Vanderbilt graduate student Anna Douglas holds one of the batteries she has modified by adding millions of quantum dots made from iron pyrite, or fool's gold. Photo: John Russell, Vanderbilt University.
Vanderbilt graduate student Anna Douglas holds one of the batteries she has modified by adding millions of quantum dots made from iron pyrite, or fool's gold. Photo: John Russell, Vanderbilt University.

If you add the nanocrystals known as quantum dots to a smartphone battery, it will charge in 30 seconds, but the effect only lasts for a few recharge cycles. A group of researchers at Vanderbilt University in Nashville, US, have now discovered a way to overcome this limitation. As reported in ACS Nano, by utilizing quantum dots made out of iron pyrite, commonly known as fool's gold, they have produced batteries that charge quickly and work for dozens of cycles.

"Researchers have demonstrated that nanoscale materials can significantly improve batteries, but there is a limit," said Cary Pint, assistant professor of mechanical engineering at Vanderbilt University. "When the particles get very small, generally meaning below 10nm, 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."

Together with colleagues at Vanderbilt, Pint decided to explore this ‘ultrasmall’ regime in iron pyrite, which is one of the most abundant materials in the Earth's surface. They did so by adding millions of iron pyrite quantum dots of different sizes to the kind of standard lithium button batteries used to power watches, automobile key remotes and LED flashlights. They got the most bang for their buck when they added ultrasmall nanocrystals about 4.5nm in size, which substantially improved both the batteries' cycling and rate capabilities.

Further research revealed this was due to iron pyrite changing 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," said graduate student Anna Douglas.

"You can think of it like vanilla cake," explained Pint. "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 favor 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," said Douglas.

A key observation of this study is that these ultrasmall nanoparticles allow the iron to move to the surface while the sodium or lithium reacts with the sulfur in the iron pyrite. The researchers demonstrated that this doesn’t happen with larger particles, where the inability of the iron to move through the iron pyrite materials limits their storage capability.

Pint believes that understanding chemical storage mechanisms and how they depend on nanoscale dimensions is critical for improving battery performance. "The batteries of tomorrow that can charge in seconds and discharge in days will not just use nanotechnology, they will benefit from the development of new tools that will allow us to design nanostructures that can stand up to tens of thousands of cycles and possess energy storage capacities rivaling that of gasoline," said Pint. "Our research is a major step in this direction."

This story is adapted from material from Vanderbilt University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.