Introduction
Soft-pack battery cells, referring to cells using aluminum-plastic composite film as packaging material, have seen widespread application in the lithium-ion battery field due to their high energy density, enhanced safety, and flexible design. Below, I will provide a detailed introduction to the manufacturing process of soft-pack battery cells, covering material preparation, critical steps, and post-processing aspects.
1. Cell Design
Cell design refers to the process of assembling the cathode material, anode material, electrolyte, separator, and current collectors for the positive and negative electrodes in specific proportions and processes to meet particular electrical performance requirements. This process requires designers to understand the characteristics of various materials, possess extensive electrochemical knowledge, and have the ability to think logically and comprehensively.
1.1 Clarify Design Requirements
It's important to clarify the usage scenarios for the cell, such as whether it is for consumer batteries, energy storage batteries, or power batteries, and the application conditions, like drones, mining trucks, cars, ships, etc.
Determine the basic parameters of the cell, such as capacity, cycle performance, rate capability, high and low-temperature performance, energy density, safety, etc.
1.2 Select Materials
- Cathode Material: Determines the energy density of the cell. Commonly used cathode materials are lithium cobalt oxide (for consumer batteries), ternary materials (such as NCM, for power batteries), and lithium iron phosphate (for power batteries, which is cost-effective and has good safety).
- Anode Material: Common anode materials include artificial graphite, which can be mixed with silicon or use lithium titanate, hard carbon, etc. The choice of anode material will affect the cell's energy density, cycle life, and fast-charging capability.
- Electrolyte: Acts as ionic conductor and electronic insulator between the battery's cathode and anode. The nature of the electrolyte has a significant impact on the battery's cycle life, operating temperature range, charge-discharge efficiency, battery safety, and power density.
- Separator: Used to isolate the cathode and anode to prevent internal short circuits within the battery.
- Current Collectors: Carry active material and collect the current generated by electrode active materials for output. Commonly used current collector materials include copper, aluminum, nickel, and stainless steel.
1.3 Design Structure
Design the cell structure, such as winding or stacking, based on the characteristics of materials and the cell's needs.
Determine the cell's size and shape to meet the requirements of the usage scenario.
1.4 Optimize Performance
Improve the energy density, cycle life, and safety of the cell by adjusting material ratios and optimizing process parameters.Conduct simulation analysis and testing verification on the cell to ensure its performance meets design requirements.
1.5 Key Parameters of Cell Design
- Energy Density: The amount of energy a cell can store per unit volume or unit weight. Increasing energy density is one of the key goals in cell design.
- Cycle Life: The number of times a cell can be charged and discharged under specific conditions. The length of cycle life directly influences the battery's lifespan and cost.
- Safety: The stability and reliability of the cell under various conditions. Safety is one of the primary considerations in cell design.
2. Electrode Manufacturing
2.1 Slurry Mixing
The active materials of the cathode/anode, conductive agents, and binders are uniformly dispersed in a solvent and stirred to form a stable slurry with a certain viscosity.
The uniformity of slurry mixing has a significant effect on subsequent coating and cell performance.
2.2 Electrode Coating
Apply the uniformly mixed slurry onto a copper/aluminum foil current collector.
The coating process includes two methods: traditional wet coating and dry coating.
- Wet Coating: The slurry is coated onto the current collector and then sent into a drying chamber to evaporate the solvent, with the solvent being recovered.
- Dry Coating: Active particles and conductive agents are dry-mixed uniformly before adding a binder. A self-supporting film is formed under the fibrillation action of the binder, which is then roll-pressed onto the surface of the current collector.
- Coating Uniformity: Ensure that the slurry is evenly distributed on the current collector to avoid issues such as uneven coating or missing spots.
- Coating Precision: Control the thickness and width of the coating to meet the cell design requirements.
2.3 Electrode Roll Pressing
- Place the coated electrodes under the twin rollers of a roll press machine. By compressing the electrodes, achieve the desired thickness and interfacial uniformity.
