Analysis of the characteristics of lithium battery anode materials and the mixing process

As a negative electrode material, graphite needs to be mixed with a conductive agent, a binder and other additives, and a solvent is added to prepare a negative electrode slurry before it can be coated on a current collector to prepare an electrode. However, in the process of preparing graphite anode slurry, it is believed that many technicians have encountered large or small problems, such as too long time, graphite sedimentation, unstable viscosity, large slurry particles, many bubbles, etc. ...Then, before solving these problems, we need to do a simple sorting out of the basic properties of various materials. Only on the basis of understanding the material properties can the problems encountered be solved quickly, accurately and well. This article mainly introduces the characteristics of the materials used in the preparation of anode slurry:

1.Graphite

Graphite includes natural graphite and artificial graphite. The particle shapes are various and irregular, mainly spherical and flake. Graphite is a non-polar substance, it is easily contaminated by non-polar substances, and is easily dispersed in non-polar substances. It is not easy to absorb water and is not easy to disperse in aqueous solution. Even if the contaminated graphite is dispersed in water, it will re-aggregate. Regarding the characteristics and requirements of graphite, I believe everyone is very clear, so I will not repeat them here.

2. Conductive agent

There are many types of conductive agents, including carbon black, CNT, graphene, etc., most of which are non-polar. The role of the conductive agent is to mix between the graphite materials to form the main conductive network and reduce the internal resistance of the lithium battery. The amount of conductive agent added is generally small, and it can be said that the smaller the better, about 2% or less. As a conductive agent, it is generally necessary to have a sufficiently high specific surface area, so as to increase the electron transfer rate with as little as possible. But it will also face the problem of easy reunion and not easy dispersion.

The dispersion of conductive agents has always been an important point. As the types of conductive agents range from simple carbon black, to CNT slurry, to carbon black, CNT, and graphene mixed conductive slurry, the slurry needs to be adjusted according to its properties. The process of preparation. In the lithium battery slurry preparation article [exquisite] analysis of the properties and key influencing factors of lithium battery slurry, it is mentioned that the preparation of slurry will mainly go through the stages of dry mixing of materials, powder wetting, particle cluster crushing, and finally reaching stability. The dispersion of the conductive agent needs to be completed at the stage of powder wetting and particle clusters breaking, otherwise the conductive agent will re-aggregate and cause waste of the conductive agent. When solving the problem of dispersion of conductive agent, the mechanical force and time need to be adjusted to achieve the best state.

3.CMC

CMC (Sodium Hydroxymethyl Cellulose) is an important cellulose ether. It is a polyanionic cellulose compound obtained by chemical modification of natural fibers. It is easily soluble in cold and hot water and belongs to medium Sex material. CMC has excellent properties such as thickening, dispersion, suspension, adhesion, film formation, protective colloid and moisture protection, so it is selected as a dispersant and thickener for graphite negative electrodes.

CMC has dispersibility and binding properties, but it cannot be used alone as a binder for negative electrodes in industrial applications. In the production process of lithium batteries, it is necessary to take into account the energy density and internal resistance of the lithium battery. This requires the negative pole piece to have a certain volume density (about 1.6g/cc). In this case, it is necessary to coat The latter pole piece undergoes rolling compaction treatment, and CMC has greater brittleness, which will inevitably lead to the collapse of the pole piece structure after rolling, and phenomena such as powder drop and foil leakage. Therefore, the condition that CMC can be used alone is: 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, during the preparation of the negative electrode slurry, attention should be paid to the influence of the mixer speed on the CMC. CMC solution has pseudoplasticity, its viscosity decreases with increasing temperature and is reversible. When the mixer rotates too fast, the CMC viscosity will decrease, which will affect the suspension of the graphite anode.

4. SBR binder

SBR (styrene-butadiene rubber) is a small molecule linear chain emulsion. The latex particle unit is a core-shell structure. The shell is a cross-linked structure of copolymer molecular chains. The outer shell is a hydrophilic polar group and surface. Active agent. SBR is a substance that coexists with hydrophilicity and lipophilicity. The water-based groups combine with the groups on the surface of the foil to form a binding force, which is conducive to dispersibility and slurry stability. The oily segment combines with the negative graphite to form a binding force, thereby achieving the bonding effect.

However, SBR cannot be used alone in the graphite anode slurry, because SBR has no dispersing function, and too much SBR will also cause the pole piece to swell in the electrolyte.

In the industrial production of lithium batteries, CMC and SBR are often used at the same time. The combined use of the two can solve the problems of unstable slurry viscosity, pole piece swelling, and high brittleness. Commercial graphite materials are non-polar substances, not easy to be hydrophilic, and difficult to disperse in water systems. One function of CMC is to act as a dispersant, dispersing graphite and conductive additives. In addition, CMC will form a gel when it meets with water, which makes the slurry thicker (thickener) and improves the suspension stability of the aqueous negative electrode slurry. When the slurry is coated, because of the CMC gel structure, it can not only retain moisture but also stabilize the slurry, and maintain the uniformity of the slurry within a certain period of time, which meets the needs of large-scale industrial production. Taking into account the disadvantages of CMC, the introduction of the flexible molecular SBR emulsion that is easily soluble in water makes the slurry have better adhesion and at the same time improves the toughness of the pole piece, so that the pole piece will not be compacted under high pressure. After the powder is removed, the bonding strength of the pole piece after rolling is also high.

When CMC and SBR are used together, one thing to note is the timing of adding SBR emulsion. Because SBR is prone to demulsification under long-term high shearing force, which reduces the adhesion of pole pieces.

5. Deionized water

Deionized water is a weakly polar molecule and a solvent for the negative electrode slurry. A key parameter related to the solvent is the solid content. In the slurry preparation process, the solid content is related to the dispersion and stability of the material.