Lithium-ion battery pulping process (1) - pulp dispersion and stabilization mechanism
According to the different states of matter, the phase states of matter can usually be divided into solid, liquid and gaseous states. In addition, there is another special state of matter between solid and liquid states. Nobel Prize winner Pierre-Gilles de Gennes Such substances are called 'soft substances'. Soft substances mainly include polymers, liquid crystals, surfactants, colloids, emulsions, foams, suspensions, and biological macromolecules. Soft substances such as colloids and suspensions are also widely studied and applied dispersion systems in daily life. Dispersion system refers to one or several substances dispersed in another substance. The substance is called the dispersion medium. According to the particle size of dispersed phase, it can be divided into molecular dispersion system (particle radius <1nm), colloidal dispersion system (1nm<particle radius<100nm) and suspension (particle radius>100nm). Lithium battery slurry is mainly composed of active material, conductive agent, binder and solvent. Its dispersed phase is composed of particles with different particle sizes, shapes and densities. The corresponding dispersion medium is divided into oily solvent NMP (often used as positive electrode slurry). feed solvent) and aqueous solvent deionized water (often used as anode slurry solvent). Therefore, lithium battery slurry is also a kind of suspension like sediment, paint and ceramic slurry. In the preparation of lithium battery pole pieces, the pulping process is the front-end process, and the quality and process stability of the obtained pulp will have a significant impact on the entire production process. Therefore, the dispersion and stability of the pulp during the pulping process are studied and analyzed. The chemical mechanism is the theoretical guarantee for obtaining a slurry with high dispersion, uniform composition and stable performance.
1. Powder agglomeration
The powder materials involved in the lithium battery pulping process mainly include micron-scale active particles and nano-scale conductive agents. The binder is usually pre-stirred to obtain a binder glue. In different stages of powder material preparation, drying and post-processing, it is easy to form agglomerates with several connecting interfaces between particles. According to the agglomeration size of particles, they can be divided into primary particles, agglomerates, agglomerates and flocs.
Fig.1 Different aggregation states of powder particles
(a) Primary particles: particles of a single particle or crystal, referred to as primary particles;
(b) Condensate: the primary particles are connected by faces, and cannot be separated without external energy;
(c) Agglomerate: refers to the clusters formed by the point and angle connection between primary particles or the adsorption of small particles on large particles;
(d) Flocs: looser structures formed in order to reduce the surface energy due to the increase in the surface area of the system.
In addition, powder agglomerates can be divided into hard agglomerates and soft agglomerates according to the difference in the interaction force between the particles in the agglomerates and the differences in the agglomeration methods. Hard agglomeration is formed by strong chemical bonds between particles, and its structure is not easily destroyed during the processing and molding of powder; soft agglomeration is caused by weaker forces such as van der Waals force, electrostatic attraction, and capillary force. Chemical action or the application of mechanical action to eliminate.
There are different theories on the agglomeration mechanism of powder particles. The reasons for the soft agglomeration of powder include:
Size effect: As the particle size decreases to the nanometer level, the specific surface area of the particle increases significantly, the surface atomic ratio and active groups increase rapidly, and the active particles collide and agglomerate;
Surface electronic effect: insufficient coordination on the surface of nanoparticles, there are a large number of crystal defects and unsaturated bonds, and the accumulation of surface charges makes the surface of the particles extremely unstable and easy to agglomerate;
Surface energy effect: nanoparticles with large surface area and high surface energy are in an unstable state of energy, and are prone to aggregation and tend to be in a stable state.
Proximity effect: The distance between nanoparticles is small, the van der Waals force between them is much greater than the gravity, and the particles are easy to agglomerate through the intermolecular attraction.
Regarding the mechanism of particle hard agglomeration, there is currently no unified theory to explain this, mainly including the following theories:
Chemical bond theory: The chemical bond theory holds that the non-bridging hydroxyl groups present on the surface of the gel are the source of hard agglomeration.
Capillary adsorption theory: The capillary adsorption theory believes that the hard agglomeration is mainly caused by the capillary action generated by the exclusion of water molecules during the separation and drying process of the nano-powder.
