Poor peeling of Lipo battery negative electrode: improved by double coating

When Lipo battery is coated and dried, the adhesive will migrate to the surface due to capillary force, and as the coating speed of Lipo battery increases and the thickness of the coating increases, the migration of the adhesive will become more serious during the drying process, which will further weaken the dressing and the collection. The adhesion between fluids has a negative impact on the performance of Lipo battery. In order to solve this problem, Lipo battery double-layer coating technology came into being. Through multi-layer coating, the primer is compensated with a high ratio of SBR.

In the study, by combining two Lipo battery slurries with different SBR contents, the slurry with gradient SBR binder was double-coated. All Lipo battery electrodes consist of copper foil with a bottom layer of 50% thickness and a top layer of 50% thickness, three two-layer configurations (A+A, B1+B2 and C1+C2), at the same time, with the respective slurries A single-layer Lipo battery electrode was prepared as a comparative reference.
For the Lipo battery double-layer coating process, there may be three problems: (1) air infiltration; (2) longitudinal streaks; (3) mixing of upper and lower layers. Next, CNHL lipos, the manufacturer of Lipo battery, will introduce the content of Lipo battery coating in detail.

1 Lipo battery coating defect

Corresponding view of Lipo battery coating bead and coating film top, including Lipo battery stabilizing coating, air entrainment coating defects and bulging coating defects
The bottom layer of the Lipo battery is highlighted with an ultraviolet (UV) tracer, which glows blue under UV light, and the top layer without UV tracer is black. If the volume flow is too low, the moving contact wire becomes unstable and introduces air into the Lipo battery slurry.
These substances appear as bubbles or streaks in the Lipo battery coating. Conversely, if the volume flow rate is too high, the liquid is forced out of the coating in the direction of the coating, which leads to mixed streaks in the Lipo battery coating.

2 Factors affecting the stability of Lipo battery coating

To study the stability of Lipo battery coatings, each coating condition of Lipo battery with different coating speeds and wet film thicknesses was evaluated and classified into three categories: no defect, lower limit, and upper limit. The area between the defect-free coating and the defective coating is called the coating window.
1) Different coating speeds of Lipo battery
Coating stability at 127μm gap between coating back rolls: At 0.5m/min, the minimum wet film thickness for defect stable coating is 87μm, when the speed is increased to 20m/min, the thickness increases to 90μm, at 1m/min peak value.
2) Different wet film thickness of Lipo battery
At 0.5m/min, the maximum wet film thickness before bulge expansion was 147μm, which decreased to 133μm at 20m/min. Defects are in the stable coating region between the stability limits and wet film thickness can vary without defect coating defects. No mixing of layers occurs between these stability limits. It can be seen that the minimum wet film thickness of the defective double-layer film is higher than that of the single layer, at a coating speed of 20 m/min, the single layer is 64 μm and the double layer is 90 μm.

When the larger gap is 420 μm, the lower limit of Lipo battery defect wet film thickness is 300 μm. The upper limit of wet film thickness is 510 μm at 0.5 m/min and 450 μm at 20 m/min. The minimum wet film thickness of Lipo battery double layer wet film is also significantly higher than that of single layer. This is caused by flow conditions in the upstream meniscus. If a Couette flow forms in the gap without the superposition of Poiseuille flow, the simulated pressure balance is approximately balanced.
This is the case when the Lipo battery wet film thickness is half the gap of the single layer coating. For the bilayer coating, 50% of the corresponding wet film thickness was decisive in this study.
In the case of the Lipo battery double-layer slot die, this flow is different from the flow in the single-layer slot die, where two fluid flows are created due to the two feed ports of the double-layer slot die.
For Lipo battery stabilizing coatings with minimum wet film thickness, Couette and Poi cloud flow are superimposed in multiple layers, resulting in higher Lipo battery wet film thickness.

In addition to the proposed failure modes of Lipo battery air entrainment and swelling, there are also two-layer mixed coating defects of Lipo battery. The UV-active markers were visualized by the proposed experimental setup and one layer of mixing (a mixture of two layers, the bottom layer of Lipo battery was blue with UV tracer, and the top layer of Lipo battery was unpigmented black, which can be detected optically)

3 Process points where the Lipo battery slot die is mixed

The experimentally determined mixing process point is below the minimum wet film thickness of air infiltration, therefore, only at very low Lipo battery coating speeds of 0.2 and 0.5 m/min, the failure mode of Lipo battery mixing can be observed. No mixing was detected at coating speeds above 1 m/min and above the minimum wet film thickness. Mixing is caused by backflow within the coated beads and the resulting intense vortexing.

The literature indicates that the Lipo battery failure mode occurs when the layer thickness of the primer is less than one third of the back-roll gap. For the Lipo battery coatings used in this study, the top coat and base coat thickness ratios were 50%, which resulted in a critical bottom layer thickness well below the minimum wet film thickness in the relevant speed range, so the Lipo battery compound was outside the process window of this experiment.

4 Analysis of peeling strength of Lipo battery coating

The peel strength of Lipo battery can well characterize the bonding effect between the foil and the dressing, and can also indirectly observe the migration of the adhesive. Adhesion under different formulations of Lipo battery base coat and top coat: The adhesion is mainly determined by the SBR content near the collection foil, the greater the ratio, the greater the adhesion.
By doubling the SBR content directly on the Lipo battery foil, the adhesion also increased approximately twofold, from 23 N/m for 3.7 wt% SBR to 44 N/m for a single layer of 7.4 wt% SBR. This is evident in both single and double layers of Lipo battery.

Adhesion with a uniform distribution of binder for a single-layer coating is as high as that for a two-layer coating. For Lipo battery bilayer coating, the base coat has the same binder content as the single coat, while the top coat has much less binder, B1 (SBR 4.97%) 1+B2 (SBR 2.49%) The adhesion of C1 (SBR 7.46%) + C2 (SBR 0%) increased by 43.5% relative to A (SBR 3.73%). Therefore, Lipo battery-coated electrodes with SBR binder gradients can significantly reduce the total binder content without any negative impact on adhesion.

5 Lipo battery electrical performance analysis

When the rate is lower than 1C, there is no difference in the capacity of single-layer coating and double-layer coating. At higher rates, the Lipo battery with double-layer coating can release higher capacity, and C1+C2 has the highest capacity at high rates. . In terms of cycle performance, at 1200 cycles, the remaining capacity of A+A is 87.7%, that of B1+B2 is 87.6%, and that of C1+C2 is 89.1%.

The higher adhesion of Lipo battery multilayer electrodes contributes to long-term stability. Compared with the single-layer coating, the Lipo battery two-layer electrode has a higher discharge capacity up to 11.0%, and shows slightly better results in terms of cycle performance.
The above is the whole content of Lipo battery coating brought to you by Lipo battery manufacturers. I hope this article will help you learn more about Lipo battery. For more information on lithium batteries, please read the following:
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