Discarded silicon aids creation of flexible battery components

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Researchers at Rice University and the Université catholique de Louvain, Belgium, have developed a way to make flexible components for rechargeable lithium-ion (Li-ion) batteries from discarded silicon.

Rice lab of materials scientist Pulickel Ajayan created forests of nanowires from silicon, which absorbs 10 times more lithium than the carbon commonly used in Li-ion batteries. However, because it expands and contracts as it charges and discharges, it breaks down quickly.

The Ajayan lab has reported this week in the journal Proceedings of the National Academy of Science on its technique to make carefully arrayed nanowires encased in electrically conducting copper and ion-conducting polymer electrolyte into an anode. The material is said to give 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.

Transforming waste into batteries should be a scalable process, said Ajayan, Rice’s M and Mary Greenwood Anderson professor in mechanical engineering and materials science and of chemistry. In a statement, the researchers said they hope their devices are a step towards a new generation of flexible, efficient, inexpensive batteries that can conform to any shape.

Co-lead authors Arava Leela Mohana Reddy, a Rice research scientist, and Alexandru Vlad, a former research associate at Rice and now a postdoctoral researcher at the Université catholique de Louvain, were able to pull multiple layers of the anode/electrolyte composite from a single discarded wafer.

They are reported to have used 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.

The mask was used in metal-assisted chemical etching, in which the silicon dissolved where it touched the metal. Over time in a chemical bath, the metal catalyst would sink into the silicon and leave millions of evenly spaced nanowires, 50–70 microns long, poking through the holes.

The researchers deposited a thin layer of copper on the nanowires to improve their ability to absorb lithium and then infused the array with an electrolyte that transported ions to the nanowires and served as a separator between the anode and a 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, freestanding nanowires quickly crumble.’

The electrolyte engulfs the nanowire array in a flexible matrix and facilitates its 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.

When combined with a spray-on current collector on one side and a cathode and current collector on the other, the resulting battery showed promise as it delivered 150 milliamp-hours per gram with little decay over 50 charge/discharge cycles. The researchers are working to enhance those qualities and are testing the anodes in standard battery configurations.

‘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 of life for silicon chips.’

The abstract can be read here.