Analysis of the influence of slurry quality on coating surface fluctuation｜‌Lithium battery production process front end (mixing/coating)

Lithium battery production process - mixing/coating

The production process of lithium batteries varies among different battery manufacturers. Generally speaking, it is divided into three stages: front, middle and back.

1. Front section: pole piece production

(Mixing, coating, tableting, baking, slitting, tableting, tab forming)

(Core link: coating)

2. Middle section: battery cell assembly

(winding or lamination, pre-packaging of cells (into shells), electrolyte injection, sealing)

(Core link: winding)

3. Post-processing (activating the battery cell)

(battery cell formation, capacity division, static placement, testing, and sorting)

(Core links: formation, volume fractionation, and testing)

The process of making the front pole piece:

1. Stirring

The positive electrode (negative electrode) slurry is prepared by using the positive electrode (negative electrode) active material and the conductive agent, that is, stirring.

1. Principle of stirring

By the mutual rotation of the stirring blades and the revolving frame, a suspension is generated and maintained under mechanical stirring, and mass transfer between the liquid and solid phases is enhanced.

(II) The role of stirring

The batching process is actually to mix the various components in the slurry together in standard proportions to prepare the slurry, which is conducive to uniform coating and ensures the consistency of the electrode.

Batching generally includes five processes, namely: pretreatment of raw materials, blending, soaking, dispersion and flocculation.

(III) Slurry parameters

Viscosity

Viscosity is an important parameter of the slurry. It reflects the characteristics of friction or viscosity in the slurry and is one of the key parameters for identifying whether the slurry is suitable for coating. If the viscosity of the slurry is too high, it will lead to poor fluidity, which is not conducive to the coating process and is prone to abnormal appearance of the electrode; while if the viscosity is too low, it will lead to poor storage performance, easy sedimentation, coating and drying difficulties, coating cracking and other problems. 1. The viscosity of the general slurry is controlled between 4000 and 8000 mPa·s. In the future, with the improvement of processing technology, the viscosity may be higher.

temperature

Temperature has a significant impact on the performance of the slurry. During the preparation of the slurry, processes such as mixing, kneading and high-speed dispersion will generate a lot of heat, resulting in uneven temperature distribution inside the slurry, which will affect the fluidity, viscosity and coating effect of the slurry. Changes in temperature will cause changes in viscosity, resulting in uneven coating thickness. Therefore, the temperature of the slurry needs to be tested and controlled to ensure temperature stability.

Solid content

Solid content refers to the content of solid components in the slurry. Solid content has an important impact on the performance of the battery. For example, the solid content of the ternary material lithium slurry is marked as 622-x, which indicates different ratios of electrode slurry and its battery performance.

Granularity

The particle size corresponding to the cumulative particle size distribution percentage of the slurry sample reaches 50%

(IV) How to judge whether the mixing effect of slurry is uniform

1. Direct measurement

1) Viscosity method

Take samples from different positions of the system and measure the viscosity of the slurry with a viscometer; the smaller the deviation, the more uniform the mixing;

2) Particle size method

A. Take samples from different positions and use a particle size scraper to observe the particle size of the slurry; the closer the particle size is to the size of the raw material powder, the more uniform the mixing is;

B. Take samples from different positions and use a laser diffraction particle size tester to observe the particle size of the slurry; the more normal the particle size distribution, the smaller the large particles, and the more uniform the mixing;

3) Specific gravity method

Take samples from different positions of the system and measure the density of the slurry. The smaller the deviation, the more uniform the mixing.

2. Indirect measurement

1) Solid content method (macroscopic)

Take samples from different positions of the system, bake them at appropriate temperature and time, and measure the weight of the solid content. The smaller the deviation, the more uniform the mixing;

2) SEM/EPMA (micro)

Take samples from different positions of the system, apply them to the substrate, dry them, and use SEM (electron microscope) /EPMA (electron probe) to observe the distribution of particles or elements in the film after the slurry is dried; (the solid content of the system is usually conductive material)

(V) Mixer

The equipment needed for the mixing process is a mixer.

Domestic manufacturers of mixers include: Han's Laser, Golden Galaxy, North China Huachuang, and Pioneer Intelligent

2. Coating

The most core step in the production of front-end electrodes: positive (negative) electrode coating.

1. Principle of coating

‌Coating is a technology that evenly coats a fluid on a substrate, mainly used to make composite materials or give substrates specific functions. ‌The principle of coating involves evenly coating a paste polymer, molten polymer or polymer melt on a substrate such as paper, cloth, plastic film, etc. through a coating device to form a film layer with special functions.

2. Value of coating

In the front-end process flow, the vacuum mixer required for slurry mixing costs 200,000 to 400,000 yuan, while the coating machine required for coating costs 12-25 million yuan in China and 25-30 million yuan abroad, the roller press required for rolling costs 2-4 million yuan, the slitting machine required for slitting costs 500,000 to 1 million yuan, and the sheet making die-cutting machine required for tab forming costs 500,000 to 1 million yuan. It can be seen that coating is also the step with the highest value in the front-end process of lithium batteries.

