Thrown graphite into water, basically it will float on the water surface, because the graphite surface is hydrophobic, so for this problem, CMC needs to be used in the anode electrode slurry, and the effect of CMC is to make the graphite surface and water extremely moist to achieve the dispersion effect. In addition, in fact, there are many types of graphite, including spherical, scaly, secondary particles, and so on. There is a significant difference between the primary particles and the secondary particles, and it is more difficult for the surface of the secondary particles to be fully wetted. Therefore, our ordinary, conventional anode electrode stirring is generally suitable for primary particle graphite, and secondary particle graphite needs to use a lot of kneading and stirring that is also used in the current industry. In fact, it is high-viscosity stirring, the principle is the same as kneading dough, adding a small amount of water, and then kneading constantly, so that the surface of the material is fully wet. In this way, I want to explain two problems, one is that CMC is necessary, and the second is to choose the mixing method according to the material.
The comparison of this picture can see the intuitive effect of CMC on the slurry, after adding CMC, the fluidity of the slurry becomes better, the particles are effectively dispersed, and the coating is a world of difference. One point is emphasized here: as a special adhesive for batteries, they are basically imported, and there are very high requirements, and not just one can be done (such as CMC, there are many kinds of materials called CMC, and only specific brands will have good performance when used in batteries). Therefore, CMC is very important and commonly used in anode stirring.
Next, let's look at the role of SBR, another component of the slurry, as a binder. I am giving an example here the most commonly used, polystyrene-butadiene rubber. Of course, there are new adhesives like PAN, PAA, PI, sodium alginate, etc. In other words, even if it is SBR, we see that the monomers in it can be adjusted, some add some new monomers, some do changes in functional groups, and so on. SBR basically exists in the form of emulsion, that is, in the glass bottle above, and when dried, it is a rubber block. In the SEM, you can clearly see the SBR between the particles and the current collector, which is how to bond the effect.
This is a conventional mixing formula and process, and there are many similarities with the cathode. It is necessary to glue first, make CMC into glue, and then add conductive carbon SP and active substances. Generally, the amount of CMC is 1%~2%, and the amount of SBR is less than 3%.
SBR is microscopically a micelle, when you stir too fast, the micelle will be destroyed, and the part inside is insoluble in water, so if the speed is too fast, it will lead to the situation on the far right. And we know that when dispersing the conductive agent SP and graphite, we have to use high speed to disperse, and if we add it in front, this problem will occur. So put it at the end to join. In addition, SBR should pay attention to the problems of the coating baking process. If the baking speed is too large, the wind speed is too fast, and the solvent evaporates too quickly, it will cause the SBR to run to the surface with the solvent, so that the dispersion of SBR inside the electrode is uneven, and the adhesive in the lower layer is less, which will lead to separation from the current collector, and the adhesive on the surface is too much, and it is easy to stick to the roller in the rolling process. CMC is mainly a thickener and surfactant, and the adhesion is relatively poor, so it is still necessary to add SBR.
Here's a picture of the whole process of the glue and slurry, which is actually very similar to the cathode one. Of course, in the first picture, we can see that sometimes, some composite adhesives are used. It's important to understand that no material is perfect and is adapted to your application. Then added SP and active materials , and adjusted the viscosity, andremoved bubbles after sieve, which is the same as the cathode electrode.
After the slurry is done, how to check whether it is OK. The above five methods can be used, of course, because the conductivity of graphite is very good, the final separator resistance is applied less. The first viscosity test, the appropriate range of viscosity, can only be said to be an empirical value, the purpose is to make the slurry stable for a long time, and fluid, easy to coat. The second is solids, which is a means of post-event detection, which is mainly used by industry for batch control. The third is similar to a particle size tester, but this is simpler and faster than the Malvern tester, according to the scraping results to see whether the slurry is effectively dispersed, the fourth is the slurry stability, which is mainly used to confirm whether the previous solid content and viscosity control are appropriate, and the last one is the seprator internal resistance tester, similar to the principle of four probes, to test whether the electronic conductivity of the separator is smooth and whether the formula is optimized.
