What exactly is the role of kneading in the lithium battery slurry mixing process?

Generally speaking, the dry anode material preparation process can be broadly divided into the following steps: mixing, wetting, dispersing, and stabilizing, with the wetting stage typically requiring a slower rotation speed. The dispersing stage, however, (kneading refers to the operation of using mechanical stirring to uniformly mix paste-like, viscous, and plastic materials, including both dispersion and mixing of the materials. Simply put, stirring highly viscous materials can also be referred to as kneading, such as kneading in toothpaste. The wetting process generally does not belong to the kneading process, though this may vary depending on different companies' understanding.) often requires a certain shear force and high-speed rotation, with a linear speed exceeding 20m/s.

The main purpose of lithium-ion battery slurry dispersion is to uniformly disperse active materials, conductive agents, adhesives, etc. in a solvent in a certain mass ratio to form a stable slurry with a certain viscosity, which is used for coating the electrode sheet. The technological goal of lithium-ion battery slurry making is to prepare for the electrode sheet production. The ideal slurry requirements for electrode sheets are: (i) the active material particles are finely and uniformly dispersed without agglomeration, the conductive agent particles form a thin layer and are dispersed to form a conductive network, and the maximum amount of active material particles are interlocked and connected on the current collector; (ii) the active material particles are preferably small to ensure that the battery has a high current density.

Kneading process
Kneading principle: The high-speed rotating stirring paddle uses the friction force generated by the surface inclined at a certain angle and the material to make the material move tangentially along the paddle surface. At the same time, due to the centrifugal force, the material is thrown to the inner wall of the mixing chamber and rises along the wall. When it rises to a certain height, it falls back to the center of the impeller due to gravity, and then is thrown up again. The combination of this upward movement and tangential movement makes the material actually in a continuous spiral motion state. Due to the high rotation speed of the paddle and the fast movement speed of the material, the particles moving quickly collide and rub against each other, so that the particles or agglomerated clumps are broken, and the temperature of the material also increases accordingly, which is conducive to the adsorption of various additives by the powder.

Kneading operation generally has the following characteristics:

1. Kneading operation is often accompanied by heating or cooling process. On one hand, the unit volume of the kneader needs to have sufficient heat transfer surface. On the other hand, the moving parts should be able to stably and quickly scrape off the material adhered to the heat transfer surface and send it back to the high-shear zone to prevent the material from sticking to the wall of the equipment.
2.  In the analysis of dynamic characteristics and three-dimensional flow field simulation of differential speed stirring kneader, the high-shear zone with a small gap can generate high shear stress, dispersing the material. At the same time, the shape of the driving parts in the mixer (such as impeller shape) can ensure that the movement path and range of the material in the kneader constantly pass through the high-shear zone with a small gap, so that the material can be repeatedly sheared and evenly dispersed.
3. Compared with other mixing operations, kneading operation is more difficult, takes longer time, and can only achieve a statistically complete mixing state in the end.

The main parameters affecting the mixing and dispersion process:

1. The impact of stirring speed on dispersion speed. Generally speaking, the higher the stirring speed, the faster the dispersion speed, but the greater the damage to the material's own structure and the equipment.
2. The impact of concentration on dispersion speed and adhesive strength. Under normal conditions, the smaller the slurry concentration, the faster the dispersion speed, but too thin a concentration will lead to material waste and increased slurry sedimentation. The greater the concentration, the greater the kneading strength and adhesive strength; the lower the concentration, the smaller the adhesive strength.
3. The impact of vacuum degree on dispersion speed. High vacuum is conducive to the exhaust of gases from the material gaps and surfaces, reducing the difficulty of liquid adsorption. Under the condition of complete weightlessness or reduced gravity, the difficulty of uniform dispersion will be greatly reduced.
4. The impact of temperature on dispersion speed. Under suitable temperature, the slurry has good fluidity and is easy to disperse. Too hot slurry tends to crust, while too cold slurry will greatly reduce its fluidity.

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