The Influence of Pseudoplastic Flow Behavior on the Slot Die Process in Solid Electrolyte Production
Authors: Wiegandt Andrea , Langer Frederieke , Schwenzel Julian 
Published: February 2024

Solid-state batteries (ASSB) are the next generation of batteries for energy storage. An ion-conducting solid replaces the liquid electrolyte. The trend is toward thin, homogeneous solid electrolytes (SE) that replace the liquid electrolyte and separator. At the same time, the use of a lithium metal anode and the reduction of the layer thickness enable higher energy and power densities. SEs should have a high mechanical strength with a certain flexibility. In addition, electrochemical and chemical properties such as good electrical isolator and good ion conductor, as well as low flammability, are essential. This requirement profile for SEs increases the demand for suitable materials and their processing methods.

A promising candidate for the production of SE is the slot die process. This process is mainly controlled by three process parameters, i.e. coatings speed, dispense rate and coating gap. Compared to other production methods, this makes it easy to set parameters for targeted coating thicknesses. The process is already used in many production steps, e.g. for electrodes in the conventional lithium ion battery production. This provides a good opportunity to use and adapt this knowledge for SE production.

In general, for coating solutions, structurally viscous fluids have well suited properties. The change in flow properties during shear stress can be used to achieve a certain stability between the coating tool and the substrate to avoid defects during coating. It also ensures easy handling of the system after processing and the shape stability of the resulting coatings.

Therefore, a promising candidate as base material for SE production is Poly(ethylene oxide) (PEO). In addition to the shear thinning (or pseudoplastic) flow behavior, it also offers many advantages, such as its good availability on the global market, high solubility in solvents such as acetonitrile, and the capability to dissolve conductive salts like the alkali salt Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

The aim of this work is to investigate the influence of the pseudoplastic flow behavior during and after the coating process by varying the molecular weight (Mw) of PEO. Therefore, the coating window is evaluated as a function of material properties MW and solution concentration. From viscosity and surface tension determination, an increase in MW leading to a stronger pseudoplastic flow behavior can be observed. For the coating process, a DoE employing a constant set of the process parameters dispense rate, coating gap and coating speed is used. The produced layers are evaluated based on their coating quality, dry layer thickness and ionic conductivity. For each solution, the obtained coating window is compared to a theoretical film break-up model.

This work shows successful production of SE with an ionic conductivity of 10-6 S/cm using different MW. Further, the experimental results are comparable to the theoretical models. Moreover, the experiments show both, a dependency of MW and PEO concentrations. Within one MW, stable coating window becomes smaller with increasing concentration. Similar behavior is observed at a constant concentration with different MW. Due to the change in flow properties, the stable coating window becomes smaller with increasing MW. As a result of these correlations, the produced dry layer thickness varies depending on the use of MW and the associated PEO concentration.