Marcel Schmitt

Slot die coating of lithium-ion battery electrodes: investigations on edge effect issues for stripe and pattern coatings

An important step in the production of lithium-ion batteries is the coating of electrodes onto conducting foils. The most frequently used coating method in industry is slot die coating. This process allows the reproducible preparation of thin functional films at high velocities. A phenomenon that is often neglected in scientific studies and has attracted little attention, compared to film stability, is the inevitable presence of edge effects. Film elevations appear at the coating edges and at the start of each single patch during intermittent coating. These superelevations will cause problems at the downstream cell-assembling steps. In this study the influence of dynamic and geometric coating parameters on the shaping of coating edge effects was investigated. A quantitative measurement technique for edge profiles was developed and implemented. Film stretching has been identified in literature as a main reason for edge effects. We could show that varying the coating speed and the gap ratio did not lead to ideal edges, but affected the shape of the coating edges. The elevation height appeared to be independent from the varied process parameters, in the range of investigation. Also the gradient at the very edge of the film was not affected by these parameters. Only the edge width showed a disproportionately increasing trend towards higher applied gap ratios. The results indicate that the approach of film stretching is not sufficient to fully describe the cause of superelevations in lithium-ion battery coatings.

Fluid-dynamic properties and wetting behavior of coating inks for roll-to-roll production of polymer-based solar cells

In the last decade, semiconducting and conducting materials were developed that can be processed by solvent-based deposition to form functional layers or complete electronic devices. These materials are typically synthesized in laboratory scale quantities and tested on small spin-coated substrates, whereas the final goal is to produce them on flexible substrates in a continuous roll-to-roll process. To enable a fast scale up and optimization, fluid-dynamic properties have to be known. Here, we present viscosity and surface tension data for typical material systems, applied in polymer-based solar cells. Materials presented include water-based polymer dispersions (hole-conducting and high-conductive PEDOT:PSS types), solvent-based anorganic nanoparticle dispersions (silver nanoparticle ink, hole-blocking ZnO nanoparticle ink), and dissolved organic molecules and polymers (P3HT:PCBM photoactive blend). Predictive models are proposed to approximate viscosity and surface tension for these materials at various compositions. As well, corona treatment is used to modify the surface energy of P3HT:PCBM and described as a function of web speed and corona power. The importance of material properties is demonstrated by predicting stable conditions for a slot-die coating process. A simple drying simulation highlights the possibility of using property models to investigate wetting problems.

Analytical determination of process windows for bilayer slot die coating

Slot die coating is a film casting process with a highly diverse variety of everyday applications. As a pre-metered process it not only guarantees excellent film uniformity, but is also suitable for simultaneously applied multilayer coatings. Characteristic singularities like the behavior of the liquid–liquid interface and the impact of the additional mid-lip on film uniformity were already investigated before. However, the effect of an altered gap pressure regime on commonly used coating windows has not yet been discussed. In this work, we therefore extended available single-layer coating windows for Newtonian and power-law liquids to the bilayer case. Here, the emphasis was laid on the air entrainment limit. Subsequently, the theoretical results were compared to experimental data. It was found that the onset of air entrainment strongly depends on the top to bottom film thickness ratio for bilayer coatings. A critical film thickness ratio which delivers similar coating limits as those for single-layer coatings was derived and confirmed by experimentally gained results.

Slot die stripe coating of low viscous fluids

Slot die coating is applied to deposit thin and homogenous films in roll-to-roll and sheet-to-sheet applications. The critical step in operation is to choose suitable process parameters within the process window. In this work, we investigate an upper limit for stripe coatings. This maximum film thickness is characterized by stripe merging which needs to be avoided in a stable process. It is shown that the upper limit reduces the process window for stripe coatings to a major extent. As a result, stripe coatings at large coating gaps and low viscosities are only possible for relatively thick films. Explaining the upper limit, a theory of balancing the side pressure in the gap region in the cross-web direction has been developed.