Hydrogen bonding theory: The hydrogen bonding theory believes that hydrogen bonding is the main reason for the hard agglomeration of nanoparticles.
Crystal bridge theory: The crystal bridge theory is based on the fact that the nano powder has a certain dissolution phenomenon in the dispersion medium. Some atoms and surface hydroxyl groups dissolve and precipitate in the medium to form a crystal bridge, which makes the particles more compact.
Surface Atomic Diffusion Theory: The surface atoms of powder particles obtained after high temperature decomposition have high activity, the energy generated by surface bond breaking is much higher than the energy of atoms inside the powder, and surface atoms can easily diffuse to the surface of adjacent particles. Bonds with corresponding atoms, forming strong chemical bonds, resulting in hard agglomerations.
2. Macro and micro process of pulping
The main purpose of the lithium battery pulping process is to uniformly disperse the active materials, conductive agents, binders and other substances to obtain a uniform and stable slurry for the pole piece coating process. The ideal electrode structure is shown in Figure 3. The particles of each component are uniformly dispersed without agglomeration, and the active particles are in full contact with the conductive agent and the binder to form a good electronic and ionic conductive network. The macroscopic process of the pulping process is the dispersion and uniform mixing of different components, while the microscopic process involves the interaction between particles and the formation of a stabilizing network structure during the pulping process. The dispersion of particles in lithium battery pulping includes the following steps:
Wetting of solid particles in liquid phase;
Deagglomeration and dispersion of solid particle agglomerates under the action of mechanical force;
The depolymerized slurry is stabilized to prevent re-agglomeration.
2.1 Wetting of powder particles
Wetting is the process of slowly adding powder into the liquid system so that air or other impurities adsorbed on the surface of the powder are replaced by liquid. The wetting of the electrode material surface is mainly determined by the degree of polarity difference between the surface of the liquid phase and the surface of the particles. The wettability of the powder in the liquid phase is an important prerequisite for the uniform dispersion of the powder. Agglomeration and agglomeration will affect the subsequent dispersion and mixing. The wetting properties of powder particles and solvents are usually characterized by the wetting angle, which is related to the size of the solid-liquid interfacial tension. According to the size of the wetting angle, the wettability of powder and solvent can be divided into four grades: θ=0, strong hydrophilicity; 0<θ<40°, weak hydrophilicity; 40<θ<90°, Weak hydrophobicity; θ>90°, strong hydrophobicity. In addition, the wetting heat can also be used to characterize the wettability. The larger the wetting heat, the better the wettability of powder and solvent.
2.2 Aggregate depolymerization
During the lithium battery pulping process, the particle agglomerates are depolymerized and dispersed under the action of shear force, centrifugal force, compressive stress, inertial force, etc., and the initially larger agglomerates are broken and dispersed to form smaller particles. The deagglomeration process of agglomerate particles can be further refined into three stages: erosion, rupture, and shattering. Erosion usually occurs during the low-energy stirring stage, when fine particle fragments fall off the surface of the agglomerates under shear force; with the increase of stirring intensity and time, the initially large agglomerates break down into smaller clusters, this stage In order to break; the stirring intensity increases continuously, the large agglomerates are rapidly deaggregated into fine particle aggregates, and this process is called pulverization. According to the difference of mechanical stirring intensity, the three processes can be carried out gradually or simultaneously.
2.3 Slurry stabilization
After the slurry is dispersed, it is necessary to prevent the particulate matter from agglomerating again, so it is very important to maintain the dispersion stability of the slurry during the pulping process. Whether the slurry is re-agglomerated after dispersion is closely related to the interaction force between particles. At present, different theoretical models have emerged about the dispersion stabilization mechanism of the slurry, as shown in Figure 8, mainly including electrostatic interaction stabilization mechanism and steric hindrance stabilization mechanism. , Electrostatic steric hindrance stabilization mechanism. The theoretical basis of different mechanisms is directly related to the interaction force between particles. The interaction force between lithium battery slurry particles is analyzed and summarized below.
3. Interaction between particles in the slurry
There are various interaction forces between each component particle in the lithium battery slurry, including van der Waals force, electrostatic repulsion force, steric resistance, vacancy force, hydration force, etc. The size of the interaction force between particles determines whether they agglomerate.
