Abstract
Moisture is considered to be one of the most critical contaminants in Lithium-ion batteries (LIBs), as it impairs the electrochemical performance and poses high safety risks. To reduce the moisture in the cells below a critical level, electrodes and separators for LIBs need to be post-dried before cell assembly. However, the moisture adsorption, desorption and re-adsorption behavior of electrodes is defined by the chosen material system, the manufacturing route and the microstructure of the coatings. The microstructure, in turn, is significantly determined by the calendering step, which predominantly aims at increasing the specific density and consequently, the volumetric energy density of the electrode coatings. As the calendering step decisively changes the microstructure of the electrode coatings, it can be assumed that it accordingly must have an impact on the moisture sorption behavior of the electrodes during further processing. The sorption equilibrium of electrode coatings mainly depends on the surface of the particles, the sorption in the binder system and, to a certain extent, on the capillary condensation. LIB cathodes are most commonly manufactured with polyvinylidene fluoride (PVDF) as a binder. PVDF is very hydrophobic, which is why we can assume that the moisture sorption of these cathodes is fundamentally determined by sorption on the surface and, to a lesser degree, on capillary condensation. In contrast, the moisture sorption of LIB anodes produced with a carboxymethyl cellulose/styrene-butadiene (CMC/SBR) binder system is assumed to be mainly defined by moisture sorption of the very hygroscopic binder system. Changes to the surface should only have a small influence.
Hence, it is expected that the impact of the calendering step on the moisture sorption is far more pronounced for the cathodes than for the anodes. To prove this theory and analyze the impact of the calendering step in detail, the influence of different densities on the moisture sorption behavior, microstructural properties and electrochemical performance of NCM622 cathodes with PVDF binder and graphite anodes with CMC/SBR binder system is investigated. Therefore, the electrodes are calendered to three different densities in dry room atmosphere, post-dried in vacuum and assembled into coin cells.
Focusing on the cathode, after calendering an increasing moisture content is observed for increasing density, measured by Karl Fischer Titration and proved by sorption measurements. Although post-drying significantly reduces the moisture content of the cathodes, the course trend is still the same with the lowest moisture at a low density and the highest moisture at high density. Via SEM analysis, a rising amount of active material particle cracks is detected with increasing density. BET measurements show that this phenomenon is accompanied by an increasing specific surface area. Hence, the increased moisture uptake of cathodes with higher density can be mainly traced back to their higher specific surface area, caused by particle cracking during calendering. Further analytics display that the chosen post-drying parameters lead to an increase of cohesion strength for all cathodes, whereas the influence on the electrical resistance depends on the density of the cathode. The electrochemical testing shows that the increased electrochemically active surface area of the cathodes with higher density results in a good performance during formation and at lower C-rates. However, the reduced porosity lowers the ionic conductivity and leads to capacity losses at higher C-rates. [1]
Concerning the anode, as expected, the influence of the calendering step on the moisture sorption is much smaller. Although the specific surface area of the anodes also increases with rising density, a significant increase of moisture is not observed. Hence, the influence of the changes in microstructure is overlapped by the hygroscopic binder system of the anodes. Post-drying increases the adhesion strength of all anodes, whereas the electrical resistance seems to decrease to a small degree. The electrochemical analysis of the anodes shows a deterioration of performance with increasing density, due to limitations of the ionic conductivity at lower porosities.
Altogether, we will show that for NCM622 cathodes with PVDF binder, the moisture uptake increases with increasing density after calendering, whereas there is no change in moisture content at graphite anodes with CMC/SBR binder system. These findings make an important contribution to a better process understanding and consequently, improved moisture management along the entire process chain of Lithium-ion batteries.
[1] F. Huttner, A. Diener, T. Heckmann, J. C. Eser, T. Abali, J. K. Mayer, P. Scharfer, W. Schabel, A. Kwade, Increased Moisture Uptake of NCM622 Cathodes after Calendering due to Particle Breakage, J. Electrochem. Soc. 168 090539 (2021), doi.org/10.1149/1945-7111/ac24bb, CC BY 4.0