What are the lithium-ion battery homogenization processes and equipment involving NMP solvent?
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In the previous article, we learned about the mixing technique in the homogenization process of Li-ion batteries using NMP solvent. So do you know what equipment is used in the homogenization process? Today, let’s learn about the homogenization process and the equipment used.
【Homogenization process: wet method, dry method, kneading】
The wet homogenization process is a routine operation, which is used by 80% of enterprises. The specific operation is, sol (without NMP solvent) → disperse the conductive agent → mix the main material (lithium cobalt oxide, graphite and other raw materials). In the wet homogenization process, dispersing the conductive agent is a very important step, and it takes the longest time. Because SP is agglomerated by particles with a D50 of only 40nm into 150-200nm aggregates, if no dispersant is added, it will take more time to uniformly disperse the conductive agent. In order not to damage the equipment, the revolution of the machine should be carried out at a slow speed, because power = torque × angular velocity, and the power of the motor is constant.
Here, I will introduce the double planetary mixer, as the mainstream equipment for lithium-ion battery homogenization, also known as PD mixer. It is equipped with a low-speed stirring part: Planet, and a high-speed dispersing part: Disper.
The low-speed stirring part has 2 bending style stirring paddles, which are rotated by planetary gears, and the stirring paddles will also autobiography when revolving → → to promote the material to move in directions such as up, down, left, right, etc., so as to achieve the ideal in a short period of time. blending effect.
The high-speed dispersing component is generally a toothed dispersing disc, which revolves together with the planet carrier, and also rotates at a high speed → → can make the material subject to strong shearing and dispersing. Such a mixing effect is several times that of an ordinary mixer. High-speed dispersing parts are divided into single dispersing shaft and double dispersing shaft.
Double planetary mixer slurry preparation often uses the shear force generated by hydrodynamics, which is controlled by the flow shear rate, cluster cross-sectional area, and hydrodynamic viscosity. The preparation of the slurry generally has two processes: the breakup of the clusters and the reorganization of the suspended agglomerates.
【Definition of Cluster Breakup and Reorganization of Suspended Aggregates】
Cluster fragmentation is a complex process, including: abrasion, fracture, breaking and so on.
The cluster rupture mainly depends on the interaction between particles and particles, the interaction between the slurry solvent (the solvent of the negative electrode does not use NMP solvent) and the particles, and the shear force;
The shear force in turn depends on the viscosity and velocity of the solvent.
Abrasion usually occurs when the energy is low, and the small fragments are gradually sheared from the large aggregate by the effect of abrasion.
When the stirring energy is high, the clusters are fractured and divided into several parts.
Fragmentation is a special variation of fracture, whereby a cluster is divided into a large number of small fragments at the same time.
The balance of cluster recombination and dispersion velocity dominates the equilibrium size of clusters in the slurry; there is a critical size below which the cluster dispersion velocity is small.
It has been reported in the literature that the size of the agglomerates cannot be less than 100 nanometers when the slurry prepared by hydrodynamic shearing and stirring is carried out under the appropriate processing time and stirring energy.
Therefore, only when the size of the primary particles (primary particles refers to the grain size of the crystal particles; the secondary particles refers to the particle size of the agglomerated particles) is not less than 100 nanometers, this kind of stirring is possible to completely disperse the powder → →Up to the primary particle size. Complete dispersion of nanoparticles is impossible to achieve.
Dispersion of mesoscale clusters to 40-60 nm is also possible when using a Ramond high speed mixer.
Therefore, this method is not suitable for the dispersion of nanomaterials.
In addition, surfactants can alter the balance of agglomerate composition and dispersion, potentially making the size of the slurry clusters smaller.
Introduction of specific process technology (negative electrode material):
(1) First, use a double planetary mixer to disperse and dissolve CMC in deionized water at a high speed. The concentration of CMC is about 1.5%, and the dissolution time is about 3-4h.
(2) Add conductive agent to the prepared glue, start stirring, first slow speed and then high speed, the time is about 30-60min.
(3) Add the negative electrode material in two times, stir at a slow speed for 15 minutes first, and then continue stirring for 2-3 hours after scraping the edges.
(4) Add an appropriate amount of water to dilute the slurry, and adjust the viscosity to 2000-5000cP.
(5) Add SBR solution, stir for 1-2h, and then slowly stir and vacuum degassing.
The dry homogenization process is prized for its time saving!
Using the dry process to homogenize can increase the solid content and reduce the loss of solvent; that is, it can save the time of the sol (the sol is often used for several hours), which is considerable. Of course, this still has certain requirements for equipment and process technology.
Typical dry process: dry powder mixing – wetting – dispersion – stabilization.
Obviously, dry powder mixing is much less difficult than wet mixing. But the wetting step is very important, and it is very difficult to do it well.
The reason why wetting is the core is that if the agglomerates are left behind, it is more difficult for the trial coating to be broken by dispersing shear force.
Because the linear speed of the mainstream planetary mixer is only 20m/s, it can’t reach the optimal 23m/s at all, and the bigger the jar, the harder the control is.
The kneading homogenization process is also more complicated.
1. Kneading is to use the high-speed rotating stirring slurry to make the material move tangentially along the slurry surface with the help of the friction force generated by the surface inclined at a certain angle and the material;
2. 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;
3. After rising to a certain height, due to the action of gravity, it falls back to the center of the impeller, and then is thrown up again.
The combination of this upward movement and the tangential movement makes the material actually in a continuous spiral movement state.
Due to the high speed of the blade, the moving speed of the material is also very fast, and the fast moving particles collide and rub against each other →→ make the particles or the agglomerates suspected to be together broken; at the same time, the temperature of the material also increases accordingly, which is beneficial to Adsorption of various additives by powder.
[The kneading operation generally has the following characteristics]
1. The kneading operation is often accompanied by a heating or cooling process. On the one hand, the unit volume of the kneader should have enough heat transfer surface; on the other hand, the moving parts should be able to scrape off the material adhering to the heat transfer surface stably and quickly, and send it back to the high shear zone to prevent the material hang on the wall.
2. In the analysis of the dynamic characteristics of the differential stirring kneader and the high shear zone with a small gap in the three-dimensional flow field simulation kneader, high shear stress can be generated to disperse the material; at the same time, the shape of the driving parts in the mixer (such as The shape of the impeller) can ensure that the moving path and moving range of the material in the kneader continuously pass through the high shear zone with small gaps, so as to withstand repeated shearing and disperse evenly.
3. Compared with other mixing operations, the kneading operation is difficult, the mixing time is long, and only a complete mixing state in the statistical sense can be obtained in the end.
The above is a superficial explanation of the homogenization process and the corresponding equipment using this process. If there is any inappropriateness, please point out. Of course, the more specific homogenization process does not stop there. In a later article, we will continue to discuss this aspect. Looking forward to continuing to follow.