Dispersant, thickener and binder for negative electrode paste of CMC lithium battery

CMC is an important cellulose ether. It is a kind of water-soluble polyanionic compound obtained by chemical modification of natural fiber. It is easily soluble in hot and cold water. CMC material has the functions of dispersion, thickening and adhesion. It is used in different types of electrode materials such as natural graphite, mesophase carbon microspheres (MCMB), lithium titanate, silicon-based anode materials and lithium iron phosphate cathode materials In addition, CMC has good thermal stability, conductivity and electrochemical properties. The main mechanisms of CMC improving the performance of Li ion batteries are as follows：
1. The adhesive property of CMC is stable, which creates a necessary prerequisite for obtaining stable battery performance;
2. CMC has good electronic and ionic conductivity, which can promote the transfer of Li
3. Dispersive materials and conductive agents
4. Thickening and anti settling of negative paste
5. Improve the adhesive strength of dressing and fluid collector
6. CMC has good compatibility with SBR latex and other materials.

SPECIFICATION OF AFT-CMC350/500

CMC Production process

① Alkalization
Cellulose reacts with liquid alkali to form alkali cellulose with high reactivity
② Etherification
The alkali cellulose with high reactivity reacts with sodium chloroacetate to form CMC and sodium chloride

Function and working principle of CMC

1. Dispersed anode active material and conductive agent: there is repulsive force between CMC anions distributed on the surface of anode material
2. Thickening and anti settling of negative paste
3. Improve the adhesive strength of dressing and fluid collector

Dissolution of CMC

Because the dissolution of CMC particles is very fast, the particles will gather and stick together when they meet with water. Once agglomerated, dissolution takes a lot of time.
In order to prevent the occurrence, there are the following methods:
1) Aim at the center of vortex formed by mixing and put CMC powder.
2) While stirring violently near the water surface, the powder can be put in as little as possible through a coarse mesh sieve.
3) Mix with insoluble powder before putting into water.
4) CMC was dispersed in water-soluble solvent (ethanol, etc.) and then put into water.

Suggestions for use

1. When CMC dissolves, it will swell first. If a large amount of CMC is added in batch dissolution and the stirring speed is not fast, it is easy to agglomerate. It is suggested that high speed dispersion should be used, and it can be heated to about 40 ℃ when conditions permit.
2. Or the CMC is wetted and dispersed with appropriate amount of alcohol in advance, and then added with the required amount of water and dissolved by high-speed stirring.
3. Due to the small particle size, dust and wall hanging should be prevented when adding and dissolving, so as to avoid particle phenomenon when mixing into slurry at later stage.
4. After dissolution, the slurry can be sealed and stored at room temperature for 1-5 days before putting into use, so that even if there is a small amount of CMC not completely dissolved, it can be fully dissolved during the storage period, ensuring the stability and reliability of use. CMC aqueous solution is a pseudoplastic fluid. When it is fully dissolved and kept still, the molecular chains will cross due to the force between the active groups, resulting in the increase of viscosity, but the viscosity will return to normal immediately after stirring. Therefore, for lithium-ion battery negative paste, it is best to keep stirring at a low speed before coating.
4. The conductive agent can be mixed with CMC powder and dissolved and dispersed at the same time.
5. Small particles are easy to be hydrated and have advantages in high solid content and high viscosity mixing process.
6, there should be no transition metal or two valence metal ions in the process to avoid gel formation.
7. The addition amount of graphite anode is 0.8 ~ 1.2%.

Common Problem

1. Is the viscosity test too large or too small?
a) Improper product storage. CMC has strong water absorption, so it should be stored in a dry environment to avoid moisture absorption, especially in the case of Kaifeng, it should be used as soon as possible.
b) Test method problems:
1) Because CMC generally contains less than 10% moisture, if the moisture is not converted, the liquid-solid content of the actual test glue will be less than 1%.
2) The viscosity of CMC solution is related not only to the degree of polymerization, but also to the shear rate and temperature. CMC viscosity is generally measured at 30 rpm at 25 ℃.
2. What are the reasons for pinholes in negative electrode coating?
a) The foil is not clean;
b) The conductive agent is not dispersed;
c) The main material of negative electrode is not dispersed;
d) There are impurities in some components of the formula;
e) The particles of conductive agent are uneven and difficult to disperse;
f) The negative electrode particles are uneven and difficult to disperse;
g) The formula material itself has quality problems;
h) The mixing pot was not cleaned, resulting in dry powder residue in the pot.