Slot extrusion coating technology is an advanced pre-measured coating technology, coating, the fluid sent to the extrusion die all formed on the substrate coating.Therefore, through the slurry feeding speed and coating Speed changes can be precisely controlled wet coating surface load.Coating process shown in Figure 1, a certain flow of slurry from the extrusion head feed into the die cavity, and the formation of a stable pressure, slurry Finally, at the exit of the die slit, it is sprayed on the foil and the coating is dried in an oven.
During the coating process, due to the fluid properties of the slurry, the beginning of the coating, the termination point and the edges on both sides tend to form a half-moon shape as shown in Figure 1. In the coating process, the thickness of the edge of the electrode plate suddenly increases Topography is known as the "thick edge." This thick edge phenomenon is undesirable and can cause problems with battery performance and battery performance and consistency.
Figure 1 extrusion coating schematic
On extrusion coating flow field characteristics and thick edge coating, previously published articles to summarize, read as follows:
(1) Analyze the flow field characteristics of slot extrusion coating of lithium ion battery pole piece
(2) Lithium battery electrode plate extrusion thick edge phenomena and solutions
The coating of lithium ion battery pole pieces usually requires the production of strip pole pieces, which are mainly designed through the gasket fixed between the upper and lower die to form the flow channel, so that strip coating is prepared (as shown in FIG. 2) The shape of the gasket will affect the velocity distribution of the fluid in the die and will eventually affect the morphology of the coating, especially the edge of the coating. Optimizing the exit shape of the slit gasket will change the direction and size of the slurry flow, Reduce the stress state of the edge slurry and weaken or eliminate the thick edge of the coating.The optimization of the shape of the gasket in this paper provides a reference for solving the thick edge phenomenon of the pole piece.
Figure 2 Example of extrusion coating gasket
Gui Hua Han et al. Designed four kinds of gaskets shapes, and studied the effect of gaskets shape on the outlet velocity distribution and the coating window of the die by taking the Newtonian fluid as an example by a combination of computer simulation and experiment. Only consider the shape optimization of the gasket middle piece (Figure 2), the four kinds of gasket specifications corresponding to the die flow passage shown in Figure 3:
case1: gasket width size of 10mm unchanged, corresponding to each channel size 20mm unchanged;
case2: gasket width in the exit from the 5mm expansion to 10mm and then maintain a parallel width;
case3: gasket width at the exit from the 5mm direct expansion to 10mm;
case4: The width of the gasket shrinks from 15mm to 10mm near the exit and then retains a parallel width.
The experimental fluid was an aqueous glycerol solution (80:20, wt%) with a viscosity of 0.045 Pa ∙ s, a surface tension of 0.066 N / m and a density of 1210 kg / m3.
Figure 3 four gasket shape corresponding to the flow channel:
(a) case1, (b) case2, (c) case3, (d) case4
Fig. 4 shows the velocity distribution along the width direction of the die at the die exit of the four kinds of standard spacers obtained by computer simulation:
case1: the flow channel size remains unchanged, the width of the die exit is relatively balanced;
case2: gasket expansion, the flow channel contraction, the fluid at the edge of the die speed speed increases;
case3: gasket expansion, the flow channel contraction, the fluid at the edge of the die speed speed increases, and more obvious than the case2 increase;
Case 4: Shim contraction, the flow channel expansion, fluid at the edge of the die speed speed decreases.
The velocity distribution of the die outlet will inevitably affect the thickness of the coating. Since the properties of the lithium ion battery slurry itself easily lead to the thick edge of the coating, it can be seen from the above speed distribution that when the speed of the lithium ion battery slurry is reduced at the edge of the case 4, Inhibit or even eliminate the thick edge phenomenon.In actual production, you can refer to the above gasket design, according to the actual situation of the process parameters to improve to solve the thick edge phenomenon.
Figure 4 four kinds of specifications in the die at the exit of the die along the width direction of the speed distribution
Figure 5 shows the strain rate distribution of the fluid in the flow path corresponding to the four kinds of gaskets. Compared with (a), the flow paths in (b) and (c) are wider and the overall fluid strain rate is lower. However, ) The overall flow path is narrower, the fluid strain rate is higher, and the fluid pressure is greater, but in (b) (c) (d) there is a region where the strain rate is locally large, and these regions are responsible for the non-Newtonian lithium ion battery slurry Depending on the material, changes in strain rate may change the properties of the slurry, such as viscosity.
Figure 5 four types of gasket corresponding flow channel fluid strain rate distribution
In addition, according to the flow field analysis between the die and the foil, the slurry leakage occurs easily when the liquid level of the fluid upper flow channel is close to the outer side of the lip of the die (as shown in FIG. 6 a), and the liquid level of the upper flow channel is close to the die When the inner mouth of the lip mouth is expelled, it is easy to cause the instability of the flow field and collapse of the flow field (as shown in Fig. 6b). Judging the coating window according to the liquid level of the upper runner, four kinds of size gaskets are found, As shown in FIG. 7, case 2, case 3, and case 4 both narrow the coating window, which corresponds to a small range of stable coating process parameters. If the coating is not operated at the coating window, the coating is more likely to appear more inhomogeneous phenomenon.
Fig.6 Schematic diagram of the flow field between die and foil: (a) The liquid level of the upper runner is close to the outside of the lip of the die and leaking material; (b) The liquid level of the upper runner is close to the outlet of the inner side of die lip,
Figure 7 four specifications of the gasket corresponding to the coating window
