The mainstream mixing equipment used by lithium-ion battery manufacturing companies is the Double Planetary Mixer. The Double Planetary Mixer used in the lithium battery industry, also known as the PD Mixer, is equipped with low-speed stirring components (Planet) and high-speed dispersing components (Disper). The low-speed stirring components consist of two curved frame-type stirring blades, driven by planetary gears. As the blades revolve, they also rotate on their own axis, causing the material to move both vertically and horizontally, achieving the desired mixing effect in a short time. The high-speed dispersing components typically consist of toothed dispersing discs, which revolve in sync with the planetary frame, while also rotating at high speed. This creates intense shear and dispersion forces on the material, making it several times more effective than ordinary mixers. The dispersing components can be configured with either a single dispersing shaft or double dispersing shafts.
01Introduction to pulping process
1.1 Importance of pulping process
The upper limit of lithium-ion battery performance is determined by the chemical system used (positive electrode active material, negative electrode active material, electrolyte) , while the actual performance depends on the microstructure of the pole piece , which is mainly determined by the microstructure of the slurry and the coating process , among which the microstructure of the slurry is dominant. Therefore, there is a widely recognized saying that among the effects of the manufacturing process on the performance of lithium-ion batteries, the influence of the front-end process accounts for at least 70% , and the influence of the pulping process in the front-end process accounts for at least 70% , that is, the influence of the pulping process accounts for about half.
1.2 Slurry composition and ideal dispersion state of each component
The electrode materials of lithium-ion batteries include three main components: active materials, conductive agents and binders. The active materials account for the vast majority of the total weight, generally between 90% and 98%, while the conductive agents and binders account for a smaller proportion, generally between 1% and 5%. The physical properties and sizes of these main components vary greatly. The particles of active materials are generally between 1 and 20 μm, and the conductive agents are mostly nano-carbon materials. For example, the primary particle diameter of commonly used carbon black is only tens of nanometers, and the diameter of carbon nanotubes is generally below 30 nm. The binders are polymer materials, some of which are soluble in solvents, and some of which form microemulsions in solvents.
The electrodes of lithium-ion batteries need to achieve good electron and ion transmission , which requires that the distribution state of active materials, conductive agents and binders in the electrodes meet certain requirements. The ideal distribution state of each material in the electrode is shown in Figure 1, that is, the active material is fully dispersed, the conductive agent is fully dispersed and in full contact with the active material to form a good electronic conductive network, and the binder is evenly distributed in the electrode and bonds the active material and the conductive agent to make the electrode a whole.
Figure 1 Ideal distribution of materials in lithium-ion battery electrodes
In order to obtain the electrode microstructure that meets the above requirements, it is necessary to obtain a slurry with a corresponding microstructure in the slurry making process. That is to say, the active material, conductive agent and binder in the slurry must be fully dispersed, and the conductive agent and the active material, the binder and the conductive agent/active material need to form a good bond, and the dispersion state of each component in the slurry must be stable.
Slurry is actually a suspension formed by solid particles suspended in a liquid. There are many forces between particles in the suspension. Among them, the attraction between particles formed by van der Waals force is the main reason for particle agglomeration. To prevent this agglomeration, it is necessary to make the particles have a certain repulsive force. Common repulsive forces include electrostatic repulsion and steric hindrance formed by polymer chains. A classic theory to describe the stability of colloidal dispersions is the DLVO theory (Deryaguin-Landau-Verwey-overbeek theory), which takes into account the combined effect of double-layer electrostatic repulsion and van der Waals attraction (see Figure 2). It can be seen from Figure 2 that the total energy composed of electrostatic repulsion and van der Waals force at a certain distance will reach a maximum value Gmax. This maximum value forms an energy barrier that can prevent the particles from getting closer to form hard agglomerations (gprimary).
Figure 2 Variation of the interparticle interaction energy consisting of double layer repulsion and van der Waals attraction with the distance between particles in the DLVO theory
In lithium-ion battery slurry, the steric hindrance formed by the molecular chain of the binder adsorbed on the surface of the particles plays a very important role in the stability of the slurry. When the binder molecules are adsorbed on the surface of the particles to form an adsorption layer, when the adsorption layers on the surfaces of two particles are close to each other, the steric hindrance will generate interaction energy. The steric hindrance force, the double layer repulsion and the van der Waals attraction together constitute the total interaction energy between the particles, as shown in Figure 3.
