NSF-DFG MISSION: INCUBATOR - Exploring the Interfacial Chemistry in Sulfide-Based Solid-State Batteries by Soft and Hard X-ray Spectroscopies Under Operating Conditions

• Applicants: Professor Dr. Philipp Adelhelm; Professor Dr.-Ing. Marcus Bär; Elmar Kataev, Ph.D.; Katherine Mazzio, Ph.D.
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Solid State and Surface Chemistry, Material Synthesis
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 542015736

Solid state batteries (SSBs) provide high energy densities by using Li metal anodes and improve safety by eliminating liquid electrolytes. Layered oxides have been widely used as the cathode active materials (CAMs) in SSBs, but they are approaching their performance and stability limits. Sulfide CAMs are attractive alternatives because while they suffer from lower voltages, they make up for it through increased capacity via reversible anion sulfur redox reactions. We have recently investigated CuS as a CAM in SSBs due to its high energy density. CuS exhibits a displacement reaction, where the reaction products reversibly travel large distances under an electric field. This is thought to be due to the similar ionic radii of Li+ and Cu+, but the details of the underlying redox mechanism are still unclear. This and the perspective of using Li metal anodes requires incisive tools to probe redox processes with elemental sensitivity. Synchrotron-based analytical tools enable operando studies at buried interfaces by X-ray spectroscopies, such as HAXPES, XAS, and RIXS. Probing SSBs operando is essential because post-mortem analysis can introduce structural and/or chemical changes due to relaxation effects and the reactive nature of S- and Li-containing compounds. No common sample platform exists for SSBs that allows operando studies with this combination of X-ray spectroscopies. It is very challenging to create “windows” for X-rays and electrons to pass through while maintaining structural integrity in SSBs because they must be cycled at elevated pressures (typically 5-70 MPa) in order to achieve reliable performance. In INCUBATOR we will develop mechanistic understandings of the redox processes occurring in sulfide CAMs. Achieving this goal necessitates developing a multi-modal operando cell that enables experiments with high energy resolution on different length scales by HAXPES, XAS, and RIXS. With these methods, an atom-specific understanding of electrochemical phenomena and degradation processes at electrode/electrolyte interfaces will be achieved, including 1) Distinguishing sulfur anion redox in CAMs from decomposition processes of the solid electrolyte. 2) Determining the reaction pathway in CuS CAMs during cycling. 3) Understanding Li deposition in an anode-less configuration from an over-lithiated CAM (Li2S+Cu or Li2S+Cu2S). Addressing these objectives is challenging, but also critical for drafting design principles for CAMs with reversible sulfur anion redox that form stable SEIs. Ultimately, this will open new approaches for high-energy density Li-metal SSBs with enhanced long-term performance. The workflow and multi-modal operando platform developed in this project can be used to further the development of different solid-state electrochemical systems, while also offering the possibility to extend its use to other techniques such as SEM or XRD with minimal design changes.

• DFG Programme: Research Grants
• International Connection: USA
• Partner Organisation: National Science Foundation (NSF)
• Co-Investigator: Professorin Dr. Annika Bande
• Cooperation Partners: Professor Dr. Jinghua Guo; Wanli Yang, Ph.D.