The coating process is an important step in the manufacturing process of the lithium battery electrode, and it is the process of transferring the electrode slurry from the liquid state to the electrode solid living substance. After the electrode slurry is prepared, it is passed through a screw pump, a filter, a screen, or the like, and is coated on the current collector after being coated via an extrusion coating head or a transfer roll. After oven drying, the liquid slurry solidifies on the surface of the current collector and has stronger adhesion.
In the oven drying process, there is no obvious anomaly on the surface, but during the rolling process after coating, it is sometimes found that after the battery electrode passes over the roller, the material drop, the leakage foil, or the pressure roller abnormally darkens. phenomenon.
This may not be due to a problem with the binder ratio during the preparation of the electrode paste, but may be due to the fact that the binder floats to the surface during drying of the electrode coating.
1.The reason for the binder floating
Lithium-ion battery pastes are mainly composed of active materials and conductive agent particles, solvents, binders, etc. In the process of drying, surface temperature and internal temperature of the electrodes will form a slight temperature gradient, and the surface temperature will be higher than the internal temperature.
During the drying process, the surface is first dried and the concentration of the substance is higher than the internal concentration. Under such a driving force, the binder in the electrode gradually migrates to the surface as the solvent volatilizes, and precipitates on the surface of the active material and the conductive agent. Therefore, the drying process will affect the pore structure and binder distribution of the electrode.
The reasons for the binder floating are mainly the drying rate is too fast and the oven temperature range is set unreasonably.
2.Disadvantages of binder floating
After the slurry coating is completed, the product needs to be inspected. The inspection items are generally pole load, electrode thickness, peel strength, resistivity, etc., and may also evaluate the Lipf6 electrolyte infiltration effect of the electrode. The float of the adhesive will reduce the peel strength of the battery electrode.
3.How to lower the adhesive float
The effect of drying temperature and oven setting on the peel strength of the electrode is shown in Fig. 1:
Figure1: Comparison of peeling strength of cathode electrode at different temperature intervals
In the figure, the Z axis is the peel strength value, the X axis is the temperature range distribution, and the Y axis is the interval temperature setting. Here, the 2/2 on the X-axis represents the drying section consisting of a four-section oven, where the vacuum drying oven temperature setting is the same for each two sections.
In order to prevent the floating problem of the binder in the drying process, the control of the temperature zone will select a three- or four-stage or even multi-stage temperature gradient distribution, generally low temperature to prevent the sprayed electrode from drying after entering the high temperature zone. The defect caused by the fast, interruption is the high temperature range. In this interval, PVDF has a high degree of crystallinity, has a good adhesion, and has a positive effect on the internal resistance and cycle performance of the manufactured battery.
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