Inline Thickness Measurement of Coatings on Battery Foils

The coating material is usually applied to copper or aluminum foils as a liquid called »Slurry«. To ensure optimum battery performance, the coating must meet high requirements in terms of layer thickness and homogeneity. Until now, the industry has used beta and X-ray radiation to determine the layer thickness. These have the disadvantage that they only measure the total thickness, i.e. the film including the coating, and therefore require differential measurements before and after the individual process steps. This means that the measurement errors add up drastically over the course of the process, especially as measurements are not always taken at the same strip position.

New Sensor Technology Makes Flexible

Our terahertz technology measures in reflection and thus directly records the thickness of the coating; it takes less than five milliseconds to perform a measurement. The new sensor technology can be flexibly adapted to the respective task in terms of the number and position of the measuring heads and the measuring frequency. Optimization for frequencies between 50 GHz and 1 THz, frequency stabilization through improved driver electronics and adapted signal processing enables precise thickness measurements on thin, highly absorbent and electrically conductive layers.

While cathodes are characterized by low conductivity, anodes have a significantly higher conductivity, which makes them difficult to measure. With our photonic terahertz FMCW systems, which are specially optimized for this application, we are nevertheless able to generate precise information from the films, which are several 10 μm thick.

There are a number of stages in the production process of both cathodes and anodes where terahertz sensors can play a role in process control and scrap reduction (see figure). An optimal in-line configuration could measure the coating thickness and feed back these parameters to control the coating speed and gap. Of additional value are these measurements both before and after drying and calendering (densification). The coating thicknesses measured with terahertz at the various process steps are therefore the decisive factors for controlling and optimizing the coating process.

New Sensor Technology Makes Flexibledemonstration of Inline Coating Thickness Measurement

Terahertz measurement technology has successfully proven itself on an industrial roll-to-roll coating system at the Center for Digitized Battery Cell Production at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart.

This coating system already has laser-based and capacitive distance sensors for indirect coating thickness measurement - both in the wet (L1 / C1) and dry state (L2 / C2), which have typically been used for this application to date. The additional integration of a terahertz sensor allows the different measurement methods to be compared directly during the coating process.

In the field test, around 22 meters of film were coated with lithium iron phosphate (LFP) - a frequently used cathode material. The film thickness can be varied by adjusting the pump output at the slot die and the distance between the die and the film. However, short-term changes to these parameters can lead to film breaks or strong inhomogeneities, which explains the outliers in the measurement curve.

The entire coating process can be divided into seven phases:

• The first and seventh phase: film uncoated
• Phases two to six: Application of LFP with different layer thicknesses

It is noticeable that the measurements at L2 and C2 drift in the course of the measurements and show a clear deviation of approx. -20µm at the end of the measurement series (phase seven). This uncontrolled drift is a frequent and critical problem with indirect measurement methods. In comparison, the deviation to the terahertz measurement is explained by the fact that this direct method does not require re-referencing. The changes in layer thickness between the phases are clearly recognizable.

The subsequent calendering (densification) of the film reduces the surface roughness and improves the electrical properties of the cathode. The film thickness also decreases as a result of this process.

Several manufacturers are already working on the dry coating of battery films in order to shorten the drying process. This results in a significant reduction in CO2, which improves the environmental balance of the production process. The use of terahertz measurement technology is particularly advantageous here, as beta and X-ray radiation cannot be used due to the process requirements.

Advantages of our Measurements  

• Anodes and cathodes can be measured
• Can be used in both wet and dry processes
• Continuous monitoring of production
• Ensure product quality

Technical Specifications  

• Contactless and non-destructive
• Measuring range from 10 μm to 300 μm
• Measurement rate up to 1 kHz
• Reproducibility better than 1 μm