As a relatively new approach to battery research, solid-state technology is much discussed as it promises cells with higher energy density. This reduces CO2 emissions and brings cost benefits, while production and composition are associated with risks. For this reason, employees of the Fraunhofer Institute for Production Technology IPT and the Fraunhofer Research Unit Battery Cell Manufacturing FFB cooperated with the aim of shedding light on the challenges in the development of solid-state batteries. The result is the now available technology study "Red Brick Walls of All-Solid-State Technology". In this context, red brick walls are technological obstacles that cannot yet be overcome according to the current state of knowledge.

To take stock of the situation, an internal workshop was held at Fraunhofer FFB in which the participants developed an interview guideline for the subsequent broader survey. In the process, the challenges in the guide could be divided into three categories: (1) overarching, (2) product-related, and (3) production-related. The product-related hurdles within the battery cell can be broken down into its individual components. Here, the interfaces of anode, electrolyte, and cathode are regarded by many experts as particularly critical.

On the other hand, the production-related red brick walls are mainly in the area of electrode and electrolyte production. This is considered a particularly critical process step in the manufacture of solid-state batteries. In this context, the processability of lithium metal anodes and their long-term stabilization in operation is a particular sticking point. Another challenge: is the production of electrode-electrolyte composites at high productivity as well as a lack of compatibility with the existing plant technology.

Using the developed guideline, the researchers conducted 25 interviews with experts from research and industry. More than 240 product- and production-related challenges emerged from the interviews. These can be assigned to 14 red-brick-wall categories, which were then discussed in in-depth interviews. From the results, initial recommendations for action were derived in two thrust directions under the guiding questions:

• Top-down: How does Fraunhofer FFB become a relevant player in the field of solid-state batteries?
• Bottom-up: How will Fraunhofer FFB be enabled to produce solid-state batteries? 

From both questions, both a short-term (for the years 2024-2025) and a long-term (from 2027) recommendation for action were derived. The study was able to help Fraunhofer FFB gain a comprehensive overview of the hurdles to solid-state battery production: The definition of the red brick walls made it possible to identify key technologies and starting points that can accelerate the industrialization of solid-state batteries at Fraunhofer FFB. At the same time, it became clear that a strong partner network is needed to achieve rapid progress in this field.

[1] Challenges that cannot or can hardly be overcome within a defined target framework according to the current state of knowledge are classified as "red brick walls". The term was coined by the German Engineering Federation. Cf. VDMA Battery Production: "Roadmap Battery Production Equipment 2030", Frankfurt am Main: VDMA Verlag 2014.

[2] Cf. https://www.isi.fraunhofer.de/content/dam/isi/dokumente/cct/2022/SSB_Roadmap.pdf