Mixing process
The mixing process is the first step in producing Lithium-Ion Battery-Slurries. It is crucial for battery quality and has a significant impact on the cell's performance. In the mixing process, active material, binder, and conductive additives are mixed with a dispersion agent, like water or solvent, to form the battery-slurry.
The mixing tools must distribute the particles homogeneously throughout the entire volume. These mixing tools must also fulfill other important requirements: breaking up potential agglomerates, wetting out and coating of particles, and avoiding local material accumulation on a microscopic level.
Currently, many conventional planetary mixers are used for mixing battery slurries for cathodes and anodes. They use similar geometries and there is almost no significant difference in the design of these machines.
But is the execution of these mixers the best and most efficient method for mixing battery masses?
In a battery mixing plant, mixing is divided into the Cathode Line and the Anode Line. The most critical mixing is for the anode due to the higher viscosity and the potential damage to the binder structure. The current mixing times are between 4-6 hours. During a validation step, a potential customer, shared their experiences with an Asian mixer supplier, which had a total mixing time of 270 minutes (4.5 hours).
During tests in the NETZSCH lab, the mixing time was significantly reduced by a factor of 2. The NETZSCH Planetary Mixers (PMH) mixed the slurry in 120 minutes and delivered even better quality performance. So, why were the NETZSCH Planetary Mixers so much better, and how does a Planetary Mixer work?
Working Principle
The NETZSCH PMH (Planetary Mixer High Speed) operates with a planetary gearing mechanism. The self-rotating mixing tools, Low-Speed as axial cross beam and High Speed as butterfly tool, perform a rotary movement in a stationary tank and pass through the entire mixing product.
Increasing the diameter of the mixing tools dramatically increases the power input, leading to faster, more efficient, better mixing and a better product quality. The NETZSCH PMH tools have a much larger diameter compared to conventional planetary mixers.
Not only the diameter but also the design is crucial. Conventional planetary mixers can damage the binder due to the high shear forces of the dispersion disc (Figure 1 – left). The NETZSCH mixing tools are specially designed to reduce shear force and generate softer and better kneading. This allows the mixing tool to run at higher speeds without damaging the crucial binder structure of the battery slurry.
A practical example of advanced processing techniques can be observed in the preparation of water-based anode slurry. This slurry incorporates carboxymethylcellulose (CMC), characterized by its long polymer chains and high molecular mass. Conventional planetary mixers, equipped with high-shear dispersion discs, often induce mechanical stress that fragments the CMC chains. This fragmentation results in undesirable viscosity changes, such as shear thickening, rendering the slurry unsuitable for subsequent coating processes.
In contrast, the NETZSCH mixing tools provide a gentler, more efficient kneading action. This approach prevents the degradation of CMC chains, ensuring consistent viscosity and facilitating uniform mixing resulting in a uniform coating of the active material. Consequently, this method enhances the electrode's energy density, ultimately contributing to the development of high-performance batteries. These batteries exhibit extended range and faster charging capabilities, underscoring the significance of optimized slurry preparation in advancing battery technology.
Overall, the increased power input results in significantly shorter mixing times of 120 minutes for anode slurries. For cathode mixing, which typically requires 480 minutes (8 hours), NETZSCH mixing reduces the time to an overall mixing time of 180 minutes (3 hours).
How to measure the quality?
Besides shorter mixing times, the quality is also improved. But how do you measure quality in slurry mixing? The next important step after slurry making is coating. For the coating process, the flowability of the slurries is crucial to achieve a homogeneous and processable film on the current collector foil. This can be determined by viscosity.
The NETZSCH Analyzing & Testing Business Unit measures flow behavior with the Rotational Rheometer Kinexus. The dependence of viscosity on different shear rates is important and provides significant information about quality. One key factor is the stability of the produced slurry. During production, waiting times can occur and direct feeding of the coater is not always guaranteed. It's important to have a slurry that doesn't sediment quickly and has a longer storage time, indicated by higher viscosity at lower shear rates (Figure 2). This is due to better wetting and dispersion of the particles, resulting from higher energy input due to the unique design. Moreover, the coating process improves, yields sharper edges, and avoids smearing.
Another important factor is processability and flow rate. The slurry is transferred via a slot die to the collector foil. To prevent blockage, a shear-thinning effect is important. The slot die generates high shear rates and a steep viscosity slope is needed for a fast coating process. It's important to have a high slope of the viscosity curve, resulting in lower viscosity at higher shear rates, as shown in Figure 2.
Summary
In conclusion, the NETZSCH Planetary Mixers significantly enhance lithium-ion battery slurry mixing efficiency and quality through innovative design and optimized power input. However, the PMH is not limited to a single technology. Its special design and high flexibility in changing mixing tools allow the planetary mixer to be used for various other applications in the battery industry, ensuring process security. One mixer or plant can cover multiple applications, including: