Electric vehicles are a promising candidate for an emission free future especially in urban areas. The main challenge to a more widespread use of electric vehicles is to store a sufficient amount of electricity with respect to weight and size limitations. Many new concepts for future battery technologies have emerged over the past years, but so far the lithium ion battery (LIB) technology is the most widely used. As each car battery consists of thousands of individual LIB cells the manufacturing is extremely labour intensive and every advancement can tremendously improve cost and efficiency.
LIBs consist of layers of anodes, cathodes, separator foils and conductor foils. These stacks are filled with liquid electrolyte. During production the most time consuming step is to fill the battery with electrolyte as it has to first penetrate and wet the solid phase, followed by filling up the pores and forming the solid/electrolyte interface (SEI), which is very important for the electrochemical processes.
Battery pack for electric vehicle
In order to improve the time efficiency of filling the battery with electrolyte it is important to understand the interaction between battery components and electrolyte. So far, contact angle measurements have been used to characterise the wettability of the separator membranes, which can estimate the wetting behaviour of the membranes qualitatively. Besides, penetration measurements with the Wilhelmy plate method were also commonly used to analyse separator and electrodes regarding their surface wettability.
However, both approaches cannot reflect the electrolyte's penetration into battery cells well enough to draw firm conclusions. Many other advanced methods were developed for studying the filling process of electrolytes, but most of them can only be applied to certain systems. Up to now there is no generally accepted method available to assess the electrolyte's penetration behaviour in LIB cells.
Recently, Beyer et al. reported a new commonly available method that provides a better way to understand the wetting and penetration behaviour in LIBs . In this research they combined tensiometry with chronoamperometry to monitor the penetration of liquid electrolytes in the LIBs model cells as described in the following.
Schematic illustration of a lithium-ion battery
For their study of electrolyte penetration rates they chose a model system consisting of anode, cathode and separator fixed between two glass slides. Diethyl carbonate (DEC) was selected as model liquid because of its common use in LIBs. The sample was suspended into DEC and the weight increase was recorded by the tensiometer over time. Based on the modified Washburn equation, surface tensions, as well as the complex geometries of electrodes and separators a penetration model was developed. Different separators were studied showing that the penetration rates into ceramic coated separators, which have a high surface roughness and porosity, is significantly higher than for smooth polyolefin separators.
The authors could find that surface roughness and the surface chemistry of the solid have a significant influence on the wetting and penetration process. Moreover, the results from the tensiometric measurements were validated by chronoamperometric measurements. When comparative measurements with the real electrolyte solution were carried out, the results were comparable to that from the DEC model liquid.
In conclusion, the authors could show that tensiometric measurements of the wetting process in LIB model cells reflect the electrolyte's penetration behaviour very well. The measurements showed a very good reproducibility. This research provides a deeper insight into how the surface characteristics affect the penetration rate of electrolytes into LIB model cells and therefore has proven to be a valuable pre-evaluation tool for battery components.
The tensiometric measurements in this study were conducted with a dynamic contact angle measuring device and tensiometer of the DCAT series. For more information please refer to the original publication:
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