Mass Transport in the Stefan–Knudsen Transition Region during Vacuum Drying at Different Pressures in a Porous Structure Resembling Battery Electrodes

A precise understanding of the mass transport kinetics of water inside the porous structure of battery electrodes is crucial to understanding and optimizing their post-drying process. This process and the moisture management during the production of Li-ion battery electrodes adjust and remove residual water from electrodes and are cost intensive. Furthermore, the amount of residual moisture in the electrode affects device performance. Mass transport phenomena in the Stefan and Knudsen transition affect these processes. In this manuscript, we investigate the mass transport in the interparticle gas phase of a porous structure gravimetrically by a magnetic suspension balance with a conditioned (humidity, temperature, and pressure) measurement cell. Emphasis lies on the pressure, porosity, and mass transport distance dependency of the desorption process. Comparing experimental data with a simulation of the interparticle gas phase shows that the mass transport close to ambient pressure can be described by Stefan diffusion through the porous structure. The experiments show the significance of Knudsen diffusion during the mass transport toward lower pressure. A proposed diffusion-coefficient model describes the Stefan and Knudsen region with a transition function, taking the mass transport phenomena overlap into account by lower and upper limits as transition values.