For the development of safe and long-lasting lithium-ion batteries we need electrolytes with excellent ionic transport properties. Argyrodite-type Li 6 PS 5 X (X: Cl, Br, I) belongs to a family of such a class of materials offering ionic conductivities, at least if Li 6 PS 5 Br and Li 6 PS 5 Cl are considered, in the mS cm À1 range at room temperature. Although already tested as ceramic electrolytes in battery cells, a comprehensive picture about the ion dynamics is still missing. While Li 6 PS 5 Br and Li 6 PS 5 Cl show an exceptionally high Li ion conductivity, that of Li 6 PS 5 I with its polarizable I anions is by some orders of magnitude lower. This astonishing effect has not been satisfactorily understood so far. Studying the ion dynamics over a broad time and length scale is expected to help shed light on this aspect. Here, we used broadband impedance spectroscopy and 7 Li NMR relaxation measurements and show that very fast local Li ion exchange processes are taking place in all three compounds. Most importantly, the diffusion-induced NMR spinlattice relaxation in Li 6 PS 5 I is almost identical to that of its relatives. Considering the substitutional disorder effects in Li 6 PS 5 X (X = Br, Cl), we conclude that in structurally ordered Li 6 PS 5 I the important inter-cage jump processes are switched off, hindering the ions from taking part in long-range ion transport. † Electronic supplementary information (ESI) available: Rietveld refinements and structural data, further NMR data. See Li 6 PS 5 I are also included. The lower part of the graph shows s DC T(1/T); the values given represent activation energies. For the sake of clarity, data of Li 6 PS 5 Br 0.75 I 0.25 (solid line, grey) have been plotted using an offset of +1 on the log scale.Li 6 PS 5 Cl 0.11(1) 0.18(2) 0.17(4) Li 6 PS 5 Br 0.06(1) 0.09(1) 0.10(4) Li 6 PS 5 I 0.23(1) 0.38(2) 0.18 (5) Paper PCCP

Electrolytes with excellent ionic conductivity are needed for the development of safe and long-lasting all-solid-state batteries. Perfect candidates are Argyrodites, like Li 6 PS 5 X (X: Cl, Br, I), that show an ionic conductivity in the mS/cm range. Translational Li + diffusion in ceramic electrolytes is influenced by many factors such as crystal structure and defect chemistry. Considering thiophosphates, only little information is, however, available about the effects of rotational anion dynamics on Li + jump processes. Here, we used 31 P spin-lattice relaxation nuclear magnetic resonance (NMR) to find out whether rapid Li + ion dynamics in Li 6 PS 5 X is affected by rotational jumps of the PS 4 3− units. NMR revealed that in Li 6 PS 5 I, having an ordered anion sublattice, PS 4 3− rotational dynamics are much faster than Li + self-diffusion. At 223 K, the rotational correlation rate, which is obtained from NMR relaxation rate peaks, takes a very high value in the order of 10 9 events per second. It clearly decreases when anion disorder is introduced. At the same time, anion substitution contracts the lattice when we go from Li 6 PS 5 I to Li 6 PS 5 Br and further to Li 6 PS 5 Cl as the ionic radii of X decrease along this series. While for Li 6 PS 5 I, with its soft lattice, the rotational jumps of the PS 4 3− units are decoupled, that is, independent from Li + translation, in Li 6 PS 5 Cl, which has a smaller lattice constant, the much slower rotational dynamics seem to influence Li + intercage hopping. The corresponding rotational jumps do occur almost on the same time scale as the Li + exchange processes take place. For Li 6 PS 5 Br, on the other hand, an optimal balance between lattice properties, site disorder, and fast rotational jumps seems to be established, which leads to facile translational Li + displacements. Our experiments show that rotational motions, if they are in resonance with cation exchange, should be considered when the origins of long-range Li + transport in argyrodite-type thiophosphates need to be identified.

