Lithium-ion battery pulping process (2) - pulp dispersion and stabilization mechanism

3.1 Van der Waals forces

According to the London theory, when the wave electrons are distributed around an atom or molecule, a temporary dipole is generated. This temporary dipole causes the adjacent atoms or molecules to generate dipoles, and causes van der Waals attraction between two neutral atoms or molecules. The resulting dipole always attracts two atoms to each other. The van der Waals attraction is the sum of dispersion, induction and orientation forces, and its magnitude is inversely proportional to the sixth power of the distance between particles. The attractive potential energy between two approximately spherical particles in close proximity in vacuum is:


3.2 Electrostatic force

If there is only van der Waals attraction between the particles, the particles must be agglomerated and precipitated, and the electrostatic repulsion can be avoided by charging the surface of the particles to avoid the agglomeration and precipitation between the particles. In the actual slurry, the particle surface will be charged due to self-dissociation, lattice substitution or lattice deletion, adsorption, etc. Due to the existence of surface charges on the particles, the charged particles tightly adsorb some anti-signal ions through Coulomb and other gravitational forces, forming a compact layer. In the range outside the compact layer, the positive and negative ions in the solution exhibit a certain positional distribution under the two opposing effects of electrostatic repulsion and thermal motion, and this range is called the diffusion layer. The interface between the dense layer and the diffused layer is called the Stern layer, which constitutes the electric double layer. The potential difference between the surface of the particles in the slurry relative to the solvent body is called the surface potential ϕo, the potential difference between the Stern layer and the diffusion layer is the Stern potential ϕs, the zeta potential is the potentiodynamic potential or Zeta potential, and the thickness of the Stern layer Usually denoted by δ.

When the particulate matter moves in the solvent, the zeta potential ζ of the same sign makes the particles repel each other, which can prevent the occurrence of agglomeration and keep the particles in a dispersed state. The electrostatic repulsion between particles is related to their spacing. When the diffusion layers of adjacent particles do not overlap, there is no repulsion. When the particles are close to each other and overlap with the surface diffusion layers, a strong electrostatic repulsion is formed.

For two spherical particles with the same size and the same surface potential, the electrostatic repulsion potential energy generated between the two particles (as shown in Figure 11, the electrostatic repulsion potential energy changes with the particle spacing) UR is:

It can be seen that when the Zeta potential of the particles in the slurry is the largest, the electric double layer of the particles shows the maximum repulsion, so that the particles are dispersed; when the Zeta potential of the particles is equal to zero (that is, the isoelectric point IEP), the attraction between the particles is greater than the double layer of the particles. The repulsive force between the electrical layers causes the particles to agglomerate and settle.

3.3 The steric hindrance force A certain amount of uncharged polymer compound is added to the slurry to make it adsorb around the particles to form a thicker steric hindrance layer, so that a steric repulsion force is generated between the particles. The adsorption of polymer compounds on the particle surface can be divided into three types: horizontal type, ring type and tail type.

In addition, two situations occur when the particulate matter adsorbing macromolecular polymers is close to each other:
(1) The adsorption layer is compressed without mutual penetration;
(2) Interpenetration and overlapping of adsorption layers occur.

The size of the steric hindrance potential energy between the two particles is:

3.4 Solvation force When the particle surface adsorbs organic substances or cations containing hydrophilic groups, the particle surface will form a solvation effect. At this time, if the particles are close to each other, there will be a strong repulsion between them. called the solvation energy. The solvation energy for spherical particles with radii R1 and R2, respectively, can be expressed as:

3.5 Mechanical shearing force During the preparation of lithium battery slurry, through strong mechanical stirring, the particulate matter in the slurry collides and squeezes with each other, and at the same time, the liquid flow shears break and disperse the agglomerated large particles. The direct reason for the dispersion and disintegration of aggregates is the effect of shear stress and pressure, and shear stress plays a very important role in the dispersion process.

4. Lithium battery slurry stabilization mechanism

The relationship between the van der Waals energy, electrostatic repulsion energy, solvation energy and steric hindrance energy between particles in the slurry is the main factor determining the dispersion stability of the lithium battery electrode slurry. The theoretical criterion for slurry dispersion and agglomeration can be expressed by the following formula:

When the mutual attraction energy between particles in the slurry is less than the repulsive energy, it is a stable dispersion state; otherwise, the slurry will agglomerate. Lithium battery slurry belongs to suspension dispersion system, and the dispersion stabilization mechanism of slurry can be explained by referring to the stabilization mechanism of colloid. In colloidal dispersion systems, widely used stabilization mechanisms include DLVO theory (electrostatic stabilization theory or electric double layer stabilization mechanism), steric stabilization mechanism, and electrostatic steric stabilization mechanism.

