Development and investigation of a simultaneous double-sided coating process


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In the future, electricity storage systems will become an increasingly important aspect of the energy industry, especially for the upcoming e-mobility and for everyday use. Due to their properties such as high-energy density, high-power density and in addition high cycle stability, lithium-ion batteries (LIB) are regarded as the basis for electrification. LIBs are manufactured in production lines that are supposed to be highly efficient and have a high throughput. For a more efficient assembly and saving of inactive materials, e.g. expensive copper, both sides of the substrate foils are coated. It is state-of-the-art technology that electrodes are produced sequentially in two separate coating and drying steps, in which the electrode slurry is first coated and dried on the upper side and then on the lower side of the substrate. In the future, a simultaneous double-sided coating and drying process can optimize efficiency and energy costs. As a result, alternative coating methods need to be applied and scientifically investigated in addition to the conventional slot-die coating process. Moreover, the efficiency of the electrode production can be increased with an intelligent manufacturing process by minimizing waste by removing the side edges and optimizing the quality of electrodes.
In this work a flexible and modular coating system will be conceptualized and constructed. The aim is to compare different coating methods such as slot-die coating in bead mode and in tensioned-web mode. Subsequently, slot-die coating in tensioned-web mode and its influence on the quality and homogeneity of battery coatings will be investigated. To compare the quality of the tensioned-web mode with the slot-die coating process in bead mode, it is mandatory to examine process windows of the tensioned-web mode. The next step is to investigate the influence of process parameters and material properties, e.g. the viscosity of the electrode slurry, on the edge quality. In the end, both coating methods are integrated into the coating system and are intended to produce a high-quality electrode coated on both sides.
At the beginning of the investigations, the slot die coating was examined with regard to coating quality. Coatings were produced in the stable range of the process window. The dimensionless gap width G* and the process speed uw were varied. Results in literature could be reproduced for the material system used. It was shown that the reduction of G* to the value of 1 has a positive effect on the formation of the side edges of the electrode coating. However, the close gap adjustment can lead to further challenges. Therefore, the adjustment of the gap distance is not optimal to reduce edge elevations. Furthermore, the influence of the process speed on the edge elevations, which is known from literature, was proven. The edge height is independent of the process speed.
Another approach to reduce the edge height is the adjustment of the internal geometry of the slot die with constant process conditions. This is also known from literature. However, this fact has only been investigated for newtonian fluids. In this work, the focus was on shear thinning electrode slurries. The internal geometry of the slot die was adjusted that the flow at the edge of the coating was influenced. The change of the flow profile directly influences the formation of the edges. With an optimal design of the inner geometry of the slot die, an elimination of the edge elevations for the material system used was achieved.
In further work, the influence of the material properties on the formation of the edges will be investigated. In addition, the influence on the edges must also be investigated for other coating methods (e.g. tensioned-web mode) and compared with the conventional bead mode in order to be able to produce double-sided electrodes with constant quality.

The authors would like to acknowledge financial support of the Federal ministry of Education and Research (BMBF) via the ProZell cluster-projects “HiStructures“ (Grantnumber: 03XP0243C) and “Sim4Pro” (Grant number: 03XP0242C).

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Keywords: slot-die coating, tensioned-web slot-die coating, lithium-ion battery

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