Battery technology: Is the drying process definitely better than the wet process?

In the manufacturing process of lithium-ion batteries, the homogenization process is extremely important as the first step in the front stage. It is the most important link in the entire production process of lithium-ion batteries to affect the quality of the product. Regardless of the homogenization process, the final positive and negative electrode slurries are basically composed of positive and negative electrode active materials, binders, conductive agents, solvents and other trace additives.

There are roughly three types of mixing processes for electrode slurries:

(1) Ball milling process: It was previously used in lithium battery manufacturing in Japan and South Korea. The method is to mix all the powders into a ball mill and use the friction of the ceramic balls in the ball mill to grind and mix the powders. This method is time-consuming and has poor results and has been eliminated;

(2) Wet homogenization process: The basic process is sol-mixed conductive agent-active material-dilution. This is the current mainstream process in China.

(3) Dry mixing process: The basic process is to mix the active material, conductive agent and binder dry powder-add an appropriate amount of solvent to moisten-add solvent to disperse and crush at high speed-dilution to adjust the viscosity.

The requirements for battery slurry are ultimately to meet the electrochemical properties of the battery, with uniform dispersion, no uneven dispersion, agglomeration and sedimentation. At the same time, it must also have good rheological properties to meet the requirements of the electrode coating process and obtain a coating with uniform thickness. The slurry characterization generally includes: solid content, viscosity, fineness, etc.

The most commonly used dry mixing process is:

(1) Add the active material, conductive agent, and binder powders together into a stirring kettle, and start the mixer to mix the dry powder;

(2) Add an appropriate amount of solvent to wet the powder particles so that the surface of the particles adsorbs the solvent. At the same time, stir at this high viscosity to start to form a large shear force, and fully mix and wet the powder particles;

(3) Continue to add solvent to disperse the particle agglomerates under the action of high-speed shear force to evenly distribute the conductive agent;

(4) Continue to add solvent to dilute the slurry and adjust the viscosity to make it suitable for the coating process.

Among them, the wetting of the powder is the core step of the process. There is a critical point in the amount of solvent. If the solvent is too little and not enough to wet all the powder, the dry powder will inevitably clump, and it will be difficult to open it later. Moreover, in the case of overdrying, the slurry in the double planetary mixer is easy to "climb the pole" and cannot achieve the stirring effect; if the solvent is too much, the slurry is easy to flow, the shear force effect of the stirring paddle is reduced, and the kneading and stirring cannot achieve the effect of crushing and agglomeration. The effect of the actual slurry can be judged by fineness and viscosity: under the same conditions, the smaller the viscosity and the smaller the fineness, the better the dispersion effect.

In theory, the dry mixing process significantly shortens the mixing process time, and the slurry stability and dispersion uniformity are also better. However, the dry homogenization process has two shortcomings compared with the wet homogenization process:

1. High equipment requirements: Since dry mixing is solid-solid mixing, the shear force required for stirring is very large, and the equipment power, design structure, speed and stability are required to be high. To a certain extent, it increases the difficulty and cost;

2. Difficult process control: In the dry mixing process, the mixing speed and mixing time, and the selection of solvent amount at different stages are important key quality control points, which are closely related to the particle size, size distribution, specific surface area, etc. of the raw materials. If these parameters change slightly, the corresponding solvent amount and mixing process conditions also need to be adjusted. Especially the control of the amount of solvent added in the first step: if too much solvent is added, the particle agglomerates are not easy to disperse, and the quality problems of large slurry fineness and uneven distribution of conductive agent appear. If too little solvent is added in the first step, the wetting and stirring force is large, and the binder cannot be fully dispersed and dissolved or the long chain of the binder is destroyed, resulting in problems with the viscosity and stability of the slurry.

So which is better, wet or dry homogenization? The conclusion is that each has its pros and cons, depending on the advanced factory equipment and automation level, the production technology quality management level, and the production capacity and shipment volume. Leading high-tech first-line enterprises such as CATL, SVOLT, LG, and EVE are fully capable and qualified in all aspects, and the dry process is basically the first choice. Some second-tier and third-tier small and medium-sized enterprises are more suitable for wet homogenization.

In a very simple sentence, theoretically speaking, the dry method is better than the wet method, but it depends on whether you can control it.

The article is reproduced from the WeChat: Lithium Battery Frontier.