Iron pyrite, commonly known as fool’s gold, can produce batteries that charge quickly and work for dozens of cycles.

Text: Bhushan Mhapralkar

The quantum of electronics in automobiles is growing. Supporting this rise are new age batteries. In case of hybrids and Electric Vehicles (EV) it is the Lithium Ion batteries. Looked upon as superior to the traditional lead acid batteries, Lithium Ion batteries offer an EV or a hybrid vehicle to perform more efficiently. They do not suffer the tendency to drop voltage or turn less efficient until they have run out of charge. The challenges surrounding Lithium Ion batteries therefore is the range they can help an EV or a hybrid vehicle . Quantum Dots made from fool’s gold are set to help the battery boost its performance. This is going beyond the fact that quantum dots, nanocrystals 10,000 times smaller than the width of a human hair, added to a smartphone battery will enable it to charge in 30 seconds, the effect of which will however last only for a few recharge cycles.

Anna Douglas holding one of the batteries that she has modified by adding millions of quantum dots made from iron pyrite, fool’s gold

Anna Douglas holding one of the batteries that she has modified by adding millions of quantum dots made from iron pyrite, fool’s gold

A group of researchers at Vanderbilt University, in their report, released recently in the journal ACS Nano, have claimed to have found a way to overcome this problem. They have found out that making the quantum dots out of iron pyrite, commonly known as fool’s gold, can produce batteries that charge quickly and work for dozens of cycles. 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 on the surface of earth. It is produced in raw form as a byproduct 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. According to Pint, researchers have demonstrated that nanoscale materials can significantly improve batteries, but there is a limit. When the particles get very small, generally below 10 nanometers (40 to 50 atoms wide), they begin to chemically react with the electrolytes and can only charge and discharge a few times. The size regime is forbidden in commercial lithium-ion batteries. Pint added that aided by Dougla’s expertise in synthesizing nanoparticles, the team set out to explore the ‘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 4.5 nanometers in size. These substantially improved both the batteries’ cycling and rate capabilities.

The researchers discovered that they got this result because iron pyrite has a unique way of changing form into an iron and a lithium-sulphur (or sodium 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. According to Pint, one could think of it like a 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,” he added.

As a result, the rules that forbid the use of ultrasmall nanoparticles in batteries no longer apply. In fact, the scales seem to tip in favour of very small nanoparticles. Explained Douglas, that instead of simply 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.
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 sodium or lithium reacts with the sulphurs in the iron pyrite. Pint’s team has 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. He believed that the understanding of 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 new tools that will allow us to design nanostructure 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,” he concluded. ACI


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