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.

The sodium-ion conducting family of Na 3 PnS 4 , with Pn = P, Sb, has gained interest for the use in solid-state batteries due to their high ionic conductivity. However, significant improvements to the conductivity have been hampered by the lack of aliovalent dopants that can introduce vacancies into the structure. Inspired by the need for vacancy introduction into Na 3 PnS 4 , the solid solutions with WS 4 2− introduction are explored. The influence of the substitution with WS 4 2− for PS 4 3− and SbS 4 3− is monitored using a combination of X-ray diffraction, Raman, and impedance spectroscopy. With increasing vacancy concentration, improvements resulting in a very high ionic conductivity of 13 ± 3 mS•cm −1 for Na 2.9 P 0.9 W 0.1 S 4 and 41 ± 8 mS•cm −1 for Na 2.9 Sb 0.9 W 0.1 S 4 can be observed. This work acts as a stepping-stone toward further engineering of ionic conductors using vacancy injection via aliovalent substituents.

Inspired by the recent interest in fast ionic conducting solids for electrolytes, the ionic conductivity of a novel ionic conductor Na1+xTi2−xGax(PS4)3 has been investigated. Using X‐ray diffraction and impedance spectroscopy the sodium ionic conductivity in this compound was demonstrated, in which bond valence sum analysis suggests a tunnel diffusion for Na+. Substitution with Ga3+ leads to an increasing Na+ content, an expansion of the lattice and an increasing conductivity with increasing x in Na1+xTi2−xGax(PS4)3. Given the relation to the NASICON family, upon replacement of the phosphate by a thiophosphate group, a rich structural chemistry can be expected in this class of materials. This work demonstrates the potential for making NaTi2(PS4)3 an ideal system to study structure‐property relationships in ionic conductors.

We have investigated three-dimensional (3D) MoS2 nanoarchitectures doped with different amount of Ni to boost the hydrogen evolution reaction (HER) in alkaline environment, where this reaction is normally hindered. As a comparison, the activity in acidic media was also investigated to determine and compare the role of the Ni sites in both media. The doping of MoS2, especially at high loadings, can modify its structural and/or electronic properties, which can also affect the HER activity. The structural and electronic properties of the Ni doped 3D-MoS2 nanoarchitecture were studied by X-ray diffraction (XRD), Raman spectroscopy, scanning and transmission electronic microscopy (SEM; TEM), and X-ray photoemission Spectroscopy (XPS). XPS also allowed us to determine the Ni-based species formed as a function of the dopant loading. The HER activity of the materials was investigated by linear sweep voltammetry (LSV) in 0.5 M H2SO4 and 1.0 M KOH. By combining the physicochemical and electrochemical results, we concluded that the Ni sites have a different role in the HER mechanism and kinetics in acidic and in alkaline media. Thus, NiSx species are essential to promote HER in alkaline medium, whereas the Ni-Mo-S ones enhance the HER in acid medium.