Methodology for the characterization and understanding of longitudinal wrinkling during calendering of lithium-ion and sodium-ion battery electrodes

"The manufacturing of lithium-ion battery (LIB) cells is following a complex process chain in which the individual process steps influence the subsequent ones. Meanwhile, increasing requirements especially concerning the battery performance, sustainability and costs are forcing the development of innovative battery materials, production technologies and battery designs. The calendering process directly affects the volumetric energy density of an electrode and therefore of a battery cell. Calendering is still challenging as it causes high stresses in the electrode that lead to defects and thus increased rejection rates. The interaction between electrode material and process as well as the formation of defects is still not fully understood, especially when new material systems are used. In this context, the sodium-ion battery (SIB) is one post-lithium battery system that is a promising option to overcome the limitations of conventional LIBs. Therefore, this paper presents a first material and machine independent methodology to describe and understand the defect type longitudinal wrinkle, which mostly appears at the uncoated current collector edge of an electrode and in running direction. The aim is to systematically characterize the longitudinal wrinkles according to their geometry. The automatic data acquisition is carried out with a laser triangulation system and a 3D scanning system. The geometry values are calculated from the raw data and correlated to selected process parameters. The methodology is applicable regardless of the material as shown by exemplary results of NMC811 cathodes for LIB and hard carbon anodes for SIB. By using two different pilot calenders it is shown, that the data acquisition can be carried out independently of the machine. The presented methodology contributes to finding solutions for the avoidance of longitudinal wrinkling in any battery electrode and therefore to reducing the rejection rate.