3. What is the impact of coating process on lithium battery performance?

Pole coating is of great significance to the capacity, consistency, safety, internal resistance, cycle life, etc. of lithium batteries. The selection of coating method and control parameters have an important influence on the performance of lithium-ion batteries, mainly manifested in:

1. Coating and drying temperature control

The coating drying temperature should be controlled neither too high nor too low. If the drying temperature is too low during coating, the electrode cannot be completely dried. If the temperature is too high, the organic solvent inside the electrode may evaporate too quickly, causing cracks and peeling of the electrode surface coating.

2. Coating surface density

The coating surface density also needs to be precisely controlled. If the coating surface density is too small, the battery capacity may not reach the nominal capacity. If the coating surface density is too large, it will easily cause waste of ingredients. In severe cases, if the positive electrode capacity is excessive, lithium will precipitate, forming lithium dendrites that pierce the battery separator and cause a short circuit, causing safety hazards.

3. Coating size

If the coating size is too small or too large, the positive electrode inside the battery may not be completely covered by the negative electrode. During the charging process, lithium ions are embedded from the positive electrode and move to the electrolyte that is not completely covered by the negative electrode. The actual capacity of the positive electrode cannot be efficiently utilized. In severe cases, lithium dendrites will form inside the battery, which can easily pierce the diaphragm and cause a short circuit inside the battery.

4. Coating thickness

If the coating thickness is too thin or too thick, it will affect the subsequent electrode rolling process and cannot guarantee the performance consistency of the battery electrode.

5. Slurry viscosity

Relationship between basic physical properties of slurry and coating: In the actual process, the viscosity of slurry has a certain influence on the coating effect. The viscosity of slurry prepared by different electrode raw materials, slurry ratio and binder type is also different. When the slurry viscosity is too high, coating often cannot be carried out continuously and stably, and the coating effect is also affected. The uniformity, stability, edge and surface effects of the coating liquid are affected by the rheological properties of the coating liquid, which directly determines the quality of the coating.

6. Surface tension of copper foil and aluminum foil

The surface tension of the copper and aluminum foil must be higher than the surface tension of the coated solution, otherwise the solution will be difficult to spread evenly on the substrate, resulting in poor coating quality.

7. Static electricity

On the coating line, a lot of static electricity will be generated on the surface of the substrate when the coating is unrolled and passes through the roller. The generated static electricity can easily absorb the air and the dust layer on the roller, causing coating defects. In the process of static discharge, static electricity will also cause static appearance defects on the coating surface, and more seriously, it may even cause fire. If it is in winter with low humidity, the static electricity problem on the coating line will be more serious. The most effective way to reduce such defects is to keep the ambient humidity at a relatively high state, ground the coating line, and install some anti-static devices.

8. Cleanliness

Impurities on the surface of the substrate can cause some physical defects, such as protrusions, contamination, etc. Therefore, the cleanliness of the raw materials needs to be better controlled in the production process of the substrate.

The effect of slurry stirring on the fluctuation of the coating surface is mainly reflected in the fluidity and rheology of the slurry. Poor fluidity of the slurry will lead to mud rubbing, peeling, scratches and other phenomena during the coating process, while excessive fluidity will lead to sagging and flowing, which will affect the coating quality and surface flatness.

The influence mechanism of slurry fluidity on coating surface fluctuation

Poor fluidity: When the fluidity of the slurry is poor, it is easy to cause mud, peeling, scratches, etc. during the coating process. This is because the slurry with poor fluidity cannot be fully and evenly distributed on the surface of the substrate after coating, forming a layer of coating with poor leveling and hanging, resulting in an uneven surface.

‌ Too good fluidity ‌: Slurry with too good fluidity will sag and flow during the coating process, resulting in excessively thick coating and poor coating quality. Repeated coating defects will appear, seriously affecting the quality of the product ‌.

The influence mechanism of slurry rheology on coating surface fluctuation

The rheology of the slurry determines its viscosity and fluidity, which directly affects the uniformity and stability of the coating process. If the rheology of the slurry is poor, it may cause defects such as uneven coating or bubbles, which will affect the performance and quality of the battery. In addition, the particle distribution and characteristics within the slurry will also affect the rheology, which in turn affects the processability of the coating process.

Methods to improve slurry stirring effect

‌Control thinner and solvent content‌: Control the content of thinner and ISO solvent in the ink formula to adjust the viscosity and fluidity of the slurry‌.

‌Moderate grinding‌: Use appropriate grinding methods and grinding media to adjust the size, distribution and shape of particles to improve the fluidity of the slurry‌.

Adding leveling agent: Adding leveling agent helps to improve the fluidity and leveling of the slurry, increase the surface tension of the coating liquid, control the flow of the coating on the surface, and evenly form a layer of coating.

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