How to avoid the problem of coating dark marks on lithium batteries
The coating process of lithium batteries is to coat the slurry on the current collector, and drying is to remove all the solvent in the slurry. Lithium battery pole piece coating is more prone to dark marks, streaks and coating wrinkles, which is obviously different from general coating coating: the slurry wet coating is thicker, the slurry is a Newtonian high-viscosity fluid, and the fluid flow is all The required shear stress changes with the change of the flow rate. It is difficult to adjust the coating thickness according to a standard method, which is also a difficulty in the coating process of the coater. It requires high levels of operator proficiency and experience.
The accuracy of the pole piece coating is very high, and the thickness deviation is about 3um, so the equipment is very demanding.
(1) To ensure the consistency of coating as much as possible, pay attention to the change of slurry viscosity, the change of liquid level on the hopper, and the coating speed during operation. The change;
(2) The thin layer of slurry on the aluminum foil enters the oven to dry and remove the NMP solvent (NMP boiling point 202℃, flash point 95℃). If the drying is too slow, the coating surface has fluidity, the thickness is unstable, and the drying is too fast. A PVDF film is formed on the surface. The volatilization of the internal NMP solvent will cause the surface layer to wrinkle. The middle section has the highest temperature. The positive electrode is generally 100-130℃, if the negative electrode adopts an aqueous system, the drying temperature is generally 75-90℃; in the drying process, NMP volatilizes, and the composition of the coating slurry is constantly changing, so the amount of NMP solvent used in the preparation of the slurry is increased. The method and heating time have an impact on this;
(3) The heating temperature or time is not enough, it is difficult to remove the liquid in the slurry, so that part of the binder is dissolved, causing the active material to peel off; the heating temperature is too high If it is high, the binder will crystallize and the active material will also peel off, which will cause a short circuit in the cell. In addition, improper drying temperature and drying time will cause aluminum foil oxidation and pole piece wetness;
(4) Coating and drying must be the first three inspections-thickness, quality, and size meet the requirements , And regularly check the corresponding drawing speed, baking time, gap, etc.
Explanation of coating streaks and dark marks:
Dark marks are caused by uneven thickness on the surface of the pole piece during coating, resulting in chromatic aberration. The display appearance is a dark mark. The uneven thickness may be caused by (1) the surface thickness of the pole piece is uneven due to foreign matter on the knife edge or uneven surface of the knife edge (with gaps) during the drawing process; (2) before or after rolling There are linear traces. It is mostly caused by particles that have not opened the knife edge, or a small dry slurry stuck on the knife edge.