Many of the most studied nanostructured materials for metal-ion batteries, such as silicon, exhibit alloying or intercalation reactions where the metal ions shuttle through a solid-electrolyte interphase (SEI) layer to store charge within the interior of the storage material. In these materials, reports have emphasized the detrimental effect of ultrafine nanoscale materials due to the dominance of the SEI layer on inhibiting reversible storage.

In the particular case of silicon, researchers have emphasized that SEI layer growth upon cycling in ultrasmall nanostructures can deactivate the active storage material, leading to lower capacity and shorter cycling lifetimes than larger nanoparticles. Similar observations have been made with other intercalation or alloying storage materials as well, including antimony and tin nanocrystals, indicating that nanostructures that are too small may not be advantageous for metal-ion storage.

… In contrast to intercalation or alloying reactions, where metal atoms are reversibly shuttled in and out of a host lattice, conversion reactions involve the chemical transformation of one or more of the atomic species into a host lattice to form a new compound. … In this spirit, the metal sulfides, which exhibit conversion reactions with lithium or sodium metal atoms, have been at the forefront of both fundamental and applied research in battery systems.

… Whereas most studies on metal sulfides are focused on the transition metal dichalcogenides, FeS₂ (cubic pyrite, “fool's gold”) is a highly promising material that has not been widely studied for secondary battery systems. FeS₂ is particularly attractive for energy storage technology due to its earth abundance, low toxicity, and low raw material cost. … In this work we explore the sodium and lithium conversion of ultrafine FeS₂ nanoparticles, with a tight size distribution centered around ∼4.5 nm, which is in the size regime where strong quantum confinement effects are observed.

—Douglas et al.

The Vanderbilt team attributed the performance of the iron pyrite quantum dots to a nanoparticle size comparable to or smaller than the diffusion length of Fe during cation exchange, yielding thermodynamically reversible nanodomains of converted Fe metal and Na_xS or Li_xS conversion products.

This is compared to bulk-like electrode materials, where kinetic and thermodynamic limitations of surface-nucleated conversion products inhibit successive conversion cycles.

The research team headed by Assistant Professor of Mechanical Engineering Cary Pint and led by graduate student Anna Douglas added millions of iron pyrite quantum dots of different sizes to standard lithium button batteries. They found the best results with ultrasmall nanocrystals that were about 4.5 nanometers in size. These substantially improved both the batteries’ cycling and rate capabilities.

The researchers discovered that iron pyrite has a unique way of changing form into an iron and a lithium-sulfur (or sodium-sulfur) compound to store energy.

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 (LD) represents the distance iron atoms have to move through the iron pyrite to reach the surface. (Pint Lab / Vanderbilt) Click to enlarge.

As a result, the rules that discourage 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.

—Anna Douglas

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 sulfurs 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 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.

We demonstrate that ultrafine FeS₂ nanoparticles bring mechanistic advantages for batteries that store charge through chemical conversion reactions. … Not only does this provide a route to match low-cost materials with high-capacity sodium sulfur (or lithium sulfur) based conversion storage reactions, but it opens up a pathway toward a new size regime for the design of chemical storage systems.

As a significant effort has been placed on understanding the manifestation of quantum mechanical phenomena in nanocrystals and quantum dots toward applications in recent years, we anticipate an exciting research area existing at the intersection of quantum-confined nanostructures and energy storage or conversion processes where kinetics and thermo- dynamics ultimately dictate performance. This is compounded by our results that demonstrate, for the first time, that mechanistic processes occurring during sodium sulfur and lithium sulfur chemical conversion reactions are enhanced by nanostructures that have features at this length scale.

—Douglas et al.

Coauthors of the paper with Pint and Douglas include mechanical engineering graduate students Rachel Carter and Adam Cohn and interdisciplinary materials science graduate students Keith Share and Landon Oakes. The research was funded in part by National Science Foundation grant EPS 1004083 and NSF’s graduate research fellowship program grant 144519.

Resources

• Anna Douglas, Rachel Carter, Landon Oakes, Keith Share, Adam P. Cohn, and Cary L. Pint (2015) “Ultrafine Iron Pyrite (FeS₂) Nanocrystals Improve Sodium–Sulfur and Lithium–Sulfur Conversion Reactions for Efficient Batteries” ACS Nano doi: 10.1021/acsnano.5b04700