402030 battery

Why is the dry mixing process for 402030 battery slurry better?

Lithium-ion batteries are a complex system engineering project. The performance of batteries is affected by many factors such as raw materials, battery design, manufacturing equipment and processes, and the environment. Any defect may lead to the collapse of battery products. Therefore, although new materials, new designs, and new processes for lithium batteries have emerged in large numbers, their industrialization process is very slow, and lithium batteries have not seen major technological innovations.

Materials are the basis of lithium batteries, and manufacturing processes are also very important. Among them, the mixing process has an impact on the quality of the product by more than 30% in the entire production process of lithium-ion batteries, and is the most important link in the entire production process. In the manufacture of 402030 battery electrodes, the positive and negative electrode slurries are basically composed of active substances, polymer binders, conductive agents, etc. There are roughly three types of mixing processes for electrode slurries:

(1) Ball milling process, originally derived from the coating industry;

(2) Wet mixing process, the basic process is sol-mixed conductive agent-mixed active substances-dilution. This is the current mainstream process in China.

(3) Dry mixing process. The basic process is to mix the active material, conductive agent and binder dry powder - add appropriate amount of solvent to moisten - add solvent to disperse and crush at high speed - dilute to adjust viscosity.

The requirements for battery slurry are: first, uniform dispersion. If the slurry is unevenly dispersed and there is serious agglomeration, the electrochemical performance of the battery will be affected; second, the slurry needs to have good sedimentation stability and rheological properties to meet the requirements of the electrode coating process and obtain a coating with uniform thickness.

Advantages of dry mixing process

At the beginning, the manufacture of lithium battery slurry borrowed from the coating industry. In 1999, Koreans began to study the influence of feeding sequence on slurry properties and battery performance. They adopted four feeding processes as shown in Figure 1 for mixing. Using the same materials and formula, only changing the feeding sequence can change the properties of the slurry. The degree of mixing of the slurry depends on the particle size, particle size distribution, shape, specific surface area, solvent absorption rate of the particles, etc. The time required from the start of stirring to the stability of viscosity is most related to the specific surface area of the materials added in sequence.

Method 1: The active material does not absorb the liquid sufficiently. The specific surface area of the conductive agent is much larger than that of the active material particles. The surface absorbs a large amount of liquid, and the liquid is trapped in the conductive agent and cannot flow easily.

Method 2: The active material has a small specific surface area and is easier to release liquid. The conductive agent is added later and begins to absorb the solvent, and the viscosity is stable for a longer time.

Method 4: The active material and the conductive agent absorb the liquid at the same time and wet the solid particles. This method absorbs the solvent most fully and the slurry has the best dispersion. Therefore, the slurry viscosity is the lowest under the same solid content conditions.

The research results show that the time for the slurry viscosity to reach stability using the fourth process is acceptable, and the prepared slurry has the lowest viscosity, as shown in Table 1, with the best dispersion, and the half-battery cycle test results show that the battery prepared by this process has the smallest discharge capacity decay (as shown in Figure 2).

Qian Long et al. adopted a pre-mixing process of active material, conductive agent, and binder dry powder - ultra-high viscosity stirring process. Compared with the traditional wet process, the negative electrode slurry produced by this process has better performance. The viscosity, particle size and solid content stability of the slurry are better than those obtained by the fluid dispersion process. The resistivity of the prepared membrane is lower, the adhesion is higher, and the capacity retention rate of the prepared battery cell is higher. When ultra-high viscosity stirring is used, the large shear force can more fully disperse the conductive agent with fine particles that are easy to agglomerate, and it is also more conducive to the dissolution and stability of the binder, thereby improving the battery performance.

Basic process of dry mixing process

The basic process of solid powder dispersion in liquid is shown in Figure 3, which is divided into:

(1) Wetting of powder, replacing the air attached to the powder with liquid medium. A powder must be wetted to be dispersed in a liquid. The wettability of the solid surface is determined by its chemical composition and microstructure. The larger the free energy of the solid surface, the easier it is to be wetted by the liquid; vice versa, the wettability can be expressed by the size of the contact angle. (2) The rupture and dispersion of particle agglomerates. There are three main forces for breaking agglomerates: mechanical force during the rotation of the equipment, force generated by collisions between particles, and high-speed dispersion shear force. (3) Stabilization of suspended solids to prevent dispersed particles from agglomerating. Dispersion stabilization includes electrostatic stabilization and steric hindrance stabilization.

The typical dry mixing process is as follows:

(1) Add active material, conductive agent, and binder powders to the stirring kettle and mix the dry powders evenly; (2) Add an appropriate amount of solvent to wet the powder particles so that the surface of the particles adsorbs the solvent. At the same time, stir under this high viscosity to start to form a large shear force to fully mix and wet the powder particles;

(3) Continue to add solvent to disperse the particle agglomerates under the action of high-speed shear force to evenly distribute the conductive agent;

(4) Continue to add solvent to dilute the slurry and adjust the viscosity to make it suitable for the coating process.

