September 14, 2012 – Researchers at Rice U. and the Université catholique de Louvain in Belgium have devised a method for converting discarded silicon into a key material for lithium-ion batteries.

Silicon’s superior lithium absorbtion (10× better than the carbon/graphite typically used in Li-ion batteries) has put it in the spotlight as a better material for Li-ion batteries, but among its drawbacks is that it expands and contracts (and quickly breaks down) during charging cycles.

This new technique, published in Proceedings of the National Academy of Science, adds a new wrinkle: start with a scrap silicon wafer, and build arrayed nanowires encased in copper and ion-conducting polymer electrolyte into an anode.

The researchers made a silicon corrosion mask by spreading polystyrene beads suspended in liquid onto a silicon wafer; the beads self-assembled into a hexagonal grid and stayed put when shrunken chemically. A thin layer of gold was sprayed on and the poylstyrene removed, and the silicon was chemically etched away, leaving a gold mask with evenly spaced holes on top of the wafer. During that metal-assisted chemical etch bath, over time the metal catalyst sunk into the silicon and left a forest of evenly spaced nanowires (50-70μm long) poking up through the holes. A layer of copper was deposited on those nanowires to help them better absorb lithium, and an electrolyte was added to both transport ions to the nanowires and separate the anode and later-applied cathode.

"The bottleneck for battery applications had always been taking nanowires off the silicon wafer because pure, free-standing nanowires quickly crumble," explains paper co-author Arava Leela Mohana Reddy. Engulfing the nanowire array with the electrolyte facilitates its easy removal — "we just touch it with the razor blade and it peels right off" — and the mask is left on the unaffected wafer to etch a new anode.

Combining a spray-on current collector on one side and a cathode and current collector on the other side, the resulting battery delivered 150mAh/g "with little decay" over 50 charge/discharge cycles, the researchers report. They are currently tweaking the process to test the anodes in standard battery configurations.

The process of transforming waste silicon into batteries should be scalable, noted Prof. Pulickel Ajayan, leader of the materials science lab. "We hope the present process will provide a solution for electronics waste management by allowing a new lease on life for silicon chips," added Reddy.

As silicon is dissolved in a chemical bath, a gold mask sinks to the bottom, leaving
silicon nanowires ~100nm wide poking through the holes. (Credit: Alexandru Vlad/Rice U.)