Anja Bielefeld

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.

Composite electrolytes attract significant attention in solid-state battery research due to their potential to combine the advantages of both inorganic and polymer electrolytes. Ion transport in these electrolytes is frequently investigated using electrochemical impedance spectroscopy, where each impedance contribution is typically assigned to a charge migration and polarization process related to a specific phase or interface present within the composite. Here, we provide fundamental insights into the interrelation between impedance spectra and charge transport using electric network simulations. We demonstrate that for 3D composite electrolytes, the assignment of impedance contributions to individual phases or interfaces is often physically not justified. The spatial distribution of the alternating current in these materials depends on excitation frequency, mesostructure, and the electric and dielectric properties of all constituents and their interfaces. This leads to Maxwell-Wagner-type impedance contributions that have no simple correspondence to direct current conduction mechanisms, i.e., leads to impedance contributions that depend on the electric and dielectric properties of multiple phases. We discuss interrelations with classical equations of effective medium theory, e.g., the Maxwell-Wagner model, the impact of particle size, and practical implications for the impedance analysis of ceramic/polymer-based composite electrolytes as one important model case. Among others, the widely studied LLZO-PEO system is considered as a specific example. The conclusions drawn are general and extend to other heterogeneous systems like composite cathodes.

Research into the development and understanding of solid‐state batteries often relies on pelletized press cells due to their comparative ease of use. However, these model cells are prone to comparability and reproducibility issues. This study examines the extent to which the cathode composite preparation influences the cell performance of a reference system comprising LiNi_0.82Mn_0.07Co_0.11O₂ as the cathode active material, Li₆PS₅Cl as the solid electrolyte, carbon nanofibers as the conductive additive, and an indium–lithium foil anode. The cathode composite is prepared either via hand mortaring or in a mini vibrating mill. The mixing process is found to be critical for the reproducibility of cell performance and accounts for many of the discrepancies observed in the capacities of different cells made with identical materials and following the same cell assembly protocol. The open‐circuit relaxation method is implemented to quantify active mass utilization in the cathode in situ, which depends on the mixing process and correlates with the cell performance. This approach allows for a quantitative differentiation between static and kinetic capacity losses during the discussion of specific capacity values. The results demonstrate the significance of cathode composite mixing and the necessity of quantifying the mixing quality for reliable electrochemical data acquisition and interpretation.

Hybrid battery cells that combine a garnet-type Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte with other solid, polymer or liquid electrolytes are increasingly investigated. In such cells with layered electrolytes, ensuring a low-resistive heteroionic interface between neighboring electrolytes is crucial for preventing major additional overpotentials during operation. Electrochemical impedance spectroscopy is frequently used to extract such parameters, usually on multilayer symmetrical model cells that contain the different electrolytes stacked in series. Unfortunately, the impedance contributions of the heteroionic interfaces often overlap with those of the electrolyte|electrode interfaces, necessitating the use of sophisticated four-point cells that probe the electrochemical potential away from the polarization source. In this work, an alternative solution to this problem is demonstrated by taking advantage of the inherent fast charge transfer kinetics of LLZO with its parent metal electrode. The "resistance-free" nature of a reversible Li|LLZO interface enables a precise evaluation of the heteroionic interface impedance in symmetric two-point cells of the type Li|LLZO|electrolyte|LLZO|Li with negligible electrode contribution. This is exemplified for symmetric multilayer cells containing tantalum-doped LLZO and a poly(ethylene oxide) (PEO)-based dry polymer electrolyte. Validation and comparison of impedance data with results from symmetric four-point cells and two-point cells with ion-blocking electrodes demonstrate the advantage of the proposed method. Overall, this study presents a simple and reliable method for studying heteroionic interface impedances in LLZO-containing multilayer cells.

The kinetics of composite cathodes for solid-state batteries (SSBs) relies heavily on their microstructure. Spatial distribution of the different phases, porosity, interface areas, and tortuosity factors are important descriptors that need accurate quantification for models to predict the electrochemistry and mechanics of SSBs. In this study, high-resolution focused ion beam-scanning electron microscopy tomography was used to investigate the microstructure of cathodes composed of a nickel-rich cathode active material (NCM) and a thiophosphate-based inorganic solid electrolyte (ISE). The influence of the ISE particle size on the microstructure of the cathode was visualized by 3D reconstruction and charge transport simulation. By comparison of experimentally determined and simulated conductivities of composite cathodes with different ISE particle sizes, the electrode charge transport kinetics is evaluated. Porosity is shown to have a major influence on the cell kinetics and the evaluation of the active mass of electrochemically active particles reveals a higher fraction of connected NCM particles in electrode composites utilizing smaller ISE particles. The results highlight the importance of homogeneous and optimized microstructures for high performance SSBs, securing fast ion and electron transport.