Lithium-rich anti-perovskites (LiRAPs) have attracted a great deal of attention as they have been praised as another superior group of solid electrolytes that can be used to realize all-solid-state batteries free of flammable liquids. Despite several studies that have reported on the properties of LiRAPs, many questions remain unanswered. In particular, these include fundamental ones concerning the structure, stability, and Li-ion conductivity and diffusivity. Moreover, it is not clear whether some of the previously reported compounds do really exist. To untangle the current picture of LiRAPs, we synthesized "Li 3 OCl" and Li 2 OHCl polymorphs and applied a wide spectrum of methods, such as powder X-ray diffraction (PXRD), powder neutron diffraction (PND), nuclear magnetic resonance spectroscopy, and impedance spectroscopy to carefully shed some light on LiRAPs. Here we self-critically conclude that the cubic polymorph of the two compounds cannot be easily distinguished by PXRD alone as the lattice metrics and the lattice parameters are very similar. Furthermore, PXRD suffers from the difficulty of detecting H and Li. Even Rietveld refinement of our PND data turned out to be complicated and not easily interpreted in a straightforward way. Nevertheless, here we report the first structural models for the cubic and a new orthorhombic polymorph containing also structural information about the H atoms. In situ PXRD of "Li 3 OCl", intentionally exposed to air, revealed rapid degradation into Li 2 CO 3 and amorphous LiCl•xH 2 O. Most likely, the instability of "Li 3 OCl" explains earlier findings about the unusually high ion conductivities as the decomposition product LiCl•xH 2 O offers an electrical conductivity that is good enough for some applications, excluding, of course, those that need aprotic conditions or electrolytes free of any moisture. Considering "H-free Li 3 OCl" as well as Li 5 (OH) 3 Cl 2 , Li 5 (OH) 2 Cl 3 , Li 3 (OH) 2 Cl, and Li 3 (OH)Cl 2 , we are confident that Li 4 (OH) 3 Cl and variants of Li 3−x (OH x )Cl, where x > 0, are, from a practical point of view, so far the only stable lithium-rich anti-perovskites.

Solid electrolytes are at the heart of future energy storage systems. Li-bearing argyrodites are frontrunners in terms of Li + ion conductivity. Although many studies have investigated the effect of elemental substitution on ionic conductivity, we still do not fully understand the various origins leading to improved ion dynamics. Here, Li 6+ x P 1– x Ge x S 5 I served as an application-oriented model system to study the effect of cation substitution (P 5+ vs Ge 4+ ) on Li + ion dynamics. While Li 6 PS 5 I is a rather poor ionic conductor (10 –6 S cm –1 , 298 K), the Ge-containing samples show specific conductivities on the order of 10 –2 S cm –1 (330 K). Replacing P 5+ with Ge 4+ not only causes S 2– /I – anion site disorder but also reveals via neutron diffraction that the Li + ions do occupy several originally empty sites between the Li rich cages in the argyrodite framework. Here, we used 7 Li and 31 P NMR to show that this Li + site disorder has a tremendous effect on both local ion dynamics and long-range Li + transport. For the Ge-rich samples, NMR revealed several new Li + exchange processes, which are to be characterized by rather low activation barriers (0.1–0.3 eV). Consequently, in samples with high Ge-contents, the Li + ions have access to an interconnected network of pathways allowing for rapid exchange processes between the Li cages. By (i) relating the changes of the crystal structure and (ii) measuring the dynamic features as a function of length scale, we were able to rationalize the microscopic origins of fast, long-range ion transport in this class of electrolytes.

Lithium-thiophosphates have attracted great attention as they offer a rich playground to develop tailor-made solid electrolytes for clean energy storage systems. Here, we used poorly conducting Li6PS5I, which can be converted into a fast ion conductor by high-energy ball-milling to understand the fundamental guidelines that enable the Li+ ions to quickly diffuse through a polarizable but distorted matrix. In stark contrast to well-crystalline Li6PS5I (10–6 S cm–1), the ionic conductivity of its defect-rich nanostructured analog touches almost the mS cm–1 regime. Most likely, this immense enhancement originates from site disorder and polyhedral distortions introduced during mechanical treatment. We used the spin probes 7Li and 31P to monitor nuclear spin relaxation that is directly induced by Li+ translational and/or PS4 3– rotational motions. Compared to the ordered form, 7Li spin–lattice relaxation (SLR) in nano-Li6PS5I reveals an additional ultrafast process that is governed by activation energy as low as 160 meV. Presumably, this new relaxation peak, appearing at T max = 281 K, reflects extremely rapid Li hopping processes with a jump rate in the order of 109 s–1 at T max. Thus, the thiophosphate transforms from a poor electrolyte with island-like local diffusivity to a fast ion conductor with 3D cross-linked diffusion routes enabling long-range transport. On the other hand, the original 31P nuclear magnetic resonance (NMR) SLR rate peak, pointing to an effective 31P-31P spin relaxation source in ordered Li6PS5I, is either absent for the distorted form or shifts toward much higher temperatures. Assuming the 31P NMR peak as being a result of PS4 3– rotational jump processes, NMR unveils that disorder significantly slows down anion dynamics. The latter finding might also have broader implications and sheds light on the vital question how rotational dynamics are to be manipulated to effectively enhance Li+ cation transport.