Identification of morphology-critical stages during drying of particulate electrode slurries

The evolution of film morphology during the drying of disperse coating media is a process that is still insufficiently understood in spite of its paramount importance for film properties. Up to date, experimental drying processes are often based on empirical parameter studies and not on sound scientific understanding of the basic processes. Lithium-ion battery electrodes have been an intensely discussed subject in recent years. In industrial manufacturing, slurries are commonly cast continuously on a substrate and dried in subsequent belt dryers. During drying, the film solidifies and a complex, porous structure evolves. The properties of the film and, therefore, the battery cell strongly depend on the component distribution set during this process. Electrode slurries consisting of graphite, carbon black as well as two different binder systems were applied on a copper substrate by doctor blade and dried homogeneously and isothermally under well-defined boundary conditions in a unique and optimized impingement drying set-up. The drying rates adjusted by the film temperature and the mass transfer coefficient fell within the range applied in industrial drying processes. ... mehrAn experimental approach was developed, which allowed for the identification of critical drying stages, in which, for instance, binder migration occurs. Therefore, the transfer coefficients in the gas phase were systematically changed after defined time intervals during drying. In extensive studies, the adhesive force, which served as an indirect measure for the binder concentration at the copper/film interface, was determined by conducting a 90° peel test. The binder concentration at the film surface as well as the copper/film interface of delaminated films was characterized using energy dispersive X-ray spectroscopy (EDX). A comparison of the results with experimental drying curves revealed critical stages during drying. This valuable knowledge can be incorporated in future designs of customized drying processes for battery electrodes.