Direct Coating of Separator on Electrode Foils
Authors: Wuersig Andreas , Durst Franz , Dreger Henning , Zink Oliver 
Published: February 2020

The manufacturing process of lithium cells is a complex series of chemical and physical manufacturing and assembly steps. The value chain for lithium cells and batteries is divided into three production areas:

electrode coating line (mixing, coating and compaction )

cells production line

battery production line

As part of a project funded by the German government S-Protrak was focusing especially on the electrode coating and parts of the cell production (separator). The developed technologies were than integrated in prototypes. For the process development existing test facilities were used in order to minimize the material costs and the expense of production integration in the available project period.  Each of the project partners worked on different tasks. Jonas & Redmann developed and evaluated system concepts and important characteristic data for inspection technology during coating processes. FMP developed solutions for integrating coating and drying systems optimized for the installation space based on diffusion-optimized convection drying. The Fraunhofer ISIT designed procedures for the direct coating of separators on electrode foils using existing patents, and the Technical University of Braunschweig focused primarily on the continuous manufacturing of electrode pastes using extrusion to increase economic and environmental efficiency.  Headed by the Fraunhofer Institute for Silicon Technology (ISIT) the S-Protrak consortium developed technologies for industrial production of cost-effective high-quality lithium cells and manufacture prototypes of these technologies. The projects plays an important role in the development of battery-powered electric vehicles and, through their work and research results, help implement the German federal government’s national development plan for electromobility and establish a German battery supplier market.

Lithium batteries have undergone a significant expansion of application fields in recent years. Besides its use in electric and hybrid vehicles, this also includes stationary energy storage systems. The cell capacity has at the same time grown significantly and is now typically between 20 and 40 Ah. This is accompanied by significantly increased demands on manufacturing quality and a strong price pressure. The separator concept developed in this project takes this into account by focusing on the improvement of the quality (waste reduction) and cost reductions especially with increasing production efficiency. In addition, a significant improvement in the cell performance was expected. This can establish other fields of applications, for instance starter batteries.

Core of this innovation is a direct coating of the separator on the electrode foil. This can be done either in a wet on wet process (WoW) directly on the undried electrode foil or wet on dry (WoD) after the electrodes has dried. For the development of this process, enhancements of existing coating technology, especially for the coating head as well as for the separator slurry formulation were necessary.

The basic separator concept was developed and patented by Fraunhofer ISIT already in 1999. It consists of a lithium conductive ceramic that is bound by a PVDF polymer. Acetone is used as a solvent. In order to be used for a direct coating process on electrodes the slurry formulation had to be adapted, especially by increasing the viscosity without losing its performance. Therefore the mixing procedure had the be changed. Using the coating facility of Fraunhofer ISIT and a slot die coating system from FMP both processes (WoW and WoD) were developed and different coated electrode foils were produced.

With these foils test cells (full and half cells) were assembled for the electrochemical characterization of the electrodes. It was possible to show a stable cycling behavior for the WoW as well as the WoD process. It is therefore possible to use this technology, especially WoW, in order to enhance the production efficiency and to reduce the costs.

Figure 1