Common Problems and Solutions for Binder  (PVDF, SBR, CMC, PAA, etc.)

What about slurry settlement?

Reason:

l  The selected CMC type is not applicable, and the degree of substitution and molecular weight of CMC will affect the stability of the slurry to a certain extent, such as poor hydrophilicity of CMC with low substitution, good wettability to graphite, but poor suspension capacity of slurry;

l  The amount of CMC is small, and the slurry cannot be effectively suspended;

l  Too much CMC is involved in the kneading process, resulting in insufficient CMC free between particles and in suspension, which often leads to poor slurry stability;

l  High mechanical forces and fluctuations in the acidity or alkalinity of the slurry can lead to breakage of the SBR, which can cause the slurry to settle;

Solutions:

l  Change or match CMC with high substitution degree and high molecular weight, for example, match WSC with CMC2200 in mass production formula, WSC itself has low molecular weight and low substitution degree, good wetting of graphite and weak suspending ability, after matching with CMC2200, the stability of the slurry has been greatly improved;

l  Increasing the amount of CMC is one of the most effective means of improving paste stability, but it is important to find a balance between process capability and the low temperature performance of the cell;

l  Reducing the amount of CMC used in kneading and increasing the amount of free CMC can improve the stability of the paste to a certain extent;

l  After SBR is added to the slurry system, the stirring speed of the rotation should be reduced;

What should I do if the hole is blocked during filtration and cannot be filtered?

Reason:

l  poor wetting of the active substance, no dispersion;

l  Failure to filter due to SBR emulsion breakage;

Solution:

l  Using kneading process;

l  After SBR is added to the slurry system, the stirring speed of the rotation should be reduced to prevent the occurrence of demulsification;

What should I do if there is gel in the slurry?

Reason:

Gel production is mainly divided into two types, one is a physical gel and the other is a chemical gel.

l  Physical gel: Cathode active materials, SP, solvent NMP have absorbed water, or the water content in the environment exceeds the standard, which is easy to form physical gels.This is because the polymer chain of PVDF is wrapped around the particles, and when the content of the slurry exceeds the standard, the polymer chain movement is blocked, and the polymer chains are entangled with each other, which reduces the fluidity of the slurry and the gel phenomenon occurs.

l  Chemical gels: Chemical gels are prone to occur in the process of preparing high nickel or highly basic active materials or in stationary processes.This is because PVDF in the high pH environment of the base (as shown in the figure below), the polymer backbone is easy to de-HF to form double bonds, and the moisture present in the slurry or the amines in the solvent attack the double bonds and form crosslinks, which seriously reduces production capacity and deteriorates battery performance. Generally, with the increase of alkalinity of the active material, the more serious the slurry gel phenomenon.

Solution:

l  Physical gels: can be controlled by tightly controlling the moisture in the raw material and environment, and can be stirred at an appropriate speed when the slurry is stored.

l  Chemical gels: can be controlled by:

(1)   The active substance and conductive carbon need to be baked before dispersion to remove the adsorbed moisture; Improve NMP purity;

(2)   Strict control of ambient moisture during homogenization process;

(3)   The incoming material reduces the free Li on the surface of NCM particles in order to reduce the alkalinity of NCM materials;

(4)   To develop Anti-gel PVDF, the development idea is as follows, grafting other groups to replace H/F in the unit group - CH2-CF2- to inhibit the continuous HF de-HF reaction in the polymer and reduce the proportion of crosslinking sites.At present, the grafted groups or modified groups used are mostly vinyl ether, hexafluoropropylene, tetrafluoroethylene and other monomers.

(5)  Development of non-PVDF cathode binders, because the above solutions can not completely inhibit PVDF HF removal reaction, if the development of highly basic cathode materials (high nickel materials, NCA) or the addition of functional additives (Li2CO3, alkaline), there is still a risk of slurry gel, so non-PVDF cathode binders are being developed to completely solve this problem.

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