Elaborate the Lithium-ion Battery Manufacturing Process 5 - Winding/Stacking

 Winding Process

1. Principle and Process

• Principle: The winding process involves the use of a fixed winding needle to wind and compress the pre-processed anode sheets, separator, and cathode sheets in sequence to form a cylindrical or elliptical shape.
• Process: The raw materials are stacked in the sequence of anode, separator, cathode, separator. Then, they are wound into cylindrical or elliptical shapes and placed into metal casings, either square or cylindrical. Specific steps include the unwinding of the anode and cathode sheets and separators, automatic alignment, automatic tension detection and control. The anode and cathode sheets are fed into the winding section by a clamp feeding mechanism, which, together with the separator, undergoes automatic winding according to specified process requirements. After the winding is completed, the machine automatically switches workstations, cuts the separator, attaches the end seal tape, and the finished bare cell is automatically discharged. After pre-pressing, the cell is transported to the discharge outlet by a pull belt.
• Application Scenario: The winding process is mostly used in square and cylindrical batteries.

2. Key Parameter Control

• Tension Parameters: Tension is crucial for ensuring the formation of the wound cell and the interface of the electrodes after the formation process. If the tension is too low, the cell may be loose, and the electrodes may shift during transportation. If the tension is too high, the cell will be tightly wound, which may lead to wrinkles in the electrodes.
• Separator Cutter Temperature: The separator cutter temperature is determined by experimental comparison of cutting effects at different temperatures. After determining the type and thickness of the separator, the separator chamber provides an approximate heat resistance temperature. This temperature acts as the upper limit for the hot-pressing and drying temperatures. Exceeding this limit will cause increased shrinkage of the separator, affecting the coating dimensions and potentially leading to closed pores.
• Winding Needle Circumference: The standard for the winding needle circumference comes from the design process. Theoretically, the circumference of the winding needle equals (cell width - cell thickness) × 2. However, after the cell undergoes hot-pressing, the corners of the cell are not semi-circular but more trapezoidal in shape. Additionally, to accommodate fluctuations in material thickness, the Teflon adjustment results in the winding needle circumference being slightly smaller than the theoretical value.
• Anode Cutter Lifespan: The anode cutter's lifespan is primarily determined by customer requirements. Even if there are burrs at the cutting position, since the negative electrode is wrapped around the positive electrode at the beginning and end of the wound cell, any burrs that puncture the separator will still overlap with the negative electrode. Therefore, burr control may not be necessary.
• Separator Free Revolution Numbers at Winding Head and Tail: The current design specifies 1.5 revolutions for the winding head and 1.25 revolutions for the winding tail, based on design drawings. Adjustments to the winding head revolutions need to mainly verify the impact on core extraction and the thickness of the cell. The number of revolutions at the winding tail is primarily considered with respect to the sealing glue, cell QR code position, and scanning effects, which are closely related to the assembly and welding methods.

3. Equipment and Technical Requirements

• Automation Equipment: The winding process is typically completed using automated equipment to ensure uniformity and consistency in the winding. These devices are usually characterized by high precision, high speed, and high reliability.
• CCD Inspection Device: During the winding process, a CCD inspection device is used to monitor alignment in real-time, ensuring that the anode and cathode sheets are uniformly and tightly bonded with the separator, and that the wrapping is appropriately done.
• Strict Control of Process Parameters: Several key parameters must be strictly controlled during the winding process, including tension, separator cutter temperature, and the circumference of the winding needle, to ensure the quality and performance of the cell.

4. Quality Inspection and Testing

After the winding process is completed, a series of quality inspections and tests are required to ensure that the cell meets the design specifications and quality standards. These checks and tests typically include appearance inspection, electrical performance testing, and safety performance testing.

Stacking Process

1. Principle and Process

• Principle: The coated anode and cathode layers are first cut into predetermined sizes, and then the anode layer, separator, and cathode layer are sequentially bonded together. Multiple "sandwich" structure layers are then stacked in parallel to form a cell core that can be packaged. The continuity of the stacking process relies on the "Z" shaped bending of the separator, which sequentially stacks the anode and cathode onto the separator, with the separator "zig-zagging" through them to separate the electrodes. Finally, the stack is enclosed in an outer casing for packaging.
• Process: The anode and cathode sheets are fed into the stacking machine via an automatic conveyor line, with material boxes for automatic loading and returning. The separator is unwound automatically and is guided through tension control and alignment mechanisms before being introduced into the stacking station. The stacking station moves the separator back and forth to place the electrodes. Two sets of robotic arms with suction cups pick up the anode and cathode sheets from their respective material boxes and, after precise positioning by the pre-positioning system, place them onto the stacking station. After stacking, the robotic arms move the cell to the end-winding and glue application station, where the tail is automatically wound. The separator is cut, and glue is applied to the sides of the cell. Meanwhile, the automatic stacking of the next cell begins. The completed cells, with applied glue, are automatically transferred to the cell transport line's following fixture and then moved to the next process.

• Application Scenario: This process is commonly used for square and soft-pack batteries, and it is especially suited for the production of high-rate batteries, large-size batteries, and custom-shaped batteries.

2. Core Equipment of the Stacking Process

The stacking machine is one of the key pieces of equipment in lithium battery production and is typically composed of the following components.

• Feeding Mechanism: Used to place the anode, cathode sheets, and separator.
• Electrode Sheet Material Box: Used to store and transport the anode and cathode sheets.
• Electrode Positioning Mechanism: Ensures the accurate placement of the electrode sheets during the stacking process.
• Feeding Mechanism: Picks up the electrode sheets from the material boxes and conveys them to the stacking station.
• Stacking Station: Used to support and stack the anode and cathode sheets along with the separator.
• Glue Application Mechanism: Applies protective glue to the finished cells.
• Discharge Mechanism: Removes the finished stack of the cells from the stacking station.

3. Advantages of the Stacking Process

• Improved Battery Performance: The stacking process can significantly enhance the energy density, safety, and cycle life of the battery. Compared to wound batteries, stacked batteries have a higher volumetric energy density limit, more stable internal structures, and longer cycle life.
• High Adaptability: The stacking process is more suited for producing high-rate batteries, large-sized batteries, and custom-shaped batteries, meeting the diverse performance requirements across different fields.
• Higher Material Utilization: In the stacking process, material waste is minimized, as only single sheets need to be removed. In contrast, the winding process often results in waste of entire sheets, or even both the anode and cathode sheets. Therefore, the stacking process offers higher material utilization.

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