Lithium battery cell slurry stirring is the most important link in the whole production process, which is the most important link in the whole production process of lithium ion battery.
Composition of positive electrode slurry for lithium ion battery
The positive electrode slurry of lithium ion battery is composed of binder, conductive agent, positive electrode material, etc.; the negative electrode slurry is composed of binder, graphite carbon powder, etc. The preparation of positive and negative slurries includes a series of technological processes such as mutual mixing, dissolution and dispersion between liquid and liquid, liquid and solid materials, and these processes are accompanied by changes in temperature, viscosity, and environment. In the positive and negative electrode slurries, the dispersion and uniformity of the granular active material directly affects the movement of lithium ions between the two poles of the battery, so the mixing and dispersion of the slurry of each pole piece material is very important in the production of lithium ion batteries. , The quality of slurry dispersion directly affects the quality of subsequent lithium-ion battery production and the performance of its products.
In the traditional process, ultra-fine dispersion is carried out because: through traditional mixing and stirring equipment, only large powder clusters in the solution can be dispersed and evenly distributed; however, the powder form exists in the form of fine powder clusters. In the solution, only the processing requirements of macroscopic dispersion are met. The slurry after macroscopic stirring and dispersion can further disperse and homogenize the fine powder or solid particle agglomerates in the solution under the action of the strong mechanical cutting force of the ultra-fine dispersing and homogenizing equipment to obtain sufficiently fine solids. The particles are uniformly distributed in the solution to achieve the effect of microscopic ultrafine dispersion and homogeneity, which can significantly improve the comprehensive performance of the slurry.
The current traditional lithium battery slurry process is:
- Solution preparation:
a) The mixing ratio and weighing of PVDF (or CMC) and solvent NMP (or deionized water);
b) The stirring time, stirring frequency and times of the solution (and the surface temperature of the solution);
c) After the preparation of the solution is completed, the inspection of the solution: viscosity (test), degree of dissolution (visual inspection) and shelving time;
d) Negative electrode: SBR+CMC solution, stirring time and frequency.
a) When weighing and mixing, monitor whether the mixing ratio and quantity are correct;
b) Ball milling: the ball milling time of the positive and negative electrodes; the ratio of the agate beads to the mixture in the ball milling barrel; the ratio of the large balls to the small balls in the agate balls;
c) Baking: setting of baking temperature and time; test temperature after cooling after baking.
d) Mixing and stirring of active substance and solution: stirring mode, stirring time and frequency.
e) Sieve: pass through 100 mesh (or 150 mesh) molecular sieve.
f) Testing, inspection:
The following tests are carried out on the slurry and mixture: solid content, viscosity, mixing fineness, tap density, and slurry density.
In addition to clarifying the traditional craftsmanship, it is also necessary to understand the basic principles of lithium battery slurry.
The main effect leading to the agglomeration of colloidal particles is from the van der Waals force between particles. To increase the stability of colloidal particles, there are two ways: one is to increase the electrostatic repulsion between the colloidal particles, and the other is to generate steric potentials between the powders. In these two ways, the agglomeration of the powder is blocked.
The simplest colloidal system is composed of a dispersed phase and a dispersed medium, and the size of the dispersed phase ranges from 10-9 to 10-6 m. The substances in the colloid exist in the system and need to have a certain degree of dispersibility. According to the difference of solvent and dispersed phase, various colloid forms can be produced, such as: mist is aerosol in which droplets are dispersed in gas, toothpaste is sol in which solid polymer particles are dispersed in liquid.
The application of colloids abounds in life, and the physical properties of colloids vary depending on the dispersion phase and dispersion medium. Observing colloids from a microscopic point of view, colloidal particles are not in a constant state, but move randomly in the medium, which is what we call Brownian motion. Above absolute zero, colloidal particles will undergo Brownian motion due to thermal motion, which is the dynamic characteristics of microscopic colloids. The collision of colloidal particles due to Brownian motion is an opportunity for aggregation, and colloidal particles are in a thermodynamically unstable state, so the interaction force between particles is one of the key factors of dispersion.
Electric double layer theory
The electric double layer theory can be used to explain the distribution of charged ions in colloids and the potential problems on the surface of particles. In the 19th century, Helmholtz proposed the parallel capacitor model to describe the electric double layer structure. It is simply assumed that the particles are negatively charged, and the surface is like an electrode in a capacitor. However, this theory ignores the diffusion behavior of charged ions due to thermal motion.
Therefore, at the beginning of the 20th century, Gouy and Chapman proposed the diffusion electric double layer model, in which the counter ions in solution will be adsorbed on the surface of charged particles due to electrostatic interaction, and at the same time, they will diffuse around the particles due to thermal motion. Therefore, the distribution concentration of counter ions in the solution will decrease with the distance from the particle surface. In 1924, Stern combined the two models of parallel capacitor and diffused electric double layer to describe the structure of electric double layer. Stern believes that counter ions will form a tight adsorption layer on the surface of the particle, also known as the Stern layer. As the distance from the particle surface increases, the potential of the particle will decrease linearly. At the same time, there is also a diffusion layer outside the Stern layer, and the particles will diffuse The potential in the layer decreases exponentially with distance.
