The coating and drying of lithium battery pole pieces accounts for 8% to 10% of the entire manufacturing cost of lithium batteries, which is mainly due to the low thermal efficiency of the pole piece drying process. In addition, the residual solvent in the pole piece drying process has a great impact on the stability, capacity and cycle life of the subsequent processing of the pole piece. The pole piece drying process not only affects the manufacturing cost of the battery, but also indirectly determines the manufacturing process level and safety.This paper studies the drying characteristics of the coating layer of the positive electrode of LiFePO4 battery under static drying and hot air tunnel drying conditions, and analyzes its drying rules to provide a reference for optimizing the hot air drying process and the development of corresponding drying equipment.
1. Experimental part
1.1 Instruments and Materials
In the experiment, the static drying method was first used to study the drying characteristics of lithium battery pole pieces, and then the hot air drying method was used to study the drying performance of lithium battery pole pieces. The static drying electric constant temperature blast drying oven has a temperature control range of room temperature + 10~220℃. During coating, a four-sided film applicator was used to prepare slurry films of different thicknesses. The four-sided film applicator is mainly used to apply a wet film of a specified thickness. The coating thickness is 100, 200, 300 and 400um. The hot air dryer can adjust the drying temperature and wind speed. When heating, there is no need to take out the dried material, and the wet material mass can be read directly. The morphology of the coating after drying is analyzed by scanning electron microscopy.
Lithium iron phosphate, PVDF, conductive carbon black, NMP, graphene, aluminum foil.
1.2 Experimental procedures and methods
Prepare PVDF adhesive into PVDF glue, add conductive agent carbon black and graphene to the whole PVDF glue and premix for 30 minutes to obtain the first premix. Add lithium iron phosphate to the first premix and continue mixing to obtain a mixed slurry. Add the remaining PVDF glue to the mixed slurry and mix again to obtain lithium iron phosphate slurry. Note that during the slurrying process, lithium iron phosphate is always in an environment rich in NMP solvent, which is conducive to fully absorbing the solvent. Obtain lithium iron phosphate coating slurry.
When making the pole piece, first, use a knife to cut out aluminum foil of the same size, use a key to take out the lithium iron phosphate slurry and evenly apply it on the aluminum foil, and then the required film applicator to scrape it from top to bottom, and the pole piece is ready. When drying the pole piece, use tweezers to hold the finished pole piece and put it in a constant temperature box to dry.
The experiment adopts the single variable method. When the coating thickness is 300um, the pole piece coating is dried at hot air temperatures of 130, 150, 160, 170, and 180℃. It was found that the pole piece dried at 170℃ had no cracks and the drying time was also short. In order to study the effect of thickness on pole piece drying, pole pieces of 100, 200, 300, and 400um were dried at a temperature of 170℃. The specific method is as follows: Start and set the temperature required for the experiment, run for 1h, and keep the temperature constant. Weigh the mass of the cut aluminum foil and the mass of the pole piece coated with slurry. Use tweezers to place the prepared pole piece in a constant temperature box, take out the pole piece every 1min and weigh the mass of the pole piece until the mass of the pole piece remains unchanged. Process the data and draw a pole piece drying curve.
The production of the electrode in the hot air convection drying experiment is consistent with the constant temperature drying experiment. When drying the electrode, use tweezers to put the prepared electrode into the digital tunnel drying experimental device, and read the weight of the wet material every 30 seconds until the weight of the wet material remains unchanged. The specific method is as follows: dry the electrode at hot air temperatures of 90, 100, 105, 110, and 115°C, measure the weight of the wet material of 300 and 400um electrode at different temperatures, and make a drying curve.
2 .Results and Discussion
2.1 Static drying
Through a large number of experiments, it was found that the 300um electrode did not fall off or crack in a large area when dried at 130, 150, 160, 170, and 180°C. Therefore, 130, 150, 160, 170, and 180°C were selected as the experimental temperatures.
According to the experimental parameters, group experiments were conducted to measure the mass of dried materials at different times for electrodes of the same thickness and different temperatures, and a drying curve was drawn. Figure 1 shows the drying rate curve of 300um electrode coating at different temperatures.
As shown in Figure 1, when the pole piece coating is dried at 130°C, the required drying time is 45 minutes, and the slope of the drying curve goes through three stages: increasing speed, constant speed and decreasing speed. When the pole piece coating is dried at 150°C, the required drying time is 35 minutes, which is shorter than the drying time at 130°C, and the slope of the curve is large. When the pole piece coating is dried at 160°C, the required drying time is 25 minutes. It takes less time than drying at 150°C. When the coating is dried at 170°C, the required drying time is 16 minutes. The slope of the curve is larger than that of 130, 150, and 160°C, and the trend is relatively smooth. When the pole piece coating is dried at 180°C, the required drying time is 13 minutes.
When drying 300um electrodes at different temperatures, the dry basis NMP content gradually decreases, eventually approaches zero, and remains unchanged. When the drying temperatures are 130, 150, 160, 170, and 180°C, the time taken is 45, 35, 25, 16, and 13 minutes. The higher the temperature, the shorter the time required for the electrode to reach the equilibrium NMP content from the initial NMP content. Conversely, the longer the drying time. The drying time of the electrode at 170°C differs by 3 minutes from that at 180°C, and the drying time of the electrode at 170°C differs by 29 minutes from that at 130°C. It can be seen that the temperature rises from 130°C to 170°C, which effectively reduces the drying time. If the temperature continues to rise, the drying time will be significantly reduced.
