Local charge inhomogeneity and lithium distribution in the superionic argyrodites Li₆PS₅<i>X</i> (<i>X</i> = Cl, Br, I)

The lithium-argyrodites Li6PS5X (X = Cl, Br, I) exhibit high lithium-ion conductivities, making them promising candidates for use in solid-state batteries. These solid electrolytes can show considerable substitutional X-/S2- anion-disorder, with greater disorder typically correlated with higher lithium-ion conductivities. The atomic-scale effects of this anion site-disorder within the host lattice—in particular how lattice disorder modulates the lithium substructure— are not well understood. Here, we characterize the lithium substructure in Li6PS5X (X = Cl, Br, I) as a function of temperature and anion site-disorder, using Rietveld refinements against temperature-dependent neutron diffraction data. Analysis of these high-resolution diffraction data reveals an additional lithium position previously unreported for Li6PS5X argyrodites, suggesting that the lithium conduction pathway in these materials differs from the most common model proposed in earlier studies. Analysis of the Li+ positions and their radial distributions reveals that greater inhomogeneity of the local anionic charge, due to X-/S2- site- disorder, is associated with more spatially-diffuse lithium distributions. This observed coupling of site-disorder and lithium distribution provides a possible explanation for the enhanced lithium transport in anion-disordered lithium argyrodites, and highlights the complex interplay between anion configuration and lithium substructure in this family of superionic conductors

Thermal Conductivities of Lithium-Ion-Conducting Solid Electrolytes

Solid electrolytes and solid-state batteries have gathered attention in recent years as a potential alternative to state-of-the-art lithium-ion batteries, given the promised increased energy density and safety following the replacement of flammable organic electrolytes with solids. While ongoing research focuses mainly on improving the ionic conductivities of solid electrolytes, little is known about the thermal transport properties of this material class. This includes fundamental studies of heat capacities and thermal conductivities, application-oriented investigations of porosity effects, and the modeling of the temperature distribution in solid-state batteries during operation. To expand the understanding of transport in solid electrolytes, in this work, thermal properties of electrolytes in the argyrodite family (Li6PS5X with X = Cl, Br, I, and Li5.5PS4.5Cl1.5) and Li10GeP2S12 as a function of temperature and porosity are reported. It is shown that the thermal conductivities of solid electrolytes are in the range of liquid electrolytes. Utilizing effective medium theory to describe the porosity-dependent results, an empirical predictive model is obtained, and the intrinsic (bulk) thermal conductivities for all electrolytes are extracted. Moreover, the temperature-independent, glass-like thermal conductivities found in all materials suggest that thermal transport in these ionic conductors occurs in a nontextbook fashion

A Lattice Dynamical Approach for Finding the Lithium Superionic Conductor Li3ErI6

Driven by the increasing attention that the superionic conductors Li3MX6 (M = Y, Er, In, La; X = Cl, Br, I) have gained recently for the use of solid-state batteries, and the idea that a softer, more polarizable anion sublattice is beneficial for ionic transport, here we report Li3ErI6, the first experimentally-obtained iodine-based compound within this material system of ionic conductors. Using a combination of synchrotron and neutron diffraction, we elucidate the structure, the lithium positions and possible diffusion pathways of Li3ErI6. Temperature-dependent impedance spectroscopy shows low activation energies of 0.37 and 0.38 eV alongside promising ionic conductivities of 0.65 mS·cm-1 and 0.39 mS·cm-1directly after ball milling and the subsequently annealed Li3ErI6, respectively. Speed of sound measurements are used to determine the Debye frequency of the lattice as a descriptor of the lattice dynamics and overall lattice softness, and Li3ErI6 is compared to the known material Li3ErCl6. The softer, more polarizable framework from the iodide anion leads to improved ionic transport, showing that the idea of softer lattices holds up in this class of materials. This work provides Li3ErI6 as an interesting novel framework for optimization in the class of halide-based ionic conductors.</p

On the Local Charge Inhomogeneity and Lithium Distribution in the Superionic Argyrodites Li6PS5X (X = Cl, Br, I)

The lithium-argyrodites Li6PS5X (X = Cl, Br, I) exhibit high lithium-ion conductivities, making them promising candidates for use in solid-state batteries. These solid electrolytes can show considerable substitutional X−/S2− anion-disorder, with greater disorder typically correlated with higher lithium-ion conductivities. The atomic-scale effects of this anion site-disorder within the host lattice—in particular how lattice disorder modulates the lithium substructure—are not well understood. Here, we characterize the lithium substructure in Li6PS5X (X = Cl, Br, I) as a function of temperature and anion site-disorder, using Rietveld refinements against temperature-dependent neutron diffraction data. Analysis of these high-resolution diffraction data reveals an additional lithium position previously unreported for Li6PS5Xargyrodites, suggesting that the lithium conduction pathway in these materials differs from the most common model proposed in earlier studies. Analysis of the Li+ positions and their radial distributions reveals that greater inhomogeneityof the local anionic charge, due to X−/S2− site-disorder, is associated with more spatially-diffuse lithium distributions. This observed coupling of site-disorder and lithium distribution provides a possible explanation for the enhanced lithium transport in anion-disordered lithium argyrodites, and highlights the complex interplay between anion configuration and lithium substructure in this family of superionic conductors

