34 This is carried out in a dry room with a dewpoint of -40 C, although even lower temperatures are required for more moisture-sensitive chemistries, such as NMC811 or lithium metal. At this stage, electrodes are cut and assembled in their cases. Although this process varies according to the cell format (pouch, prismatic or cylindrical), there are three main activities in this block. Notching For pouch cells, the next step is a cutting process that converts the coated rolls into individual electrode sheets. The cutting machine (which differs for anode or cathode production) unrolls the foil and produces rectangular electrodes with an uncoated area left, which will be the tabs required later in the assembly. The cutting process can be performed with two types of technologies: mechanical cutting, formed by a die with blades; or laser cutting. While the mechanical system usually reduces the cost, it requires regular sharpening and replacement of the blades. Laser cutting avoids direct contact with the electrodes and offers more flexibility. Stacking Once the sheets are produced, they go through a stacking process, which is usually the trickiest and often a bottleneck in cell assembly. This is the first stage in which the cathode and anode lines are combined. The goal is to alternately stack anode layers – separators and cathodes – while leaving the uncoated tabs exposed. The typical method is Z-stacking, where the separator is folded over each electrode layer in a zigzag movement. Alignment is key, as misalignment can cause the electrodes to extend beyond the separator, creating a short-circuit once the cell is completed. Another alternative is stacking through lamination. This method, similar to single-sheet stacking, joins two of the components together (separator/ anode/separator) and these are later stacked inbetween cathode layers in an alternating manner. Pouch assembly Once stacking is complete, the exposed electrode tabs must be attached to the main terminals through a welding process, which can be laser-based. The cell is then placed into a preformed packing material and sealed, leaving an open edge for the electrolyte filling. After vacuum sealing the remaining edge, the product is left to soak for hours prior to forming, ageing and the test phase. Sensors Sensors are key for an effective manufacturing process. The biggest thing we see is that manufacturers need to monitor the complete line, from the mixing to the coating, to provide continuous data. At the start of the process, the anode or cathode slurry has the correct solid loading, as that is crucial for uniform coatings on the thin foil and ensuring its always of the same thickness. This is best with continuous monitoring or inline, as batch sampling for quality control does not tie in well with knowing what happens during the process. Senors provide data points for the monitoring software to check conditions, and also to change the process and control the amount of liquid in the slurry in real time. The inline viscosity sensors detect the homogeneousness of the slurry in a storage tank as it circulates to avoid sedimentation. The sensor is a balanced, torsional resonator that measures the product of density and viscosity, with the magnitude of the noise in the signal giving the dispersion. The co-axial resonator twists the two ends of the sensors in opposing directions, cancelling out any torque when mounted on the equipment and making the sensors more stable in the mounts to provide more accurate measurements. During the mixing of the slurry, unneeded agitation deteriorates and degrades internal structures with time. The target is to achieve thorough mixing of the constituents with maximum homogeneity and without the particles breaking up. Density control ensures correct material composition and constituent fraction, and viscosity control ensures consistency of the slurry preparation process. A high-viscosity slurry causes problems in the coating process and poor dispersibility results in low film uniformity. Uniformity of the coating thickness and the layer density are crucial to guarantee control over the lifetime (recharge cycle time) and ion-transfer rate of the battery. Regulating the layer thickness Tech focus | Battery cell manufacturing January/February 2025 | E-Mobility Engineering The role of extruders in lithium-ion cell manufacturing (Image courtesy of Thermofisher Scientific)
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