Silicon along with carbon nanotubes
has been used to develop a nanostructure to make an effective design for
battery anodes. CNT is used to strengthen the material and modify the way the
silicon interacts with lithium, which has been extensively used in electrical
cars and other devices.
Scientists at the U.S. Department
of Energy's Pacific Northwest National Laboratory have used a novel way to use silicon.
Silicon, used in computer chips is attractive as it can hold 10 times the
electrical charge per gram compared to graphite. The trouble is, silicon
expands greatly when it encounters lithium, and it is too weak to withstand the
pressure of electrode manufacturing.
To tackle these issues, a unique
nanostructure that prevents silicon's expansion when it is mixed with carbon. Scientist
work, which was recently published in the journal Nature Communications, could
inform new electrode material designs for other types of batteries and
eventually help increase the energy capacity of the lithium-ion batteries in
electric cars, electronic devices, and other equipment.
How they do it!!
A conductive and stable form of
carbon, graphite is well suited to packing lithium ions into a battery's anode
as it charges. Silicon can take on more lithium than graphite, but it tends to expand
about 300 percent in volume, causing the anode to break apart. The researchers
created a porous form of silicon by aggregating small silicon particles into
microspheres about 8 micrometers in diameter.
The electrode with porous silicon
structure exhibits a change in thickness of less than 20 percent while
accommodating twice the charge of a typical graphite anode. However, unlike
previous versions of porous silicon, the microspheres also exhibited
extraordinary mechanical strength, thanks to carbon nanotubes that make the
spheres resemble balls of yarn.
The researchers created the
structure sequentially, starting by coating the carbon nanotubes with silicon
oxide. Next, the nanotubes were put into an emulsion of oil and water. Then
they were heated to boiling. The coated carbon nanotubes condense into spheres
when the water evaporates. Then aluminum and higher heat is used to convert the
silicon oxide into silicon, followed by immersion in water and acid to remove
by-products. What emerges from the process is a powder composed of the tiny
silicon particles on the surface of carbon nanotubes.
