Exploring the mixing process of lithium-ion batteries---mixer

Understand the principle of lithium-ion battery mixing equipment in one article

Lithium-ion battery mixing equipment plays a vital role in lithium battery production. Its working principle involves many complex and delicate links .

Mechanical stirring principle

The stirring equipment mainly achieves the stirring of materials through the mutual rotation of the stirring blades and the revolving frame. During the operation, the stirring blades rotate at high speed, like a strong arm, stirring the materials vigorously. The revolving frame carries the stirring blades to make a circular motion around the center of the container, further expanding the stirring range, so that the materials can be fully stirred at different positions.

Under the action of mechanical stirring, a suspension is generated and maintained in the container, so that solid particles that might have precipitated or agglomerated are evenly suspended in the liquid medium. This process greatly enhances the mass transfer between the liquid and solid phases, allowing the various components to fully contact and mix, thus laying a good foundation for the subsequent battery manufacturing process. For example, when making positive electrode slurry, solid particles such as active materials and conductive agents can be evenly dispersed in the solvent under the action of stirring equipment, ensuring the smooth progress of subsequent coating and other processes.

Wetting, dispersion and stability

1. Wetting

Wetting is an important basis for the stirring process, which refers to the phenomenon that when a liquid contacts a solid, it expands along the solid surface and replaces the gas to occupy the solid surface. In the stirring of lithium-ion battery slurry, commonly used solvents such as NMP and deionized water need to wet solids such as positive and negative electrode active materials and conductive agent particles. When the solvent can spontaneously wet the solid well, such as NMP and graphite, NMP can quickly remove the gas on the graphite surface, forming a solid-liquid interface, isolating the particles from each other, which is conducive to subsequent dispersion. On the contrary, if the solvent cannot wet the solid well, such as deionized water and graphite, the graphite particles will tend to agglomerate and it is difficult to disperse evenly.

2. Dispersion

The purpose of dispersion is to break up the agglomerated particles and distribute them evenly in the solvent. The stirring equipment crushes the agglomerates of active substances and conductive agents through strong mechanical forces such as shear force and friction force, and refines the size of the stirred substances. At the same time, the arrangement of active substances, conductive agents and binders is made more reasonable, ensuring that the slurry is evenly dispersed at the microscopic level. For example, through the high-speed operation of the stirring equipment, the conductive carbon black particles that were originally gathered together are broken up and evenly distributed in the slurry, thereby improving the conductivity of the slurry.

3. Stability

Stability is to ensure that the particles of the slurry will not re-agglomerate or settle during storage and use, and maintain a uniform dispersion state. This requires appropriate formulation design and stirring process, such as adding appropriate stabilizers and controlling the viscosity of the slurry. Stable slurry is crucial for subsequent coating, rolling and other processes, and can ensure the consistency of the pole piece and the stability of battery performance.

The mixing process is roughly divided into the following stages

1. Prepare raw materials and ingredients (ingredients stage)

At this stage, the staff needs to accurately prepare various raw materials such as positive electrode materials, negative electrode materials, electrolytes, etc., which are basically mechanical operations now, but manual operation of electric hoists is required to lift the ton bags to the feeding station, and use gravity or pneumatic transmission to send them to the main silo.

The production design has already finalized the formula. For example, if the amount of main materials or auxiliary materials is added more or less, although it may not seem to make much difference, in the subsequent battery performance test, it will be found that the cycle life of the battery is also greatly shortened. This fully illustrates the importance of accurately preparing raw materials and strictly controlling the proportion of ingredients.

2. Feeding process control (feeding stage)

Feeding process control is a key step to ensure uniform mixing of materials. The order and speed of adding materials have a significant impact on the mixing effect. In actual operation, some solid raw materials that are not easily dissolved or dispersed, such as active substances, conductive agents, etc., are usually added to the stirring equipment first, and then liquid components such as solvents and binders are added. The correct order of adding materials can effectively improve the mixing effect of the materials.

The feeding speed also needs to be strictly controlled. If the feeding speed is too fast, some materials may accumulate locally and fail to be fully mixed with other materials in time, thus affecting the overall mixing quality. On the contrary, if the feeding speed is too slow, the production cycle will be extended and production efficiency will be reduced. Therefore, it is necessary to reasonably adjust the feeding speed according to the performance of the mixing equipment and the characteristics of the materials to ensure that the materials can be added to the mixing equipment evenly and stably.

