Janis Kevin Eckhardt

Reliable assessment of buried solid–solid interfaces is essential for understanding and improving electrochemical devices including solid-state batteries. Electrical measurements such as electrochemical impedance spectroscopy provide an attractive (operando) diagnostic but probe interfacial contact only indirectly through its influence on charge transport. Consequently, the extent to which interfacial morphology can be inferred from the electrical response remains unclear. Here, we use 3D electric network simulations to study how interfacial morphology governs the impedance detectability of non-ideal contacts at constriction-dominated interfaces. The resulting general principles are discussed using metal dissolution at interfaces with fast charge-transfer kinetics as a representative case. We show that no universal detection threshold exists. Instead, detectability depends on the remaining contact area, the lateral distribution of contact and non-contact regions, and the sample thickness. In limiting cases with small, finely distributed pores, contact losses exceeding 90% may remain largely undetectable, whereas localized contact loss produces pronounced impedance features at substantially higher remaining contact areas. These findings reveal a morphology-induced diagnostic blind spot in impedance-based interface analysis and demonstrate that the absence of a measurable interface signal does not necessarily imply intimate or stable interfacial contact. This diagnostic blind spot therefore challenges the reliability of conclusions and material parameters derived from electrical measurements without independent knowledge of the underlying interface morphology.

Alkali metal solid-state batteries (Li, Na, K) suffer from contact loss at the metal|solid electrolyte interface during stripping (discharge), driven by vacancy accumulation and pore formation. This reduces contact area, increases resistance, and promotes dendrite growth during subsequent plating, eventually leading to battery failure. While lithium has been studied extensively, sodium and potassium remain comparatively underexplored despite their high abundance. Their lower yield strength, larger molar volume, and higher homologous temperature strongly affect interfacial contact evolution. Operando electrochemical dilatometry is used to determine the formed pore volume in sodium and potassium metal anodes, enabling evaluation of the vacancy injection ratio (VIR) and comparison with lithium. At low stack pressure, pores form immediately upon stripping for all three metals. The formed pore volume scales with the molar volume across lithium, sodium, and potassium. Increasing current density enhances contact loss and leads to a weaker increase in the formed pore volume in sodium and potassium, with a fraction of the formed pore volume in potassium located inside the metal bulk. In contrast, lithium shows an initial reduction in the formed pore volume (partial pore collapse), followed by a stronger increase at higher current densities. These findings reveal systematic trends among lithium, sodium, and potassium, suggesting lower yield strengths and smaller molar volumes as favorable material properties. Based on these results, a generalized mechanism for pore formation in alkali metal anodes is proposed.

Impressive electrochemical performance has been demonstrated with sulfide-based solid-state batteries. However, these results are predominantly obtained under high stack pressures—up to hundreds of MPa. While such conditions are valuable for probing fundamental material properties and intrinsic performance limits, the resulting cell metrics are not easily transferable to practical applications. Consequently, there is a growing imperative for application-oriented evaluation of SSBs under more realistic, low-pressure conditions in the single-digit MPa range. In this study, we demonstrate that the assessment of low-pressure performance can be significantly influenced by the chosen cell architecture. By comparing the cycling performance of sulfide-based SSBs—utilizing an NMC composite cathode and an In/(InLi) _x anode with argyrodite-type Li ₆ PS ₅ Cl as electrolyte—in both pouch-cell and press-cell configurations, we reveal significant discrepancies in electrochemical performance at stack pressures ranging from 0 MPa to 25 MPa. Notably, the developed pouch cells exhibit superior electrochemical performance at low stack pressures of 1 MPa compared to their press-cell counterparts despite the same materials used. Using three-electrode pouch cells, we identify In/(InLi) _x anode delamination as a primary low-pressure failure mechanism. Overall, our findings emphasize that a realistic assessment of SSB cell performance requires cell architectures that mirror targeted applications.

Reliable assessment of buried solid–solid interfaces is essential for understanding and improving electrochemical devices including solid-state batteries. Electrical measurements such as electrochemical impedance spectroscopy provide an attractive (operando) diagnostic but probe interfacial contact only indirectly through its influence on charge transport. Consequently, the extent to which interfacial morphology can be inferred from the electrical response remains unclear. Here, we use 3D electric network simulations to study how interfacial morphology governs the impedance detectability of non-ideal contacts at constriction-dominated interfaces. The resulting general principles are discussed using metal dissolution at interfaces with fast charge-transfer kinetics as a representative case. We show that no universal detection threshold exists. Instead, detectability depends on the remaining contact area, the lateral distribution of contact and non-contact regions, and the sample thickness. In limiting cases with small, finely distributed pores, contact losses exceeding 90% may remain largely undetectable, whereas localized contact loss produces pronounced impedance features at substantially higher remaining contact areas. These findings reveal a morphology-induced diagnostic blind spot in impedance-based interface analysis and demonstrate that the absence of a measurable interface signal does not necessarily imply intimate or stable interfacial contact. This diagnostic blind spot therefore challenges the reliability of conclusions and material parameters derived from electrical measurements without independent knowledge of the underlying interface morphology.

