There is a great need for rechargeable batteries that are safer and more efficient than the current standard, lithium-ion batteries. One promising area of research is in solid-state electrolytes such as superionic crystals, in which part of the material is solid and part is liquid at the same time, which allows electrical flow. These superionic crystals can also help harvest waste heat to produce electricity with thermoelectric modules. In this work, a superionic crystalline material, CuCrSe2, was bombarded with x-rays at the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS), and with neutrons at the DOE’s Spallation Neutron Source (SNS), to reveal how the copper ions could behave as a liquid. With a better understanding of superionic materials, researchers can get closer to developing better and safer personal electronics.
Rechargeable lithium-ion batteries are currently the best option for use in portable electronics, everything from phones to laptops. In these batteries, liquid lithium electrolyte flows from the negative to the positive electrode during discharge and from the positive to the negative during charging. While these batteries are essential for modern electronics, they are not perfect for their job. Lithium-ion batteries have difficulty retaining a charge and operating at high and low temperatures, and they have been known to catch fire or explode. As a result, researchers are continually looking for better electrolytic materials that are safer, less expensive, longer-lived, more energy dense, and faster charging.
One substance of great interest are superionic crystals, a rare class of materials that is a cross between a liquid and a solid. At a given temperature, some of the atoms in these materials retain a rigid crystalline structure, while others liquify and can flow through the solid crystalline structure. These materials show differences in atomic dynamics; the atoms that flow in a liquid follow stochastic diffusive dynamics, while remaining affixed to well-defined lattice sites in a crystalline solid.
In this study, published in Nature Physics, researchers used DOE Office of Science User Facilities to conduct experiments and simulations to better understand the atomic dynamics of one superionic material, CuCrSe2. The researchers powdered samples of the material and heated it to above 190o F. The researchers then performed inelastic neutron scattering experiments on an 8-g sample with the CNCS and ARCS time-of-flight spectrometers at the SNS at Oak Ridge National Laboratory. The powerful neutrons revealed a wide-scale view, in which solid Cr and Se vibrated in the atomic structure and Cu ions jumped randomly within the Cr and Se structure. This flow appears to occur at about the same speed as liquid water molecules move.
Inelastic x-ray measurements were performed on minute single crystals of CuCrSe2 (Fig. 1) at the HERIX spectrometer at Sector 30 of the APS at Argonne National Laboratory. These high-resolution x-rays depicted a narrower but more detailed view of the structure. Scaffolded vibrations enabled shear waves to propagate, revealing that the structure was solid, even in the presence of the liquid sublayer.
Subsequent simulations at the DOE’s National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory allowed the researchers to directly compare the results of the neutron and x-ray spectroscopy experiments. The simulations showed that below 190o F, the Cu was trapped, vibrating in pockets of the scaffold structure. Above 190o F, the Cu hopped randomly between available sites, allowing it to flow through the solid crystal. Additional research is needed to reveal the interactions of the Cu ions as the scaffold sites become occupied.
These findings will allow researchers to better understand the atomic dynamics that occur in matter in the intermediate state between liquid and solid and guide them in their quest to create high-performance solid-state electrolytes. Superionic compounds with ultralow conductivities will be useful for both thermoelectric applications and in rechargeable batteries that are safer and more efficient for use in personal electronics. ― Dana Desonie
See: Jennifer L. Niedziela1,2‡*, Dipanshu Bansal2**, Andrew F. May1, Jingxuan Ding2, Tyson Lanigan-Atkins2, Georg Ehlers1, Douglas L. Abernathy1, Ayman Said3, and Olivier Delaire2***, “Selective breakdown of phonon quasiparticles across superionic transition in CuCrSe2,” Nat. Phys. 15, 73 (January 2019). DOI: 10.1038/s41567-018-0298-2
Author affiliations: 1Oak Ridge National Laboratory, 2Duke University, 3Argonne National Laboratory,
‡Present address: Oak Ridge National Laboratory
We are grateful to J. Z. Tischler (APS) for algorithms enabling deconvolution of the energy resolution from the inelastic x-ray phonon scattering data. We would also like to acknowledge technical support from D. Dunning, T. Russell and S. Elorfi at the SNS. JJ.L.N., J.D., and T.L.-A. were supported as part of the S3TEC EFRC, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE) Office of Science-Basic Energy Sciences under award no. DE-SC0001299. D.B. and O.D. were supported by the U.S. DOE Office of Science-Basic Energy Sciences, Materials Sciences and Engineering Division, under the Early Career Award no. DE-SC0016166 (principal investigator O.D.). A.F.M. was supported by the U.S. DOE Office of Science-Basic Energy Sciences, Materials Sciences and Engineering Division. The research at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Basic Energy Sciences, U.S. DOE. Ab initio molecular dynamics calculations were performed using resources of the National Energy Research Scientific Computing Center, a U.S. DOE Office of Science User Facility supported by the Office of Science of the U.S. DOE under Contract no. DE-AC02-05CH11231. Density functional theory simulations for this research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. DOE under Contract no. DE-AC05-00OR22725. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.
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