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.

Solid‐state lithium‐sulfur batteries (SSLSBs) have the potential to cause a paradigm shift in energy storage. The use of emerging highly‐conductive solid electrolytes enables high energy and power densities. However, the need for an intimate mixture of electrolyte and conductive additives to compensate for the insulating nature of cathode active materials S8 and Li2S induces intense electrolyte degradation. Thus, it is paramount to understand better the electrochemical and transport properties of the cathode composite with extremely high interface density among cathode components. Here, by utilizing a ball‐milled composite of the lithium argyrodite Li6PS5Cl and carbon as a model electrode, the stability, reversibility, and transport in the composite as functions of cathode loading, the volume fraction of conducting phase, temperature, and applied potentials are comprehensively investigated. Comparing the onset potentials of electrolyte degradation and the sharp drop in the effective ionic conductivity of the composite determined through transmission‐line model analysis, successful enhancement of the capacity retention of SSLSBs is demonstrated by balancing between the attainable capacity and effective carrier transport, achieving a high areal capacity of 3.68 mAh cm−2 after 100 cycles at room temperature. The here‐observed analysis is applicable to any solid‐state composite with electrically insulating active materials.

potentials in all-solid-state Li-S batteries. ChemRxiv. Preprint.Owing to a remarkably high theoretical energy density, the lithium-sulfur (Li-S) battery has attracted significant attention as a candidate for next-generation batteries. While employing solid electrolytes can provide a new avenue for high capacity Li-S cells, all-solid-state batteries have unique failure mechanisms such as chemo-mechanical failure due to the volume changes of active materials. In this study, we investigate all-solid-state Li-S model cells with differently processed cathode composites and elucidate a typical failure mechanism stemming from irreversible Li 2 S formation in the cathode composites. Reducing the particle size is key to minimizing the influence of volume changes and a capacity of over 1000 mAh g sulfur -1 is achieved by ball-milling of the cathode composites. In addition, the long-term stability of the ball-milled cathode is investigated by varying upper and lower cut-off potentials for cycling, which results in unveiling the significantly detrimental role of the lower cut-off potential. Preventing a deep-discharge leads to a reversible capacity of 800 mAh g sulfur -1 over 50 cycles in the optimized cell. This work highlights the importance of mitigating chemo-mechanical failure using microstructural engineering as well as the influence of the cut-off potentials in all-solid-state Li-S batteries. File list (2)download file view on ChemRxiv revised manuscript.pdf (3.56 MiB) download file view on ChemRxiv Supporting Information.pdf (3.38 MiB)

A high theoretical energy density makes lithium‐sulfur (Li−S) batteries promising candidates for energy storage systems of the post‐lithium‐ion generation. As the performance of Li−S cells with liquid electrolytes is impaired by the solubility of reaction intermediates, solid‐state cell concepts represent an auspicious approach for future electrochemical energy storage. However, the kinetics of Li−S solid‐state batteries and high charge/discharge rate still remain major challenges, and in‐depth knowledge of the charge carrier transport in solid‐state composite sulfur cathodes is still missing. In this work, the charge transport and cyclability of composite cathodes consisting of sulfur, Li6PS5Cl and carbon, with varying volume fractions of ion‐ and electron‐conducting phases is investigated. The limiting thresholds of charge transport are elucidated by comparing the battery performance with effective transport properties of the cathode composite. Although both the effective electronic and ionic conductivities indicate high tortuosity factors, ionic transport is identified as a critical bottleneck. This work underscores the importance of quantitative transport analysis as a tool for cathode optimization.

Commercialization of solid‐state batteries requires the upscaling of the material syntheses as well as the mixing of electrode composites containing the solid electrolyte, cathode active materials, binders, and conductive additives. Inspired by recent literature about the tremendous influence of the employed milling and dispersing procedure on the resulting ionic transport properties of solid ionic conductors and the general performance of all solid‐state batteries, in this review, the underlying physical and mechanochemical processes that influence this processing are discussed. By discussing and combining the theoretical backgrounds of mechanical milling with regard to mechanochemical synthesis and dispersing of particles together with a wide range of examples, a better understanding of the critical parameters attached to mechanical milling of solid electrolytes and solid‐state battery components is provided.

State-of-the-art oxides and sulfides with high Li-ion conductivity and good electrochemical stability are among the most promising candidates for solid-state electrolytes in secondary batteries. Yet emerging halides offer promising alternatives because of their intrinsic low Li + migration energy barriers, high electrochemical oxidative stability, and beneficial mechanical properties. Mechanochemical synthesis has enabled the characterization of LiAlX 4 compounds to be extended and the iodide, LiAlI 4 , to be synthesized for the first time (monoclinic P 2 1 / c , Z = 4; a = 8.0846(1) Å; b = 7.4369(1) Å; c = 14.8890(2) Å; β = 93.0457(8)°). Of the tetrahaloaluminates, LiAlBr 4 exhibited the highest ionic conductivity at room temperature (0.033 mS cm –1 ), while LiAlCl 4 showed a conductivity of 0.17 mS cm –1 at 333 K, coupled with the highest thermal and oxidative stability. Modeling of the diffusion pathways suggests that the Li-ion transport mechanism in each tetrahaloaluminate is closely related and mediated by both halide polarizability and concerted complex anion motions.