Copper flow: Liquid metal in a crystal

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  • Published: Oct 15, 2018
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Copper flow: Liquid metal in a crystal

Build a better battery

An artist's rendition of the superionic crystalline structure of CuCrSe2, which has copper ions that move like liquid between solid layers of chromium and selenium, giving rise to useful electrical properties. Credit: Oak Ridge National Laboratory/Jill Hemman

We might be able to build a battery thanks to research that reveals how copper ions can flow like liquid through certain crystalline materials, according to scientists in the USA.

Researchers from Duke University, Oak Ridge National Laboratory (ORNL) and Argonne National Laboratory (ANL) have used neutrons and X-rays to probe a superionic crystal containing copper, chromium and selenium (CuCrSe2). The study reveals that the copper ions inside the material can adopt a liquid-like character. The team describes details in the journal Nature Physics in October. The explanation for this physical phenomenon could underpin a promising new approach to developing novel electrical and thermal properties in such superionic materials. Ultimately, the insights that emerge could be used to make rechargeable batteries safer and more efficient than the current class based on lithium ions.

Superionic crystals are sometimes described as lying between the liquid and the solid state. The bulk of such materials exists in a rigid, crystalline structure as in countless solids, but above a certain temperature some elements within that structure are able to flow as if they are in the liquid state without loss of the overall solid nature of the material.

Substantial paradox

“When CuCrSe2 is heated above [about 90 degrees Celsius], its copper ions fly around inside the layers of chromium and selenium about as fast as liquid water molecules move,” explains senior author on the current paper Olivier Delaire. “And yet, it’s still a solid that you could hold in your hand. We wanted to understand the physics behind this phenomenon.”

In order to understand this seeming paradox, the team probed the characteristics of the copper ions using the Spallation Neutron Source at ORNL and the Advanced Photon Source at ANL. Data from each experiment provided important information that feeds into the model of this phenomenon. The neutron study gave the researchers a wide-scale view of the material’s atomic structure and dynamics, revealing both the vibrations of the stiff scaffold of chromium and selenium atoms as well as the random jumps of copper ions within. The X-ray study looked more closely at the details of those vibration modes revealing how scaffold vibrations enable shear waves to propagate, a hallmark of solid behavior.

The team merged the data using quantum simulations of the material’s atomic behaviour at the National Energy Research Scientific Computing Center to explain their findings. Below the phase transition temperature, the copper atoms vibrate around isolated sites, trapped in pockets of the material’s scaffold structure. But above that temperature, they are able to hop randomly between multiple available sites. This allows the copper ions to flow throughout the otherwise solid crystal.

No more flaming laptops

The team concedes that more work is now needed to understand how the copper atoms interact with each other but the findings do offer important clues as to how we might exploit related materials for future electronic applications.

“Most commercial lithium ion batteries use a liquid electrolyte to transfer ions between the positive and negative terminals of the battery,” Delaire explains. “While efficient, this liquid can be dangerously flammable." There have been a number of high-profile cases where laptop, smart phones, and other devices using lithium ion batteries have overheated and burst into flames. This is not a characteristic of a device that one perches on ones lap or holds close to one's head.

“There are variants of superionic crystals that contain ions like lithium or sodium that behave like the copper in CuCrSe2,” Delaire adds. “If we can understand how superionic crystals work through this study and future research, we could perhaps find a better, solid solution for transporting ions in rechargeable batteries.”

Related Links

Nature Phys 2018, online: "Selective breakdown of phonon quasiparticles across superionic transition in CuCrSe2"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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