• Energy Research

When fabricating battery electrodes, their properties are strongly determined by the adjusted drying parameters. This does not only affect their microstructure in terms of adhesion, but also influences cell performance. The reason is found to be the binder transported to the surface during drying. Herein, it is shown that when thicker electrodes are processed, new challenges arise. On the one hand, loss of adhesion associated with certain drying conditions becomes a more serious problem; on the other hand, cracking occurs at a certain drying rate and with increasing electrode thickness.

Abstract Produktionslinien für Batterieelektroden bestehen aus hoch produktiven aber starren Prozessen. So können Änderungen nur langsam umgesetzt werden, und eine Reaktion auf neue Bedingungen ist kaum möglich. Im Projekt ReFlexBat (Regelbare Beschichtungseinheit für die flexible Beschichtung von Batterieelektroden) wird deshalb eine autonome Beschichtungseinheit entwickelt, um den Anfahrprozess zu beschleunigen, auf neue Randbedingungen zu reagieren und allgemein den Ausschuss im Beschichtungsprozess zu reduzieren. Bisher erfolgte zum einen die Erarbeitung des Konzepts, zum anderen wurde die notwendige Präzision zum Erreichen der Qualitätsziele bestätigt.

Understanding and reducing edge elevations at the lateral edges are crucial aspects to reduce reject rates during electrode production for lithium‐ion batteries (LIB). Herein, different process conditions to reduce edge elevations at the lateral edges of water‐based, shear‐thinning coatings in the production of LIB electrodes are presented. The reduction of edge elevations is transferred from state‐of‐the‐art electrodes to high‐capacity electrodes. The developed process configuration greatly reduces reject caused by cutting off the edge areas in the industrial roll‐to‐roll process for electrode production. Compared with state‐of‐the art electrodes, the reject rate for high‐capacity electrode production is significantly higher because the edge geometry in crossweb direction of the electrodes is wider. An optimization can be achieved by a combined adjustment of the coating gap and the slot‐die angle to the substrate (angle of attack) to affect the pressure field in the coating bead. Therefore, a systematic investigation and optimization of these process parameters are presented. In addition, the investigation of the process stability of the coating is required. Based on this optimization, a reduction of edge elevations for high‐capacity electrode coatings (5 mAh cm−2) of 69% and ultrathick high‐capacity electrode coatings (7 mAh cm−2) of 48% is possible.

AbstractIn the battery industry, very thin primer layers are used to improve electrode adhesion on substrates or act as blocker layers to prevent corrosion in case of aqueous cathodes. For these material configurations, high-speed coating is mandatory to ensure the economic viability of the process. One way to realize high-speed coating is a set-up including a slot die and a vacuum box to stabilize the coating bead. Knowledge and prediction of the coating window of thin wet film thicknesses is crucial to design the production process. Therefore, the influence of coating gap and viscosity of shear-thinning fluids on the coating window is investigated with the help of various model fluids. In addition, a prediction model for the calculation of the coating window for high-speed slot-die coating with vacuum box is developed. This model is shown to be valid for the prediction of the coating window for the investigated material systems and coating gaps over the investigated range of coating speeds up to 500 m min−1. For a material system, which corresponds to a real material system for adhesive primer coatings, it is possible to reach a target wet film thickness of 20–25 µm. This would correspond to a layer thickness of 0.5 µm for a solid content of 2–2.5 wt%.

A reduction of the inactive components can increase the energy density and reduce production cost of Li‐ion batteries. But an effective reduction of the binder amount also negatively affects the adhesion of the electrode. Herein, slot‐die coating of a primer layer for Li‐ion anodes is investigated. It is shown that the use of a primer layer with only 0.3 g m−2 can increase the adhesive force by the factor of 5 as well as the cell performance for anodes with low binder content. The process limits for a stable, defect‐free primer coating are investigated at coating speeds of up to 550 m min−1. The limits coincide both for a setup without vacuum box and with vacuum box with theory‐based equations. By using a vacuum box, the minimum wet film thickness can be reduced by half.

Porous, nanostructured particles ensure the wetting of electrolyte up to the particle core and shortened diffusion paths, which is relevant not only for lithium‐ion batteries but also for postlithium systems like sodium‐ion batteries. The porous structure leads to a high C‐rate capability. However, compared to conventional compact NCM, porous NCM shows a reduced adhesion force but no or only slight negative influence on C‐rate capability by binder migration at higher drying rates. Herein, a multilayer concept is used to increase the adhesion force with equal or better electrochemical performance compared to single‐layer electrodes. Compact particles of high volumetric energy density and porous particles with high C‐rate capability are combined in a simultaneously coated multilayer electrode. Multilayers with compact NCM toward the current collector and porous NCM with reduced binder content toward the separator side show an about 16‐times higher adhesion force at lower drying rate and an about ten‐times higher adhesion force at increased drying rate compared to electrodes produced of porous NCM only. The specific discharge capacity of the multilayers is increased by 88% at the lower and 67% at the higher drying rate for a discharge rate of 3C compared to a single layer with compact NCM.

The drying of electrodes represents a critical process step in the production of lithium‐ion batteries. In this process step, unfavorably adjusted drying conditions can result in deteriorated electrode properties. Furthermore, the process speed is restricted by limited heat and mass transfer in purely convective drying. To counteract those effects, energy input by near‐infrared (NIR) radiation is a promising approach. Herein, analytical considerations are carried out to demonstrate the suitability of infrared radiation with regard to achievable electrode temperatures and drying rates. In an experimental approach, aqueous processed graphite anodes are dried with an NIR module, varying the power and the amount of convection for different experiments. The temperature profiles of the electrodes and the drying rates are measured and analyzed, and the electrodes are subsequently characterized using adhesion measurements. The results obtained show that energy input by NIR radiation during the drying of electrodes can lead to an increased drying speed, electrode temperature during drying, and adhesion force of the dry electrode. These findings indicate that the binder distribution during NIR drying is advantageous in terms of electrode adhesion, compared with convectively dried electrodes produced at comparable drying rates, positioning the process promisingly with regard to high throughput rates.