4.1 DLVO theory

DLVO theory is a theory developed by Deriaguin, Landon, Verwey and Overbeek in the 1940s to study the stability of charged colloidal particles. The relationship between the charged charge and the stability of the colloidal system is also called the electric double layer repulsion theory. The DLVO theory holds that whether a sol exists stably or agglomerates under certain conditions depends on the competition between the mutual attractive force and electrostatic repulsion between particles. If the repulsive force is greater than the attractive force, the sol is stable; The interaction potential energy curve between two charged particles, when the particles are far apart, there is no interaction between the particles and VT is zero; when the particles begin to approach each other, the van der Waals gravitational potential VvaW increases rapidly, while the electric double layer repulsive potential Vdl increases Relatively slow, at this time, the total potential energy of z is negative; when the particles continue to approach, the repulsive potential energy curve of the electric double layer rises sharply, and the total potential energy rises to a positive value. When the two particles are close to a certain distance, the total potential energy reaches the maximum value, which is called "potential barrier". The height of the potential barrier is considered to be the activation energy that must be overcome for the particles to adhere. When the particle energy is high enough to overcome the repulsive barrier, it causes the nanoparticles to collide together and agglomerate. Although the DLVO theory ignores the steric hindrance force formed by the adsorption of high-molecular polymers, this theory successfully explains the stabilization behavior of dilute suspensions. In addition, according to this theory, by adjusting the pH value of the slurry and adding electrolytes, the surface double electric charge can be increased. The layer thickness and zeta potential can increase the barrier between particles, thereby improving the dispersion stability of the slurry. Commonly used dispersants for electrostatic stabilization are generally electrolytes with small molecular weight and high ionic charge, such as sodium pyrophosphate, sodium hexametaphosphate, and citrate.

4.2 Steric hindrance stabilization mechanism

When using the DLVO theory to explain the stabilization mechanism between particles, the steric hindrance produced by the polymer material is ignored, and the total potential energy curve corresponding to the steric repulsion is considered. It can be seen that the existence of steric hindrance significantly changes the total potential energy curve between particles, and the steric hindrance potential increases the energy barrier that must be overcome for particle aggregation, which is beneficial to the long-term stability of the slurry. Dispersants that are purely sterically hindered are non-ionic polymers with high molecular weight, such as gum arabic, gelatin, peach gum, carboxymethyl cellulose, polyvinyl alcohol, polyethylene glycol, herring oil, etc.

4.3 Electrostatic steric hindrance stabilization mechanism

A certain amount of polymer polyelectrolyte is added to the slurry to make the surface of the particles adsorb the polyelectrolyte. At this time, the polyelectrolyte can not only repel the surrounding particles through its own charge, but also prevent the surrounding particles from approaching through its steric hindrance effect. The combined effect of the two can achieve the effect of composite stable dispersion (as shown in Figure 18). The commonly used electrostatic steric hindrance dispersants are ammonium polyacrylate, sodium polyacrylate, sodium alginate, ammonium alginate, sodium lignosulfonate, sodium petroleum sulfonate, ammonium polyacrylate, hydrolyzed ammonium acrylate, phosphate ester, ethoxylate, etc. .

5. Summary

Lithium battery slurry is a multiphase composite suspension, and there are various interactions between the particulate matter in the slurry, including van der Waals attraction, electrostatic repulsion and steric hindrance. Whether the particles in the slurry are uniformly dispersed or agglomerated is closely related to the total potential energy between the particles. To achieve uniform dispersion of each component of the slurry, it is necessary to increase the size of the potential barrier between the particles and reduce the Brownian motion of the particles to cross the potential barrier and agglomerate. Considering the direction of improving the repulsion of slurry particles, strategies to improve the dispersion uniformity of lithium battery slurry include:

Improve the mechanical dispersion strength, when the mechanical shear force is increased, the particle agglomerates are fully depolymerized and dispersed;
Adjust and control the pH value of the slurry or add inorganic electrolyte to increase the Zeta potential of the particle surface and improve the electrostatic repulsion;
Add dispersant or surfactant to improve the strength of steric hindrance by surface adsorption of polymer compounds.