The stability of coating slurries used for the production of anodes and cathodes was tested using the dispersion stability analysis system MultiScan MS 20. Analyzing the time-dependent and position-dependent transmission and backscattering intensity of different slurries, unstable slurry formulations could be identified within a short period of time which is valuable information in the development and optimization of battery slurries.
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Electro mobility is one of the key elements for climate-friendly transportation and has the potential to render our environment cleaner as well as to improve quality of life especially in urban areas. The success of electro mobility strongly depends on the affordability and efficiency of the utilized battery systems. Tremendous efforts are undertaken in labs all around the world aiming at the improvement of different components of the batteries, such as the anodes and cathodes .
Lithium batteries are the most widespread used mobile battery systems . Their electrodes are made up of multi-component mixtures that are manufactured from dispersions of micro- or nano-scaled powders in highly viscous polymer solutions. These ‘coating slurries’ contain a large percentage of solid particles of different composition, size and shape.
In order to facilitate the electrode production and guarantee products of reproducible quality it is crucial that the coating slurries are homogeneous and stable regarding the distribution of their ingredients.
Hence, during the development process of the coating slurries, it is essential to study the dispersion stability. Challengingly, the separation of individual components is very often invisible to the naked eye for weeks, or even months, which makes technical help a must have for a fast and timely product development process. The dispersion stability analysis system MultiScan MS 20 from DataPhysics Instruments (see Fig. 2) with its matching software MSC, is the ideal partner for the thorough stability optimizations required in the development of nano-particle based battery slurries. It is able to detect even slightest changes within dispersions and thus allows looking into and evaluating any occurring separation processes fast and objectively. Moreover, a dispersion stability analysis with the MS 20 can also provide information on possible destabilization mechanisms which is helpful to eventually eliminate the instabilities.
A study of two different nano-particle based battery coating slurries with the MS 20 and a comparison of the results will be presented throughout this application note.
Fig. 1 left: Battery pack for electric vehicleright: Battery slurry 1 in sample container after 26 hours
For analyzing a dispersion with the MultiScan MS 20, the dispersion is filled into a, standard sample container (max. volume 27 ml) which is then placed into one of the system’s ScanTowers. The towers incorporate a scanning unit, composed of a transmission and a backscattering LED along with a detector, which moves up and down the vertical side of the sample container (along the z-axis) in customer-defined time intervals. This allows to detect the transmission and backscattering intensities both position-resolved and time-resolved, which is represented as multiple intensity profiles in intensity–position diagrams in the MSC software (see figure 3, figure 4 and figure 5)
In the experiment described here the stability of two battery slurries with different formulations was studied. For this purpose 20 ml of each battery slurry was poured into a standard sample container and put into a scan tower in which temperature had been set to T = 25 °C. A measurement routine was set up which scheduled scans every 5 min for a total measuring time of 26 hours (slurry 1) or 120 hours (slurry 2), respectively. The scanned z-axis range was between 0 mm (bottom of the sample container) and 50 mm (fill level).
Figure 1 right shows the sample container filled with unstable battery coating slurry 1 at the end of the experiment.
Fig. 2: DataPhysics Instruments MS 20 stability analysis system
Figure 3 shows the plots of the transmission and backscattering intensities against the position for battery slurry 1. The color-coding indicates the time at which the individual intensity profiles were recorded (red: t = 0 s, to purple: t = 26 h).
The transmission profiles of slurry 1 (see figure 3, top) show a constant mean intensity value of ITr = 0 %, which does not change throughout the whole experiment. This can be explained by the turbidity of the battery coating slurry that prevents the transmission of any incident light.
The backscattering diagram (see figure 3, bottom), on the other hand, shows a clear time-dependent and position-dependent change of the intensity signal.
Fig. 3: Transmission (top) and backscattering (bottom) intensity vs. position diagrams for battery slurry 1, with intensity profiles color-coded from red to violet from the first to the last scan, respectively
This indicates that battery slurry 1 is not stable over the investigated time period but some destabilization processes are occurring. This becomes even more obvious in a relative plot where the change of the backscattering intensity compared to the one of the very first scan is shown (see figure 4).
Looking closer at the shape of the backscattering profiles in figure 4 one can see an increase of the backscattering at the bottom of the sample container along with a backscattering decrease at the top of the sample. This indicates a sedimentation process with a sedimentation layer building up at the bottom and the sample clearing up from the top (see figure 1 right). Hence, a possible further analysis step could be the determination of the sedimentation velocity using the Migration Front analysis option of the MSC software.
Fig. 4: Relative backscattering intensity vs. position diagram with profiles illustrating the intensity relative to the intensity measured at t = 0 s
For the second studied sample, battery slurry 2, the situation looks completely different. As can be seen in figure 5, for battery slurry 2 both the transmission and the backscattering intensities did not change during the whole measurement time of 120 hours. This indicates that slurry 2 is very stable and hence from this perspective a great candidate for battery coating.
Fig. 5: Relative transmission (top) and backscattering (bottom) intensity vs. position diagrams for battery slurry 2 (intensities relative to the one measured at t = 0 s)
Using the dispersion stability analysis system MultiScan MS 20 from DataPhysics Instruments and the corresponding MSC software the stability of two different battery slurries was studied and compared. Recording transmission and backscattering intensity profiles for a period of 26 and 120 hours, respectively, one of the slurries (sample 2) could be identified as stable while the other one (sample 1) clearly turned out to be unstable.
Due to the turbidity of the two slurries this was not seen in the transmission data, but became directly obvious looking at the (relative) backscattering profiles which show distinct changes for battery slurry 1, after a couple of hours, while for slurry 2 there are no changes during the whole measuring time of 5 days.
Due to the shape of the evolving backscattering profiles of battery slurry 1 one can furthermore deduce that the predominant destabilization process is apparently sedimentation as a migrating sedimentation front and a growing sedimentation layer are seen.
The opportunity to observe position-resolved and time-resolved even smallest changes of a sample’s backscattering and transmission within a very short period of time enables the producers of battery coating slurries (as well as producers of all kinds of dispersions) to fast and objectively carry out stability analysis. Thus, the dispersion stability analysis system MultiScan MS 20 from DataPhysics Instruments helps to quickly anticipate long term stability and thus guarantees a time and cost efficient product development.
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