Can NMP be used in negative electrode slurry of lithium battery? !

Edited the information before, we collected some information and learned from the company’s engineering team that NMP solvent generally play a role in the positive electrode slurry of lithium batteries, and participates in the stages of slurry stirring, coating, coating and baking of lithium batteries. To collect information until today, the editor found that NMP solvent can also act in the negative electrode slurry of lithium batteries?

Let’s review firstly,w, what are the negative electrode materials of lithium batteries?

The negative electrode ingredients are mainly: graphite, deionized water, cmc (sodium carboxymethyl cellulose), sp (conductive carbon black), sbr (styrene-butadiene rubber).

The mixing process is as follows: cmc is first mixed with water to configure glue, then sp is added to configure conductive glue, then graphite is added, stirred evenly, and finally sbr is added and stirred evenly.

In fact, our team discussed this issue yesterday, and it seems that NMP can also participate in the negative electrode of lithium batteries. However, since we are not the R&D and production team of lithium batteries, we cannot know the details. So no conclusion can be drawn. But today, the editor found the journal literature and finally determined that the NMP solvent can really act on the negative electrode of the lithium battery, and can also have a certain impact on the performance of the lithium battery.

The original text of the journal article: In the current lithium-ion battery manufacturing process, a small amount of NMP is usually added to the negative electrode slurry to increase the flexibility of the pole piece during coating and reduce cracks and powder drop;Add NMP to the aqueous cathode slurry to improve flexibility and adhesion.

Not only that, some professionals who have researched on this also said that during the preparation of lithium batteries, NMP (N-methylpyrrolidone) solvent will be added to the negative electrode slurry.It is to reduce the cracking problem of the negative electrode sheet in the coating and baking area during the baking process.

During the drying process of the negative electrode coating of lithium batteries, the edges are easily rolled up and cracked. At this time, if a small amount of NMP is added, this problem can be alleviated.

However, it is also mentioned in the journal literature that when NMP solvent is added to the negative electrode slurry of lithium battery, NMP cannot be completely removed when the negative electrode sheet is heated and dried;The residual NMP is adsorbed on the surface of the negative electrode material graphite, which will make the SEI film and graphite weakly combined.

In the journal article, the researchers used water as the solvent to make two slurries to test:

1. A slurry is added with NMP solvent, the components are graphite, SP, CMC, SBR, water, NMP, and the mass of NMP is 1% of the total mass of the slurry;
2. The other slurry does not add NMP, the composition is graphite, SP, CMC, SBR, water (ratio 95.7:0.6:1.7:2.0).

The mass fraction of the non-volatile matter of the two kinds of slurries is the same. After coating, drying, rolling and die-cutting, two kinds of negative electrode sheets are obtained.

After testing one by one and comparing the results, it is found that adding NMP to the negative electrode slurry has no effect on the initial performance of the battery such as capacity, internal resistance, rate, and storage;

However, it will affect the cycle performance of the battery, mainly manifested as accelerated decay in the late cycle.

Maybe it is the reason that the solvent used in the negative electrode slurry of the lithium battery is replaced by deionized water? ! The researchers suggested in the literature that ethylene carbonate (EC), EC also has the effect of improving the flexibility and adhesion of the pole piece, so EC is also one of the necessary components of the electrolyte. Even if EC remains in the pole piece, it will not have a negative effect on the battery performance.

Of course, which solvent is more suitable to put in the negative electrode slurry in the end is also different due to the composition and preparation method of the battery.