Optimization of electrolyte filling by surface modification of separators and electrodes
The filling of lithium-ion batteries with liquid electrolyte still represents a significant bottleneck in the production chain of lithium ion batteries. Uniform electrolyte distribution is decisive for cell performance and is currently being realized by a multi-stage filling and subsequent storage for several hours up to days. The aim of the joint project “Optilyt” of Fraunhofer IKTS and Leibniz Institute of Polymer Research IPF was therefore, first of all, to develop suitable measuring set-ups allowing to extend the knowledge about this filling process based on reliable and reproducible results. The second goal as well as challenge was to reduce the real filling time by developing suitable modification methods for the main components of lithium ion batteries (anode, cathode and separator).
Within the project, two measurement methods (tensiometric, electrical) that allow to monitor the filling behavior with electrolyte were used and tailored. The tensiometric method with a special measuring set-up followed the penetration of model electrolytes into all components of the cell (anode, cathode, separator), and into model stacks (anode/separator/cathode). The penetration rate was quantitatively described using the Washburn equation and the differences between the components were described by the parameters of the Washburn equation. On the other hand, a chronoamperometric method allowed to follow the filling of a lab size pouch cell with electrolyte. It was shown that the current curve characteristics were representative for the electrolyte wetting process. Both methods yielded comparable results although they used different basic concepts. Structure-property relationships could be derived.
In addition, strategies for morphological and chemical modification of the individual components were developed and studied aiming to influence the surface of the cell components and the 3D pore network to accelerate the electrolyte wetting. In the morphological modification, structures were introduced into the individual components which can act as capillaries during the filling process. Thus, the filling time could be significantly reduced. The grafting of polar monomers onto separators by either in situ or subsequent grafting did not yield significant improvements in the wetting properties, whereas the coating of the electrodes with oxides resulted in a reduction in filling time. The extensive results obtained help to better understand the filling process of lithium ion batteries and at the same time open up new areas of interest that are relevant to the complete understanding of the filling process.