Detailed explanation of material particle state changes during the preparation of electrode slurry

The thickness of the pole piece of lithium battery is generally about 40-200μm, and the thickness of the pole piece varies according to the type of battery (high energy, high power, etc.). If you want a lithium battery to have good electrochemical performance, the pole piece must have the same thickness in the front, middle and back, and have a smooth surface without defects. Under the condition that the coating process is excluded, the preparation of the slurry is the key procedure to determine its quality. The viscosity of a good lithium battery slurry is stable and 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, during the slurry preparation process, there are two main aspects that need to be paid attention to: one is to disperse the initial materials evenly; the other is to prevent the secondary agglomeration caused by the interaction between the raw materials. Therefore, the purpose of the mixing process is to disperse and mix the original materials evenly, and at the same time prevent the particles from reuniting.

Lithium battery pole pieces are divided into positive pole pieces and negative pole pieces. The active materials, conductive agents, binders, and solvents used in the two pole pieces vary with the battery system. To mix such a complex multiphase system into a homogeneous suspension slurry requires more practical experience and theoretical basis. The smaller the particle size of the material, the more strange the shape is, the more difficult it is to disperse, especially the conductive agent carbon black, which often agglomerates into large pieces during the stirring process, not only does not play a good conductive role, but also affects the specific energy of the battery . Carbon black can not only be used in lithium batteries but also in plastics, polymer and other industries, so there are many studies on the dispersion of carbon black. According to the current research report, there are several ways to disperse agglomerates:

The first dispersion on the left (erosion) occurs under the action of relatively weak mechanical force. At this time, the material flows with the mechanical force to form agglomerates of particles of different sizes. During this process, the small fragments of material are gradually peeled off from the large aggregates, forming secondary particles. When the external mechanical force exceeds a certain critical value, the rupture of the particles will occur suddenly, as shown in the middle of the above figure. If the mechanical energy reaches a certain level and the time is extended, the particles will continue to break apart, but the probability of this phenomenon is low.

During the preparation of lithium battery slurry, the contact form between materials may have the three states shown in the figure above. First, the structure of carbon black is not easy to be changed, and its distribution is relatively uniform, existing as structure1. As the mechanical stirring progresses, the depolymerization and dispersion of the conductive agent will occur simultaneously, and finally the conductive agent is completely coated on the surface of the living substance.

It is a very important process from the existence of raw material particles to become a homogeneously mixed substance. The whole process can be divided into solid powder state (I), mixed wet state (II) and finally formed suspended 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 equipment, the mechanical force and the particle size of the living material all affect the dispersion of the conductive agent. In addition, the agglomeration of materials is greatly affected by air humidity.

The wetting state starts when the solvent is added to the solid powder for the first time. The degree of wetting depends on the degree of saturation of the powder to the liquid and the state of adhesion and stress. In this process, dispersion and agglomeration are constantly taking place, while changing the mixing state of the mixture. When the gel point or fully saturated state is reached, there is no longer a large force input and the interaction force between the particles is also reduced, and then it can be diluted to reduce it to the desired solid content. In the stage of diluting the suspension, the fluid mechanics and shear force of the slurry are involved. Under fluid flow conditions, the shear force exerted by the mixer on the particles promotes the material to be in a stable state.

Different processing and control processes exert different forces on the material and raw materials, which will cause the uniformity of the particles. Especially for the carbon black conductive agent, the varying stress intensity has a very good effect on the electronic conductivity, battery capacity, and current density of the pole piece. Great influence, so it is very necessary to understand the state of the material mixing stage in the slurry preparation process.