Test & measurement
the wet active layer, the coating is air-dried. Coated electrode foils pass at speeds of up to 80m/min through a sequence of heated chambers. These have a temperature profile ranging from 50°C up to 160°C where the solvent evaporates and is recovered for later use. As with the previous coating phase, foil tension must be accurately measured before and after the drying oven. This prevents breakages while maintaining sufficient tension as the foil passes through the chambers. A particular requirement is to monitor foil tension with great accuracy at elevated temperatures. Calendering is performed after completing the coating and drying processes. The aluminium or copper foils pass at speeds up to 100m/min between a pair of rotating compression rollers. This process compacts the applied coating layer, ensuring a consistent thickness while reducing its porosity. It directly influences the electrodes’ wetting properties when the electrolyte is subsequently introduced.
A precisely defined line pressure of up to 2,500N/mm is required between the rollers for calendering. This is measured using strip tension cells. If compaction pressure between the rollers is too high, squeezing of the substrate and its coating can lead to the formation of stress cracks. After coating, drying, and calendaring, each
‘mother’ foil roll is cut into several narrower ‘daughter’ rolls, typically with a width of between 60mm and 300mm according to the battery’s design and intended use. The slitting process is conducted with
rolling knives or a laser - and typically sees the foils carried at even higher linear speeds up to 150m/min. To maximise battery quality, each daughter roll must be cut precisely, with no burring on the cut edges.
Accurate web tension control needs to be maintained throughout the slitting and rewinding processes, before the cut and cleaned daughter rolls are then vacuum dried to expel residual moisture and any remaining solvent traces in preparation for cell assembly.
CELL ASSEMBLY
Accurate tension control is also critical to ensuring correct alignment during the cell assembly stage. As dictated by the type of battery being produced, anode and cathode foils are either wound or stacked together. Winding is commonly used for producing cylindrical batteries. Here, the anode overlays the cathode, with the insertion of a microporous separator sheet between the two electrodes. Accurate alignment and tension control minimise the risk of issues such as short- circuits and electrode fracturing. An alternative assembly technique is stacking, where the anode and cathode sheets
are alternately laid on top of each other with the laminated separator between them. As with winding, assurance of quality and reliability of the finished battery relies on accurate control of both alignment and tension throughout the cell assembly process.
Accurate monitoring of tension, flatness and thickness is critical to maintaining production capacity and continuity, as well as contributing directly to the safety and performance of the finished product that powers hundreds of millions of devices every day. Across all these procedures, force measurement solutions must combine high accuracy with minimised calibration and maintenance requirements.
ABB
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Instrumentation Monthly March 2024
29
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