I. The significance of coating
Slurry coating is the next step after slurry preparation. Its main purpose is to uniformly coat the positive and negative electrode current collectors with a slurry that exhibits good stability, viscosity, and flowability. Electrode coating is of great significance to lithium-ion battery packs, primarily in the following aspects:
1. It is of great significance to the capacity of the finished battery.
If the coating thickness of the positive and negative electrode slurry is inconsistent in the front, middle and rear sections of the electrode sheet during the coating process, it can easily lead to the battery capacity being too low or too high, and it is more likely to form lithium plating during battery cycling, affecting battery life.
2. It is of great significance to the safety and performance of the battery.
Before coating, the 5S process must be followed to ensure that no particles, debris, dust, or other contaminants are mixed into the electrode sheet. If impurities are introduced, it can cause micro-short circuits inside the battery, and in severe cases, lead to a fire or explosion. Battery manufacturers are particularly concerned about significant differences in capacity and cycle life within a batch of batteries; therefore, it is crucial to ensure consistent electrode parameters before and after coating.
3. It is of great significance to battery life.
Significant differences in the coating before and after slurry application, dust mixed into the electrode, uneven thickness of the electrode on both sides, etc., all affect the quality of the battery's electrochemical performance.
II. Coating Method
It is understood that coating equipment mainly consists of a winding and unwinding unit, a feeding unit, a tension control system, a coating head, and an oven. Coating can be divided into two types: transfer coating and extrusion coating.
1. Transfer coating
The coating roller rotates, carrying the slurry. The slurry transfer rate is adjusted by changing the doctor blade gap. The slurry is then transferred onto the substrate using the rotation of the back roller or coating roller. The coating thickness is controlled according to process requirements to achieve the desired weight. Simultaneously, drying and heating remove the solvent from the slurry spread on the substrate, ensuring good adhesion of the solid material. Its advantages include low viscosity requirements for the slurry, easy adjustment of coating parameters, and no clogging. Disadvantages include poor coating precision for power lithium batteries, making it difficult to guarantee electrode consistency. The slurry is exposed to air between the rollers, which partially affects its properties.
2. Extrusion coating
The feeding system delivers the coating to the screw pump, which then power-feeds the slurry to the extrusion head. The slurry is extruded into a liquid film, which is then coated onto a moving current collector. After drying, a uniform coating is formed, as shown in the figure. Its advantages include highly uniform and precise electrode plates after coating, smooth coating edges, a closed operating system unaffected by foreign objects, and suitability for mass production. Its disadvantages include high equipment precision requirements, demanding maintenance requirements, strict requirements on the slurry viscosity range, and the need to replace gaskets when changing specifications.
III. Issues to be aware of during coating
Point defects
During the coating process, air bubbles inside the slurry are sprayed onto the electrode. When dried in an oven, these bubbles burst, forming white circular spots on the electrode. The active material coating is relatively thin in these areas, making them most susceptible to micro-short circuits during battery charging and discharging. When foreign matter is present on the electrode, the coating around the particles forms a low surface tension region, causing the liquid film to migrate radially outwards, creating point defects.
