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Characterization of calandered cathodes of li-ion battery cells using confocal and electron microscopy

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Summary:

The capacities and volumetric energy densities of li-ion battery cells can be raised by applying higher compressions on the electrodes. The calendering process does not only have a notable effect on the coating, it also has an effect on the current collector foil. In this study the focus is on the cathode and in particular on the aluminium current collector foil, where the coating does does not cover the edge regions. With increasing compressions due to the calendering process, wrinkle formation raises in the edge regions, especially in the transition of the coated to the uncoated region. At some point of compression, the wrinkles lead to problems in the further processing of the electrode as exceeding tolerance ranges via laser cutting or delaminations due to the roll to roll process at the edge region. As a result, the degree of compression with this type of electrode manufacturing process is limited by the formation and intensity of wrinkles. In order to achieve the necessary increases in capacity and power density, higher compressed coatings are necessary while at the same time avoiding the formation or at least less distinct wrinkles.

For a better understanding of the wrinkle formation we focus on calendered cathodes by application of confocal laser scanning microscopy (LSM) to achieve informations about topography and impact of compressed state of the art NCM coatings on the current collector. With scanning electron microscopy (SEM) on metallographical cross-sections of cathodes we quantify characteristics in high-resolution as the phase fraction of NCM in the coating or the thickness of the calendered current collector. As there are currently no standardized test methods for these issues, we show how the deformations of the aluminium foil can be quantified from the particle impressions on the surface of the current collector to the grain orientation in cross-sections. As a result of these investigation we indicate the impact of the compressed coating containing spherical particles like NCM to the aluminium current collector and how it likely affects wrinkle formation.

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