Eddy current methods for inline monitoring of drying, calendering, and slitting

In the manufacturing process of electrodes for lithium-ion batteries, drying is a crucial factor for subsequent cell performance. This process is divided into different drying stages, to which the temperature profile in the drying channel (drying profile) should be continuously adjusted. This prevents defects such as binder migration. Since the electrical conductivity of the electrode material changes during the various drying stages, eddy current testing has been identified as a suitable method for this type of defect.

In recent years, Fraunhofer IKTS has continuously developed its eddy current-based testing systems (EddyCus® Pro-II series). These powerful and modular systems are particularly suitable for integration into the automation process of a production line. Thanks to multi-channel technology across the entire process width, they measure without contact and without damaging the material and capture integral conductivity information in real time due to the very high sampling rates.

© Fraunhofer IKTS

Initial recognizable correlations between the drying stages during the drying of the battery electrodes (top [1]), percolation (bottom left [2]), and the amount of complex impedance from the eddy current signal (bottom right [3]).

Fraunhofer ZESS is testing how the EddyCus® Pro-II testing system can be used to monitor the drying process inline. To achieve this, the sensors are integrated at several different positions in the drying tunnel. Initial tests show promising correlations between the drying behavior and the eddy current signal. In order to significantly increase the information content about the current drying stage, the existing eddy current testing system will be supplemented with a near-infrared (NIR) spectroscopic sensor. This will also allow the residual moisture in the battery electrodes to be determined. Such a combined system is currently under development.

The EddyCus® Pro-II testing system can also be used after drying, calendering, and splitting to detect defects such as layer inhomogeneities (thickness, composition, conductivity) across the entire coating width.

Characteristic features of the modular EddyCus® Pro-II eddy current array are:

• Cascadable system with sensor multiplex rate up to 100 kS/s
• Up to 100 MHz eddy current excitation
• Temperature compensation
• USB 2.0 / RS-485 connection for external device communication
• Integrated incremental encoder inputs
• Analog and digital I/O for integration into existing measurement systems
• Auto-calibration function specifically for inhomogeneous materials
• Customized eddy current array sensor technology
• Measurements possible in transmission or reflection mode, depending on the application

[1] S. Jaiser, L. Funk, M. Baunach, P. Scharfer, W. Schabel, Experimental investigation into battery electrode surfaces: The distribution of liquid at the surface and the emptying of pores during drying, Journal of Colloid and Interface Science 494 (2017) 22–31.

[2] Patsora, Iryna; Hillmann, Susanne; Heuer, Henning; Foos, Bryan C.; Calzada, Juan G.: High-frequency eddy current based impedance spectroscopy for characterization of the percolation process of wet conductive coatings. Annual Review of Progress in Quantitative Nondestructive Evaluation (2014).

[3] M. Wild, A. du Barét, S. Reuber, M. Schulze, Prof. H. Heuer, M. Wolter. Prof. A. Michaelis: Analysis and monitoring of the electrode films during the drying process using eddy current technology. International Battery Production Conference (Conference Presentation), Braunschweig (2019).