In the search for novel solid electrolytes for solid-state batteries, thiophosphate ionic conductors have been in recent focus owing to their high ionic conductivities, which are believed to stem from a softer, more polarizable anion framework. Inspired by the oft-cited connection between a soft anion lattice and ionic transport, this work aims to provide evidence on how changing the polarizability of the anion sublattice in one structure affects ionic transport. Here, we systematically alter the anion framework polarizability of the superionic argyrodites LiPSX by controlling the fractional occupancy of the halide anions (X = Cl, Br, I). Ultrasonic speed of sound measurements are used to quantify the variation in the lattice stiffness and Debye frequencies. In combination with electrochemical impedance spectroscopy and neutron diffraction, these results show that the lattice softness has a striking influence on the ionic transport: the softer bonds lower the activation barrier and simultaneously decrease the prefactor of the moving ion. Due to the contradicting influence of these parameters on ionic conductivity, we find that it is necessary to tailor the lattice stiffness of materials in order to obtain an optimum ionic conductivity.

Solid-state batteries with inorganic solid electrolytes are currently being discussed as a more reliable and safer future alternative to the current lithium-ion battery technology. To compete with state-of-theart lithium-ion batteries, solid electrolytes with higher ionic conductivities are needed, especially if thick electrode configurations are to be used. In the search for optimized ionic conductors, the lithium argyrodites have attracted a lot of interest. Here, we systematically explore the influence of aliovalent substitution in Li 6+x P 1−x Ge x S 5 I using a combination of X-ray and neutron diffraction, as well as impedance spectroscopy and nuclear magnetic resonance. With increasing Ge content, an anion site disorder is induced and the activation barrier for ionic motion drops significantly, leading to the fastest lithium argyrodite so far with 5.4 ± 0.8 mS cm −1 in a cold-pressed state and 18.4 ± 2.7 mS cm −1 upon sintering. These high ionic conductivities allow for successful implementation within a thick-electrode solid-state battery that shows negligible capacity fade over 150 cycles. The observed changes in the activation barrier and changing site disorder provide an additional approach toward designing better performing solid electrolytes.

Li+- and Na+-conducting thiophosphates have attracted much interest because of their intrinsically high ionic conductivities and the possibility to be employed in solid-state batteries. Inspired by the recent finding of the influence of changing lattice vibrations and induced lattice softening on the ionic transport of Li+-conducting electrolytes, here we explore this effect in the Na+ conductor Na3PS4–x Se x . Ultrasonic speed of sound measurements are used to monitor a changing lattice stiffness and Debye frequencies. The changes in the lattice dynamics are complemented by X-ray diffraction and electrochemical impedance spectroscopy. With systematic alteration of the polarizability of the anion framework, a softening of the lattice can be observed that leads to a reduction of the activation barrier for migration as well as a decreased Arrhenius prefactor. This work shows that, similar to Li+ transport, the softening of the average vibrational frequencies of the lattice has a tremendous effect on Na+-ionic transport and that ion–phonon interactions need to be considered in solid electrolytes.

