Editor's note
In the preparation process of the slurry, there are two main aspects to be noted: on the one hand, the first material is uniformly dispersed; on the other hand, the secondary agglomeration caused by the interaction between the raw materials is prevented.
The thickness of the lithium battery pole piece is generally about 40-200 μm, and the thickness of the pole piece varies depending on the type of battery (high energy, high power, etc.). If you want a lithium battery with good electrochemical performance, the pole piece must have the same thickness before, after, and with a smooth surface and no defects. Under the conditions of the coating process, slurry preparation is the key procedure to determine its quality. The viscosity of a good lithium battery slurry is not easy to change, the particle size is small, there is no obvious graininess after coating and drying, and there is no foreign matter such as bubbles. Then, in the preparation process of the slurry, there are two main aspects to be noted: on the one hand, the first material is uniformly dispersed; on the other hand, the secondary agglomeration caused by the interaction between the raw materials is prevented. Therefore, the purpose of the agitation process is to disperse and mix the materials in the original state, and also to prevent re-agglomeration between the particles.
The lithium battery pole piece is divided into a positive pole piece and a negative pole piece. The living materials, conductive agents, binders, solvents and the like used in the two types of pole pieces vary with the battery system. Mixing such a complex multiphase system into a homogeneous slurry with good homogeneity requires more practical experience and theoretical basis. The smaller the particle size of the material, the more difficult it is to disperse the shape. In particular, the conductive agent carbon black is often agglomerated into large pieces during the stirring process, which not only does not play a good conductive role, but also affects the specific energy of the battery. . Carbon black can be used not only in lithium batteries but also in plastics, polymers, etc., so there are many studies on the dispersion of carbon black. According to current research reports, the dispersion of agglomerates can be done in the following ways:
The first type of dispersion on the left occurs under the action of weaker mechanical forces, at which point the material flows with mechanical forces, forming agglomerates of different sizes. During this process, small pieces of material are gradually peeled off from the large agglomerates to form secondary particles. When the external mechanical force exceeds a certain critical value, the rupture of the particles suddenly occurs, as shown in the middle of the figure above. If the mechanical energy reaches a certain level, and the time is extended, the particles will continue to break, but this phenomenon has a low probability of occurrence.
During the preparation of the lithium battery slurry, the contact form between the materials may have the three states shown in the above figure. First, the structure of carbon black is not easily changed, and its distribution is relatively uniform, and exists in structure1. As mechanical agitation proceeds, depolymerization and dispersion of the conductive agent occur simultaneously, and finally the conductive agent is completely coated on the surface of the living material.
It is an important process from the separate presence of raw material particles to the formation of a homogeneously mixed substance. The whole process can be divided into solid powder state (I), mixed wetting state (II) and finally formed suspension state (III). The process control is shown in the figure below.
The characteristics, advantages and disadvantages of each stage in this process are shown in the following table:
In the dry mixing stage, the input energy of the device, the mechanical force, and the particle size of the active material all affect the dispersion of the conductive agent. In addition, the agglomeration of materials is greatly affected by the humidity of the air.
The wetting state begins with the first addition of solvent to the solid powder, the degree of wetting depending on the degree of saturation of the powder to the liquid and the state of adhesion and stress. During this process, dispersion and agglomeration continue to occur while changing the mixing state of the mixture. When the gel point or full saturation is reached, there is no longer a large force input and the interaction between the particles is reduced, which can then be diluted to the desired solids content. In the stage of diluting the suspension, the hydrodynamics and shearing forces of the slurry are involved. Under the fluid flow conditions, the shearing force exerted by the mixer on the particles causes the material to be in a stable state.
Different processing control processes exert different forces on the material raw materials, which will cause the dispersion of the particles to be uniform. Especially for the carbon black conductive agent, the varying stress intensity is uniform to the electron conductivity, battery capacity and current density of the pole piece. Great impact, so it is necessary to understand the state of the material mixing stage in the slurry preparation process.
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