The lithium‐conducting, rare‐earth halides, Li3MX6 (M = Y, Er; X = Cl, Br), have garnered significantly rising interest recently, as they have been reported to have oxidative stability and high ionic conductivities. However, while a multitude of materials exhibit a superionic conductivity close to 1 mS cm−1, the exact design strategies to further improve the ionic transport properties have not been established yet. Here, the influence of the employed synthesis method of mechanochemical milling, compared to subsequent crystallization routines as well as classic solid‐state syntheses on the structure and resulting transport behavior of Li3ErCl6 and Li3YCl6 are explored. Using a combination of X‐ray diffraction, pair distribution function analysis, density functional theory, and impedance spectroscopy, insights into the average and local structural features that influence the underlying transport are provided. The existence of a cation defect within the structure in which Er/Y are disordered to a new position strongly benefits the transport properties. A synthetically tuned, increasing degree of this disordering leads to a decreasing activation energy and increasing ionic conductivity. This work sheds light on the possible synthesis strategies and helps to systematically understand and further improve the properties of this class of materials.
Summary Low lithium-ion migration barriers have recently been associated with low average vibrational frequencies or phonon band centers, further helping identify descriptors for superionic conduction. To further explore this correlation, here we present the computational screening of ∼14,000 Li-containing compounds in the Materials Project database using a descriptor based on lattice dynamics reported recently to identify new promising Li-ion conductors. An efficient computational approach was optimized to compute the average vibrational frequency or phonon band center of ∼1,200 compounds obtained after pre-screening based on structural stability, band gap, and their composition. Combining a low computed Li phonon band center with large computed electrochemical stability window and structural stability, 18 compounds were predicted to be promising Li-ion conductors, one of which, Li 3 ErCl 6 , has been synthesized and exhibits a reasonably high room-temperature conductivity of 0.05–0.3 mS/cm, which shows the promise of Li-ion conductor discovery based on lattice dynamics.
The lithium-argyrodites Li 6 PS 5 X (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 − /S 2− 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 Li 6 PS 5 X (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 Li 6 PS 5 Xargyrodites, 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 − /S 2− 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. File list (2) download file view on ChemRxiv revised manuscript.pdf (2.52 MiB) download file view on ChemRxiv Revised Supporting Information.pdf (1.35 MiB)
In recent years, ternary halides Li3MX6 (M = Y, Er, In; X = Cl, Br, I) have garnered attention as solid electrolytes due to their wide electrochemical stability window and favorable roomtemperature conductivities. In this material class, the influences of iso-or aliovalent substitutions are so far rarely studied in-depth, despite this being a common tool for correlating structure and transport properties. In this work, we investigate the impact of Zr substitution on the structure and ionic conductivity of Li3InCl6 (Li3-xIn1-xZrxCl6 with 0 ≤ x ≤ 0.5) using a combination of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy.Analysis of high-resolution diffraction data shows the presence of an additional tetrahedrally coordinated lithium position together with cation site-disorder, both of which have not been reported previously for Li3InCl6. This Li + position and cation disorder lead to the formation of a three-dimensional lithium ion diffusion channel, instead of the expected two-dimensional diffusion. Upon Zr 4+ substitution, the structure exhibits non-uniform volume changes along with an increasing number of vacancies, all of which lead to an increasing ionic conductivity in this series of solid solutions.
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