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.

All-solid-state lithium-ion batteries (ASSBs) are expected to represent a future alternative compared to conventional lithium-ion batteries with liquid electrolytes (LIBs). The excellent performance of today's LIBs relies to a large extent on the development of liquid electrolytes that form stable, or at least slowly degrading, interfaces (interphases) with both anodes and cathodes. This has not yet been achieved in ASSBs, and degradation of anode and cathode interfaces of solid electrolytes (SE) is one of the key issues to be solved. Unlike investigations of liquid/solid interfaces, the degradation of interfaces between the solid electrodes and the SE is challenging since (i) solid/solid interfaces are less easily accessed analytically, (ii) interface compounds may contribute only in very low concentrations to spectroscopic or spectrometric data, and (iii) a high spatial resolution is required to determine the local component distribution. Typically, solid/solid interface investigations are primarily based on electrochemical experiments, diffraction studies, electron microscopy, or on theoretical calculations to obtain sufficient information. Interestingly, the prospects of recent advanced analytical tools such as time-of-flight secondary-ion mass spectrometry (ToF-SIMS) are not fully exploited yet; therefore, we demonstrate in this paper that ToF-SIMS can provide valuable insights into the interphase composition and microstructure of ASSBs. For this purpose, we combine local compositional information from ToF-SIMS and complementary X-ray photoelectron spectroscopy measurements to characterize and visualize the degradation mechanism in the LiNi 0.6 Co 0.2 Mn 0.2 O 2 /Li 6 PS 5 Cl-composite cathode of an ASSB. Our results indicate that sulfates and phosphates play an important role in the formation of a solid electrolyte interface (SEI), whereas transition-metal chlorides, phosphides, and sulfides can be neglected. Furthermore, to the best of our knowledge, we show for the first time the local structure and morphology of the SEI layer on the basis of information about the chemical composition using ToF-SIMS analysis.

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 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.

Li7La3Zr2O12 (LLZO) garnet-based oxides are a promising class of solid electrolytes used as the separator in all-solid-state batteries (ASSBs). While LLZO is considered to have a wide electrochemical stability window, its intrinsic stability in contact with lithium metal is not sufficiently well understood, and there is still a debate on the key question of whether LLZO does or does not form passivation layers before and during cycling. Utilizing both in situ and operando X-ray photoelectron spectroscopy techniques, we reveal the presence of a kinetic barrier to the reduction of LLZO by Li metal, with the extent of oxygen-deficient interphase (ODI) formation depending sensitively on the energetics of Li metal arriving at the Li|LLZO interface. Despite the clear presence of a kinetic barrier to reduction, the electrochemical response of the Li|LLZO interface is unchanged by the presence of the ODI, indicating that ODI formation during electrochemical cycling does not hinder charge transfer across the Li|LLZO interface. Overall, these results reveal that the reactivity of LLZO with Li metal depends not only on the material properties of the adjoining phases (i.e., surface purity and active contact) and their resulting thermodynamic stability but also on the energy input at the interface and the resulting reaction kinetics. Furthermore, the presence of a kinetic barrier to reduction highlights the additional complexities governing the reactivity of solid-state interfaces in ASSBs and underscores the importance of operando characterization of interfacial stability to design more robust, high-performance protection strategies for solid electrolytes in contact with reactive electrodes.

Solid‐state batteries with a lithium metal anode (LMA) are promising candidates for the next generation of energy storage systems with high energy and power density. However, successful implementation of the LMA requires deeper insight into the lithium metal–solid electrolyte (Li|SE) interface. Since lithium is highly reactive, reaction products form when it comes into contact with most solid electrolytes (SEs) and the resulting interphase can have detrimental effects on cell performance. To better understand the formation of interphases, Li|SE interfaces are studied with time‐of‐flight secondary‐ion mass spectrometry (ToF‐SIMS), which provides chemical information with high sensitivity in 2D as well as 3D and is a valuable complement to commonly used techniques. To investigate the interphase, lithium is deposited in situ on SE pellets either through lithium vapor deposition or electrochemical lithium plating. Subsequent depth profiling provides information about the stability of the Li|SE interface and about the microstructure of the formed interphase. At the Li|Li6PS5Cl interface of lithium metal with argyrodite‐type Li6PS5Cl, an apparently covering Li2S‐rich layer is found as major part of the interphase. Independent of the deposition method, a combination of ToF‐SIMS and atomic force microscopy indicates a thickness of about 250 nm for the Li2S‐rich interlayer.

A non-ideal contact at the electrode/solid electrolyte interface of a solid-state battery arising due to pores (voids) or inclusions results in a geometric constriction effect that severely deteriorates the electric transport properties of the battery cell. The lack of understanding of this phenomenon hinders the optimization process of novel components, such as reversible and high-rate metal anodes. Deeper insight into the constriction phenomenon is necessary to correctly monitor interface degradation and to accelerate the successful use of metal anodes in solid-state batteries. Here, we use a 3D electric network model to study the fundamentals of the constriction effect. Our findings suggest that dynamic constriction as a non-local effect cannot be captured by conventional 1D equivalent circuit models and that its electric behavior is not ad hoc predictable. It strongly depends on the interplay of the geometry of the interface causing the constriction and the microscopic transport processes in the adjacent phases. In the presence of constriction, the contribution from the non-ideal electrode/solid electrolyte interface to the impedance spectrum may exhibit two signals that cannot be explained when the porous interface is described by a physical-based (effective medium theory) 1D equivalent circuit model. In consequence, the widespread assumption of a single interface contribution to the experimental impedance spectrum may be entirely misleading and can cause serious misinterpretation.

To overcome current challenges of lithium metal anodes (LMAs), which hinder their wide industrial application, the chemical composition of the lithium metal surface is an important factor. Due to its high reactivity and depending on the pre-treatment during processing, lithium is covered with a passivation layer composed of mainly Li 2 CO 3 , LiOH, and Li 2 O, what is mostly neglected in later electrochemical studies. Here, we investigate the effect of storage time and conditions on the surface passivation layer of commercial lithium foils, based on lithium surface characterization with X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, finding that only sealed pouch bags can prevent lithium surface changes effectively. Otherwise, the passivation layer thickness increases steadily, even in gloveboxes with a low degree of contaminations. Testing the stored lithium foils in solid-state batteries with LLZO as model solid electrolyte, it is demonstrated that the solid electrolyte roughness and the applied pressure have a huge impact on the obtained impedance. While the passivation layer has no major effect on the interface resistance with a rough LLZO pellet and at high pressure, it clearly affects the interface resistance with smoother LLZO surfaces and at lower pressure. Consequently, the lithium passivation layer may hinder the application of the LMA in a solid-state battery what we discuss in depth. By reactivity experiments with model lithium surfaces, we show that water residuals are the main reason for the aging of lithium foil in gloveboxes. Additionally, nitrogen reacts with fresh lithium surfaces and lithium foils with an incomplete or damaged passivation layer. The results demonstrate that storage conditions are important factors for the surface state of lithium metal and consequently for the application as an anode material.