Solid electrolytes (SE) for all solid-state batteries (ASSB) are becoming more important in recent years.
Due to constantly increasing energy and power densities, the trend is towards thinner homogeneous SEs. At the same time, SEs should have a high mechanical strength with a certain flexibility. Furthermore, properties such as good electrochemical performance and low flammability are essential. These constantly growing requirements for SEs increases the demand for suitable materials and their processing methods. In order to achieve these requirements, the slot die process is a suitable production method for homogeneous, dense, thin layers. This process is mainly controlled by three process parameters, which simplifies scaling. Further, it is already used in many production steps, e.g. for electrodes in the conventional lithium ion battery production. This offers a good opportunity to use this knowledge and adjust it for SE production.
Poly(ethylene oxide) (PEO) based SE make a good model material as PEO offers many advantages, such as its good availability on the global market, high solubility in solvents like acetonitrile, and the capability to dissolve conductive salts like the alkali salt Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Furthermore, PEO has a wide range of flow properties due to the use of different molecular weights (MW). This provides the opportunity to produce an optimal coating solution for the process offering the advantages of simple processing while maintaining mechanical and electrochemical properties.

The aim of this work is to investigate the influence of the process parameter on the layer quality. Therefore, the coating window is evaluated as a function of material properties, i.e. MW (100.000 g/mol to 300.000 g/mol) of PEO and solution concentrations (0.05 g/cm³ to 0.30 g/cm³). For each solution, the necessary material properties, viscosity and surface tension, are determined. Due to the variation of MW, the solution properties vary strongly, i.e. an increase in MW results in increasing viscosities at constant concentrations. All coatings are produced with the same design of experiment (DoE), varying the process parameters dispense rate, coating gap and coating speed. The produced layers are evaluated based on their qualitative coating quality, dry layer thickness and ionic conductivity. For each solution, a coating window is determined and compared to a theoretical film break up model. With increasing MW, the coating window of the stable coating processes becomes smaller using the same DoE. In general, the slot die process is well suited for low MW solutions due to its wider processing window that allows the use of higher concentrated coating solutions while reducing the solvent amount.