The manufacturing of lithium-ion battery electrodes involves two main processes: the positive electrode slurry, composed of binders, conductive agents, and positive electrode materials; and the negative electrode slurry, composed of binders and graphite carbon powder. The preparation of both positive and negative electrode slurries involves a series of processes, including mixing, dissolving, and dispersing liquids and solids, all of which are accompanied by changes in temperature, viscosity, and environment. The dispersion and uniformity of the particulate active materials in the positive and negative electrode slurries directly affect the movement of lithium ions between the battery electrodes. Therefore, the mixing and dispersion of the slurries for each electrode material is crucial in lithium-ion battery production. The quality of the slurry dispersion directly impacts the quality of subsequent lithium-ion battery production and the performance of the final product.
Further ultrafine dispersion is performed on top of traditional processes because: traditional mixing and stirring equipment can only break up large agglomerates in the solution and distribute them evenly; however, the powder remains in the form of fine agglomerates in the solution, only meeting the processing requirements of macroscopic dispersion. After macroscopic stirring and dispersion, the slurry, under the strong mechanical cutting force of ultrafine dispersion homogenizers, can further break up and homogenize the fine agglomerates or solid particle clusters in the solution, obtaining sufficiently fine solid particles that are evenly distributed in the solution, achieving the purpose of microscopic ultrafine dispersion homogenization, which can significantly improve the overall performance of the slurry.
The current traditional pulping process is:
(a) Ingredients:
1. Solution preparation:
a) The mixing ratio and weighing of pVDF (or CMC) and solvent NMp (or deionized water);
b) Stirring time, frequency, and number of times the solution was stirred (and the surface temperature of the solution);
c) After the solution is prepared, the solution is inspected: viscosity (test), degree of solubility (visual inspection), and standing time;
d) Negative electrode: SBR + CMC solution, stirring time and frequency.
2. Active substances:
a) Monitor the mixing ratio and quantity during weighing and mixing to ensure they are correct;
b) Ball milling: Milling time for both positive and negative electrodes; ratio of agate beads to mixed material in the milling barrel; ratio of large to small agate balls;
c) Baking: Setting the baking temperature and time; testing the temperature after cooling following baking.
d) Mixing and stirring of active substances and solutions: stirring method, stirring time and frequency.
e) Sieving: Pass through a 100-mesh (or 150-mesh) molecular sieve.
f) Testing and inspection:
The following tests were performed on the slurry and mixture: solid content, viscosity, fineness of mixture, tap density, and slurry density.
In addition to understanding the traditional manufacturing process, it is also necessary to understand the basic principles of lithium-ion battery slurry.
Colloid theory
The important reason for the aggregation of colloidal particles is the van der Waals force between the particles. To increase the stability of colloidal particles, there are two ways: one is to increase the electrostatic repulsion between colloidal particles, and the other is to create steric hindrance between the powder particles. These two methods can prevent the aggregation of powder particles.
The simplest colloidal system consists of a dispersed phase and a dispersion medium, with the dispersed phase ranging in size from 10⁻⁹ to 10⁻⁶ m. For substances within a colloid to exist within the system, they must possess a certain degree of dispersion. Depending on the solvent and dispersed phase, various colloidal forms can occur, such as mist (aerogel where droplets are dispersed in a gas) and toothpaste (sol-gel where solid polymer particles are dispersed in a liquid).
Colloids have numerous applications in daily life, but their physical properties vary depending on the dispersed phase and dispersion medium. From a microscopic perspective, colloidal particles are not in a constant state but move randomly within the medium; this is known as Brownian motion. Above absolute zero, colloidal particles undergo Brownian motion due to thermal motion, which is the dynamic characteristic of microscopic colloids. Collisions caused by Brownian motion are the catalyst for aggregation. Since colloidal particles are thermodynamically unstable, the interaction forces between particles are one of the key factors in dispersion.
Double layer theory
The electric double layer theory can be used to explain the distribution of charged ions in colloids and the potential problems that occur on particle surfaces. In the 19th century, Helmholtz proposed the parallel capacitor model to describe the electric double layer structure. It simply assumes that the particles are negatively charged and that the surface is like the electrodes in a capacitor. Positively charged counterions in the solution are adsorbed onto the particle surface due to the attraction between opposite charges. However, this theory ignores the diffusion behavior of charged ions due to thermal motion.
Therefore, in the early 20th century, Gouy and Chapman proposed the diffused double layer model. In this model, counterions in solution are electrostatically adsorbed onto the surface of charged particles and diffuse around the particles due to thermal motion. Thus, the concentration of counterions in the solution decreases with increasing distance from the particle surface. In 1924, Stern combined the parallel capacitor and diffused double layer models to describe the double layer structure. Stern proposed that counterions form a tight adsorption layer on the particle surface, called the Sternlayer. With increasing distance from the particle surface, the particle's potential decreases linearly. Simultaneously, a diffuse layer exists outside the Sternlayer, and the particle's potential in the diffuse layer decreases exponentially with increasing distance.
