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.