The power of wasteful thinking - silicon for rechargeable batteries of any shape

Materials World magazine
,
29 Oct 2012

Discarded silicon is being used in place of carbon/graphite or tin-based compounds to create flexible components for rechargeable lithium-ion (Li) batteries. The project could lead to inexpensive batteries that conform to any shape.  

Silicon can absorb up to 10 times more lithium than carbon, which is commonly used in Li batteries, however, challenges arise when it expands and contracts during charges and discharges, leading to material break down. To combat this, a collaborative group at Rice University, Texas, USA, and the Université Catholique de Louvain, Belgium, has devised a repeatable etch-infiltrate-peel cycle.  

The key phase of the work involves transforming bulk and hard to recycle waste silicon chips into forests of carefully arrayed nanowires encased in electrically conducting copper and ion-conducting polymer electrolyte into an anode. This method gives nanowires the space to grow and shrink as needed and prolongs their use.  

Using colloidal nanosphere lithography, the team began by creating a silicon (Si) corrosion mask by spreading polystyrene beads suspended in liquid onto a silicon wafer. The beads on the wafer were self-assembled into a hexagonal grid – and stayed put when shrunken chemically. A thin layer of gold was sprayed on and the polystyrene was removed, leaving a fine gold mask with evenly spaced holes on top of the wafer.  

The mask was then used for metal-assisted chemical etching, in which the silicon was dissolved where it touched the metal. While in a chemical bath, the metal catalyst is sunk into the silicon and leaves 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 not only transported ions to the nanowires but also served as a separator between the anode and a later-applied cathode.  

Dr Leela Mohana Reddy at Rice University explains, ‘The next task was to find a way of removing the nanowires from the substrate. The first thing that came to mind was that polymer and polymer composites are flexible and have excellent mechanical properties. So the next step was to find a polymer that would stabilise the entire structure and enable delamination from the original substrate, while at the same time allow the passage of Li for proper functioning of the Li-battery. While the choice of polymer was easy (a PvDF-HFP copolymer), the method of applying this polymer to properly infiltrate the dense Si nanowire arrays gave us nightmares. Finally, we were able to adjust the technique, and apply layer-by-layer coatings through a modified painting technique,’ explains Reddy.  

The final aim was to find a way to properly peel the Si/polymer composite without destroying its integrity. This was achieved using a simple razor blade. Although Reddy adds that this task could be easily automated or replaced with laser cutting techniques, he admits the risk of rupturing a nanowire at its base is still higher than they would like.  

The next stage will be to improve the delamination of the Si nanowires and polymer composite membranes, and investigate large-scale fabrication methods.