Methods and advantages and disadvantages of wet and dry pulping processes for lithium batteries

Electrode slurry preparation and pole piece coating are undoubtedly the most basic content and the most critical process of electrode manufacturing, and the nature of electrode slurry directly affects the efficiency and quality of coating.

In the production of Lithium Iron Phosphate Battery, electrode manufacturing, cell assembly and packaging, and battery pre-charging and activation are the three main working stages, that is, the previous process, the middle process and the latter process in the Lithium Batteries population. Electrode manufacturing is the flesh of Lithium Battery production, cell assembly and packaging is to shape the skeleton of LiFePO4 Battery, lithium battery pre-charged into activation is the soul of the soul, the three are closely related and inseparable whole, in the production process of lithium ion battery China plays an important role. Any one of these factors, such as raw materials, battery design, manufacturing equipment and processes, environment, etc., slight defects may cause poor performance of Lithium Iron Phosphate Battery products.

In the pole piece manufacturing process stage, it can be subdivided into five processes: slurry preparation, slurry coating, pole piece rolling, pole piece cutting, and pole piece drying. Of course, according to the different manufacturing processes, each company manufacturer will change or delete part of the process order. Electrode slurry preparation and pole piece coating are undoubtedly the most basic content and the most critical process of electrode manufacturing, and the nature of electrode slurry directly affects the efficiency and quality of coating.

Lithium battery paste is divided into positive and negative electrode pastes. The active materials, conductive agents, binders, solvents, etc. used in the two pastes vary with the battery system. The preparation of positive and negative electrode slurries includes a series of processes such as liquid to liquid, liquid to solid material mixing, dissolution, and dispersion, and in this process are accompanied by changes in temperature, viscosity, and environment. Regardless of whether the water system or the oil system is used, the current stirring process used by lithium battery manufacturers is mainly divided into two kinds of wet process and dry process. Let's analyze the two stirring processes:

1. Wet pulping process

The difference between wet pulping and dry pulping processes is mainly reflected in the difference in the solid content of the pulp at different stages. The wet pulping process is characterized by low solids content in the finished pulp, while the dry pulping is just the opposite. The main process of wet pulping is to mix and stir the binder, conductive agent and other materials, then add the active material to fully stir and disperse, and finally add an appropriate amount of solvent to adjust the viscosity to suit the coating. The state of the binder is mainly powdered and colloidal. Some companies will use the glue first, which is convenient for the role of the binder, and some companies directly use the powdered binder. The selection of the binder state of the positive and negative electrodes also depends on the situation. For example, for the commonly used positive electrode binder PVDF, it is not recommended to use powder directly for higher molecular weights. Instead, it should be made into a glue and then slurried. . Material preparation. The dissolution of PVDF takes at least 2-3 hours at room temperature and pressure to completely dissolve. Otherwise, the viscosity of the slurry will fluctuate greatly, which will also greatly affect the change in the surface density of the coating.

The wet pulping process is widely used in domestic lithium battery manufacturers. In the wet blending process, the double planetary vacuum mixer is generally selected as the mainstream lithium battery slurry mixing equipment. The characteristics of the wet pulping process are that the conductive adhesive that is uniformly dispersed is prepared first, and then the active material and the conductive adhesive are mixed and dispersed. The process takes less time, the process is simpler, the slurry has good fluidity and fewer bubbles . The less mixing time is conducive to the improvement of production efficiency. The process has strong adaptability and can be accepted even when there is a small difference in quality of living materials and conductive agents. The prepared slurry will not affect the existing coating process and the quality of the pole piece. In addition, the time required for evacuation of the slurry is short. However, the problem with the wet agitation process is that due to the large surface area of the conductive agent, it is easy to absorb the solvent, resulting in poor fluidity of the solvent. After adding the active material, it is difficult to achieve a uniformly dispersed state. Under the same solid content, the viscosity of the slurry is higher than that of the dry pulping process.

2. Dry pulping process

The process of dry pulping is to pre-mix powder materials such as living materials and conductive agents at a certain speed. After mixing, add the binder, mix and stir, gradually add the solvent to mix and disperse, and finally add a certain amount of solvent for dilution to adjust to the viscosity required for coating. In the dry pulping process, the mixing and dispersion process of the lithium ion battery slurry can be divided into a macro mixing process and a micro dispersion process. These two processes will always be accompanied by the entire process of preparing the lithium ion battery slurry. The dry pulping process can be visually represented by the following diagram (pictures are from the Internet):

① Mix dry powder. In the dry powder mixing stage, the material particles are in contact with each other in the form of dots, dots, faces, and lines, and the contact form is different according to the type of material. At this stage, the internal porosity of the material is high, and the friction of the powder surface is small, and the mixed contact of most materials can be achieved in a short time.

②Material wetting and mixing stage. In this stage, after the dry powder is mixed uniformly, the binder liquid or solvent is added, and the raw materials are wetted and made into a muddy state. After the strong stirring of the mixer, the material is sheared and rubbed by mechanical force, and at the same time there will be internal friction between the particles. Under each force, the raw material particles tend to be highly dispersed. This stage has a crucial influence on the particle size and viscosity of the finished slurry.

③Dilution and dispersion stage. After the second stage is completed, slowly add a solvent to adjust the viscosity and solid content of the slurry for further dispersion. At this stage, dispersion and agglomeration coexist, and finally reach stability, and the slurry forms a stable suspension state. At this stage, the dispersion of materials is mainly affected by mechanical force, frictional resistance between powder and liquid, high-speed dispersion shear force, and impact interaction force between slurry and container wall.

The difference between the dry pulping process and the wet process is that there is a large internal friction between the materials in the second stage and between the equipment and the materials. Under all forces, the materials can reach a good dispersion state. The high degree of dispersion of the slurry is beneficial to the quality of the pole piece. At the same solid content, the viscosity of the slurry prepared by the dry method is lower. The lower the viscosity of the slurry, the shorter the time to maintain stability and the easier the delamination. During the coating process, the slurry can easily settle at the corners, causing the settlement to cause fluctuations in the coating accuracy. Secondly, the optimal state of the materials in the dry pulping process is more difficult to control. With the changes in raw material particle size, pH value, specific surface area and other factors, the solid content in the optimal state will change and the process adjustment needs to be re-adjusted Will affect the production efficiency and consistency between different batches of pole pieces.

Above, the methods and advantages and disadvantages of the wet pulping process and the dry pulping process are summarized. The so-called ruler has some advantages and disadvantages. Both processes have their own characteristics. If in order to ensure the quality of the pole piece and the slurry will not be stored for a long time, a dry pulping process with better dispersion can be used. If you want to ensure the consistency of the quality of the pole pieces and pay more attention to the efficiency and cost of production, you can use the wet pulping process. The key is to improve the process in continuous practice to meet and exceed the quality requirements for lithium battery paste.