Ions are charged atoms or molecules. How easily they move within a solid material determines how batteries, fuel cells and sensors work. Solid electrolytes are especially interesting: they could make batteries more capacious and safer compared with conventional ones, where a liquid carries the ions.
A group of scientists led by the U.S. Department of Energy's (DOE) Argonne National Laboratory tested a new method for observing ions. It was named 4D-QENS - four-dimensional quasielastic neutron scattering. In essence, it is an improved way to illuminate a sample with neutrons and, from how they are deflected and change energy, understand how atoms move inside. The results were published in the journal PRX Energy.
The experiments were conducted at the Spallation Neutron Source, a Department of Energy facility at Oak Ridge National Laboratory (ORNL). The method's distinguishing feature is that the single crystal is rotated during irradiation. This makes it possible to collect volumetric data and see not only individual ion hops but also their coordinated movements.
The idea of rotating a sample at pulsed neutron sources has existed for about 15 years, but it had not previously been applied to quasielastic scattering, explained Raymond Osborn, a senior physicist at Argonne National Laboratory and lead author of the work.
To test the method, the researchers used single crystals of strontium chloride. This is a salt that, at temperatures above 1200 degrees Fahrenheit, becomes a fast ion conductor. It is not suitable for future technologies, but its properties are well studied, making it convenient as a model.
The method confirmed a known picture: ions hop from one site in the crystal lattice to another. But in addition, it was possible to directly see correlated motion - when ions influence each other. Such interactions can both aid movement and create bottlenecks that are harmful to a battery. It is important to understand: correlation here is a statistical relationship in the data, not proof that one phenomenon causes another.
Neutrons are convenient for such experiments because they have no electric charge and their mass is close to that of atoms. Even a small change in a neutron's speed after colliding with the sample helps reconstruct the picture of movements. That is precisely why the experiments ran at the world's most powerful pulsed neutron source.
The more data, the higher the computing requirements. After the recent upgrade of the source at ORNL, the volume of measurements grew, and supercomputers, such as the Argonne Leadership Computing Facility (ALCF), help process them.
Just a few years ago, questions about ion conductivity were far narrower, noted Stephan Rosenkranz, a group leader in Argonne's Materials Science Division. Now scientists measure three-dimensional volumes plus energy transfer - yielding four dimensions.
The authors emphasize that this is still a laboratory method, not a ready-made instrument for production. But in the future it can be applied to more complex materials that are directly suitable for solid electrolytes and other energy technologies.