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Institute Electrochemical Energy Storage

NaSeFest

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Operando Characterization of Solid-State Sodium-Sulfur Batteries

The solid-state sodium-sulfur (ssNa||S) battery is a promising candidate in the long line of “beyond lithium-ion” batteries. It addresses the demand for high energy densities by combining a sulfur cathode and a sodium metal anode (enabled by the use of a solid-state electrolyte) with their high theoretical specific capacities (CS = 1672 mAh g-1, CNa 1165 mAh g-1). At the same time, the ssNa||S cell chemistry is more independent from critical resources than lithium-ion batteries (LIB) (e.g. lithium, nickel, cobalt, and graphite). Thus, ssNa||S batteries represent a promising alternative that could coexist with LIBs in the market, offering increased flexibility based on application requirements, resource availability, and economic considerations.

Solid-state Na||S batteries face challenges due to sulfur's large volume change during cycling and its low conductivity, which limits rate capability and conversion kinetics. Thus, a deeper mechanistic understanding is crucial. Operando techniques provide real-time insights into structural changes, phase transformations, and kinetic limitations, aiding in the optimization of cell components.

The “NaSeFest” project (“Natrium-Schwefel Festkörperbatterien“ / “Sodium-Sulfur Solid-State Batteries“), funded by the German Federal Ministry of Education and Research (BMBF), aims to systematically further develop the ssNa||S technology on material level and fundamental understanding as well. Besides our group at HZB, the following project partners are involved: Prof. Zeier group (University of Münster), Prof. Adelhelm group (Humboldt-University of Berlin), Prof. Janek group (Justus-Liebig University Gießen).

As part of this project, a novel operando setup has been developed in our group for the multimodal characterization of solid-state batteries. The cell allows for high operating pressures (up to 50 MPa), which are crucial for sufficient interfacial contact when using sulfide-based electrolytes. To monitor the cell stack pressure upon assembly and in operando, a force sensor is integrated in the setup, which is especially useful to investigate cell systems with more pronounced volume changes like the sulfur cathodes of ssNa||S cells. Another pivotal part is the incorporation of a window (material: Vespel®) for X-ray techniques like imaging. A mobile heating platform ensures constant (possibly elevated) temperatures at any measurement station.

Initially developed for the FestPoLiS and the NaSeFest projects, the setup can be applied to other solid-state cell systems as well, providing crucial mechanistic insights and enabling rich collaborations.