Among them, the wetting of the powder is the core step of the process. There is a critical point in the amount of solvent. If the solvent is too little and is not enough to wet all the powder, the dry powder will inevitably clump, and it will be difficult to open it later. Moreover, in the case of overdrying, the slurry in the double planetary mixer is easy to "climb the pole" and cannot achieve the stirring effect; if the solvent is too much, the slurry is easy to flow, the shear force effect of the stirring paddle is reduced, and the kneading and stirring cannot achieve the effect of kneading force to crush the agglomeration. The effect of the actual slurry can be judged by fineness and viscosity: under the same conditions, the smaller the viscosity and the smaller the fineness, the better the dispersion effect.

High-intensity dry powder mixing process

In recent years, the dry mixing process has been further optimized in the first step of dry powder mixing, and there have been reports on the improvement of slurry and battery characteristics by high-intensity dry powder mixing process. Figure 4 is a schematic diagram of the structure of the high-intensity shear mixing equipment Nobilta?. The gap between the stirring elbow and the wall is 3mm, and there is a layer of water jacket between the inner and outer walls to cool the temperature rise caused by the heat generated during high-speed dispersion. During the mixing process, due to the high shear force generated by high-speed rotation, the solid particles are centrifugally separated by the high-speed rotation of the high-speed rotating shaft. The strength of high-speed dispersion can be characterized by the Froude number Fr (Froude-tool number), which is defined as the ratio of the centrifugal force acting on the particles to the gravity, and can be described by formula (1). When the radius of the rotor remains unchanged, Fr depends on the rotor speed ω. The higher the rotor speed, the larger the Froude number, indicating that the strength of high-speed dispersion is greater.

High-intensity dry powder shear dispersion has two effects: on the one hand, high shear force can fully break up and disperse the conductive agent agglomerates; on the other hand, under the action of high-speed dispersion, dry powder stirring can achieve microscopic mixing, and deposit a layer of conductive agent deposited by fine dispersed particles on the surface of larger active material particles, thereby forming a good conductive network. As shown in Figure 5, the general dry powder mixing strength is low, the conductive agent is not completely dispersed, and there are still agglomerations on the surface of the active material particles. The high-intensity dry powder mixing process makes the conductive agent agglomerates fully broken up and dispersed, forming a deposition layer on the surface of the active material.

Figure 5 Comparison of particle micromorphology of general dry powder mixing (left) and high-intensity dry powder mixing (right)

The main parameters of high-intensity dry powder dispersion mixing are: (1) the intensity of high-speed dispersion, which can be expressed by Froude number or rotor linear speed, and (2) the time of high-speed dispersion. Figure 6 shows the effect of high-intensity dry powder mixing on the porosity of coated pole pieces. After the pole pieces are coated, they are not rolled. High-intensity dry powder mixing can reduce the porosity of the pole pieces. When the rotation speed is constant, the longer the dispersion time, the lower the porosity. When the dispersion time is constant, the higher the rotation speed, the lower the porosity.

This is the effect of high-intensity dry powder mixing on the bonding strength of coated pole pieces. After the pole pieces are coated, they are not rolled. High-intensity dry powder mixing can improve the bonding strength of the pole pieces. When the rotation speed is constant, the longer the dispersion time, the higher the bonding strength. When the dispersion time is constant, the higher the rotation speed, the higher the bonding strength.

Therefore, the use of high-intensity dry powder mixing technology will inevitably improve the performance of the battery, as shown in Figure 8. After the electrode is coated, it is rolled to the same coating compaction density, and then the same negative electrode is assembled into a full battery to test the battery performance. Compared with batteries that do not use this process, the high-intensity dry powder mixing process can improve the rate characteristics and cycle performance of the battery.

However, if the high intensity is too high or the time is too long, the conductive agent is crushed into fine particles. Although the contact and dispersion effect between the conductive agent and the active material is increased, the long-distance conductivity of the conductive agent network is destroyed. The prepared electrode resistance will increase, and the corresponding battery performance will deteriorate, as shown in Figures 9 and 10. As the dispersion intensity increases, the electrode resistance first decreases and then shows an increasing trend, and the rate and cycle performance of the battery will also deteriorate accordingly.

Therefore, although the dry mixing process significantly shortens the mixing process time, the slurry stability and dispersion uniformity are also better. However, this process has the disadvantage of a narrow process range.

In the conventional dry mixing process, quality problems are likely to occur if the amount of solvent, stirring speed and time are not selected properly in the wetting step, which is closely related to the particle size, size distribution, specific surface area and other aspects of the raw materials. If these parameters change slightly, the corresponding amount of solvent and stirring process conditions also need to be adjusted. If too much solvent is added in the first step, the particle agglomerates are not easy to disperse, resulting in quality problems such as large slurry fineness and uneven distribution of conductive agents. If too little solvent is added in the first step, the wetting and stirring force is large, and the binder cannot be fully dispersed and dissolved or the long chain of the binder is destroyed, resulting in problems with slurry viscosity and stability. This may be the key issue that limits the widespread application of dry mixing technology.

Similarly, there is also a suitable process range for high-strength dry powder dispersion and mixing. Within the appropriate process range, the performance of the pole piece and battery will be improved, but beyond this range, the battery performance will deteriorate.

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