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Ready for the battery production of tomorrow's gigafactories

The sharp rise in sales of electric vehicles and an unprecedented surge in demand for lithium-ion batteries require battery production designed for high capacities. A new technology for the production of lithium-ion electrode slurries draws powder materials dust-free into a liquid stream under vacuum, enabling dispersion of the powder particles within milliseconds. On a production scale, the new solution achieves more than 10 times the productivity of conventional planetary mixers.

Fig. 1 Illustration of a planetary mixer and aa High Speed Dissolver (HSD), conventional mixing machinesmixing machines for electrode slurries.

A planetary mixer requires a high viscosity (several million centipoise) in order to generate the shear stress required for dispersion. As the shear rate is determined by the rotational speed of the mixer blades, the higher the viscosity, the higher the possible shear stress (Newton's axiom for the viscosity of liquids: Shear stress = viscosity * shear rate). At very high viscosities, however, the motors required for the movement need a high torque and high power. As only a fraction of the power is available for dispersion, most of the power is wasted in an inefficient process. To maintain this shear stress, all or most of the solvent must be retained, leading to wear and metal contamination from hard cathode powders. The binder is usually pre-dissolved at elevated temperature using a high speed dissolver (Fig. 1), an inefficient powder wetting technology, cooled, filtered and slowly added to the already dry mixed active and conductive materials. This step takes place in the planetary mixer, in which HSDs are also installed, whereby the viscosity decreases as the amount of binder increases. In such a process, the planetary mixer and HSDs must manage the mixing and dispersion together. The production of battery slurries using these conventional technologies is energy-intensive, slow and requires an enormous amount of space. Industrial planetary mixers for the production of electrode slurries now achieve batch sizes of around 3,000 liters. However, the mixing time is more than three hours, to which one to three hours must be added for cleaning between the individual batches. It will hardly be possible to meet the extremely strong increase in demand for storage batteries - estimates assume growth by a factor of 30 in the coming years - by scaling traditional batch mixing technology linearly.

 Illustration of Batt-TDS mixing concept Fig. 2: Batt TDS mixing concept for electrode slurry, which includes both an inline mixer for powder feeding and dispersion in a liquid stream and a 3D mixer with low shear rate in the vessel for batch homogenization.

Dispersion within milliseconds

With this in mind, mixing and dispersion technology specialist Ystral has developed the Ystral Batt-TDS mixing platform, which enables the production of battery slurries with significantly lower time, energy and space requirements. The Batt-TDS mixing concept (Fig. 2) for electrode slurries consists of a circulation process with an inline mixer, which sucks and disperses powder materials dust-free into a liquid stream, and a 3D mixer, which achieves rapid batch homogenization in the container with low energy consumption and a low shear rate. Dispersion in the inline mixer takes place under vacuum. Instead of a high viscosity, the inline mixer operates at a significantly higher shear rate and thus achieves a high shear stress that is inversely proportional to the sub-mm gap between a rotating rotor and the stator. In this dispersion range, strong speed gradients are created that correspond to the shear rate. The specific power can be increased by a factor of 10,000 and the shear stress can be concentrated to a few milliseconds of dispersion time in the active mixing area - instead of hours as in conventional mixing.

Wide process window within a single machine installation

The inline mixer of the Batt TDS mixing platform is specifically tailored to the requirements of battery slurry production, as rotor-stator systems are typically limited to viscosities lower than those relevant for electrode slurries. With a large stator diameter, the inline mixer is designed to accommodate the high viscosities and control the exposure time at high shear by decoupling the shear rate from the controlled powder and liquid flow rates.With the ability to generate shear stresses on demand regardless of viscosity, the gentle but efficient separation of agglomerates allows powders to be dispersed in near real time over a wide range of viscosities, including viscosities lower than those required for effective dispersion in a planetary mixer. A key advantage of this concept is that the shear rate can be tailored to the process requirements of an individual powder, enabling a shear stress and energy optimized process in a specific sequence without the need to use different equipment for each powder type.

Table comparing productivity and space requirements Fig. 3: Productivity and space requirements of planetary mixers and the YSTRAL Batt-TDS-Pro systems mid and supra in the production of electrode slurries on the scale of the currently planned factories, shown for NMC 622/graphite batteries.

From R&D to production scale

The Batt-TDS mixing platform is available in four sizes - from a 12-liter unit for rapid formulation and process screening (Batt-TDS Navi) to a production system for large-scale manufacturing processes for cathode or anode slurries with a batch size of 10,000 l (Batt-TDS Pro supra). Results from the test scale can be easily transferred to the production scale (scale-up). On a production scale, the new technology achieves more than ten times the productivity of conventional technologies and twice the productivity of available extruders. Fig. 3 shows a comparison of conventional planetary mixers and the Batt-TDS-Pro systems mid and supra for the production of electrode slurries on the scale of the currently planned factories (40 GWh/year), shown for NMC 622/ graphite batteries. Since large factories for cathode processes need to be air-conditioned, the ability to produce a large amount of slurry in a small space also means significant potential savings in terms of energy consumption and costs for buildings housing the machines.

Increase in long-term cycle output by 27

Tests carried out by the Karlsruhe Institute of Technology (KIT) have shown that Batt TDS cells have a better conductive morphology and therefore a lower internal resistance with increasing throughput than conventionally produced cells. For this purpose, NMC-622 cathodes were coated from slurries produced separately in 12- to 15-liter batches with a Batt-TDS Navi and, for comparison, with a planetary mixer (Inoue-TX-15-Trimix). Cells produced by KIT with Batt-TDS cathode slurry and an identical anode showed a 27 percent increase in long-term cycling performance over the planetary mixer blended materials. Metal contamination by iron (ICP-OES, detection limit: 8-ppm Fe) was not detected in the NMC slurry of the liquid-lubricated Batt-TDS process.

Cooperation with Frauenhofer FFB ikm battery cell research production project

Battery manufacturers are expected to be able to carry out tests on an R&D system with the YSTRAL Batt-TDS Navi dispersion system as part of the "Battery Cell Research Production (FFB)" project in Münster from the end of the year. The machine and plant manufacturer is supporting the project with a mixing and dispersing system in the "FFB PreFab", which is currently under construction and will be used to test manufacturing processes and new formulations for battery cell production.

[Translate to Englisch:] f

Author: David Manke
Publication date: 03/2023

About the author

David Manke is a managing partner at ystral. The mechanical engineering graduate with a passion for jazz guitar joined the company in 1995, which he took over together with his brothers George and Peter in 2005. His focus is on process engineering and he takes great pleasure in managing the project planning and commissioning of systems on site at the customer's premises.

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