Thermal Conductivities of Lithium-Ion-Conducting Solid Electrolytes

Solid electrolytes and solid-state batteries have gathered attention in recent years as a potential alternative to state-of-the-art lithium-ion batteries, given the promised increased energy density and safety following the replacement of flammable organic electrolytes with solids. While ongoing research focuses mainly on improving the ionic conductivities of solid electrolytes, little is known about the thermal transport properties of this material class. This includes fundamental studies of heat capacities and thermal conductivities, application-oriented investigations of porosity effects, and the modeling of the temperature distribution in solid-state batteries during operation. To expand the understanding of transport in solid electrolytes, in this work, thermal properties of electrolytes in the argyrodite family (Li6PS5X with X = Cl, Br, I, and Li5.5PS4.5Cl1.5) and Li10GeP2S12 as a function of temperature and porosity are reported. It is shown that the thermal conductivities of solid electrolytes are in the range of liquid electrolytes. Utilizing effective medium theory to describe the porosity-dependent results, an empirical predictive model is obtained, and the intrinsic (bulk) thermal conductivities for all electrolytes are extracted. Moreover, the temperature-independent, glass-like thermal conductivities found in all materials suggest that thermal transport in these ionic conductors occurs in a nontextbook fashion

Pressure Dependence of Ionic Conductivity in Site Disordered Lithium Superionic Argyrodite Li6PS5BrLi_6PS_5Br

The understanding of transport in Li+ solid ionic conductors is critical for the development of solid-state batteries. The influence of activation volumes on ion transport in solid electrolytes has recently garnered renewed research interest, due to the need to control the ion dynamics that influence the ionic conductivity in solid electrolytes. Microscopic activation volumes are believed to correspond to the volume change in the atomic structure of a material that occurs during an ion jump and can be determined thermodynamically from pressure dependent conductivity measurements. However, it remains unknown if and how this external pressure can affect the structure and transport properties of Li+ solid electrolytes. The lithium argyrodites Li6PS5Br have shown high ionic conductivities, influenced by their Br−/S2− site disorder, which is associated with more spatially diffuse lithium-ion distributions. Herein, impedance spectra were acquired over a pressure range of 0.1 GPa to 1.5 GPa and revealed the activation volumes for Li+ migration. Specifically, activation volumes for Li+ migration increase with increasing degrees of Br−/S2− site disorder in Li6PS5Br and with more spatially distributed lithium-ions. Furthermore, estimations of the corresponding migration volumes, which are thought to be a constant of the diffusing mobile ion in the structure are here found to change significantly among different Br−/S2− site disorders. These observations motivate further investigations on how the thermodynamic activation volume in superionic Li+ conductors may provide novel insights to the influences of structure on ion transport in fast ionic conductors

Relating critical phonon occupation to activation barrier in fast lithium-ion conductors

Phonon-based (vibrational) theories of ion transport are likely key to developing new design strategies for solid-state ionic conductors. However, they are not often utilized because it is difficult to ascertain which vibrational frequencies are important, even in describing fundamental parameters such as the activation barrier. This is perpetuated by the fact that it is difficult to tune vibrational frequencies directly, without changing the chemical structure, in order to study underlying phonon relations. Using isotopic substitution of 6Li for 7Li, we are able to change the mobile ion vibrations in the two exemplary ion conductors Li10SnP2S12 and Li6PS5Cl without altering their structure. Using a combination of nuclear magnetic resonance spectroscopy and ab initio molecular dynamics simulations to characterize temperature-dependent ion transport, it is demonstrated that the isotopic substitution of 6Li for 7Li increases the activation barrier for Li-ion transport. The magnitude of this isotope effect cannot be explained by changes in the zero-point vibrational energy alone. Therefore, we propose that the observed change in the activation barrier is related to the differences in the average critical phonon occupation needed to overcome the activation barrier. Our hypothesis is supported by an analytical model, based on the physics of quantum harmonic oscillators, that gives good agreement with the experimental results. Thus, isotopic substitution provides unique insights into the vibrational perspective and frequency dependence of the activation barrier in fast Li-ion conductors