A rapidly approaching theoretical limit of Li-ion batteries pushes the desire for next-generation energy storage devices [1]. One of the promising candidates is the all-solid-state battery with inorganic solid ion conductors. By replacing the currently employed liquid electrolyte, this battery architecture is thought to pave the way for a significant enhancement in the energy density with a Li-metal anode, as well as increase the battery safety [1][2][3][4]. The superior thermal stability of solid electrolytes enables operation without cooling, leading to a further gain in energy density when it comes to the device integration. Utilization of Na-ions may even enhance environmental friendliness. The solid ionic conductor performs the function of the separator, as well as the electrolyte in the electrode composites. Therefore, when replacing the liquid-solid interfacial contact that already proves cumbersome in today's lithium-ion batteries, solid-solid interfaces will show different degradation kinetics [5,6]. Although the diffusion kinetics in electrode materials matters in practice [7], as only one type of charge carrier is available effectively leading to cation transference numbers of unity, fast-charging seems possible due to minute cell polarization at high currents [8]. However, instability of the ionic conductors towards the electrodes makes protection concepts necessary [9][10][11][12]. While the interfacial stability and battery architecture are still open questions in the field, high ionic conductivity is paramount for all-solid-state battery operation [2,13]. Conductivities at room temperature above 1 mS cm −1 are typically considered to be sufficient for building research-based devices, whereas likely higher conductivities >10 mS cm −1 will be needed for high energy densities with thick electrode configurations and fast charging/discharging [14].The process of ionic conduction in solids has received attention over decades due to the possible application of oxide ion conductors in sensors and fuel cells and cation conductors in batteries, for instance as electrode materials [15][16][17][18][19][20]. The understanding of lithium and sodium solid-state ionic conductors grew when Na β″alumina, the NASICON (Na SuperIonic CONductor), and LISICON (Li SuperIonic CONductor) structures were found [21][22][23]. However, grain boundaries and mechanical brittleness of these materials have limited the

All-solid-state batteries are often expected to replace conventional lithium-ion batteries in the future. However, the practical electrochemical and cycling stability of the best-conducting solid electrolytes, i.e. lithium thiophosphates, is still a critical issue that prevents long-term stable high-energy cells. In this study, we apply a stepwise cyclic voltammetry approach to obtain information on the practical oxidative stability limit of Li 10 GeP 2 S 12 , two different Li 2 S−P 2 S 5 glasses, as well as the argyrodite Li 6 PS 5 Cl solid electrolytes. We employ indium metal and carbon black as the counter and working electrodes, respectively, the latter to increase the interfacial contact area to the electrolyte as compared to the commonly used planar steel electrodes. Using a stepwise increase in the reversal potentials, the onset potential of oxidative decomposition at the electrode−electrolyte interface at 25 °C is identified. X-ray photoelectron spectroscopy is used to investigate the oxidation of sulfur(-II) in the thiophosphate polyanions to sulfur(0) as the dominant redox process in all electrolytes tested. Our results suggest that in later cycles the crystalline solid electrolyte itself is not the major redox active phase, but rather that only after the formation of such electrolyte decomposition products is significant redox behavior observed. Indeed, the redox behavior of the decomposition products is an additional contributor to the overall cell capacity of solid-state batteries. The stepwise cyclic voltammetry approach presented here shows that the practical oxidative stability at 25 °C of thiophosphate solid electrolytes against carbon is kinetically higher than predicted by thermodynamic calculations and that the decomposition products dominate the redox behavior of cathode composites. The method serves as an efficient guideline for the determination of practical, kinetic stability limits of solid electrolytes with respect to the employed electrode materials.

All-solid-state batteries are promising candidates for next-generation energy-storage devices. Although the list of candidate materials for solid electrolytes has grown in the past decade, there are still many open questions concerning the mechanisms behind ionic migration in materials. In particular, the lithium thiophosphate family of materials has shown very promising properties for solid-state battery applications. Recently, the Ge-substituted Li6PS5I argyrodite was shown to be a very fast Li-ion conductor, despite the poor ionic conductivity of the unsubstituted Li6PS5I. Therein, the conductivity was enhanced by more than 3 orders of magnitude due to the emergence of I–/S2– exchange, i.e., site disorder, which led to a sudden decrease of the activation barrier with a concurrent flattening of the energy landscapes. Inspired by this work, two series of elemental substitutions in Li6+x P1–x M x S5I (M = Si and Sn) were investigated in this study and compared to the Ge analogue. A sharp reduction in the activation energy was observed at the same M4+/P5+ composition as previously found in the Ge analogue, suggesting a more general mechanism at play. Furthermore, structural analyses with X-ray and neutron diffraction indicate that similar changes in the Li sublattice occur despite a significant variation in the size of the substituents, suggesting that in the argyrodites the lithium substructure is most likely influenced by the occurring Li+–Li+ interactions. This work provides further evidence that the energy landscape of ionic conductors can be tailored by inducing local disorder.

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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