Comparison of Longitudinal Wrinkle Formation During Calendering of NMC811 and LFP Cathodes

Increasing customer demands for lithium-ion batteries (LIB) and measures to meet the climate targets will require better batteries in terms of performance, sustainability, safety and product cost. Minimizing production scrap is an important factor in reducing manufacturing costs. Battery cell manufacturing involves many complex process steps. One of these is calendering, which takes place at the end of the electrode manufacturing process. Calendering is necessary to adjust the thickness of the electrode and therefore its density. This contributes significantly to the volumetric energy density of the final battery cell. Apart from not achieving the required target density, scrap is caused by the formation of defects. One such defect is longitudinal wrinkling, which can occur in electrodes in the areas of uncoated substrate foils. In addition to web tears, longitudinal wrinkles cause severe problems in the further processing of the electrode. Therefore, the aim of this paper is to analyze and compare the characteristic geometric features of longitudinal wrinkles formed during calendering of NMC811 and LFP cathodes, using a statistical experiment design. The influence of the varying process parameters on the previously determined geometric features is investigated. The process parameters density, temperature and web tension are varied in three levels. In addition, the mechanical behavior in terms of bending stiffness and strain during calendering is investigated. All three process parameters influence the geometry of the longitudinal wrinkles in a complex and different way. Furthermore, the material properties of the electrodes are changed by calendering and correlations to the geometry are identified. These findings provide a deeper insight into the complex material-process interactions. Furthermore, this work contributes to a better understanding of the formation of longitudinal wrinkles and the calendering process in general.