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Superionic compound with liquid-like dynamics shows promise as solid-state battery electrolyte

Superionic materials are a class of materials that simultaneously present properties that are characteristic of solids and liquids. Essentially, a set of ions in these materials exhibits liquid-like mobility, even if the materials' underlying atomic structure maintains a solid-like order.

Due to their unique ionic conductivity patterns, superionic materials could be promising for developing solid-state batteries. These are batteries that contain electrolytes based on solid materials instead of liquid electrolytes.

While various past studies have explored the potential of superionic materials as solid-state electrolytes, the physics underpinning their rapid ionic diffusion is not yet fully understood. Specifically, it is unclear whether this property results from liquid-like motion in the material or from the conventional lattice phonons (i.e., atom vibrations) in the material.

Researchers at Duke University and other institutes recently carried out a study exploring the mechanisms underlying the ion mobility in the superionic compound Li₆PSâ‚…Cl, which could be a promising solid-state electrolyte. Their findings, in Nature Â鶹ÒùÔºics, unveils that the ionic motion in this material is linked to its liquid-like dynamics, while also opening new possibilities for the optimization of solid-state batteries.

"Our research group has been interested in understanding the atomic dynamics in energy materials for some time," Olivier Delaire, senior author of the paper, told Â鶹ÒùÔº.

"We have found, across many classes of materials, strong deviations from the textbook models of harmonic lattice dynamics, which are described in terms of phonon in solid-state physics. In the case of solid-state electrolyte materials, one seeks a solid framework that enables very high diffusivities of mobile ions (such as lithium or sodium), comparable to those of liquid electrolytes, while also offering good chemical and thermal stability."

The primary objective of this recent study by Delaire and his colleagues was to better understand how Li⁺ cations can diffuse so rapidly in the complex crystalline framework of the lithium argyrodite Li₆PSâ‚…Cl. In addition, the researchers wished to determine whether the dynamics of these ions could be understood in the context of phonons in a crystalline material, even if they predicted the presence of strong anharmonic effects.

"To probe the motions of ions in superionic materials, we performed neutron scattering experiments at the DOE's Oak Ridge National Laboratory and atomistic computer simulations, based on so-called first-principles methods and augmented with machine-learning techniques," explained Delaire.

Neutron scattering, the main technique that the researchers used to examine Li₆PS₅Cl, is a powerful tool designed to probe atomic vibrations and thus uncover atomic dynamics with frequencies in the GHz-THz range. Using this technique, Delaire and his colleagues were able to determine whether ions vibrate around local potential wells or hop between multiple sites in the sample material's structure instead.

"Because the neutron scattering cross-section is well understood, we can directly compare the neutron data with the results of molecular dynamics simulations," said Delaire.

"Recent advances with machine-learning to train surrogate force-fields on accurate quantum mechanical energies/forces are opening the possibility of directly matching the experimental resolution. However, we need sufficiently large, simulated trajectories and therefore run our simulations on high-performance computers, such as the NERSC supercomputing center."

Combining neutron scattering experiments with machine learning-generated molecular dynamics simulations, the researchers could observe how the vibrational spectra of mobile Li⁺ ions in Li₆PSâ‚…Cl evolved from a crystal-like to a liquid-like phase. Overall, their findings gathered new important insight into the nature of rapid ionic motion in superionic materials.

"Superionic materials have quite complex atomic dynamics and often quite complex structures as well," Delaire said. "Designing materials with superionic diffusivities and good stability from the ground up has remained challenging.

"Our methods and results identify an important role of combined dynamics of the diffusive ions and the vibrational modes of the host crystal framework. Since we have experience and tools to systematically investigate and predict phonons in rather complex structures, we hope to gain new insights by going beyond a static view of materials."

The recent work by Delaire and his colleagues highlights the promise of Li₆PS₅Cl as a solid-state electrolyte. In the future, it could inform the design of new solid-state batteries, opening new avenues for the optimization of superionic materials, which could help to improve the energy storage and power-conversion efficiencies of solid-state batteries. The design of materials with superionic diffusivities could also inform the development of other devices beyond solid-state batteries, including fuel cells and neuromorphic computing hardware.

"We are broadening the range of superionic material compositions we are investigating. We are very open to new collaborations," added Delaire.

"Future developments of neutron instruments could drastically accelerate our ability to probe superionic dynamics, for instance, in-situ or very small samples, and with higher throughput to screen many compositions. We are now leveraging artificial intelligence and machine learning to guide our scientific workflow and data analysis, and we see great potential in further integration of computer simulations with neutron or X-ray scattering experiments."