As moisture presents a critical contamination in lithium-ion batteries (LIBs), electrodes and separators need to be post-dried before cell assembly. The moisture adsorption, desorption and re-adsorption of electrodes during processing is strongly dependent on their material system, manufacturing route and microstructure. The microstructure, in turn, is significantly defined by the coating density, which is adjusted by calendering. As a consequence, the calendering step is expected to directly influence the moisture sorption behavior of electrodes. This is why the influence of different coating densities and structural properties on the moisture content of NCM622 cathodes was investigated in this study. For increasing density, an increasing moisture content was detected by Karl Fischer Titration and sorption measurements. SEM and BET analyses showed an increasing amount of NCM622 particle breakage, accompanied by a rising surface area. Hence, the increased moisture uptake of cathodes with higher density is mainly caused by a higher surface area, which results from particle cracking and breakage during calendering. Electrochemical analysis showed that the increased active surface area of cathodes with higher densities leads to a good performance during formation and at low C-rates. However, the reduced porosity impairs the ionic conductivity and causes capacity loss at higher C-rates.

/pdf/increased-moisture-uptake-of-ncm622-cathodes-after-3ooyjp4kpy.pdf

Increased Moisture Uptake of NCM622 Cathodes after Calendering due to Particle Breakage

Calendering is the state-of-the-art process for electrode compaction in lithium-ion battery manufacturing through which the final electrode structure is defined. As the electrode structure determines the electrical und electrochemical transport properties, it is essential to develop a deep understanding of the interdependence between the calendering process and the deformation behavior of the electrodes. Therefore, a novel method for the investigation of the deformation behavior of electrodes, especially the springback (SB) effect, through direct-gap detection within the calendering machine, is developed. With this installation, it is possible to directly access the maximum compression of the electrodes during calendering. With this approach, two different variations of cathodes, a variation of mass loading and a variation of binder weight content, based on lithium nickel manganese cobalt oxide (NMC622), are manufactured and investigated. Moreover, an empirical model is introduced, allowing the estimation of the SB for NMC622 cathodes considering different mass loadings and cathode compositions. Due to more space for particle rearrangement, cathodes with higher mass loading show a lower SB effect. Adding binder leads to higher plastic deformation and thus lowers also the SB effect of the electrodes. 

Evaluation of deformation behavior and fast elastic recovery of lithium‐ion battery cathodes via direct roll gap detection during calendering

In the past years the research and development in lithium-ion batteries has continuously grown in various fields. The possible negative effect of moisture content has been investigated and it is given a key importance for a high quality product. This research requires the reliable detection of the water content. A commonly used method is the indirect coulometric Karl Fischer Titration (icKFT). The correct usage of the icKFT in lithium-ion battery technology is urgent to detect correct measurement results. This article describes parameter settings, handling issues and possible mistakes, which need to be understood while using the icKFT. The disclosure of fundamental information about the measurement process like stop criterion, shell temperature, environmental conditions or others mentioned in the article need to be provided to compare and evaluate measurement results of icKFT. As a practical application the effect of coating compaction due to calendering on the moisture content of anode and cathode material is investigated. It could be shown that the calendering process directly influences the detected water content of the electrodes. The results can be helpful for a better understanding of possible sources of water contamination and process design for electrode production. 

Correct water content measuring of lithium-ion battery components and the impact of calendering via Karl-Fischer titration

Investigation on mechanical and microstructural evolution of lithium-ion battery electrode during the calendering process

For batteries with high energy density and good fast-charge capability, NCM cathode active materials with ≥80 mol% nickel are promising due to their high specific capacities. Unfortunately, the increase in nickel content is accompanied by a high susceptibility to moisture. Therefore, nickel-rich NCM is coated or doped by the manufacturers to increase its stability. However, it is unclear if special requirements regarding ambient humidity must still be met during the whole production chain, or only after post-drying and during cell assembly. Therefore, the structure and properties of three different nickel-rich NCM active materials (one doped monocrystalline, two coated polycrystalline materials) processed at ambient atmosphere were investigated. At every process step, moisture content and microstructure were examined. Prior to cell assembly, two different post-drying procedures were applied and investigated. As validation, electrochemical tests were performed. Both polycrystalline cathodes demonstrated good physical and electrochemical properties, despite the ambient process atmosphere. Higher moisture reduction led to improved electrochemical performances at higher C-rates. Finally, a comparison between dry and normal atmosphere of the best performing material indicates that a production of high-quality nickel-rich electrodes at ambient atmosphere is possible if their exposure to moisture is short and well-designed post-drying techniques are applied.
 
 
 
 


https://iopscience.iop.org/article/10.1149/1945-7111/ac7358/pdf

Investigation of Moisture Content, Structural and Electrochemical Properties of Nickel-Rich NCM Based Cathodes Processed at Ambient Atmosphere

In order to reduce the residual moisture in lithium-ion batteries, electrodes and separators need to be post-dried prior to cell assembly. On an industrial scale, this is often conducted batch-wise in vacuum ovens for larger electrode and separator coils. Especially for electrodes, the corresponding post-drying parameters have to be carefully chosen to sufficiently reduce the moisture without damaging the sensitive microstructure. This requires a fundamental understanding of structural limitations as well as heat transfer and water mass transport in coils. The aim of this study is to establish a general understanding of the vacuum post-drying process of coils. Moreover, the targeted design of efficient, well-adjusted and application-oriented vacuum post-drying procedures for electrode coils on the basis of modelling is employed, while keeping the post-drying intensity as low as possible, in order to maintain the sensitive microstructure and to save time and costs. In this way, a comparatively short and moderate 2-phase vacuum post-drying procedure is successfully designed and practically applied. The results show that the designed procedure is able to significantly reduce the residual moisture of anode and cathode coils, even with greater electrode lengths and coating widths, without deteriorating the sensitive microstructure of the electrodes.

Design of Vacuum Post-Drying Procedures for Electrodes of Lithium-Ion Batteries

In this manuscript, a method to reduce superelevations of lateral edges in cross-web direction during slot die coating of shear-thinning slurries for Li-ion battery electrodes (LIB) was developed. Therefore, the impact of the inner slot die geometry on the edge elevations was investigated. These elevations of the coating could be almost eliminated by optimizing the flow profile at the outlet of the slot die by modification of the internal geometry. This adaption is an essential step in optimizing the coating quality of slot die coating for battery electrodes to significantly reduce coating edges and, hence, the resulting production reject during the coating step of the industrial roll-to-roll process. It was also shown that lateral edges of the coating can be influenced explicitly by process parameters such as volume flow and gap between slot die and substrate. This correlation has already been shown for other shear-thinning material systems in previous works, which is now confirmed for this material system. At the beginning, the influence of different internal geometries on the formation of the edge elevations was shown. Finally, for the shear-thinning electrode slurry used in this work, optimal dimensions of the previously determined inner geometry for the slot die outlet were found. The optimization was performed for a state-of-the-art electrode area capacity (approximately 2.2 mAh cm−2). The results enable a significant reduction of defects and reject in the coating step of large-scale production of LIB electrodes in the future, adding to a more sustainable battery production.

/pdf/investigation-of-edge-formation-during-the-coating-process-c9vi0iok28.pdf

Thilo Heckmann

Papers

Increased Moisture Uptake of NCM622 Cathodes after Calendering due to Particle Breakage

Design of Vacuum Post-Drying Procedures for Electrodes of Lithium-Ion Batteries

Investigation of edge formation during the coating process of Li-ion battery electrodes