I tried to make slurry for NCM cathode but there was problem in coating process. The detail of my experiment is following;
The ratio between active material(NCM811), conductive additives(Super-P) and PVDF is 80:10:10. PVDF was dissiolved in NMP and ratio was 5w%
Then I mixed them by plentary mixer with 2000rpm for 2 times. First mixing was conducted for 5min and second minixing time was 10 min.
Finally I coated it on Al foil with 300, 350, 400 micrometer by doctor blade.
The humidity was high due to heavy rain at that time. I think humidity makes viscosity of slurry high. But I don't understand why slurry isnt coated well in the middle of foil while near starting point and ending point showed quite good coating.
It looks like dewetting of the ink, due to probably surface contamination on that particular spot. You can easily check this by measuring contact angle of water on the metal. Properly cleaning the surface first with water and a soap, rinse with water, clean with ethanol and/or heptanol and finally give a UV-ozone or O2 plasma for extremely good cleaning. Probably the last step (UV-Ozone or O2 plasma is not required).
Hi, often lesser mixing time creates this problem for me. Though I am not entirely sure what caused this problem for you material, but a well mixing of even 5-6 hour (PVDF - NMP) results in better slurry and coating for me. Hope it helps. Keep us updated.
There might be a number of issues, but I will try to address some of the possible ones as per my knowledge in slurry making.
First of all, the problem can lie either in substrate or in slurry. But before that, you need to make sure that the environment is relatively dry, and both the slurry and substrate must be free of water, as water is a non-solvent for PVDF. This can cause phase inversion of PVDF, which makes slurry useless. Also, you can try not store slurry for too long after preparation, as the solvent NMP absorbs water from moist air slowly. you can try to use freshly pepared slurry for coating. These are some of the things that might happen because of the environment.
Now, moving on to the materials and methods, If the foil is not rough or etched, coating may not happen anyway, but in that case, the uncoated portions will be there because of high contact angle of the slurry with substrate. you can see that easily by just seeing it or measuring the contact angle of substrate with slurry drop.
But in this case, as it is there in the image, those uncoated portions have streak marks, which might be an indicator of two things:
First is that there might be some significant moisture content in it (See whether the slurry is already like gel before coating). As water is a non-solvent for NMP, a low amount of water can make slurry to phase separate while retainin some NMP inside of it, giving it a gel like consistency instead of a uniform slurry) So you can try to keep the slurry away from water and moisture. You can try to use fresh slurry everytime you coat. If the slurry is too thick, you can use more NMP. If you think that NMP might have been contaminated by moisture, you can either use new batch of NMP solvent or you can dry the existing one with the help of molecular sieves.
The other reason for this thing to happen might be the improper mixing of materials. In that case, aggregated particles just brush against the substrate and move along the doctor blade while coating instead of getting deposited evenly. In that case, as Brinti Mondal suggested, the slurry can be mixed for a longer duration, preferably in a homogenizer, to get uniform slurry. If a homogenizer is not available in the lab, one can use stirring and sonication to get the best possible results.
Hope this might solve your issue. Please keep us updated.
i agree with the moisture point. We actually dry our PVDF in vaccum oven prior to use. and also we use freshly prepared solution. So i guess this can be a useful parameter to check as mentioned by Chitransh Upreti.
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