Published on September 10th, 2012 | by James Ayre2
Discarded Silicon Gets New Life In Lithium-Ion Batteries
A new way to create the flexible components of rechargeable lithium-ion (LI) batteries from discarded silicon has been developed by researchers at Rice University and the Université catholique de Louvain in Belgium.
Silicon is able to absorb roughly 10 times more lithium than the carbon that is typically used in Li-ion batteries, but because of problems with how much it expands and contracts as it charges and discharges, it doesn’t last very long.
So the researchers created nano-wires from the “high-value but hard-to-recycle silicon” as an alternate design, to give more room for expansion and contraction. As the silicon is charged and discharged, it has the ability to expand and contract because of the spacing between the wires.
From a recent news release: “The Ajayan lab reports this week in the Proceedings of the National Academy of Sciences on its technique to make carefully arrayed nanowires encased in electrically conducting copper and ion-conducting polymer electrolyte into an anode. The material gives nanowires the space to grow and shrink as needed, which prolongs their usefulness. The electrolyte also serves as an efficient spacer between the anode and cathode.”
The researchers are hoping that their recycled devices will help in the advance towards the next generation of batteries: flexible, efficient, inexpensive, and able to conform to any shape.
“They used an established process, colloidal nanosphere lithography, to make a silicon corrosion mask by spreading polystyrene beads suspended in liquid onto a silicon wafer. The beads on the wafer self-assembled into a hexagonal grid — and stayed put when shrunken chemically. A thin layer of gold was sprayed on and the polystyrene removed, which left a fine gold mask with evenly spaced holes on top of the wafer.”
Metal-assisted chemical etching was then done using the mask, dissolving the silicon where it was touching the metal. After some time in the chemical bath, the metal catalyst sinks into the silicon and leaves behind millions of evenly spaced nanowires, around 50 to 70 microns long, rising through the holes.
The researchers then put a thin layer of copper over the nano-wires in order to increase their ability to absorb lithium. After that, they then “infused the array with an electrolyte,” which serves the dual purpose of transporting ions to the nano-wires and also works as a separation between the anode and the later-applied cathode.
“Etching is not a new process,” Reddy said. “But the bottleneck for battery applications had always been taking nanowires off the silicon wafer because pure, free-standing nanowires quickly crumble.” The electrolyte engulfs the nanowire array in a flexible matrix and facilitates its easy removal. “We just touch it with the razor blade and it peels right off,” he said. The mask is left on the unperturbed wafer to etch a new anode.
“The novelty of the approach lies in its inherent simplicity,” Reddy said. “We hope the present process will provide a solution for electronics waste management by allowing a new lease on life for silicon chips.”
Source: Rice University
Image Credit: Alexandru Vlad/Rice University