- The rolling process can increase the compaction density of the electrodes, thereby improving the energy density of the cell.
- Roll Pressing Pressure: Select the appropriate roll pressing pressure to ensure the electrodes achieve the desired thickness and compaction density.
- Roll Pressing Speed: Control the roll pressing speed to avoid overheating, deformation, or damage to the electrodes.
2.4 Electrode Slitting
- Cut the wide electrodes, after roll pressing, into narrow strips.
- The slitting process has strict requirements for the consistency of electrode width, the size of burrs on the cut edges, and the morphological characteristics, all of which can affect the quality of cell winding or stacking.
- Slitting Precision: Ensure the width consistency of the electrodes and that the size of burrs on the cut edges meets requirements.
- Slitting Speed: Match the speed to the material's ductility to prevent material damage.
2.5 Process Optimization Directions
- Improve Coating Uniformity: Use advanced coating equipment and techniques to enhance coating uniformity and reduce cell performance fluctuations.
- Reduce Roll Pressing Costs: Optimize the roll pressing process and equipment to improve pressing efficiency and lower costs.
- Enhance Slitting Quality: Use high-precision slitting equipment and techniques to improve slitting quality, ensuring consistency and safety of the cells.
- Promote Process Innovation: Continuously explore and try new electrode processes and technologies, such as dry process methods, to reduce cost, improve energy density, and enhance safety.
3. Cell Assembly
3.1 Stacking & Winding
- Stacking Process: Arrange battery separators, cathodes, and anodes in a specific order and method to form the cell structure. This process is mainly used for the production of pouch batteries, offering advantages such as high energy density, easier control of dimensions and structure, and flexible manufacturing processes. During stacking, it is essential to strictly control the alignment of electrodes and separators, the number of layers, and the stacking pressure parameters to ensure the uniformity and stability of the cell's internal structure. There are various stacking methods, such as Z-folding and integrated cutting-stacking machines, with different stacking equipment having unique methods and characteristics.
- Winding Process: Roll the cathode, anode, and separator into cylindrical or prismatic cell structures in a specific sequence, controlling winding speed and tension. This process is mainly used for large batteries, like those for electric vehicles, and offers benefits such as high production efficiency and low costs.
Comparison of Stacking & Winding:
- Energy Density: The stacking process has higher energy density as it can make full use of the battery's corner space, while the winding process has relatively lower energy density due to space waste during winding.
- Structural Stability: The stacking process offers better structural stability and does not have "C-corner" issues, leading to longer cycle life. In contrast, the winding process is less stable as electrodes and separators may bend during winding.
- Safety: The stacking process provides higher safety due to efficient heat dissipation and stable internal structure. The winding process is relatively less safe because the "C-corner" in wound cells is prone to deformation and twisting.
- Production Efficiency: The stacking process has relatively lower production efficiency because it requires precise control during stacking, whereas the winding process has higher production efficiency as winding can be performed quickly.
- Cost: The stacking process generally has higher costs due to the complexity of stacking equipment and techniques; the winding process tends to be cheaper because its equipment and procedures are simpler.
In this process, the positive and negative electrode sheets of the cell are connected to tabs through welding to ensure the conduction of internal currents and stability of the battery structure. This process significantly impacts the performance, safety, and lifespan of the battery.
- Cleaning Treatment: Thoroughly clean the tabs and welding areas of pouch cells to remove oil, moisture, and impurities, ensuring a clean and dry welding surface. This helps reduce defects during the welding process and improves welding quality.
- Tab Positioning: Precisely position the tabs on the stacks according to design requirements, ensuring the gaps between tabs are appropriate. Avoid excessively large or small gaps that could impact welding effectiveness.
- Welding Method: Choose an appropriate welding method, such as laser welding or ultrasonic welding, and prepare corresponding fixtures, molds, and protective sheets as auxiliary tools.
