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.
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.
The drying of electrodes is a crucial and often limiting process step in the manufacturing chain of lithium-ion batteries. [1] While the coating step can be carried out at high coating speeds, as shown by Diehm et al., the application of high drying rates still challenges the throughput in electrode production. [2] High energy demand on the one hand and drying condition-dependent electrode properties on the other demand the necessity of an optimally conducted drying process. [3,4] The negative influence on the product properties thereby primarily occurs at high drying rates. [5] The application of high drying rates can lead to a migration of additives like binder and carbon black to the surface of the electrode layer, resulting in low adhesion between active material and current collector foil. [6][7][8][9][10] Furthermore, the electrochemical performance deteriorates compared with gently dried electrodes due to the accumulation of insulating binder and carbon black at the surface, resulting in an increased electrical resistance. [6,11] Binder migration is attributed to the capillary pressure-driven induction of concentration differences during the emptying of the porous electrode structure, leading to an inhomogeneous distribution of components over the electrode height. [5] Nevertheless, it has been shown that these effects can be compensated for by selecting suitable process parameters. Isothermal drying tests indicate that a high film temperature and a low heat transfer coefficient can have a positive effect on adhesion, therefore indicating a more homogeneous distribution of electrode components. [9] The authors reasoned that binder diffusion processes could take place to compensate for concentration differences caused by capillary pore emptying to some extent. Due to the proportionality of diffusion coefficient and temperature, as well as the antiproportionality of viscosity and temperature, higher electrode temperatures increase binder mobility and therefore adhesion. [6,9,11] In addition, the authors illustrate the influences and limitations regarding electrode temperature and drying rate in the convective drying process, which are determined by air temperature, convective heat transfer coefficient, and humidity. Based on an enthalpy balance for double-sided heat input, a maximum electrode temperature of about 50 °C is determined during the evaporation process, while the air temperature was set to 150 °C, the dew point to 15 °C, and the convective heat transfer coefficient to 80 W m À2 K À1 , resulting in a drying rate of 6.15 g m À2 s À1 . [9] These limitations in terms of electrode film temperature are due to the enthalpy transferred with the evaporation flow, which cools the electrode layer below the temperature of the supplied air.
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.
Lithium‐ion batteries are state‐of‐the‐art and still their performance is subject to constant improvement. These enhancements are based, among other things, on optimization in the electrode production process chain. High optimization potential exists for the drying process of electrodes, as aiming for high drying speeds can greatly reduce both, investment costs and operating costs of the drying. However, high drying rates without appropriate precautions go hand in hand with poorer cell performance and adhesive strength, leading to a conflict between the required performance and production costs of the electrodes. Herein, a numerical approach based on the discrete element method to describe the formation of the electrode structure during drying is presented. The focus is placed on the active material structure and the effects due to particle interactions. Herein, a direct numerical description of the fluid phase is avoided by using various fluid substitute models, so that the simulation time and the computational costs can be greatly reduced. The model is validated by simulating different electrode areal loadings and comparing the achieved layer thicknesses to experimental results of the electrode drying process. A high agreement between experiment and simulation regarding density is obtained for different areal loadings.
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