The figure below shows the Stern electric double layer model. The zeta potential (ξ, Zeta potential) is a very important parameter in the electric double layer model. The surface potential of the particle cannot be directly measured in actual measurement, but it can be calculated by the acoustic wave method or the electrophoresis method. out the zeta potential of the particle. The zeta potential exists on the shear plane between the Stern layer and the diffusion layer in the electric double layer model.
The zeta potential is closely related to the dispersion stability of the colloid. When the zeta potential is larger, the electrostatic charge on the surface of the colloid particle is more. When the zeta potential of the particle in the aqueous solution reaches ±25~30mV, the colloid has sufficient Electrostatic repulsion overcomes van der Waals forces between particles to maintain colloidal stability.
From 1940 to 1948, Deryagin, Landau, Verwey, Overbeek established the related theory of energy change when colloidal particles approach each other and its influence on colloidal stability, referred to as DLVO theory. Its theory mainly describes the relationship between the distance between colloidal particles and the change of energy.
The following figure is a schematic diagram of DLVO, indicating that there are attractive and repulsive forces between colloidal particles. The size of these two forces determines the stability of the colloidal solution. The attraction between particles is the main effect, and the particles will agglomerate; while the repulsive force In the state of greater than the attractive force, the particle cohesion can be avoided and the stability of the colloid can be maintained.
From the DLVO curve, when the distance between the particles is getting shorter and shorter, the particles will firstly attract each other, if the particles continue to approach each other, the repulsive force will be generated between the particles, and if the particles cross the repulsion barrier, the Aggregates quickly. Therefore, in order to improve the dispersion stability of particles in colloids, the repulsive force between particles must be improved to avoid agglomeration between particles.
Colloid stabilization mechanism
Colloidal particles tend to agglomerate due to their high surface energy. In order to make the colloidal system have dispersion stability, the repulsive force between particles must be improved. The stabilization mechanism between colloids can generally be divided into three types:
1) Electrostatic stabilization
2) Steric hindrance
3) Electrosteric stabilization, the stabilization mechanism is shown in the following figure:
(a) electrostatic repulsion, (b) steric barrier, (c) electrostatic steric barrier
The electrostatic stabilization mechanism uses the repulsive force caused by the surface charges of the particles. When the particles are close to each other due to attraction, the electric double layers of the colloidal particles are overlapped, and the repulsive force is generated due to the same charge on the surface of the particles.
However, the electrostatic stabilization mechanism is easily affected by the electrolyte concentration in the solution system. When the electrolyte concentration in the solution is too high, the electric double layer on the surface of the particles will be compressed, which will cause the particles to agglomerate. The stabilization mechanism of steric barriers is to use macromolecules to adsorb on the surface of colloidal particles, which will produce two different effects to enhance the repulsive force between particles:
1) Osmotic Effect
When the two colloidal particles are close to each other, the long-chain polymer adsorbed on the surface of the particles or the residual polymer in the solution will be interposed between the particles. At this time, the continuous increase of the polymer concentration between the particles will cause the change of the osmotic pressure, and the surrounding medium will enter the two colloidal particles. Between the particles, the distance from each other is arranged to achieve the effect of stable dispersion.
2) Volume restriction effect
In order to adsorb the macromolecule on the upper surface of the particle, there is a certain space barrier. When the distance between the particles is shortened, because the macromolecule cannot penetrate the particle, the macromolecule will be compressed, resulting in an increase in the elastic free energy, thus displacing the particle and achieving the effect of dispersion. .
Compared with electrostatic stabilization mechanisms, polymeric steric barriers have many advantages. The electrostatic stabilization mechanism is easily affected by the environment and loses its effect, and cannot be applied to high electrolyte environments or organic system solutions.
However, the macromolecular steric barriers are relatively insensitive to the electrolyte concentration, and have the same efficiency in aqueous solution or in organic solvents, and the macromolecular steric barriers do not affect the effect due to the colloidal solid content. When the polymer is adsorbed on the surface of colloidal particles, even if agglomeration occurs, it is still soft agglomeration, which can easily break the agglomeration phenomenon. Even if the colloidal particles are dried, they can still be dispersed in the solvent again.
Therefore, the effect of steric barriers on dispersion stability is relatively higher than that of electrostatic stabilization. The electrostatic steric stabilization has both an electrostatic stabilization mechanism and a steric barrier. The polymer grafted on the surface of the particle is charged, so that the two different stabilization mechanisms are added, so that the colloidal particles have good dispersion stability.
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