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As a cathode material, graphite needs to be mixed with conductive agent, binder and other additives, plus solvent to prepare a negative slurry, then coated on the collector to prepare electrode. However, in the process of preparing graphite negative slurry, I believe that many technical personnel have encountered large or small problems, such as time-consuming too long, graphite sedimentation, viscosity instability, slurry particles, bubbles and so on
This paper mainly introduces the properties of the materials used to prepare the negative slurry:
Graphite includes natural graphite, artificial graphite, particle shape variety and irregular, mainly spherical, flake, etc. Graphite belongs to non-polar substances, easy to be polluted by non-polar substances, easy to disperse in non-polar substances. It is not easy to absorb water, and it is not easy to disperse in aqueous solution. The contaminated graphite will reunite even after dispersing in water.
2. The conductive agent
There are a variety of conductive agents, including carbon black, CNT, graphene, etc., most of which are non-polar. The role of conductive agent is mixed in graphite materials, constitute the main conductive network, reduce the internal resistance of lithium battery. The addition of conductive agent is generally less, the less the better, about 2% below. As a conductive agent, it is generally necessary to have a high enough specific surface area to increase the electron transport rate with as little as possible. But it will also face easy reunion, not easy to disperse the problem.
The dispersion of conductive agent has always been a focus, with the type of conductive agent from pure carbon black, to CNT slurry, and then to carbon black, CNT, graphene mixed conductive slurry, with its different properties need to adjust the slurry preparation process. The preparation of slurry mainly goes through the stages of material dry mixing, powder wetting, particle cluster crushing and finally reaching stability, etc. The dispersion of conductive agent needs to be completed in the stages of powder wetting and particle cluster crushing, otherwise the conductive agent will reunite, resulting in the waste of conductive agent. It is necessary to adjust the mechanical force and time to reach the best state when solving the dispersion problem of conductive agent.
CMC (sodium hydroxymethyl cellulose) is an important cellulose ether, which is a water-soluble polyanionic cellulose compound obtained after chemical modification of natural fiber. It is easily soluble in hot and cold water and belongs to neutral substance. CMC has excellent properties such as thickening, dispersing, suspension, bonding, film forming, colloid protection and water protection, so it is selected as dispersant and thickener of graphite anode.
CMC has dispersibility and cohesiveness, but it cannot be used as a negative electrode binder alone in industrial applications. In the process of the production of lithium-ion batteries, must be both lithium battery energy density, battery internal resistance and other properties, which requires the negative pole piece has a certain volume density (1.6 g/cc), in this case need to be coated sheet after roller compaction, and CMC has great brittleness, after rolling will inevitably lead to collapse of plate structure, Appear to drop powder, leakage foil and other phenomena. Therefore, CMC can be used alone under the condition that the thickness of the pole piece is thin, the rolling process is not carried out, or the compaction density of the pole piece is not high.
In addition, in the preparation process of negative slurry, we should pay attention to the influence of mixer speed on CMC. CMC solution has pseudoplastic properties, and its viscosity decreases with increasing temperature, which is reversible. In the case of too high speed of mixer, the viscosity of CMC will decrease, thus affecting the suspension of graphite anode.
4. SBR Binder
SBR (styrene-butadiene rubber) is a small molecule linear chain emulsion, latex particle unit is a core-shell structure, the shell is a copolymer molecular chain crosslinking structure, the shell is hydrophilic polar groups and surfactants. SBR is a hydrophilic and lipophilic substance. Bonding force is formed by combining water-based groups with surface groups of foil, which is conducive to dispersion and slurry stability. Bonding force is formed by combining oily chain segments with negative graphite, so as to achieve the effect of bonding.
However, SBR cannot be used in graphite anode slurry alone because it has no dispersive function and too much SBR will cause the electrode sheet to swell in the electrolyte.
In the industrial production of lithium battery, CMC and SBR are often used at the same time. The combination of CMC and SBR can solve the problems such as unstable viscosity of slurry, pole swelling and brittleness. Commercial graphite materials belong to non-polar substances, not easy to hydrophilic, difficult to disperse in water system. One of the functions of CMC is as a dispersant, dispersing graphite and conducting additive. In addition, when CMC encounters water, it will form gel, which thickens the slurry (thickener) and improves the suspension stability of the water negative slurry. In the slurry coating, because of the existence of CMC gel structure, can not only maintain moisture can also stabilize the slurry, in a certain period of time to maintain the uniformity of the slurry, in line with the needs of large-scale industrial production. Considering the disadvantages of CMC, the flexible molecular SBR emulsion, which is easily soluble in water, is introduced to make the slurry have better cohesibility and improve the toughness of the electrode piece. In this way, the electrode piece will not drop powder under the condition of high pressure compaction, and the adhesion strength of the electrode piece after roller compaction is also high.
When CMC and SBR are used together, one thing to note is the timing of SBR emulsion. Because SBR is prone to demulsification under high shear force for a long time, which reduces the adhesion of the polar plate.
5. Deionized Water
Deionized water is a weak polar molecule and a solvent for the negative slurry. Solid content is one of the key parameters related to solvent. In the process of slurry preparation, the solid content is related to the dispersion and stability of materials.