2.2 Hot air tunnel drying
On the basis of the constant temperature drying experiment, a large number of hot air drying experiments were carried out. 90, 100, 105, 110, and 115°C were selected as the experimental temperatures to compare the drying process of electrodes with different thicknesses. Group experiments were carried out according to the experimental parameters, and the mass of the dried materials of electrodes with the same thickness and different temperatures at different time periods was measured, and a drying curve was drawn, as shown in Figure 2.
Figure 2 300um and 400um electrode coating drying curve
As shown in Figure 2 (a), the drying time of the 300um electrode is 28min, and the drying time of the 400um electrode is 32min. The drying time of the 300 and 400um electrode coatings differs by 4min. It can be seen that the thicker the electrode coating, the longer the drying time. The slopes of the drying curves of the electrode coatings with thicknesses of 300 and 400um are not much different. As shown in Figure 2 (b), at 100°C, the drying time of the 300um electrode is 20min, and the drying time of the 400um electrode is 24min. The drying time of the 300 and 400um electrode coatings differs by 4min. It can be seen that the thicker the electrode coating, the longer the drying time. The slopes of the drying curves of the electrode coatings with thicknesses of 300 and 400um are not much different.
Figure 3 (a) is a drying curve at 105°C. As shown in Figure 3a, the drying time of the 300um electrode is 14 minutes, and the drying time of the 400um electrode is 16 minutes. The drying time of the 300 and 400um electrode coatings differs by 2 minutes. It can be seen that the thicker the electrode coating, the longer the drying time. The slopes of the drying curves of the electrode coatings with thicknesses of 300 and 400um are not much different.
Figure 3 300 and 400um electrode coating drying curve
As shown in Figure 3b, when the drying temperature is 110°C, the drying time of the 300um electrode is 9 minutes, and the drying time of the 400um electrode is 11 minutes. The drying time of the 300 and 400um electrode coatings differs by 2 minutes. It can be seen that the thicker the electrode coating, the longer the drying time. The slopes of the drying curves of the electrode coatings with thicknesses of 300 and 400um differ greatly.
Figure 4 is a drying curve of 300 and 400um electrodes at 115°C. As shown in Figure 4, the drying time of 300um electrode is 7 minutes, and the drying time of 400um electrode is 8 minutes. The drying time of 300 and 400um electrode coatings differs by 1 minute. It can be seen that the thicker the electrode coating, the longer the drying time.
In summary, under different temperature drying conditions, the dry basis NMP content of the electrode with a coating thickness of 300um gradually decreases, and finally approaches zero and remains unchanged. When the drying temperatures are 90, 100, 105, 110, and 115°C, the time used is 28, 20, 14, 9, and 7 minutes, respectively. The dry basis NMP content of the electrode with a coating thickness of 400um gradually decreases, and finally approaches zero and remains unchanged. When the drying temperatures are 90, 100, 105, 110, and 115°C, the time used is 32, 24, 16, 11, and 8 minutes, respectively. It can be seen that the higher the temperature, the shorter the time required for the electrode to reach the equilibrium NMP content from the initial NMP content. Conversely, the longer the drying time. When the 300um electrode coating is drying, the drying time of the electrode at 110℃ is 2 minutes different from that at 115℃, and the drying time of the electrode at 110℃ is 19 minutes different from that at 90℃. It can be seen that the temperature rises from 90℃ to 115℃, which effectively reduces the drying time. Continuing to increase the temperature greatly improves the drying efficiency.
2.3 Scanning electron microscopy analysis
In order to further observe the dispersion of the lithium iron phosphate electrode after drying, the microstructure of the sample was observed using a scanning electron microscope. The test results are shown in Figure 5.
Figure 5 (a), (b), (c), (d), and (e) correspond to the low-magnification microstructure diagrams of 300um lithium iron phosphate pole pieces at 90, 100, 105, 110, and 115°C, respectively. The lithium iron phosphate particles are well dispersed, the pore size is consistent, and there is no agglomeration. Therefore, it can be concluded that the pole piece is dried evenly.
Figure 6 (a), (b), (c), (d), and (e) correspond to high-magnification microstructure diagrams of 300um lithium iron phosphate pole pieces at 90, 100, 105, 110, and 115°C, respectively. The larger particles in the figure are lithium iron phosphate particles, and the thin tubes are carbon nanotubes. There are many pores distributed around the particles and thin tubes. The carbon nanotubes surround the phosphoric acid particles and can strengthen the structure of the pole piece. The electrolyte can penetrate through the pores to increase the conductivity of the pole piece, indicating that the solids in the slurry are well dispersed, and the slurry and solids are mixed in the microscale range.
3 .Conclusion
The effects of drying temperature and coating thickness on drying electrodes were investigated through constant temperature drying experiments and hot air drying experiments. The experiment found that when the 300um electrode was dried at 170°C, there was no cracking or falling off. When drying electrodes of different thicknesses at the same temperature, the thinner the electrode coating thickness, the shorter the drying time, and vice versa. The hot air drying experiment was carried out on the basis of the constant temperature experiment. When the drying temperatures were 90, 100, 105, 110, and 115°C, the time taken was 28, 20, 14, 9, and 7 minutes, respectively. It was found that the higher the temperature, the shorter the drying time. At a temperature of 115°C, the drying time for electrode coatings with a thickness of 300 and 400um was 7 and 11 minutes, respectively. At a temperature of 90°C, the drying time for electrode coatings with a thickness of 300 and 400um was 28 and 32 minutes, respectively. In the hot air drying experiment, the drying time of 300um electrode at 90℃ is 28min, and the drying time at constant temperature of 130℃ is 32min. Hot air drying reduces the drying temperature, effectively improves the drying efficiency and reduces energy consumption.
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