Hydrogen incorporation strongly alters the electronic, optical, and structural properties of transition-metal oxides. In MoO3, hydrogen intercalation forms HyMoO3 bronzes that exhibit reversible color changes, crystal structure distortions, and notably higher hydrogen diffusion coefficients. Although these effects are central to electrochromic, catalytic, sensing, and energy storage applications, the mechanisms of hydrogen transport remain poorly understood, especially in the metastable β-phase. Here, we show that hydrogen diffusion in β-MoO3 is controlled by two distinct, structurally driven increases of the diffusion coefficient, each amplifying it by nearly one order of magnitude as hydrogen concentration rises. To prove this, we perform electrochemical hydrogen insertion into β-MoO3 thin films covered by a PMMA layer, only allowing incorporation via a small, well-defined uncovered stripe. Hydrogen entering through this gap diffuses laterally beneath the PMMA in a semi-infinite space. Using the electrochromic response of MoO3 in combination with the Beer–Lambert law, we monitor the hydrogen concentration in situ. These diffusion profiles are then analyzed with a deep-learning algorithm to extract concentration-dependent diffusion coefficients. In contrast to the previously implicit assumption of a single effective diffusivity, our results reveal a piecewise concentration dependence that is directly linked to structural phase transformations, as confirmed by complementary in situ Raman spectroscopy during both intercalation and deintercalation. These findings establish a quantitative connection between hydrogen-induced structural evolution and ionic transport kinetics in β-MoO3. The combination of electrochromic profiling with machine-learning-based diffusion analysis provides a generalizable experimental framework applicable to related material systems, with implications for the design of electrochromic devices and hydrogen storage systems.

Sulfide-based solid-state batteries (SSBs) offer the potential for higher energy densities compared to state-of-the-art lithium-ion batteries. Although impressive electrochemical performance has already been demonstrated, these results are predominantly obtained under high stack pressures – up to hundreds of MPa. While such conditions are valuable for probing fundamental material properties and intrinsic performance limits, the resulting cell metrics are not easily transferable to practical applications. Consequently, there is a growing imperative for application-oriented evaluation of SSBs under more realistic, low-pressure conditions in the single-digit MPa range. In this study, we demonstrate that the assessment of low-pressure performance can be significantly influenced by the chosen cell architecture. By comparing the cycling performance of sulfide-based SSBs – utilizing an NMC composite cathode and an In/(InLi)x anode with argyrodite-type Li6PS5Cl as electrolyte – in both pouch-cell and presscell configurations, we reveal significant discrepancies in electrochemical performance at stack pressures ranging from 0 MPa to 25 MPa. Notably, the developed pouch cells exhibit vastly superior electrochemical performance at low stack pressures of 1 MPa compared to their press-cell counterparts despite the same materials used. The dominant failure mechanisms at low stack pressure are investigated using three-electrode pouch cells. Overall, our findings emphasize that a realistic assessment of SSB cell performance requires practical cell architectures that mirror targeted applications.

Interfacial contact limitations challenge reversible, high-rate sodium deposition in “anode-free” sodium solid-state batteries and motivate interlayer-based interface engineering strategies. Here, we systematically investigate how microstructural properties of non-graphitic carbon interlayers, i.e., the structural arrangement of graphene domains, particle size, and network porosity, govern the early stages of sodium nucleation and growth in NaSICON-based half-cells (≤ 750 μAh·cm−2). Comparing coarse- and fine-particle hard carbon and carbon black interlayers (23–1200 nm), prepared by tape-casting, shows that finer, more compactable carbons improve solid electrolyte contact and promote uniform lateral sodium growth while suppressing dendrites. Focused ion-beam scanning electron microscopy and energy-dispersive X-ray spectroscopy confirm that sodium electrodeposition occurs predominantly at the interlayer|SE interface, with partial sodium insertion into the carbon framework. Mechanical interlocking between the compliant carbon interlayer and a rough solid electrolyte surface enhances adhesion, stabilizes interfacial contact, prevents delamination of the carbon-coated current collector, and promotes laterally extended sodium growth. These findings elucidate how tailored interlayer microstructure and interfacial mechanics govern sodium electrodeposition behavior. Carbonaceous interlayers mediate interfacial contact and homogenize current distribution at the current collector|solid electrolyte interface, thereby defining design principles for solid-state sodium batteries during electrodeposition.

The temporal evolution of the solid-electrolyte interphase (SEI) resistance in sulfide-based solid-state batteries with lithium metal anode has been shown to be well described by diffusion-controlled interphase growth. Yet, recent studies reveal that the extracted SEI rate constant estimates are highly sensitive to experimental conditions, such as stack pressure, and vary significantly depending on the electrochemical characterization method used. In this study, we evaluate SEI growth kinetics derived from symmetric cell-level impedance measurements. Through comprehensive transport simulations and experiments with the argyrodite solid-electrolyte Li₆PS₅Cl, we investigate how the characteristic contact conditions encountered in typical impedance studies affect the corresponding SEI rate constant estimates. We find that increasing stack and joining pressure leads to decreasing rate constant estimates, driven by an increase in the true contact area. Apparent saturation of SEI growth may originate from the presence of native surface passivation layers on lithium metal foil. Crucially, we highlight important uncertainties in experimental impedance data analysis and contextualize our findings by comparison with coulometric titration time analysis (CTTA). Overall, our findings contribute to the in-depth understanding of interphase growth kinetics in solid-state batteries and their quantification using impedance spectroscopy.