Figure 3 The variation of the interaction energy between particles after the polymer chains are adsorbed on the particle surface with the distance between particles
Therefore, to prevent the particles in the slurry from agglomerating, it is necessary to allow the polymer chains of the binder to adsorb to the surface of the particles to form a certain steric hindrance so that the dispersed state of the slurry can remain stable for a long time.
1.3 Microscopic process of pulping
The slurry making process of lithium-ion batteries is to evenly disperse the active material and conductive agent into the solvent, and form a stable slurry under the action of the binder molecular chain. From a microscopic point of view, the process usually includes three main stages: wetting, dispersion and stabilization (as shown in Figure 4).
Figure 4 The three main stages of pulping from a microscopic perspective
The wetting stage is the process of making the solvent fully contact with the particle surface. It is also the process of expelling the air in the particle agglomerates and replacing it with the solvent. The speed and effect of this process depend on the affinity between the particle surface and the solvent on the one hand, and are closely related to the pulping equipment and process on the other.
The dispersion stage is the process of breaking up particle agglomerates. The speed and effect of this process are related to material properties such as particle size, specific surface area, and interaction force between particles on the one hand, and are closely related to dispersion strength and dispersion process on the other hand.
The stabilization stage is the process in which polymer chains are adsorbed onto the surface of particles to prevent particles from agglomerating again. The speed and effect of this process depend on the material properties and formula on the one hand, and are closely related to the pulping equipment and process on the other.
It should be pointed out that in the entire pulping process, not all materials are processed simultaneously in the above three stages. Instead, different parts of the pulp are in different stages. For example, one part of the pulp has entered the stabilization stage, while the other part is still in the wetting stage. This situation is actually common, which is one of the reasons why the pulping process is complex and difficult to control.
1.4 Slurry dispersion equipment and process
There are two main types of equipment used for slurry dispersion. One type is equipment that uses the shear force generated by fluid movement to disperse particle agglomerates , including mixers and kneaders with various types of stirring paddles, as well as three-axis grinders and disc grinders. The other type is equipment that uses grinding beads to impact particle agglomerates to achieve a dispersion effect , mainly including stirring mills. Of course, there are some more special dispersion equipment, such as ultrasonic dispersers, which use cavitation and instantaneous microjets generated by ultrasound to disperse particle agglomerates. These different types of dispersion equipment are shown in Figure 5.
Figure 5 Different types of decentralized equipment
Not all of the above dispersing equipment is suitable for lithium-ion battery pulping. For example, the stirring mill using grinding beads has a large impact force, which can easily damage the coating layer on the surface of some positive and negative active materials, and may even break the active materials. Therefore, it is rarely used in lithium-ion battery pulping; ultrasonic dispersing equipment is not suitable for high-solid content and high-viscosity slurries, and lithium-ion battery slurries are precisely high-solid content (positive electrode slurries can reach 60% to 80%, negative electrode slurries can reach 40% to 60%) and high viscosity (20 to 200 Pa·s), which is not suitable for dispersion by ultrasonic dispersers. Therefore, in fact, the equipment used for lithium-ion battery pulping belongs to the type that uses the shear force generated by fluid movement for dispersion, including mixers, kneaders, etc. The most typical equipment is the double planetary mixer, and its structure and principle will be introduced in detail in Section 2.
The pulping process also has a great influence on the performance of lithium-ion battery slurry. The most typical example is that the slurry performance obtained by using different addition sequences can be very different. For example, there are literature reports that two different addition sequences are used to prepare the slurry of nickel-cobalt-manganese ternary positive electrode materials, and the resulting slurry characteristics and electrode performance are very different, as shown in Figure 6. The slurry obtained by the second addition sequence has a higher solid content, and the peel strength and conductivity of the electrode are much higher. The reason is that the conductive agent and the main material are first dry-mixed to allow the conductive agent to be coated on the surface of the main material, reducing the free conductive agent. As a result, on the one hand, the viscosity of the slurry is reduced, and on the other hand, the agglomeration of the conductive agent after drying is reduced, which is conducive to the formation of a good conductive network.
