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.

Hysteresis Behavior in the Sorption Equilibrium of Water in Anodes for Li-Ion Batteries.

Hysteresis in the sorption equilibrium influences the production process of many multicomponent material systems. Electrodes for Li-ion batteries consist of several materials, some of which exhibit hysteresis in their sorption equilibrium with water. The moisture content adsorbed and absorbed in the electrodes of the Li-ion battery turned out to be an issue for its electrochemical performance and is reduced in the post-drying process. During this process, hysteresis in the sorption equilibrium needs to be overcome in order to achieve a low residual moisture content of the electrode. Modeling the post-drying process requires a description of the sorption equilibria of water in the components of the battery. This paper builds on previous research about the sorption equilibria and examines the hysteresis behavior of typical graphite anodes, with the active material graphite, carbon black as the conductive additive, and sodium carboxymethyl cellulose as well as styrene butadiene rubber as polymeric binders. Moreover, the mechanisms for the occurrence of hysteresis are presented, and how sorption equilibria during drying can be described is shown by applying models from the literature on the materials of battery electrodes. Theoretical deliberations on hysteresis mechanisms are validated, investigating graphite anodes of different material compositions and their materials.