Precision, hygiene and safety are crucial

The production of battery cells is subject to the highest quality requirements. Precise adherence to the recipe and the avoidance of contamination are particularly important. In addition, dust emissions must be minimized due to the toxic and highly flammable materials. This is particularly challenging when dosing raw materials.

Powerful and efficient batteries are in demand all over the world. The course for the quality of the cells is set when the raw materials are weighed and dosed. The mixture (slurry) for electrode production must not be contaminated. Conductive particles must not get between the separator foils, otherwise there is a risk of short circuits. The machines must also be dust-tight to prevent materials such as graphite, lithium, nickel, manganese and cobalt from escaping. Last but not least, measuring and dosing accuracy is crucial for product quality. Plant engineers must resolve these conflicting objectives.

Quality standards require compromises  

High dosing accuracy is essential to ensure consistently high product quality. In the mandatory batch processes, the weigher must be very reliable. However, if the hoppers are very large, the accuracy suffers due to the high dead load. If there are vibrations in the area around the machine, the bulk material can drip. Pressure fluctuations can also affect the weighing signal and must be prevented.

Another challenge is to avoid contamination in the slurry. This can be caused by metal particles from machine parts or cross-contamination if, for example, different raw materials are weighed in one container. As moisture is also harmful to product quality, production facilities are difficult to clean. Even incoming air can have a negative effect, as some bulk materials are hygroscopic. By pressurizing the machine, it would theoretically be possible to keep dust and dirt particles away from the slurry. However, measurement accuracy suffers from unwanted pressure fluctuations.

The fact that the substances must not leak out in an uncontrolled way makes refilling the weighing containers in batch processes a particularly challenging task. To minimize the risk of contamination, the air displaced during refilling must not leak out unfiltered. Negative pressure can help to keep contaminants inside the machine. However, it can affect dosing accuracy and encourage moisture ingress.

Plant engineers resolve conflicting priorities

To prevent contamination of the slurry by metal particles, metal-to-metal contact should be avoided when designing the system. This can be achieved by using ceramic or PTFE coatings. Limiting the peripheral speed can help to reduce abrasion. This can be achieved by skillful process design of the feed screw. There should also be no dead space in the system, as material residues can lead to cross-contamination.

Moisture can also cause product damage if, for example, ambient air enters a weighing hopper when the level drops. To protect the bulk solid from moist air, the free space should be continuously filled with dry air or nitrogen. It is also advisable to keep interruptions to the batching process to a minimum. If air escapes from the machine, it must be filtered. The choice of filter depends on the containment concept.

Constant pressure conditions in the system are required to ensure that measurement accuracy is not compromised. Pressure equalization is essential when filling the weighing hoppers. Manufacturers such as Qlar have developed special controllers that coordinate the filling and refilling process and minimize pressure fluctuations. This is vital for quality and safety in production. Inaccuracies caused by trickling can be eliminated mechanically with a flap.

In summary, the requirements for measurement accuracy and containment in battery cell production are challenging but solvable. Design measures prevent metal abrasion. Intelligent control of the filling and refilling processes is crucial for quality. The bulk material must not come into contact with the ambient air. Displaced air must be filtered and pressure fluctuations minimized for the weigh feeder to work accurately. A well-designed control system is the key to success.

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