With the exit from fossil-fuel energy becoming more and more urgent, advanced batteries are highly sought after and all-solid-state batteries (ASSBs) are seen as a promising technology potentially enabling lithium metal or reservoir-free anode designs which promote the energy density. Despite high research interest in the field, ASSBs still perform below expectations, the main challenges being elevated interfacial resistances, microstructure effects and processing, particularly at large scale. While the build-up of high resistances due to void formation upon lithium stripping at the anode side is increasingly understood, the void formation and its evolution in the composite cathode as well as the effects on the cell performance are widely unknown. Why consider voids in the cathode at all? In contrast to conventional, liquid electrolytes, solid ion conductors (such as thiophosphates) do not easily infiltrate voids and cracks in the cathode¹. They possess a proper particle morphology and, depending on the solid electrolyte particle size distribution, its suitability to the active material particle sizes and morphologies the electrode structure that forms upon manufacturing can be quite different and influences the ion and electron transport as well as the charge transfer capabilities^2-5. Experimentally determined void fractions in the composite typically range from 3 to 40 vol%^6,7 and are therefore far away from being negligible. Apart from the void formation at manufacturing, scanning electron microscope (SEM) cross sections of cycled composites have also shown that the volume changes of LiNi_xCo_yMn_(1-x-y)O₂ active material can lead to contact loss at the interface of the solid electrolyte and the active material⁸ which reinforces the importance of understanding the origins and the influences of voids on the cell performance. In this talk, we will discuss these, mainly from the modeling perspective including percolation network models⁴, flux-based simulations of the effective conductivity⁵ and electrochemical charge simulations based on the finite-element method⁹. Additionally, we use focused ion beam(FIB)-SEM tomography for the reconstruction of cathodes with different microstructures obtained by changing the particle size of the solid electrolyte. We investigate the influence of the solid electrolyte particle size distribution on the void space fraction and distribution and provide guidelines for further cathode optimization. Ruess, R.; Schweidler, S.; Hemmelmann, H.; Conforto, G.; Bielefeld, A.; Weber, D. A.; Sann, J.; Elm, M. T.; Janek, J., J. Electrochem. , 2020, 167, 100532. Minnmann, P.; Quillman, L.; Burkhardt, S.; Richter, F. H.; Janek, J., Electrochem. Soc., 2021 , 168, 040537. Jiang, W.; Zhu, X.; Huang, R.; Zhao, S.; Fan, X.; Ling, M.; Liang, C.; Wang, L., Energy Mater, 2022 , 12, 2103473. Bielefeld, A.; Weber, D. A.; Janek, J., Phys. Chem. C, 2019 , 123, 1626-1634. Bielefeld, A.; Weber, D. A.; Janek, J., ACS Appl. Interfaces, 2020 , 12, 12821-12833. Shi, T.; Zhang, Y.-Q.; Tu, Q.; Wang, Y.; Scott, M. C.; Ceder, G., Mater. Chem. A, 2020 , 8, 17399-17404. Ates, T.; Keller, M.; Kulisch, J.; Adermann, T.; Passerini, S. Energy Storage Mater., 2019 , 17, 204-210. Koerver, R.; Aygün, I.; Leichtweiß, T.; Dietrich, C.; Zhang, W.; Binder, J. O.; Hartmann, P.; Zeier, W. G.; Janek, J., Mater., 2017 , 29, 5574-5582. Bielefeld, A.; Weber, D. A.; Ruess, R.; Glavas, V.; Janek, J., Electrochem. Soc., 2022 , 169, 020539. Figure 1

With research on solid‐state batteries being on the rise, also simulations and modeling studies become more and more popular. It is therefore time to reflect upon the interplay of experiments and simulations and the upcoming dilemma of drawing general conclusions from increasingly complex model approaches that naturally go along with advancements and insights in the research field. Albeit this perspective focuses on (microstructure) models for composite cathodes, it is also an invitation to contemplate the epistemological considerations and the proposed tentative agenda on how to build useful models for other sub‐fields of the solid‐state research community.

ASSBs (all-solid-state batteries) are promoted as an energy dense and safe alternative to current Li-ion batteries (LIBs) and attract great interest from academia and industry. In contrast to LIBs, which employ a liquid organic electrolyte, they utilize a solid electrolyte. This substitution promises to eliminate the flammability of the battery and to simplify the cell design. While recent research efforts have concentrated on miniaturizing and eventually even removing the anode host material in batteries, the relative portion of the cathode needs to be maximized, as cathodes are the only component that can increase energy density by increasing its fraction. In a simplified view, the cathode kinetics are determined by the cathode microstructure, the volume fractions of the constituents and the properties of electrolyte and cathode active material (CAM). Liquid electrolytes can easily penetrate porous composite cathodes, but rigid SEs can not do the same, resulting in residual porosity in the cathode. This porosity can lower active interface area between CAM and SE, and increase tortuosity of ionic and electronic charge transport pathways. Sufficient ionic and electronic transport pathways in composite cathode structures are, however, essential because cathode active material particles that are either electronically or ionically isolated cannot contribute to the charging or discharging process. We analyse the requirements for SSB cathodes and determine charge transport bottlenecks by impedance spectroscopy of a reference system consisting of a thiophosphate based solid electrolyte and a nickel rich layered CAM. Different cathode microstructures are analysed and their charge transport properties are quantified as partial conductivities. From the obtained partial conductivities, we calculated tortuosity factors and correlated them to cell performance with complementary cycling data of all-solid-state batteries in order to determine charge transport bottlenecks. We find, that ionic charge transport and consequently cathode kinetics are highly dependent on the SE particle size distribution In addition, we analyse the requirements for CAMs for SSBs and develop design principles for different CAM types that aim to further increase cathode performance.