The lithium argyrodites Li6PS5X (X = Cl, Br, and I) have been gaining momentum as candidates for electrolytes in all-solid-state batteries. While these materials have been well-characterized structurally, the influences of the static and dynamic lattice properties are not fully understood. Recent improvements to the ionic conductivity of Li6PS5I (which as a parent compound is a poor ionic conductor) via elemental substitutions have shown that a multitude of influences affect the ionic transport in the lithium argyrodites and that even poor conductors in this class have room left for improvement. Here we explore the influence of isoelectronic substitution of sulfur with selenium in Li6PS5–x Se x I. Using a combination of X-ray diffraction, impedance spectroscopy, Raman spectroscopy, and pulse–echo speed of sound measurements, we explore the influence of the static and dynamic lattice on the ionic transport. The substitution of S2– with Se2– broadens the diffusion pathways and structural bottlenecks, as well as leading to a softer more polarizable lattice, all of which lower the activation barrier and lead to an increase in the ionic conductivity. This work sheds light on ways to systematically understand and improve the functional properties of this exciting material family.

Glass–ceramic solid electrolytes have been reported to exhibit high ionic conductivities. Their synthesis can be performed by crystallization of mechanically milled Li2S–P2S5 glasses. Herein, the amorphization process of Li2S–P2S5 (75:25) induced by ball milling was analyzed via X-ray diffraction (XRD), Raman spectroscopy, and 31P magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy. Several structural building blocks such as [P4S10], [P2S6]4–, [P2S7]4–, and [PS4]3– occur during this amorphization process. In addition, high-temperature XRD was used to study the crystallization process of the mechanically milled Li2S–P2S5 glass. Crystallization of phase-pure β-Li3PS4 was observed at temperatures up to 548 K. The kinetics of crystallization was analyzed by integration of the intensity of the Bragg reflections. 7Li NMR relaxometry and pulsed field-gradient (PFG) NMR were used to investigate the short-range and long-range Li+ dynamics in these amorphous and crystalline materials. From the diffusion coefficients obtained by PFG NMR, similar Li+ conductivities for the glassy and heat-treated samples were calculated. For the glassy sample and the glass–ceramic β-Li3PS4 (calcination at 523 K for 1 h), a Li+ bulk conductivity σLi of 1.6 × 10–4 S/cm (298 K) was obtained, showing that for this system a well-crystalline material is not essential to achieve fast Li-ion dynamics. Impedance measurements reveal a higher overall conductivity for the amorphous sample, suggesting that the influence of grain boundaries is small in this case.

The ternary compound CuVS exhibits an excellent performance as anode material for sodium ion batteries with a high reversible capacity of 580 mAh g at 0.7 A g after 300 cycles. A Coulombic efficiency of ≈99% is achieved after the third cycle. Increase of the C-rate leads to a drop of the capacity, but a full recovery is observed after switching back to the initial C-rate. In the early stages of Na uptake first Cu is reduced and expelled from the electrode as nanocrystalline metallic Cu. An increase of the Na content leads to a full conversion of the material with nanocrystalline Cu particles and elemental V embedded in a NaS matrix. The formation of NaS is evidenced by Na MAS NMR spectra and X-ray powder diffraction. During the charge process the nanocrystalline Cu particles are retained, but no crystalline materials are formed. At later stages of cycling the reaction mechanism changes which is accompanied by the formation of copper(I) sulfide. The presence of nanocrystalline metallic Cu and/or CuS improves the electrical conductivity, leading to superior cycling and rate capability.

The interest in all solid-state batteries has increased notably over the last years. Reasons are, among others, the demand for higher energy densities in storage devices and considerable safety issues in classical battery systems based on liquid electrolytes. One solution is the usage of solid electrolytes in battery systems. Because the crystal structure highly correlates with ion migration, the focus of our work is a detailed determination of the structure and Li pathways in the solid electrolyte argyrodite-type Li6PS5Cl. With neutron diffraction an additional Li site was experimentally detected. The comparison of maximum entropy method and differential bond valence analysis revealed the Li ion hopping pathways. With pair-distribution function analysis, a distortion of the [PS4] 3-tetrahedra resulting in a local monoclinic structure is found. A modulation of the local monoclinic structure is averaged out on longer length scales to an overall cubic structure that is known from literature.