Recycling Silicon Chips for Lithium-ion Batteries

Your personal electronics only work as well as their batteries, so researchers around the world are racing to develop new batteries to power the next generation of technology. An innovative new design from researchers at Rice University employs a grid…

Your personal electronics only work as well as their batteries, so researchers around the world are racing to develop new batteries to power the next generation of technology. An innovative new design from researchers at Rice University employs a grid of silicon nanowires to function as an anode for a new generation of flexible, high power lithium-ion batteries.The design was motivated by a desire to re-purpose the silicon wafers from discarded electronic devices, according to one of the study’s lead authors, Arava Leela Mohana Reddy. Reddy and co-author Alexandru Vlad published their findings in the Proceedings of the National Academy of Sciences, detailing the mechanism for recycling the silicon and the advantages— high energy and high power— of the resulting material, which looks like white strips of tape. Most lithium-ion batteries today use carbon/graphite for the anode. However, silicon has a stronger affinity for lithium ions, and thousands of pounds of the valuable metalloid are discarded every year in old computer chips. Previous attempts to use silicon have failed because it experiences significant volume expansion while charging that stresses and breaks down the materials in the battery quickly.  To solve this problem, the Rice researchers created a thin layer of silicon nanowire arrays encased in an ion-conduction polymer that provides the silicon with space to expand and contract without damage.“Encasing the silicon nanowires in the polymer electrolyte was a critical innovation for the battery design,” Reddy said. “Alone, the silicon nanowires quickly crumble. Polymer electrolyte engulfs the nanowire array in a flexible matrix, facilitating both its easy removal from un-etched bulk silicon as well as allowing it room to expand during charging cycles.”Silicon wafers, salvaged from old computer chips, morph into forests of nanowires, each about 40 microns tall and only 150 nanometers wide through an established process of metal-assisted chemical etching. A thin gold mask in a hexagonal grid is built on top of the silicon wafer. In a chemical bath, the gold dissolves the silicon it touches, but the tiny wires can “grow” up through the grid as the gold sinks into the wafer. Several layers of nanowires can be grown from one recycled wafer.To enhance the electrical conductivity of the silicon wires, the researchers deposited a thin layer of copper on the silicon, like wrapping each wire in a porous web. The copper does not block the lithium ions and substantially improves the charge collection efficiency of the anode layer. This design takes three formally separate parts of the lithium-ion battery, the anode, the charge collector, and the polymer electrolyte/separator, and combines them into one functional material. According to Reddy, this design has several remarkable advantages over typical lithium-ion batteries.“When you bring the silicon in to replace the carbon, you increase the energy storage capacity”, Reddy said. “Use of polymer electrolyte in the battery offers virtually limitless design flexibility along with inherent safety.”The researchers hope that this new anode design can lead to inexpensive, flexible energy storage for the electronics of the future and provide a cost-effective means for re-using silicon. Before these anodes can become commercially viable, the design still needs to address capacity fade and rate performance issues that are a common challenge in silicon battery design, according to Paul Braun, a professor of materials science and engineering at the University of Illinois. “I think the real innovation here is the process they are using,” Braun said, citing both the etching procedure used to make the nanowires and the use of the copper coating. “It looks promising. The key is to make silicon architectures that are scalable and solve the capacity and rate issues.”  Read the abstract in from the Proceedings of the National Academy of Scienceshere.