Mixing and stirring operation (powder-liquid mixing stage)

1. Start-up and stirring time

After the addition is completed, the stirring equipment can be started. The starting process of the stirring equipment needs to be smooth to avoid damage to the equipment caused by the impact force generated by the sudden start. The control of the stirring time is crucial, which directly affects the mixing degree of the materials. Different battery materials and process requirements require different stirring times. Generally speaking, the stirring time is between tens of minutes and several hours. For example, for some high-performance lithium-ion batteries, the stirring time of the positive electrode slurry may need to reach 2-3 hours to ensure that the active material, conductive agent and binder are fully mixed to form a uniform and stable slurry system. If the stirring time is too short, the materials may not be fully mixed, resulting in agglomerated particles or uneven distribution of components in the slurry, which will affect the subsequent coating quality of the pole piece and the performance consistency of the battery. On the contrary, if the stirring time is too long, it will not only increase energy consumption and production costs, but also cause degradation or structural damage of certain components in the slurry, which will also have an adverse effect on battery performance.

2. Stirring speed control

Stirring speed is also a key factor in mixing and stirring operations. If the stirring speed is too fast, excessive shear force may be generated, resulting in the destruction of the structure of some sensitive materials. For example, the molecular chains of some polymer binders may be interrupted, thus affecting the bonding properties of the slurry. If the stirring speed is too slow, it will not provide enough energy to fully disperse and mix the materials. Usually, the stirring equipment will set different stirring speed gears, and the operator needs to adjust the stirring speed in time according to the characteristics of the materials and the stirring stage. In the early stage of stirring, in order to make the materials initially mixed evenly, a lower stirring speed can be used; as the stirring proceeds, in order to further refine the particles and improve the dispersion effect, the stirring speed can be gradually increased. For example, when preparing the negative electrode slurry, stir at a lower speed for 15-20 minutes at the beginning to make the various raw materials initially mixed, and then increase the stirring speed to an appropriate level and continue stirring for 1-2 hours to achieve a good dispersion effect.

3. Notes

During the mixing process, it is also necessary to pay close attention to the operating status of the equipment and the changes in the slurry. Operators should regularly check the temperature, pressure and other parameters of the mixing equipment to ensure the normal operation of the equipment. At the same time, pay attention to the appearance, viscosity and other indicators of the slurry to determine whether it has achieved the expected mixing effect. If the slurry is found to be abnormal, such as uneven color, abnormal viscosity, etc., the mixing should be stopped in time, the cause should be analyzed and corresponding solutions should be taken. In addition, the cleaning and maintenance of the mixing equipment should not be ignored. After each mixing operation is completed, the residual slurry inside the equipment should be cleaned up in time to prevent the residual material from affecting the quality of the next mixing. At the same time, the equipment should be inspected and maintained as necessary to ensure the long-term stable operation of the equipment.

Several parameters of the mixer that affect the key control points of the slurry (viscosity, fineness, solid content)

Stirring speed

Stirring speed is one of the key factors affecting the stirring effect. In the early stage of stirring, a lower stirring speed helps to mix various raw materials evenly and avoid splashing or agglomeration of some materials due to too fast speed. As the stirring progresses, gradually increasing the stirring speed can provide greater shear force and friction, further disperse and refine the materials, and improve the uniformity of mixing. However, there are also risks if the stirring speed is too fast, which may produce excessive shear force and destroy the structure of some sensitive materials. For example, the molecular chain of the polymer binder may be interrupted, affecting the bonding performance of the slurry. Therefore, it is necessary to reasonably adjust the stirring speed according to the characteristics of the material and the stirring stage to achieve the best stirring effect.

temperature

Temperature has a significant effect on stirring performance. On the one hand, appropriately increasing the temperature can reduce the viscosity of the slurry, improve its fluidity, and make the materials easier to mix evenly. In some cases, increased temperature can also accelerate certain chemical reactions and promote the dissolution and dispersion of the binder. On the other hand, too high a temperature may cause the solvent to evaporate too quickly, affecting the solid content and performance stability of the slurry. Some materials may decompose or deteriorate at high temperatures, so the temperature during the mixing process needs to be precisely controlled by a temperature control system to ensure that it is within an appropriate range.

Vacuum degree

During the stirring process, controlling the vacuum degree can effectively remove the gas in the slurry. The presence of gas will form bubbles in the slurry, which will not only affect the uniformity of the slurry, but also cause defects such as pinholes and voids in subsequent coating processes, reducing the performance and safety of the battery. By increasing the vacuum degree, the gas in the slurry can be more easily escaped, reducing the generation of bubbles and improving the quality of the slurry. In actual operation, it is necessary to reasonably set the vacuum degree according to the performance of the stirring equipment and the characteristics of the slurry, and keep the vacuum degree stable during the stirring process.

Common abnormalities in the mixing process and their solutions

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