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

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

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

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

Production of Nickel‐Rich Cathodes for Lithium‐Ion Batteries from Lab to Pilot Scale under Investigation of the Process Atmosphere

The selection of an appropriate cathode active material is important for operation performance and production of high-performance lithium-ion batteries. Promising candidates are nickel-rich layered oxides like LiNi$_x$Co$_y$Mn$_z$O$_2$ (NCM, x+y+z=1) with nickel contents of ‘x’ ≥ 0.8, characterized by high electrode potential and specific capacity. However, these materials are associated with capacity fading due to their high sensitivity to moisture. Herein, two different polycrystalline NCM materials with nickel contents of 0.81 ≤ ‘x’ ≤ 0.83 and protective surface coatings are processed in dry-room atmosphere (dew point of supply air T$_D$ ≈ −65 °C) at lab scale including the slurry preparation and coating procedure. In comparison, cathodes are produced in ambient atmosphere and both variants are tested in coin cells. Moreover, processing at pilot scale in ambient atmosphere is realized successfully by continuous coating and drying of the cathodes. Relevant electrode properties such as adhesion strength, specific electrical resistance, and pore-size distribution for the individual process steps are determined, as well as the moisture uptake during calendering. Furthermore, rate capability and cycling stability are investigated in pouch cells, wherein initial specific discharge capacities of up to 190 mAh g$^{−1}$ (with regard to the cathode material mass) are achieved at 0.2C

Vakuum-Nachtrocknung und Wassersorption von Elektroden für Lithium-Ionen-Batterien

Bei der Steigerung der Leistungsfähigkeit und Sicherheit von Lithium-Ionen-Batterien (LIBs) stellt insbesondere der Eintrag von Wasser während der Produktion eine große Herausforderung dar. Zu hohe Restfeuchten in LIB-Zellen können zu drastisch eingeschränkter elektrochemischer Performance führen und bergen ein großes Sicherheitsrisiko, weshalb die Zellkomponenten vor dem Zellbau nachgetrocknet werden müssen. Obwohl die Nachtrocknung sehr energieaufwändig ist und einen großen Einfluss auf die Produktqualität hat, ist sie nach wie vor nicht im Detail erforscht und verstanden. Die vorliegende Arbeit hat deshalb ein besseres Verständnis und die Optimierung des Vakuum-Nachtrocknungsprozesses von LIB-Elektroden sowie die Erforschung des Wassersorptionsverhaltens der Elektroden zum Ziel. Für ein tiefgehendes Prozessverständnis muss die Prozesstechnik über die mikroskopische Elektrodenstruktur mit den makroskopischen Elektrodeneigenschaften korreliert werden. Die im Rahmen dieser Arbeit durchgeführten Untersuchungen basieren deshalb auf dem Konzept der Prozess-Struktur-Eigenschafts-Beziehungen. Die Ergebnisse zeigen, dass hohe Nachtrocknungsintensitäten zwar einerseits niedrige Restfeuchtegehalte erzielen, andererseits aber die Mikrostruktur der Elektroden und ihre mechanischen, elektrischen und elektrochemischen Funktionseigenschaften schädigen können. Die elektrochemische Performance wird also nicht nur durch den Restfeuchtegehalt, sondern ebenfalls signifikant durch die Nachtrocknungsintensität beeinflusst. Aufbauend auf den identifizierten Prozess-Struktur-Eigenschafts-Beziehungen wird ein allgemeines Verständnis für den Prozess der Vakuum-Nachtrocknung ganzer Elektrodencoils erarbeitet und eine vergleichsweise moderate II-Phasen-Vakuum-Nachtrocknungsprozedur konzipiert und erfolgreich praktisch angewandt. Für ein besseres Verständnis des Sorptionsverhaltens von LIB-Elektroden werden zudem der Einfluss der Mikrostruktur, des Aktivmaterials und des Taupunkts beim Zellbau auf die Wassersorption untersucht und daraus Prozess-Struktur-(Eigenschafts-)Beziehungen abgeleitet. Die im Rahmen dieser Arbeit identifizierten Prozess-Struktur-(Eigenschafts-)Beziehungen bei der Nachtrocknung und dem Wassersorptionsverhalten von LIBs tragen maßgeblich zu einem besseren Prozessverständnis bei. Sie bilden die Grundlage für eine wissensbasierte Auslegung des Nachtrocknungsprozesses sowie des Feuchtemanagements über die gesamte Prozesskette und leisten somit einen wesentlichen Beitrag zur Energie- und Kosteneinsparung sowie Steigerung der Produktqualität und -sicherheit von LIBs.In order to increase the performance and safety of lithium-ion batteries (LIBs), the introduction of water during production, in particular, poses a major challenge. Excessive residual moisture in LIB cells can lead to drastically reduced electrochemical performance and represents a high safety risk. Therefore, the cell components must be post-dried prior to cell assembly. Although post-drying is energy-intensive and has a major impact on the product quality, it is still not well-researched and understood in detail. Therefore, the work presented here aims at better understanding and optimizing the vacuum post-drying process of LIB electrodes, as well as investigating the water sorption behavior of the electrodes. For better process understanding, the process technology must be linked with the microscopic electrode properties through the macroscopic electrode structure. The investigations carried out in this work are therefore based on the concept of process-structure-property relationships. The results show that although high post-drying intensities achieve low residual moisture contents on the one hand, they can damage the microstructure of the electrodes and their mechanical, electrical and electrochemical functional properties on the other hand. The electrochemical performance is therefore not only influenced by the residual moisture content, but also significantly by the post-drying intensity. Based on the identified process-structure-property relationships, a general understanding of the vacuum post-drying of whole electrode coils is developed. A comparatively moderate II-phase vacuum post-drying procedure is designed and successfully applied in practice. For a better understanding of the sorption behavior of LIB electrodes, the influence of the microstructure, the active material and the dew point during cell assembly on water sorption is investigated, and process-structure(-property) relationships are derived. The process-structure(-property) relationships generated in this work, with regard to post-drying and the water sorption behavior of LIBs, contribute significantly to a better process understanding. They form the basis for the knowledge-based design of the post-drying process and the moisture management throughout the entire process chain, and thus make a significant contribution to saving energy and costs and to increasing the product quality and safety of LIBs