Figure 6 Pulping methods with different feeding sequences
There are two types of pulping processes commonly used in the lithium battery industry, namely wet process and dry process . The difference lies in the solid content of the slurry in the early stage of pulping. The solid content of the slurry in the early stage of wet process is lower, while the solid content of the slurry in the early stage of dry process is higher. The typical process flow of these two types of pulping processes is shown in Figure 7.
Figure 7 Wet and dry processes for lithium-ion battery pulping
The process flow of wet pulping is to first mix and stir the conductive agent and the binder, then add the active material after fully dispersing them, stir and disperse them fully, and finally add an appropriate amount of solvent to adjust the viscosity for coating. The binder is mainly in powder form and solution form. It is beneficial to make the binder into a glue liquid first, but some companies directly use powdered binders. It should be pointed out that when the molecular weight of the binder is large and the particles are large, it takes a long time for the binder to dissolve, so it is necessary to make the binder into a glue liquid first.
The dry pulping process begins with the pre-mixing of active materials, conductive agents, and other powdered substances. Next, a portion of the binder solution or solvent is added, followed by stirring (kneading) under conditions of high solid content and high viscosity. The remaining binder solution or solvent is then gradually added for dilution and dispersion. Finally, an appropriate amount of solvent is added to adjust the viscosity for coating applications.
The characteristic of the dry pulping process is that the initial mixing and dispersion (kneading) must occur under high solid content and high viscosity conditions. At this stage, the material is in a thick slurry state, where the mechanical force applied by the stirring paddles is strong, and there is also significant internal friction between the particles. This promotes wetting and dispersion effectively, achieving a high degree of dispersion. As a result, the dry pulping process can shorten the pulping time and produce a slurry with lower viscosity, allowing for a higher solid content compared to the wet pulping process.
However, the optimal condition for the materials in the dry pulping process is difficult to control. When the physical properties of raw materials, such as particle size and specific surface area, change, it is necessary to adjust the solid content and other process parameters during the intermediate stages to achieve the best dispersion state. This can affect production efficiency and consistency between batches.
02 Current status of pulping equipment
At present, the traditional mixing process is still widely used in the slurrying of lithium-ion batteries at home and abroad, usually using a double planetary mixer. The working principle of the double planetary mixer is to use 2 to 3 slow stirring paddles to perform a combination of revolution and rotation, so that the movement trajectory of the paddles can cover the entire space in the mixing barrel, as shown in Figure 8.
Figure 8 The trajectory of the slow-speed paddle of the double planetary mixer when it performs a combined revolution and rotation motion
With the advancement of technology, a high-speed dispersing paddle has been added to the original slow-speed paddle. The high-speed rotation of the toothed disc forms a strong shearing effect, which can further disperse the preliminarily mixed slurry, as shown in Figure 9.
The outstanding advantage of the double planetary mixer is that it can easily adjust the process parameters such as the order of adding materials, speed and time to adapt to different material characteristics, and it can be easily reworked when the slurry characteristics do not meet the requirements. It has strong adaptability and flexibility. In addition, when switching varieties, the double planetary mixer, especially the small mixer, is relatively simple to clean.
In a double planetary mixer, the time when the material is acted on by the stirring paddle is probabilistically distributed. It takes a long time to ensure that all materials are fully mixed and dispersed. In the early days, it took more than 10 hours to prepare a batch of slurry. Later, through continuous process improvements, especially the introduction of dry pulping technology, the pulping time can be shortened to 3-4 hours. However, due to the limitation of the principle, it is difficult to further shorten the pulping time of the double planetary mixer, and its pulping efficiency is relatively low, and the unit energy consumption is high.
Canrd Brief Introduce
Canrd use high battery R&D technology(core members are from CATL) and strong Chinese supply chain to help many foreign companies with fast R&D. We provide lab materials, electrodes, custom dry cells, material evaluation, perfomance and test, coin/pouch/cylindrical cell equipment line, and other R&D services.
Email： contact@canrd.com Phone/Wechat/WhatsApp： +86 19867737979
Canrd Official Web Canrd Company Vedio Canrd Company profile
Website : www.canrud.com
Comments
Post a Comment