The shape of the extrusion coating gasket is optimized to solve the phenomenon of thick edge of the pole piece

The slit extrusion coating technology is an advanced predictive coating technology. During coating, all the fluid fed into the extrusion die forms a coating on the substrate. Therefore, the surface load of the wet coating can be accurately controlled by changing the slurry feeding speed and coating speed. The coating process is shown in Figure 1. A certain flow of slurry enters the internal cavity of the die from the feed port of the extrusion head and forms a stable pressure. The slurry is finally sprayed out at the slit outlet of the die and coated on the foil. On the material, the coating is dried in an oven.

During the coating process, due to the fluid characteristics of the slurry, it is easy to form a half-moon-shaped feature as shown in FIG. During the coating process, the appearance of a sudden increase in thickness at the edge of the pole piece is called the "thick edge" phenomenon. This thick edge phenomenon is undesirable, and will cause problems to the battery process and battery performance and consistency.

Regarding the flow field characteristics of extrusion coating and the phenomenon of thick edges of the coating, the article has been published before to summarize the relevant readings as follows:

(1) Analyze the flow field characteristics of slit extrusion coating for lithium-ion battery pole pieces

(2) Extrusion coating thick edge phenomenon of lithium battery pole pieces and solutions

The coating of lithium-ion battery pole pieces usually requires the production of strip-shaped pole pieces, which is mainly through the design of the flow channel by the gasket fixed between the upper and lower die heads, so as to realize the preparation of strip-shaped coating (as shown in Figure 2) . The shape of the gasket affects the velocity distribution of the fluid in the die, and ultimately affects the morphology of the coating, especially the morphology of the coating edge. Optimizing the outlet shape of the slit gasket can change the direction and size of the slurry flow speed, reduce the stress state of the edge slurry, and weaken or eliminate the thick edge phenomenon of the coating. The optimization of the gasket shape in the example of this article provides a reference for solving the phenomenon of the thick edge of the pole piece.

Gui Hua Han et al. designed four types of gasket shapes. Using a combination of computer simulation and experiment, taking Newtonian fluid as an example, they studied the effect of gasket shape on the slurry velocity distribution at the die outlet and the coating window. In the study, only the shape optimization of the middle piece of the gasket is considered (Figure 2). The inner runners corresponding to the four gasket specifications are shown in Figure 3:

case1: The width dimension of the gasket remains unchanged at 10mm, and the corresponding dimension of each flow channel remains unchanged at 20mm;

case2: The width of the gasket is expanded from 5mm to 10mm near the exit and then maintains a parallel width;

case3: The width of the gasket is directly expanded from 5mm to 10mm at the exit;

Case4: The width of the gasket is reduced from 15mm to 10mm near the outlet and then maintains a parallel width.

The experimental fluid is a glycerin aqueous solution (80:20, wt%), with a viscosity of 0.045 Pa∙s, a surface tension of 0.066 N/m, and a density of 1210kg/m3.

Figure 4 shows the velocity distribution of the four specifications of gaskets at the die exit along the die width direction obtained by computer simulation:

case1: The size of the runner remains unchanged, and the speed in the width direction at the die exit is relatively balanced;

case2: When the gasket expands, the flow channel shrinks, and the velocity of the fluid increases at the edge of the middle of the die;

case3: When the gasket expands, the flow channel shrinks, and the velocity of the fluid at the edge of the middle of the die increases, and the increase is more obvious than that of case2;

Case4: When the gasket shrinks, the flow channel expands, and the velocity of the fluid at the edge of the middle of the die decreases.

The speed distribution of the die outlet will inevitably affect the thickness of the coating. Due to the nature of the lithium-ion battery slurry itself, the coating thickness will easily lead to the phenomenon of thick edges. Suppress or even eliminate the thick edge phenomenon. In actual production, you can refer to the above gasket design, improve the process parameters according to the actual situation, and solve the thick edge phenomenon.

Figure 5 shows the strain rate distribution of the flow channel fluid corresponding to the four specifications of gaskets. Compared with (a), the flow channels of (b) and (c) are wider, and the strain rate of the fluid is lower, and (d) ) The overall flow channel is narrower, the fluid strain rate is higher, and the fluid pressure is also greater. However, (b), (c), and (d) all have regions with relatively large strain rates. For non-Newtonian lithium-ion battery pastes, changes in the strain rate may change the properties of the paste such as viscosity.

In addition, according to the analysis of the flow field between the die and the foil, when the upper flow channel liquid level of the fluid is close to the outside of the die lip, slurry leakage is likely to occur (as shown in Figure 6a), and the upper flow channel liquid level is close to the die head When the inner side of the lip exits, it is easy to cause instability of the flow field and collapse of the flow field (as shown in Figure 6b). The coating window was judged according to the liquid level of the upper runner, and four specifications of gaskets were found, and the corresponding coating window range has changed. As shown in Figure 7, the coating windows of case2, case3, and case4 have been reduced, corresponding to the stable coating process The parameter range is smaller. If the coating is not operated in the coating window, the coating is more prone to more obvious unevenness.

Figure 6: Schematic diagram of the flow field between the die and the foil: (a) The liquid level of the upper runner is close to the outer side of the die lip, causing leakage; (b) The liquid level of the upper runner is close to the inner outlet of the die lip, and the flow field collapses

Figure 7 Coating windows corresponding to the four specifications of gaskets