The occurrence of adhesive floating is mainly caused by drying conditions. The drying conditions mainly include drying methods (electric heating, infrared heating, or steam heating), drying temperature (temperature selection), oven settings (several oven sections), temperature gradient, etc.
Liu Ping and others from China Electronics Technology Group Corporation conducted a study on the drying method of a certain positive electrode slurry. The effects of drying temperature and oven settings on the peel strength of the electrode sheet are shown in the figure:
Comparison of peel strength of positive electrode plates in different temperature ranges
In the figure, the Z-axis represents the peel strength value, the X-axis represents the distribution of temperature ranges, and the Y-axis represents the setting of temperature ranges. Here, 2/2 in the X-axis represents that the drying section consists of four ovens, with the same temperature setting for each oven.
Similarly, 3/1 represents that the temperature settings for the three sections of the oven are the same, while the temperature settings for the other section of the oven are different. It can be clearly seen from the graph that regardless of the interval distribution, the peel strength curve shows the same trend distribution. The temperature interval containing 150 ℃ has relatively high peel strength, and the peel strength under the 2-2 interval setting is significantly higher than that in the 1-3 and 3-1 intervals.
Under the same oven temperature range setting, such as 2-2, it can be seen that the peel strength of the polarizer is highest at a temperature setting of 90 ℃ -150 ℃, and lowest at a temperature setting of 120 ℃ -150 ℃. This indicates that at a drying temperature of 120 ℃, the migration of the adhesive has a significant impact on the adhesion of the adhesive.
The above example illustrates a method for selecting the temperature and setting the temperature zone in the drying section of the coating process, which can be used as a reference for everyone. With the increasing demand for coating efficiency, the coating speed has increased from the initial tens of meters per second to the current 70-80 meters per second, and the selection of temperature and temperature zone control have become increasingly important.
In order to prevent the adhesive from floating during the drying process, the control of the temperature zone will choose a three-stage, four stage, or even multi-stage temperature gradient distribution. Generally, low temperature is used to prevent defects caused by rapid drying of the sprayed electrode after entering the high temperature zone. Interruption is in the high temperature zone, where PVDF has high crystallinity and good adhesion, which has a positive effect on the internal resistance and cycling performance of the produced battery.
In the third paragraph, lower the temperature to a lower level to prevent shrinkage and coating defects caused by sudden cooling at excessively high temperatures. Due to the different active substances and PVDF materials used by various companies, separate experiments are required to determine the optimal drying temperature and temperature zone setting conditions.
The selection of conditions needs to meet both the performance and production efficiency of lithium-ion batteries. We cannot ignore battery quality in order to improve efficiency, nor can we blindly pursue perfect performance to reduce production efficiency and increase production costs. Adopting a drying method with multiple gradient temperature settings can effectively reduce the uneven distribution of PVDF binder caused by its upward floating, while also ensuring the production efficiency of electrode sheets.
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