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Better batteries by adding quantum dots from fool's gold

Posted: 17 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Vanderbilt University? fool's gold? iron pyrite? batteries? quantum dots?

A team of researchers at Vanderbilt University has come up with a way to boost battery performance. If you add quantum dots, nanocrystals 10,000 times smaller than the width of a human hair, to a smartphone battery it will charge in 30 seconds, but the effect only lasts for a few recharge cycles. The scientists made significant headway by adding quantum dots out of iron pyrite, commonly known as fool's gold, to create batteries that charge quickly and work for dozens of cycles.

Reported in the Nov. 11 issue of the journal ACS Nano, the research team headed by assistant professor of mechanical engineering Cary Pint and led by graduate student Anna Douglas became interested in iron pyrite because it is one of the most abundant materials in the earth's surface. It is produced in raw form as a by-product of coal production and is so cheap that it is used in lithium batteries that are bought in the store and thrown away after a single use.

Despite all their promise, researchers have had trouble getting nanoparticles to improve battery performance.

"Researchers have demonstrated that nanoscale materials can significantly improve batteries, but there is a limit," Pint said. "When the particles get very small, generally meaning below 10nm (40 to 50 atoms wide), the nanoparticles begin to chemically react with the electrolytes and so can only charge and discharge a few times. So this size regime is forbidden in commercial lithium-ion batteries."

Battery with quantum dots made from iron pyrite

Anna Douglas holding one of the batteries that she has modified by adding millions of quantum dots made from iron pyrite, fool's gold. (John Russell / Vanderbilt)

Aided by Douglas' expertise in synthesizing nanoparticles, the team set out to explore this "ultrasmall" regime. They did so by adding millions of iron pyrite quantum dots of different sizes to standard lithium button batteries like those that are used to power watches, automobile key remotes and LED flashlights. They got the most bang for their buck when they added ultrasmall nanocrystals that were about 4.5nm. These substantially improved both the batteries' cycling and rate capabilities.

Cary Pint and Anna Douglas

Cary Pint and Anna Douglas examining the recharge rate of one of their modified batteries. (John Russell/Vanderbilt)

The researchers discovered that they got this result because iron pyrite has a unique way of changing form into an iron and a lithium-sulfur (or sodium sulfur) compound to store energy. "This is a different mechanism from how commercial lithium-ion batteries store charge, where lithium inserts into a material during charging and is extracted while discharging, all the while leaving the material that stores the lithium mostly unchanged," Douglas explained.

According to Pint, "You can think of it like vanilla cake. Storing lithium or sodium in conventional battery materials is like pushing chocolate chips into the cake and then pulling the intact chips back out. With the interesting materials we're studying, you put chocolate chips into vanilla cake and it changes into a chocolate cake with vanilla chips."

As a result, the rules that forbid the use of ultrasmall nanoparticles in batteries no longer apply. In fact, the scales are tipped in favour of very small nanoparticles.

"Instead of just inserting lithium or sodium ions in or out of the nanoparticles, storage in iron pyrite requires the diffusion of iron atoms as well. Unfortunately, iron diffuses slowly, requiring that the size be smaller than the iron diffusion length, something that is only possible with ultrasmall nanoparticles," Douglas noted.

Bulk iron pyrite in a battery

Putting bulk iron pyrite in a battery works poorly because the iron must move to the surface so that sodium-sulfur material (or lithium-sulfur material) can form and store energy. Iron pyrite quantum dots, by contrast, have iron close to the surface due to their small size, and this energy storage process can occur reversibly over many cycles. The diffusion length represents the distance iron atoms have to move through the iron pyrite to reach the surface. (Pint Lab/Vanderbilt)

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