Crushed, Rolled, or Slivered, Silicon Boosts Batteries

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Two different Rice University research teams are applying different but related approaches to using silicon in lithium batteries to make them more energy dense and longer-lived.

One team recently came up with a paint-on lithium battery, announced a way to use otherwise waste silicon by crushing it and spreading it on anodes in their test batteries. This has implications for manufacturing because of the simplicity of the process.

Crushed silicon anode as applied to test battery anode

Dvice.com gives one hint as to why this might be good. “In lithium-ion batteries, for example, the anode is usually graphite, because it works well and is cheap.

“You know what else is cheap? Silicon. The Earth is 28% silicon, making it the second most abundant crustal element by mass after oxygen. And it makes a fantastic battery anode, too: it can hold 10x more lithium ions than graphite. However, if you cram it full of all those ions, it swells in size [up to three times] and will damage itself, which is why batteries tend not to use it.”

As a way of getting around that nasty swelling, Rice University reports that researchers have found ways to granulate silicon from failed experiments and coat the crumbled material onto anodes in a process that could be duplicated at larger scales.

Mike Williams, reporting for the school, writes, “The team led by Rice engineer Sibani Lisa Biswal and research scientist Madhuri Thakur reported in Nature’s open access journal Scientific Reports on the creation of a silicon-based anode, the negative electrode of a battery, that easily achieves 600 charge-discharge cycles at 1,000 milliamp hours per gram (mAh/g). This is a significant improvement over the 350 mAh/g capacity of current graphite anodes.”

Researcher Sibani Lisa Biswal holds two vials, the left with 50 milligrams of crushed silicon, the right with 50 milligrams of her lab’s porous silicon powder. Photo: Jeff Fitzlow, Rice University

Since lithium needs to attach itself to electrodes for current to flow, the more lithium ions that can fit onto that electrode, the better.  Increasing the surface area of the coating gives more space on which ions can congregate and the team managed to obtain far greater surface area per weight of crushed silicon with their new approach. Going from merely crushing the waste silicon to turning it into a nano-powder increased the active surface area 50 times and gives more void space into which the silicon can expand without constraint.

The work, carried out with Lockheed Martin’s Advanced Nanotechnology Center of Excellence at Rice (LANCER), has shown better outcomes than coating anodes with thin silicon films. So far, the 3X anodes have survived 600 charge/discharge cycles without appreciable degradation. With viable cathodes under investigation, the new technique may have a commercial future.

Another group of Rice scientists, collaborating with the Universite Catholique de Louvain, Belgium, are using discarded silicon to create “forests of nanowires” that are then encased in copper and a polymer electrolyte to form an anode. The coating allows the nanowires to expand and contract and acts as a spacer between the anode and a neighboring cathode.

Since the finished product is flexible, batteries could conform to any shape, making structural batteries also capable of taking on streamlined forms, perhaps at a larger scale.

Pulickel Ajayan, Rice’s Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry along with Arava Leela Mohana Reddy of Rice, and Axelandra Vlad, a former research associate at Rice and now a postdoctoral researcher at the Universite Catholique, published their findings in the Proceedings of the National Academy of Sciences of the United States of America.

Silicon roll peeled from waste silicon wafer

 

Ajayan’s team was able to pull multiple layers of the anode/electrolyte composite from a single discarded wafer. Samples of the material look like strips of white tape or bandages.

According to Rice’s press release, “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. ‘We could do this on wafers the size of a pizza in no time,’ Vlad said.

“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 to 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.”

Although not yet achieving the power or cycle life of the other Rice innovation, the approach could be at least as practical in its application.

Described as “simple” by the researchers, the complex series of steps may find favor in a manufacturing environment, partly because it makes use of value-added materials that would otherwise be trashed.
“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.”

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