for rechargeable batteries.
Silicon is valuable -- while it makes up a huge chunk of the Earth's crust, the kind used in computers is ultra-pure and quite valuable. So throwing it into landfills is probably not the best idea.
At Rice, Pulickel Ajayan and his colleagues created forests of nanowires from the silicon. Silicon absorbs 10 times more lithium than the carbon commonly used in typical lithium-ion batteries. But silicon expands and contracts as it charges and discharges, so it breaks down quickly under the stress.
Ajayan's team found out how to encase the nanowires of silicon in electrically conducting copper and ion-conducting polymer electrolyte, making an anode. The material gives nanowires the space to grow and shrink as needed, which prolongs their usefulness. The electrolyte also serves as a spacer between the anode and cathode.
Meanwhile, co-lead authors Arava Leela Mohana Reddy, a Rice research scientist, and Alexandru Vlad, a researcher at the Université Catholique, were able to pull a number of layers of the anode/electrolyte composite from a single junked wafer.
To make the silicon nanowires, the group used a technique called colloidal nanosphere lithography. Common in the chip industry, it involves spreading polystyrene beads, which are suspended in a liquid, onto a silicon wafer. The beads self-assemble into a hexagonal pattern. When another chemical is applied the beads shrink and stay in the same configuration. After that, a thin layer of gold is sprayed on and the polystyrene is taken off. The result: a mask made of gold with evenly spaced holes sitting on top of a silicon wafer.
The next step is etching: the silicon is dissolved where it touches the gold by a chemical bath. All those evenly spaced holes where there was no gold become tiny silicon wires, 50 to 70 microns long. Given that the technique isn't that exotic, the scientist think it can be done at an industrial scale relatively easily.
The results are reported in the Proceedings of the National Academy of Sciences.
Via Rice University