Microstructure, Charge Transport and Interfaces in All-Solid-State Batteries
Authors: Hartmann Felix , Janek Jürgen 
Published: December 2024

Objective: Despite recent global efforts to establish all-solid-state batteries (ASSB) as potentially safe and stable high-energy and high-speed storage technology, long-term performance, specific power, and economic viability remain challenging. However, efficient and short ionic and electronic transport pathways and optimized interfacial contacts between the cell materials and compounds are essential. This poses a significant challenge for the microstructure of cathodes and a careful design and tailoring of anodes and separators in ASSB, which rely solely on solid-state contacts. Here, we present our recent work on ASSB with thiophosphate electrolytes conducted within the German Competence Cluster for Solid-State Batteries (FestBatt). Our work aims to investigate relevant parameters in different cell components on material, electrode, and cell level, such as the importance of particle size distributions, microstructures, and kinetic factors to optimize ASSB cathodes, separators, and anodes.

Methodology: The work will be presented in four sections, which showcase key challenges as well as optimization strategies mainly focusing on cathode composites, but also discussing selected examples for separators and anodes in ASSB with thiophosphate electrolytes. Further, an outlook for relevant production steps of ASSB electrodes and cells will be given. For these studies, we mainly conducted focused ion beam scanning electron microscopy tomography, electrochemical impedance spectroscopy, cycling experiments on ASSB cells with thiophosphate solid electrolytes.

Results: Fast conducting solid electrolytes are necessary to ensure sufficient transport in composite cathodes but does not represent the only key factor for high-quality cathode composites in ASSB. In our studies, we aimed to optimize the microstructure of composite cathodes consisting of intercalation-type active material and a thiophosphate-based solid electrolyte. We varied the particle size distribution of the solid electrolyte to create various cathode microstructures and assessed the effectiveness of electronic and ionic conduction pathways using partial conductivities. We used both electronically and ionically blocking electrodes for electrochemical impedance spectroscopy measurements to quantify the respective partial conductivities. Our analysis revealed that the particle size of the solid electrolyte significantly influences the charge transport and electrochemical performance of the cathodes. Additionally, using FIB-SEM tomography, we created detailed 3D reconstructions of the cathode microstructure and correlated the obtained partial conductivities with microstructural descriptors such as tortuosity factors, identifying possible kinetic bottlenecks. We found that minimizing residual porosity, which blocks ion and electron transport, is necessary to optimize cathode microstructures and enhance ASSB performance. Moreover, high-performance anodes, along with protective concepts, are crucial for establishing dense, high-energy ASSB cells. Lithium metal anodes may not be the ultimate solution, and silicon anodes can pose advantages, e. g. in terms of electrochemical stability for thiophosphate separators.

Discussion: Our studies demonstrate the importance of optimizing the microstructure of cathodes and carefully designing Si anodes for improved ASSB cells. By manipulating the particle size distribution of the solid electrolyte, we achieved more efficient electronic and ionic transport pathways, leading to improved battery performance. The presented work is primarily published in references 1-5, detailing key challenges and showcasing optimization strategies for these topics. Beyond that, diversity in materials, research teams, and approaches is crucial for long-term successful development of ASSB.

Acknowledgement: Scientific and administrative coordination of the BMBF Competence Cluster for Solid-State Batteries (FestBatt), FB2-Koord (03XP0431), and cell platform Thiophosphates, FB2-Thio (03XP0430A).

References

[1] J. Janek, W. G. Zeier, Challenges in speeding up solid-state battery development, Nat Energy, 2023, 8, 230–240.

[2] T. Schmaltz, F. Hartmann, T. Wicke, L. Weymann, C. Neef, J. Janek, A Roadmap for Solid-State Batteries, Adv. Energy Mater., 2023, 13, 2301886.

[3] P. Minnmann, F. Strauss, A. Bielefeld, R. Ruess, P. Adelhelm, S. Burkhardt, S. L. Dreyer, E. Trevisanello, H. Ehrenberg, T. Brezesinski, F. H. Richter, J. Janek, Designing Cathodes and Cathode Active Materials for Solid-State Batteries, Adv. Energy Mater., 2022, 12, 2201425.

[4] P. Minnmann, L. Quillman, S. Burkhardt, F. H. Richter, J. Janek, Editors’ Choice—Quantifying the Impact of Charge Transport Bottlenecks in Composite Cathodes of All-Solid-State Batteries, J. Electrochem. Soc., 2021, 168, 040537.

[5] H. Huo, M. Jiang, Y. Bai, S. Ahmed, K. Volz, H. Hartmann, A. Henss, C. V. Singh, D. Raabe, J. Janek, Chemo-mechanical failure mechanisms of the silicon anode in solid-state batteries, Nat. Mater., 2024, 23, 543–551.

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