What are some unusual uses for batteries

A milestone in the use of silicon for battery anodes

The same material found on the tip of a pencil - graphite - has long been a key component in today's lithium-ion batteries. However, as our reliance on these batteries increases, graphite-based electrodes need to be upgraded. To do this, the scientists are looking at the element that is at the heart of the digital revolution: silicon.

Scientists at the US Department of Energy's Pacific Northwest National Laboratory have come up with a new way to harness this promising but problematic energy storage component. Silicon, which is used in computer chips and many other products, is attractive because it can store ten times the electrical charge per gram of graphite. The problem is that silicon expands a lot when it comes into contact with lithium and is too weak to withstand the pressure during electrode manufacture.

To address these issues, a team led by PNNL researchers Ji-Guang (Jason) Zhang and Xiaolin Li developed a unique nanostructure that limits the expansion of silicon while simultaneously enriching it with carbon. Your work, which was recently in the magazine "Nature Communications" published, new electrode material designs for other types of batteries could inform and ultimately help increase the energy capacity of lithium-ion batteries in electric cars, electronic devices, and other equipment.

Take away the disadvantages of silicon

Graphite, a conductive and stable form of carbon, works well for packing lithium ions into the anode of a battery when charging. Silicon can hold more lithium than graphite, but it tends to expand about 300 percent by volume, causing the anode to break apart. The researchers created a porous form of silicon by aggregating small silicon particles into microspheres around 8 micrometers in diameter - about the size of a red blood cell.

"A solid material such as stone breaks if it expands too much in volume," said Zhang. "What we have created is rather spongy, where there is room inside to accommodate the expansion.

The electrode with its porous silicon structure exhibits a change in thickness of less than 20 percent, while it takes twice the charge of a typical graphite anode, the study found. However, unlike previous versions of porous silicon, the microspheres also had exceptional mechanical strength thanks to the carbon nanotubes that make the spheres look like balls of yarn.

Super strong microspheres

The researchers created the structure in several steps by first coating the carbon nanotubes with silicon oxide. Then the nanotubes were placed in an emulsion of oil and water. Then they were heated to a boil.

"The coated carbon nanotubes condense into spheres as the water evaporates," Li said. "Then we used aluminum and higher heat to convert the silicon oxide to silicon, followed by immersion in water and acid to remove by-products. What The result of this process is a powder that consists of the tiny silicon particles on the surface of the carbon nanotubes.

The strength of the porous silicon spheres was tested with the probe of an atomic force microscope. The authors found that one of the nano-sized yarn spheres "can easily yield under very high pressure and lose some porosity, but it does not break".

This is a good omen for commercialization, because anode materials have to withstand high compression when being manufactured in rolls. The next step, according to Zhang, is to develop more scalable and more economical methods of manufacturing the silicon microspheres so that they can one day find their way into the next generation of high-performance lithium-ion batteries.