In these three stages of the process, each process can be further divided into several key processes, and each step will have a significant impact on the final performance of the battery.
The electrode manufacturing process can be further divided into five stages: slurry preparation, slurry coating, electrode rolling, electrode slitting, and electrode drying. It is understood that the battery assembly process, depending on the battery specifications and models, is broadly divided into winding, casing, and welding processes. The final electrolyte injection stage includes various processes such as electrolyte injection, venting, sealing, pre-charging, formation, and aging.
Every step in the battery manufacturing process causes some waste, which can be caused by employee errors, equipment malfunctions, environmental factors, and so on. In order to ensure that the cost ratio of the product is good enough, we should try our best to ensure that the product is qualified at each step.
I. The significance of coating
Slurry coating is the next step after slurry preparation. Its main purpose is to uniformly coat the positive and negative electrode current collectors with a slurry that exhibits good stability, viscosity, and flowability. Electrode coating is of great significance to lithium batteries, primarily in the following aspects:
1. It is of great significance to the capacity of the finished battery.
If the coating thickness of the positive and negative electrode slurry is inconsistent in the front, middle and rear sections of the electrode sheet during the coating process, it can easily lead to the battery capacity being too low or too high, and it is more likely to form lithium plating during battery cycling, affecting battery life.
2. It is of great significance to battery safety.
Before coating, the 5S process must be performed to ensure that no particles, debris, dust, or other contaminants are mixed into the electrode during the coating process. If contaminants are mixed in, it can cause micro-short circuits inside the battery, and in severe cases, it can lead to battery fire and explosion.
3. It is of great significance to the consistency of battery performance.
Battery manufacturers are particularly concerned about significant differences in capacity and cycle life within a batch of batteries. Therefore, it is essential to ensure that the parameters of the electrodes are consistent before and after coating.
4. It is of great significance to battery life.
Significant differences in the coating before and after slurry application, dust mixed into the electrode, uneven thickness of the electrode on both sides, etc., all affect the quality of the battery's electrochemical performance.
Therefore, the requirements for slurry coating in this process are: with a sufficiently good slurry, the density of the active material in the electrode should be consistent throughout the coating process, and no impurities should be introduced during coating. Of course, the quality of the electrode is not solely determined by the coating effect. If the electrode exhibits severe powder shedding, poor bending resistance, or white air bubbles, then the problem lies with the slurry, and it is necessary to return to the first step to resolve it.
II. Coating Method
It is understood that coating equipment mainly consists of a winding and unwinding unit, a feeding unit, a tension control system, a coating head, and an oven. Coating can be divided into two types: transfer coating and extrusion coating, each with its own advantages and disadvantages.
1. Transfer coating
The coating roller rotates, driving the slurry. The amount of slurry transferred is adjusted by changing the doctor blade gap. The slurry is then transferred onto the substrate by the rotation of the back roller or coating roller. The thickness of the coating layer is controlled according to process requirements to achieve the weight requirements. At the same time, the solvent in the slurry spread on the substrate is removed by drying and heating, allowing the solid material to adhere well to the substrate. As shown in Figure 1.
Comprehensive Analysis of Lithium-ion Battery Electrode Manufacturing Process
Its advantages include low requirements for slurry viscosity, easy adjustment of coating parameters, and no clogging. Its disadvantages include poor coating precision for power batteries, making it difficult to guarantee electrode consistency. The slurry is exposed to air between rollers, which partially affects its properties.
2. Extrusion coating
The feeding system delivers the coating to the screw pump, which then power-feeds the slurry to the extrusion head. The slurry is extruded into a liquid film, which is then coated onto a moving current collector. After drying, a uniform coating is formed, as shown in the figure. Its advantages include highly uniform and precise electrode plates after coating, smooth coating edges, a closed operating system unaffected by foreign objects, and suitability for mass production. Its disadvantages include high equipment precision requirements, demanding maintenance requirements, strict requirements on the slurry viscosity range, and the need to replace gaskets when changing specifications.
Comprehensive Analysis of Lithium-ion Battery Electrode Manufacturing Process
III. Issues to be aware of during coating
Reducing coating defects, improving coating quality and yield, and lowering costs are important aspects that need to be studied in coating processes. Waste generated during coating mainly includes initial setup (related to the operator's skill level), coating interruptions, and the introduction of foreign matter; each shutdown results in a certain amount of waste.
Common problems encountered in coating processes include: raw material contamination, unstable coating processes, improper operation, and incorrect drying program settings. These problems often lead to the following issues with the electrode sheets:
Point defects
The main sources are air bubbles and foreign matter mixed in with the slurry. It is understood that air bubbles can originate from incomplete degassing during mixing, during the feeding process, or during coating. Foreign matter mainly stems from operational errors or environmental issues.
During the coating process, air bubbles inside the slurry are sprayed onto the electrode. When dried in the oven, these bubbles burst, forming white circular spots on the electrode. This area has a thinner active material coating, making it most susceptible to micro-short circuits during battery charging and discharging. When foreign matter is present on the electrode, the area around the particles forms a low surface tension region, causing the liquid film to migrate radially outwards, creating point defects. Measures to prevent these defects include: controlling the operating environment, optimizing slurry mixing, controlling the coating speed, and ensuring the substrate is clean.
Thick edge defect
During the rolling process, the thicker edges of the electrode sheet bear greater pressure, which not only causes differences in the transverse density of the electrode sheet but also crushes the active material particles at the thicker edges. Electrodes with thicker edge defects will exhibit severe warping after pressing, significantly impacting subsequent slitting and winding processes. The crushing of active material particles at the thicker edges extends the transport path for lithium ions and electrons during charging and discharging, leading to increased internal resistance and deeper polarization, ultimately affecting battery life and safety.
Meanwhile, the thicker edges of the electrode form stress concentration points inside the core, making them highly susceptible to lithium plating and micro-short circuits, which are extremely detrimental to battery performance. The thick edges are caused by the surface tension of the slurry, which drives the slurry to migrate towards the uncoated edges of the electrode, forming thick edges after drying.
Studies have found that coating speed has no significant effect on edge width and height, but the edge gradient increases with increasing coating speed. Reducing the gap ratio (coating gap/film thickness) can reduce edge effects. Related gap coating studies indicate that adjusting the coating gap and pressure pre-adjustment can also reduce thick edges. Adding surfactants to reduce the surface tension of the slurry can also reduce the occurrence of thick edges to some extent.
IV. Future Development Trends of Coating Technology
Extrusion coating is widely used in the power battery field due to its advantages such as high precision, uniform coating, and suitability for coating over large widths, and it is gradually replacing transfer coating machines suitable for pilot production lines. Future coating processes will develop towards higher equipment performance, higher uptime, more accurate online thickness control, and improved drying efficiency.