- Laser Welding: Laser welding is a high-precision, high-efficiency welding method suitable for welding tabs on pouch cells. During welding, the laser beam focuses on the tab welding area, generating high temperatures to melt and connect the tab materials. Laser welding offers advantages like concentrated energy, fast welding speed, and a high depth-to-width ratio of the weld, enabling high-quality welding.
- Ultrasonic Welding: Ultrasonic welding utilizes energy generated by high-frequency vibrations to melt tab materials and achieve connection. During welding, the welding head rapidly approaches and contacts the outer aluminum plastic film and tabs of the pouch cell under preset parameters. By generating local high temperatures and pressure through high-frequency vibration, the materials soften and fuse. Ultrasonic welding provides high welding strength, good sealing, and safe operation, making it suitable for welding tabs of pouch cells with different specifications and materials.
4. Packaging
4.1 Aluminum Plastic Film Forming
Pouch cells can be designed into various sizes according to customer requirements. Once the dimensions are determined, corresponding molds are needed to form the aluminum plastic film. The forming process, also known as "punching pits," involves using forming molds under heated conditions to punch a pit in the aluminum plastic film that can accommodate a rolled cell. Once the aluminum plastic film is punched and cut to shape, it is typically referred to as a pocket bag.
When the cell is relatively thin, choose to punch a single pit. For thicker cells, use double pits because excessive deformation on one side can exceed the deformation limit of the aluminum plastic film, leading to rupture. Sometimes, depending on design needs, a small pit is punched in the position of the gas bag to increase its volume.
4.2 Top and Side Sealing
First, place the welded electrode sheets into the punched pit and fold the packaging film along the dotted line. Next, after placing the rolled cell into the pit, the entire aluminum plastic film can be placed into a fixture for top and side sealing in the top-side sealing machine. The machine typically has multiple fixtures; the left workstation is for top sealing, and the right is for side sealing. Two yellow metal pieces work as the upper sealing heads, with a lower sealing head beneath them. During sealing, both sealing heads are heated (usually about 180°C) and pressed onto the aluminum plastic film, melting the PP layer of the film, which adheres together—completing the sealing.
In this process, the top seal must encompass the tabs, which are metal (aluminum for the positive and nickel for the negative). The sealing relies on a small component on the tab known as tab adhesive. During sealing, the PP in the tab adhesive melts and bonds with the PP layer of the aluminum plastic film, forming an effective sealed structure.
4.3 Gas Bag Reservation
During packaging, additional material is left in the aluminum plastic film, known as the gas bag. This is because during cell formation, a large amount of gas is generated. This reserve will be used to remove the gas in subsequent processes.
5. Formation and Aging
- Liquid Injection: Inject electrolyte into the packaged cell. The electrolyte serves as a medium to facilitate ion transport within the battery.
- Formation: Perform the first battery charge to activate the new battery. The electrolyte reacts on the electrode surface, generating gas and forming a dense solid electrolyte interface (SEI) film. The quality of the formation process directly affects the battery quality.
- Aging: Also known as curing, it's a resting process for the battery after formation, during which the battery's performance gradually stabilizes.
6. Exhaust and Second Sealing
- Exhaust: The gas generated during formation can affect battery performance, so post-formation, the battery needs to undergo exhaust treatment. During exhaust, the gas bag is pierced and the gas along with a small portion of the electrolyte is extracted.
- Second Sealing: After exhaust, the cell undergoes a second sealing to ensure its airtightness. The principle of second sealing is similar to the top and side sealing, where the PP layer of the aluminum plastic film is melted to form the seal.
7. Subsequent Processing
- Trimming and Folding: Trim and fold the edges from the first and second sealing to suitable widths to ensure the cell's dimensions remain within specifications.
- Capacity Testing: Conduct capacity testing on the cell to ensure its capacity meets the specified minimum value. Ideally, all cells must undergo capacity testing before leaving the factory.
- Appearance Inspection and Labeling: Inspect the cell’s appearance to ensure it's flawless, then apply labels.
- Packaging and Shipping: Properly package the tested cells to ensure their safety during transportation and storage before shipping.
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