The four-layer atomic structure of a-KAg3Se2, a 2D superionic conductor. The colors of the atoms in the structure match up with the colors in the name. Image: Mercouri Kanatzidis/Northwestern University and Argonne National Laboratory.
The four-layer atomic structure of a-KAg3Se2, a 2D superionic conductor. The colors of the atoms in the structure match up with the colors in the name. Image: Mercouri Kanatzidis/Northwestern University and Argonne National Laboratory.

Scientists normally conduct their research by carefully selecting a research problem, devising an appropriate plan to solve it and executing that plan. But sometimes unplanned discoveries can occur along the way.

This is exactly what happened to Mercouri Kanatzidis, a professor at Northwestern University with a joint appointment at the US Department of Energy (DOE)'s Argonne National Laboratory. While searching for a new superconductor with unconventional behavior, he and his team discovered a material only four atoms thick that can be used to study the motion of charged particles in just two dimensions. Such studies could spur the invention of new materials for a variety of energy conversion devices.

Kanatzidis’s material is a combination of silver, potassium and selenium (α-KAg3Se2) in a four-layered structure like a wedding cake. It's an example of a 2D material, having length and width but almost no thickness at only four atoms high.

Superconducting materials lose all resistance to the movement of electrons when cooled to very low temperatures. “Much to my disappointment, this material was not a superconductor at all, and we could not make it one,” said Kanatzidis, who is a senior scientist in Argonne’s Materials Science Division (MSD). “But much to my surprise, it turned out to be a fantastic example of a superionic conductor.” He and his team describe this novel material in a paper in Nature Materials.

In superionic conductors, charged ions roam about in a solid material just as freely as they do in the liquid electrolytes found in batteries. This results in a solid with unusually high ionic conductivity, a measure of the ability to conduct electricity. With this high ionic conductivity comes low thermal conductivity, meaning heat does not pass through easily. Both of these properties make superionic conductors highly promising materials for novel energy storage and conversion devices.

The researchers first clue that they had discovered a material with special properties was when they heated it up to 450–600°F, which caused it to transition into a more symmetrical layered structure. They also found that this transition was reversible when they lowered the temperature and then raised it again into the high temperature zone.

“Our analysis results revealed that, before this transition, the silver ions were fixed in the confined space within the two dimensions of our material,” said Kanatzidis. “But after this transition, they wiggled around.” While much is known about how ions move about in three dimensions, very little is known about how they do so in only two dimensions.

Scientists have been searching for some time to find an exemplary material to investigate ion movement in 2D materials, and this layered potassium-silver-selenium material appears to be just what they’ve been looking for. The team measured how the ions diffused in this solid and found it to be equivalent to that of a heavily salted water electrolyte, one of the fastest known ionic conductors.

While it is too early to tell if this particular superionic material might find practical applications, it could immediately serve as a crucial platform for designing other 2D materials with high ionic conductivity and low thermal conductivity.

“These properties are very important for those designing new two-dimensional solid electrolytes for batteries and fuel cells,” said Duck Young Chung, principal materials scientist in the MSD.

Studies with this superionic material could also be instrumental for designing new thermoelectrics that convert heat from power plants, industrial processes and even exhaust gas from automobile emissions into electricity. In addition, the studies could be used for designing membranes for environmental clean-up and desalting of water.

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