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Material scientists have studied the physical phenomenon underlying the promising electrical and thermal 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, superionic crystals are a cross between a liquid and a solid. While some of their atoms retain a rigid crystalline structure that is related to a frozen state, others become liquid above a certain temperature and are able to pass through the solid scaffold.
In a new study, scientists from Duke University, ORNL (Oak Ridge National Laboratory) and Argonne National Laboratory (ANL) surveyed one of these superionic crystals containing copper, chromium and selenium (CuCrSe2) with neutrons and X-rays to determine the ions reach their liquid properties. The results are online Oct. 8 in the journal Nature Physics.
"When CuCrSe2 is heated to over 190 degrees Fahrenheit, its copper ions fly into the chromium and selenium layers about as fast as liquid water molecules move," said Olivier Delaire, associate professor of engineering mechanical and material science at Duke and lead author. l & # 39; study. And yet, it's still a solid that you can hold in your hand. We wanted to understand the 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.
"If we can understand the operation of superionic crystals through this study and our future research, we may be able to find a stronger and more solid solution for the transport of ions in rechargeable batteries."
By sending a large sample of CuCrSe2 powder manufactured at Oak Ridge with powerful neutrons, the researchers obtained a large-scale view of the atomic structure and dynamics of the material, revealing both the vibrations of the rigid scaffold chromium and selenium atoms and random effects. leaps of copper ions in the interior.
For a more detailed but more detailed examination of the vibration modes, the researchers bombarded a tiny crystal grain of CuCrSe2 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 the two pieces of information in hand, the Delaire group carried out quantum simulations of the atomic behavior of the material at the National Center for Scientific Computing of Energy Research to explain their results. Below the phase transition temperature of 190 degrees Fahrenheit, copper atoms vibrate around isolated sites, trapped in pockets of the scaffold structure of the material. 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, which behave like copper in CuCrSe2," said Delaire. "If we can understand the operation of superionic crystals through this study and our future research, we may be able to find a stronger and more solid solution for the transport of ions in rechargeable batteries."
This research was funded by the Ministry of Energy (DE-SC0016166, DE-SC0001299, DE-AC02-06CH11357, DE-AC02-05CH11231, DEAC05-00OR22725).
QUOTE: "Selective distribution of quasiparticles of phonons during the superionic transition in CuCrSe2". Nature Physics, 2018. DOI: 10.1038 / s41567-018-0298-2
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