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<a href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hires/2018/copperionsfl.jpg" title = "An artistic interpretation of the intriguing superionic crystalline structure of CuCrSe2, whose copper ions move like a liquid between solid layers of chromium and selenium, giving rise to useful electrical properties. Credit: Oak Ridge National Laboratory / Jill Hemman ">
An artistic interpretation of the intriguing superionic crystalline structure of CuCrSe2, whose copper ions move like a liquid between solid layers of chromium and selenium, giving rise to useful electrical properties. Credit: Oak Ridge National Laboratory / Jill Hemman
Material scientists have studied the physical phenomenon underlying the promising electrical properties of a class of materials called superionic crystals. A better understanding of these materials could make rechargeable batteries safer and more efficient than the current standard lithium ion carrier.
Become a subject of popular study in the last five years only, superionic crystals are a cross between a liquid and a solid. While some of their molecular components retain a rigid crystalline structure, others become liquid above a certain temperature and can then pass through the solid scaffold.
In a new study, scientists from Duke University, Oak Ridge National Laboratory (ORNL) and Argonne National Laboratory (ANL) probed one of these superionic crystals containing copper, chromium and selenium (CuCrSe).2) with neutrons and X-rays to determine how the copper ions of the material reach their properties similar to those of a liquid. The results appear online on October 8 in the newspaper. Physical Nature.
When CuCrSe2 is heated above 190 degrees Fahrenheit, its copper ions fly in chromium and selenium layers about as fast as liquid water molecules, "said Olivier Delaire, associate professor of mechanical engineering and science materials to Duke and lead author of the study. "And yet, it's still a solid that you could hold in your hand. We wanted to understand the molecular physics behind this phenomenon. "
To probe the behavior of copper ions, Delaire and his colleagues turned to two world-class facilities: the spallation neutralization source at Oak Ridge and the advanced photon source at Argonne. Each machine provided a unique piece of the puzzle.
By sending a large sample of CuCrSe powder2 made at Oak Ridge with powerful neutrons, the researchers had a large-scale view of the atomic structure and dynamics of the material, revealing both the vibrations of the rigid scaffolding of chromium and selenium atoms as well as the random jumps of copper ions inside.
For a more detailed but more detailed examination of the modes of vibration, the researchers bombarded a tiny grain of CuCrSe.2 crystal with high resolution X-rays. This allowed them to examine how the rays scattered by its atoms and how the vibrations of the scaffolding allowed the propagation of shear waves, characteristic of solid behavior.
With both sets of information in hand, the Delaire group performed quantum simulations of the atomic behavior of the material at the National Energy Research Scientific Computing Center to explain their results. Below the phase transition temperature of 190 degrees Fahrenheit, copper atoms vibrate around isolated sites trapped in pockets of the material's scaffolding structure. But above this temperature, they can switch randomly between several available sites. This allows the copper ions to flow through the otherwise solid crystal.
Although additional work is needed to understand how copper atoms interact once both sites are occupied, the results provide clues as to how to use similar materials in future electronic applications.
"Most commercial lithium-ion batteries use a liquid electrolyte to transfer ions between the positive and negative terminals of the battery," said Delaire. "Although effective, this liquid can be dangerously flammable, as many owners of laptops and smartphones have unfortunately discovered."
"There are variants of superionic crystals containing ions such as lithium or sodium that behave like copper in CuCrSe2"If we can understand the operation of superionic crystals through this study and our future research, we could perhaps find a better and more robust solution for transporting ions in rechargeable batteries."
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More information:
Jennifer L. Niedziela et al., Selective distribution of quasi-phonon particles during the superionic transition in CuCrSe2, Physical Nature (2018). DOI: 10.1038 / s41567-018-0298-2
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