Today, we will learn about the lithium battery homogenate process and the equipment to be used.
【 Homogenizing process: wet, dry and kneading 】
Wet homogenate process is a routine operation, 80% of enterprises will use. The specific operation is, sol (does not contain NMP solvent) → disperse conductive agent → mix main material (lithium cobalt acid, graphite and other raw materials). Dispersing conductive agent is an important step in wet homogenizing process, which takes the longest time. Since SP is composed of D50 particles of only 40nm aggregated into aggregates of 150-200nm, it would take more time to disperse the conductive uniformly if no dispersant was added. In order not to damage the equipment, the revolution of the machine should be carried out slowly, because power = torque x angular speed, and the power of the motor is constant.
Double planet Mixer, as the mainstream equipment of lithium ion battery homogenate, also known as PD mixer. It contains a low-speed stirring component, Planet, and a high-speed dispersing component, Disper.
The low speed stirring component has two folding stirring paddles, which use planetary gears to rotate. The stirring paddles will rotate while rotating, so that the materials can move up and down, left and right, etc., so as to achieve the ideal mixing effect in a short time.
The high speed dispersion parts are generally toothed dispersion disks, which revolve together with the planetary shelf and rotate at high speed →→, which can make the material subject to strong shear and dispersion effect. The 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 planet mixer slurry preparation is often the use of hydrodynamic shear forces generated by the flow shear rate, cluster section area, fluid dynamics viscosity control. Slurry preparation generally involves two processes: cluster breaking and suspension aggregate recombination.
[Definition of mass groups of clusters broken and suspended aggregates]
Cluster breakage is a complex process, including: abrasion, fracture, smashing and so on.
Cluster breakage mainly depends on the interaction between particles, the interaction between slurry solvent (negative solvent does not use NMP solvent) and particles, and shear force.
The shear force depends on the viscosity and velocity of the solvent.
Abrasion usually occurs when the energy is low, and the small fragments are gradually cut off from the large aggregate by the action of abrasion.
When the stirring energy is high, the cluster will break and be divided into several parts.
Shattering is a special variation of fracture in which clusters are simultaneously broken up into a large number of small fragments.
The balance of cluster recombination and dispersion speed dominated the balance size of cluster in slurry. There is a critical size below which the clusters disperse very slowly.
It has been reported that the size of aggregates can not be less than 100 nm under appropriate treatment time and stirring energy when the slurry prepared by shear stirring through hydrodynamics is removed.
So only when a particle (a single refers to crystal grain size; Secondary particles refers to reunion particles size) size is not less than 100 nanometers, this is likely to stir fully dispersed powders - - until a particle size. The complete dispersion of nanoparticles is impossible.
Although it is possible to disperse medium-scale clusters up to 40-60 nm using the Ramond high speed mixer.
Therefore, this method is not suitable for the dispersion of nanomaterials.
In addition, surfactants can change the balance of aggregate composition and dispersion, which may make the size of the slurry clusters smaller.
Specific process technology introduction (anode material) :
(1) First, the CMC was dispersed and dissolved in deionized water at a high speed by a double planetary mixer with the concentration of about 1.5% and the dissolution time of about 3-4 hours.
(2) Add the conductive agent into the prepared glue solution, start stirring at a slow speed and then a high speed for about 30-60min.
(3) The anode material is added in two times. First, stir at a slow speed for 15min. After scraping, stir continuously for 2-3h
(4) Add 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 for vacuum defoaming.
Dry homogenate process, because of its relatively save time and respected!
Using dry process to homogenize can increase solid content and reduce solvent loss. That is, it can save the time of sol (sol time is often a few hours), quite considerable. Of course, there are certain requirements for equipment and process technology.
Typical dry process flow: dry powder mixing - wetting - dispersing - stabilizing.
Obviously, the difficulty of mixing dry powder is much less than wet mixing. But wetting is a very important step, and it's very difficult to get it right.
Wetting is the core because if you leave the aggregates behind and try to break them up with dispersed shear forces, it's difficult.
Because the mainstream planetary mixer linear speed is only 20m/s, completely not up to the best 23m/s, and the bigger the pot the more difficult to control the speed.
Kneading process is also complicated.
1. Kneading is to make the material move tangentially along the slurry surface by means of the friction generated by the surface tilted at a certain Angle and the mixing slurry rotating at a high speed;
2. At the same time, due to the effect of centrifugal force, the material is thrown to the mixing chamber wall and rises along the wall surface;
3. When it rises to a certain height, it falls back to the center of the impeller due to the action of gravity, and then it is thrown up again.
The combination of upward motion and tangential motion makes the material actually in a continuous spiral motion.
Because the blade speed is very high, the material movement speed is also very fast, the fast moving particles collide with each other, friction, so that the particles or condensed together mass broken; At the same time, the temperature of the material also rises correspondingly, which is conducive to the adsorption of various additives on the powder.
Kneading operation generally has the following characteristics.
1. Kneading is often accompanied by heating or cooling. On the one hand, the unit volume of kneader should have enough heat transfer surface; On the other hand, the moving parts should be able to steadily and quickly scrape off the material adhering to the heat transfer surface, and send it back to the high shear zone to prevent the material from sticking to the wall.
2. In the analysis of dynamic characteristics of the differential mixing kneader and the simulation of three-dimensional flow field, high shear stress can be generated and the material can be dispersed. At the same time, the shape of the driving parts (such as impeller shape) in the mixer can ensure that the movement path and movement range of the material in the kneader constantly through the high shear zone of small gap, so as to withstand repeated shear and disperse evenly.
3. Compared with other mixing operations, kneading operation is difficult and mixing time is long, and only complete mixing state can be obtained in statistical sense.