The figure below shows the Stern double-layer model. The zeta potential (ξ) is a crucial parameter in the double-layer model. While the surface potential of a particle cannot be directly measured in practice, it can be calculated using acoustic methods or electrophoresis. The zeta potential exists on the shear plane between the Stern layer and the diffuse layer in the double-layer model.
The zeta potential is closely related to the dispersion stability of colloids. The larger the zeta potential, the more static charge on the surface of the colloidal particles. When the zeta potential of the particles in the aqueous solution reaches ±25~30mV or higher, the colloid has sufficient electrostatic repulsion to overcome the van der Waals forces between the particles and maintain the stability of the colloid.
Stern double-layer model
DLVO theory
Between 1940 and 1948, Deryagin, Landau, Verwey, and Overbeek established a theory concerning the energy changes of colloidal particles when they approach each other and their impact on colloidal stability, known as the DLVO theory. This theory primarily describes the relationship between the distance between colloidal particles and the energy change; this energy is a result of the charge repulsion energy of the overlapping electric double layers of the colloidal particles and the addition of van der Waals forces.
The diagram below is a schematic of DLVO, illustrating the attractive and repulsive forces between colloidal particles. The magnitude of these two forces determines the stability of the colloidal solution. The attractive force between particles is important, as it will cause the particles to aggregate; while when the repulsive force is greater than the attractive force, it can prevent particle aggregation and maintain the stability of the colloid.
According to the DLVO curve, as the distance between particles decreases, they initially exhibit attractive forces. If particles continue to approach each other, repulsive forces will develop. If particles overcome the repulsive energy barrier, they will rapidly aggregate. Therefore, to improve the dispersion stability of particles in a colloid, it is necessary to increase the repulsive forces between particles to prevent aggregation.
DLVO schematic diagram
Colloid stabilization mechanism
Colloidal particles, due to their high surface energy, tend to aggregate. To ensure the dispersion stability of a colloidal system, the repulsive forces between particles must be increased. The stabilization mechanisms between colloids can generally be divided into three types:
1) Electrostatic stabilization mechanism
2) Three-dimensional obstacle course
3) Application of electrostatic stabilization: The stabilization mechanism is shown in the figure below.
(a) electrostatic repulsion, (b) three-dimensional barrier, (c) electrostatic three-dimensional barrier
The electrostatic stabilization mechanism utilizes the repulsive force caused by the surface charge of particles. When particles approach each other due to attraction, the double electric layers of colloidal particles overlap. Since the particle surfaces carry the same charge, a repulsive force occurs.
However, electrostatic stabilization mechanisms are easily affected by the electrolyte concentration in the solution system. When the electrolyte concentration in the solution is too high, it will cause compression of the electric double layer on the particle surface, which in turn causes particle aggregation. The stabilization mechanism of steric barriers utilizes the adsorption of polymers on the surface of colloidal particles, and its application will result in two different effects, increasing the repulsive force between particles:
1) Osmotic effect
When two colloidal particles approach each other, the long polymer chains adsorbed on the particle surface or residual polymers in the solution will be interposed between the particles. At this time, the polymer concentration between the particles will continuously increase, causing changes in osmotic pressure. The surrounding medium enters between the two particles, displacing them and achieving a stable dispersion effect.
2) Volume restriction effect
The polymer adsorbed on the surface of the particles has a certain spatial hindrance. When the distance between the particles is shortened, the polymer cannot penetrate the particles and will be compressed, causing the elastic free energy to increase, thus displacing the particles and achieving the effect of dispersion.
Compared to electrostatic stabilization mechanisms, polymeric steric barriers offer many advantages. Electrostatic stabilization mechanisms are highly susceptible to environmental influences and cannot be applied to high-electrolyte environments or organic system solutions.
However, polymeric steric hindrance is relatively insensitive to electrolyte concentration and exhibits equal efficiency in aqueous solutions or organic solvents. Furthermore, the effectiveness of polymeric steric hindrance is not affected by the colloidal solids content. When polymers adsorb onto the surface of colloidal particles, even if aggregation occurs, it is soft aggregation, which can be easily broken up. Even after the colloidal particles undergo a drying process, they can still be redispersed in the solvent.
Therefore, steric barriers are more effective than electrostatic stabilization in terms of dispersion stability. Electrostatic steric stabilization combines the effects of electrostatic stabilization and steric barriers; the polymers grafted onto the particle surface carry charges, combining the effects of these two different stabilizing mechanisms to give colloidal particles good dispersion stability.
