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)

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Lithium-ion conducting argyrodites have recently attracted significant interest as solid electrolytes for solid-state battery applications. In order to enhance the utility of materials in this class, a deeper understanding of the fundamental structure–property relationships is still required. Using Rietveld refinements of X-ray diffraction data and pair distribution function analysis of neutron diffraction data, coupled with electrochemical impedance spectroscopy and speed of sound measurements, the structure and transport properties within Li6PS5–x Se x Br (0 ≤ x ≤ 1) have been monitored with increasing Se content. While it has been previously suggested that the incorporation of larger, more polarizable anions within the argyrodite lattice should lead to enhancements in the ionic conductivity, the Li6PS5–x Se x Br transport behavior was found to be largely unaffected by the incorporation of Se2– due to significant structural modifications to the anion sublattice. This work affirms the notion that, when optimizing the ionic conductivity of solid ion conductors, local structural influences cannot be ignored and the idea of “the softer the lattice, the better” does not always hold true.

I3 8f 0.106(2) 0.087(2) 0.619(1) 1 1.23(5) Li3 4e 0 0.4218(8) 0.25 1 1.0(7)

The quantitative influence of microstructure, porosity, surface area, and changes in the crystal lattice on the electric conduction mechanisms in cathode-active materials for lithium ion batteries and therefore on the performance of a battery cell is largely unknown. To correlate the transport properties of LiNi1/3Co1/3Mn1/3O2 (NCM-111) as model type layered cathode material with its structural properties, a systematic study of the temperature dependence of the impedance of the material was performed on a set of sintered NCM-111 pellets. By variation of the sintering temperature from 850 to 1000 °C, the porosity of the material was tuned between 2 and 45%, while the grain size of the primary particles in the pellets varied between 50 nm and 1.5 μm. A careful analysis of the impedance spectra using selectively blocking electrodes allowed for the separation of the electronic and ionic partial conductivities of NCM-111. Depending on porosity and grain size, strong variations of the electronic partial conductivity were found ranging from 1.4 × 10–6 to 6.8 × 10–9 S cm–1 accompanied by an increase in the activation energy from 0.37 to 0.61 eV. The ionic transport properties exhibit similar behavior. Rietveld refinement of the X-ray diffraction (XRD) patterns of the pellets reveals that the increase in activation energies correlates with the volume of the unit cell. A Meyer–Neldel behavior is observed for both the ionic and the electronic partial conductivities, allowing for the evaluation of the defect formation enthalpies for lithium vacancies (1.74 ± 0.56 eV) and electron holes (1.36 ± 0.59 eV). These findings illustrate the complex relationships among microstructure, morphology, and transport characteristics, highlighting the need for careful design of active materials.

Next‐generation thermal management requires the development of low lattice thermal conductivity materials, as observed in ionic conductors. For example, thermoelectric efficiency is increased when thermal conductivity is decreased. Detrimentally, high ionic conductivity leads to thermoelectric device degradation. Battery safety and design also require an understanding of thermal transport in ionic conductors. Ion mobility, structural complexity, and anharmonicity have been used to explain the thermal transport properties of ionic conductors. However, thermal and ionic transport are rarely discussed in direct comparison. Herein, the ionic conductivity of Ag+ argyrodites is found to change by orders of magnitude without altering the thermal conductivity. Thermal conductivity measurements and two‐channel lattice dynamics modeling reveal that the majority of Ag+ vibrations have a non‐propagating diffuson‐like character, similar to amorphous materials. It is found that high ionic mobility is not a requirement for diffuson‐mediated transport. Instead, the same bonding and structural traits that can lead to fast ionic conduction also lead to diffuson‐mediated transport. Bridging the fields of solid‐state ionics and thermal transport, it is proposed that a vibrational perspective can lead to new design strategies for functional ionic conducting materials. As a first step, the authors relate the so‐called Meyer–Neldel behavior in ionic conductors to phonon occupations.

The lithium-argyrodites Li₆PS₅<i>X</i> (<i>X</i> = 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 <i>X</i>⁻/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 Li₆PS₅<i>X</i> (<i>X </i>= 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 Li₆PS₅<i>X</i>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 inhomogeneityof the local anionic charge, due to <i>X</i>⁻/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.

<p>Owing to highly conductive solid ionic conductors, all-solid-state batteries attract significant attention as promising next-generation energy storage devices. A lot of research is invested in the search and optimization of solid electrolytes with higher ionic conductivity. However, a systematic study of an <i>interlaboratory reproducibility</i> of measured ionic conductivities and activation energies is missing, making the comparison of absolute values in literature challenging. In this study, we perform an uncertainty evaluation via a Round Robin approach using different Li-argyrodites exhibiting orders of magnitude different ionic conductivities as reference materials. Identical samples are distributed to different research laboratories and the conductivities and activation barriers are measured by impedance spectroscopy. The results show large ranges of up to 4.5 mScm^-1 in the measured total ionic conductivity (1.3 – 5.8 mScm^-1 for the highest conducting sample, relative standard deviation 35 – 50% across all samples) and up to 128 meV for the activation barriers (198 – 326 meV, relative standard deviation 5 – 15%, across all samples), presenting the necessity of a more rigorous methodology including further collaborations within the community and multiplicate measurements.</p>