Future pivotal discoveries could be made in fuel cells, batteries, and other devices that convert heat to electricity.

Scientists usually conduct their research by selecting the right research problem and devising a plan to solve it. Unplanned discoveries may occur along the way.

Mercouri Kanatzidis is a professor at Northwestern University and has a joint appointment at the U.S. Department of Energy (DOE) Argonne National Laboratory. When he discovered it, he was looking for a superconductor that would exhibit unusual behavior. This material is just four atoms thick and allows you to study the motion of charged particles only in two dimensions. These studies could lead to the creation of new materials used in various energy conversion devices.

Kanatzidis was looking for a combination of selenium, silver, and potassium (a-KAg 3Se 2) in a four-layered structure similar to a wedding cake. These 2D materials are fragile at four atoms in height and have length and width.

When cooled to shallow temperatures, superconducting materials lose their resistance to electron movement. Kanatzidis is a senior scientist at Argonne’s Materials Science Division. It was a remarkable example of a superionic conductor, much to my delight.

Superionic conductors allow charged ions to roam freely in solid materials, just like in liquid electrolytes in batteries. This creates a solid that has a high ionic conductivity. This is a measurement of its ability to conduct electricity. This high ionic conductivity is accompanied by low thermal conductivity. Heat cannot pass through this material easily. These properties make superionic conducting materials excellent materials for energy storage or conversion devices.

A-KAg3Se2, a 2D superionic conductor, has a four-layer atomic structure. The names of the atoms correspond to the colors of the particles. Credit: Image by Mercouri Kanatzidis/Northwestern University and Argonne National Laboratory

The team realized it had unique properties when they heated the material to 450-600 degrees Fahrenheit. The material transformed into a more symmetrical, layered structure. This transition was also reversible, according to the team. They lowered the temperature and then raised it again to the high-temperature zone.

Kanatzidis stated, “our analysis results showed that the silver ions were held in the confined space of the two dimensions within our material before this transition.” “But after the transition, they wiggled about.” Although much is known about ions’ movement in three dimensions, little is known about their movements in two.

Scientists have searched for an excellent material to study ion movement within 2D materials for some time. This layered potassium-silver-selenium material is one. The team measured the diffusion of ions in the solid and found that it was comparable to the distribution of heavily salted water electrolytes, one of the fastest known ionic conductors.

Although it’s too early to know if this superionic material will find practical use, it could be a key platform for designing other 2D materials with high thermal and ionic conductivity.

Duck Young Chung is the principal materials scientist at MSD. “These properties are essential for those who design new two-dimensional solid electrolytes for fuel cells and batteries.”

This superionic material may also help design new thermoelectrics, which convert heat into electricity in power plants, industrial processes, and exhaust gas from car emissions. These studies could also be used to design membranes that can clean up the water and salt it.

This research was published in a Nature Materials paper titled “A two-dimensional type 1 superionic conductor.” Researchers from Argonne and Northwestern, DOE’s Oak Ridge National Laboratory at University College London, Duke University, and University College London are part of the team.