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 thrown away after a single use.
Despite all their promise, researchers have had trouble getting nanoparticles to improve battery performance. Pint said: "Researchers have demonstrated that nanoscale materials can significantly improve batteries, but there is a limit. When the particles get very small, generally 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."
The team set out to explore this ‘ultrasmall’ regime by adding millions of iron pyrite quantum dots of different sizes to standard lithium button batteries. They claim they got the best results when they added ultrasmall nanocrystals that were about 4.5nm in size. These were said to improve both the batteries' cycling and rate capabilities.
This result was reached because the team found that iron pyrite has a unique way of changing form into an iron and a lithium-sulphur 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.
Pint added, "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 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, and 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 explained.
A key observation of the team's study was that these ultrasmall nanoparticles are equipped with dimensions that allow the iron to move to the surface while the sodium or lithium reacts with the sulphurs in the iron pyrite. They demonstrated that this isn't the case for 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 to enable the evolution of battery performance at a pace that stands up to Moore's law and can support the transition to electric vehicles.
"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 tools that will allow us to design nanostructures that can stand up to tens of thousands of cycles and possess energy storage capacities rivalling that of gasoline," said Pint. "Our research is a major step in this direction."
