First of all, let's take a look at the first situation - the electrode shedding, from the picture on the left we can see the shedding of the electrode, the photo in the lower left corner is more obvious. The coating on the surface of copper foil has shedding severely; There are two main reasons for the shedding of the electrode: the first is the problem of the formula. When making new materials, we need to optimize the formula according to the granularity and BET of the material to ensure that the adhesion of the electrode is sufficient.
The second reason is that the baking process is too hot. Because the anode electrode slurry generally uses SBR, which is the emulsion, if the electrode baked too fast, the SBR is easy to floats, and the baking temperature should not be too high. Let's also compare it with mapping from the cross-section. Normally, SBR is evenly distributed in all areas of the coating; In the abnormal case, the SBR is concentrated on the surface of the coating, so that there is less adhesive between the current collector and the coating, which of course affects the bonding and causes material shedding. This problem is often encountered in experiments.
Coating scratch. It is common during the coating process. That is, there will be large agglomerated particles on the coating roller, which hinders the adhesion to the roller. There are two main reasons for coating scratches, one is that when the slurry is sieved, it fails to remove large agglomerated particles; Second, during the coating process, the edge slurry dries out and forms a fine block, blocking the knife edge. At this time, it is necessary to use the corresponding feeler gauge to clean the gap between the scrapers.
Third, electrode cracking. If the electrode cracks, it is generally caused by excessive drying. So our oven temperature should not be too high to prevent cracking of the electrode. If the electrode needs to stay for a long time due to the abnormality of the coating equipment, it is necessary to open the oven door to cool down the electrode to prevent it from cracking the electrode.
Electrode curling. Here are two common reasons: one is due to the low solid content, which leads to excessive solvent volatilization, and the electrode is prone to curling; The other is due to the large shrinkage of the binder when it dries, or the baking speed is too fast.
The next problem is the coating white spots. This white spot problem is mainly caused by bubbles, sometimes in the industry because of the vacuum problem, the bubbles can not be completely removed, and in the static process after coating, it can see bubbles rising up. When the bubbles burst, they form white pits one by one. Many students in the laboratory sometimes encounter it, because there is no good device to remove bubbles. In this case, we recommend that you lower the viscosity of the slurry so that the bubbles can easily escape. In addition, it can also be stirred slowly for a long time, which is also conducive to the bubbles running away.
Coating cratering can also result in white spots. We can see the picture on the far left, this is the actual picture, and there is also a round hole on the electrode. We observed these electrodes under SEM and found that there were some foreign bodies in the center of the white dot, and after some EDS analysis, they were often some organic matter containing C, which was likely to be some dust and debris in the air. Therefore, many factories now install some dust covers on the head of the coating machine.
As for the cause of cratering, it is mainly caused by the difference in surface tension. The white spot is centered on the foreign body, forming different surface tensions, so the cratering is formed, which is why we can find some foreign bodies in the center of the white point.
We have also done an interesting experiment before, sprinkle some water on the newly coated electrode of the anode electrode coating, and we also find that some cratering holes are formed. Although the solvent of the anode slurry is also water, there is a difference in surface tension between the water we sprinkle and the solvent that has formed the slurry, so this phenomenon also occurs.
Electrode bright spots. Here is the cathode electrode, and the appearance of the electrode after coating is not obvious, but after cold pressing, some bright spots can be seen. The bright spots after rolling are like the state of the particles crushed, they will shine and feel protruding, and we have also done some analysis for these bright spots.
This was taken without SEM, directly with a light microscope, and was compared by 200x and 1000x, respectively. If you look closely, the particles in the normal area are still relatively evenly dispersed. However, in the bright spots, it is still obvious that there are a lot of fine particles that have been reunited. Therefore, we think that this bright spot is mainly caused by the fact that the material is not well dispersed during mixing.
Finally, let's look at the issue of coating stripes.
For the anode electrode, CMC is not as much as possible, otherwise the viscosity of the slurry is too high, resulting in the poor leveling of the slurry, so that the slurry is not easy to spread evenly on the surface of the foil, and sometimes there will be light and dark stripes like this.
Finally, we have a summary of coating anomalies, as well as cause analysis and improvement measures, in order to get a good electrode, the stirring process is very critical, only the formula is optimized, the active material is dispersed well, the slurry stability and fluidity are good, and then combined with the baking control of the coating process, can a good electrode be made.
