Lithium-ion batteries can be classified into pouch Cell, prismatic and cylindrical batteries according to the packaging method and appearance. From the perspective of internal molding process, pouch cell and prismatic batteries can use the winding or lamination process. Cylindrical batteries have curvature everywhere and can only be rolled. Winding process.
Although the winding process has been developed for a relatively long time, with mature technology, low cost and high yield, with the promotion and development of new energy vehicles, the lamination process has high volume utilization, stable structure, small internal resistance, long cycle life, etc. Advantages and characteristics have become a rising star.
The winding process is to roll the divided positive and negative electrode sheets and the separator together by controlling the speed, tension, relative position, etc. of the electrode sheets. The characteristics of the process allow it to only prepare lithium batteries with regular shapes. The lamination process The positive and negative electrode sheets and isolation films are staggered and stacked through a sheet feeding mechanism to form a stacked core, which can prepare regular or special-shaped lithium batteries with higher flexibility.
Winding/Stacking Process Video
1.Laminated structure space utilization is higher
The pole set of the lamination process has a laminated structure. There is no bending phenomenon in the same pole piece, and there is no “C angle” problem in the winding process. The corner space of the inner shell can be fully utilized, and the capacity per unit volume is higher. Compared with For winding structure batteries, the energy density of lamination structure can be increased by about 5%.
2.The laminated structure has good stability, long cycle life and high safety.
During the charging and discharging process of the winding structure, the stress on the inner and outer layers of the “C angle” pole piece is inconsistent, and the expansion and contraction are different at different positions. The pole piece has the risk of deformation and lithium precipitation. However, there is no “C angle” in the laminated structure, and the pole piece is flat. The expansion force of each layer of pole pieces is uniform, the interface consistency is good, and the cycle performance is better.
3.The laminated structure has small internal resistance, good rate performance and low temperature rise.
The laminated structure has multiple layers of pole pieces. Each layer of pole pieces leads to a tab, which is then ultrasonically welded together through tab lead. This is equivalent to connecting several small pole pieces in parallel, with smaller internal resistance, while the traditional winding structure is The negative electrode plates each have only one layer, usually only one tab, which results in a longer electron transmission path, large internal resistance, poor magnification, and increased temperature. In order to reduce the internal resistance of the winding structure, middle-lug or multi-lug winding technology can be used, but this puts higher requirements on equipment capabilities and quality control.
4.The laminated structure has low efficiency, high cost and poor consistency.
The winding production efficiency can generally reach more than 12ppm, which is high efficiency, and the equipment occupies a smaller area, while the lamination efficiency is generally 6~8ppm, which is low efficiency, and the lamination machine has more mechanical structures and the equipment occupies a large area.
In order to improve the efficiency of lamination machines, lamination processes and lamination equipment have been continuously improved. The traditional “Z”-shaped stacking machine stacks the die-cut pole pieces together, uses a robot to grab the pole pieces for positioning, and then stacks them layer by layer on top of the separator. The stacking time for one battery cell is about 3 to 5 minutes. The efficiency is extremely slow.
Based on the traditional “Z”-shaped stacking machine, an all-in-one cutting and stacking machine has been developed, which integrates a die-cutting machine and a glue hot press. That is, the die-cut pole pieces do not need to be re-stacked, but can be cut into pieces and stacked. One piece improves lamination efficiency and reduces the risk of pole piece collision damage during transportation.
Whether it is traditional “Z”-shaped lamination or integrated cutting and lamination technology, the diaphragm is stretched during the lamination process, and there is a risk of deformation and wrinkles of the diaphragm. In order to solve this problem, thermal composite lamination technology came into being, that is, the four-layer structure of separator (belt-shaped)/negative electrode sheet (sheet-shaped)/separator (belt-shaped)/positive electrode sheet (sheet-shaped) is heated and laminated through a hot roller. Then it is cut into battery units (Bi-cells) of separator/negative electrode sheet/separator/positive electrode sheet. Multiple Bi-cells are directly stacked and then hot-pressed to form a stacked core, which eliminates the problem of separator deformation, but the process becomes more complicated.
Due to the extremely fast winding speed of the winding structure, people thought of the advantages of combining lamination and winding, and the integrated winding and laminating machine was born. Its working principle is to first cut out small pieces of positive and negative electrode pieces, and then thermally combine them on the isolation film. Since the thickness gradually increases during the winding process, the spacing between adjacent electrode pieces should also gradually increase. Finally, adopt It is rolled into shape using the winding process, which is currently the most efficient “lamination” solution.
5.The investment in lamination equipment is large and the threshold is high
From the perspective of a single production line, a production line requires 10 winding machines, with a unit price of about 450,000 US dollars per unit. The cost of a production line winding machine is 5 million US dollars. Equivalently converted into lamination equipment, the cost is 5 million – 15 million US dollars, the amount increased by 1 to 3 times.
The specific price depends on the size of the equipment!
6.The problem of burrs on the pole pieces of the laminated structure is prominent and the process control is difficult.
Laminated batteries need to cut off the pole pieces several times, resulting in a longer cross-section than the wound structure, increasing the risk of burrs. At present, the pole pieces of the laminated structure are mainly cut by die punching and laser cutting.
Mold punching uses a hardware knife to cut off the pole pieces. The punching efficiency is high and the technology is mature. It is the current mainstream cutting method for pole pieces in laminated structures. However, this physical cutting method will cause the blade to wear and the tool needs to be replaced regularly. Maintenance costs are high. Laser cutting does not have the problem of changing tools. When switching models, you only need to modify the software size parameters, which is highly flexible. However, laser cutting produces a large amount of dust and has thermal effect problems. The technical bottleneck has yet to be broken.
In addition to hardware die-cutting and laser cutting, there is another pole piece cutting method which is ultrasonic processing. Ultrasonic energy is generated by the front and back vibration of an ultrasonic vibrator installed behind the spindle. However, under such high-frequency vibration, the burr level must be controlled. It is extremely difficult, so it is difficult to implement ultrasonic cutting in pole piece cutting.
In summary, since laser cutting has problems with dust, burrs, and molten beads, there is a safety risk of internal short circuit, while hardware die-cutting has controllable burrs, fast efficiency, and mature technology, and will be the mainstream method of pole piece cutting in the next few years.
In summary, since laser cutting has problems with dust, burrs, and molten beads, there is a safety risk of internal short circuit, while hardware die-cutting has controllable burrs, fast efficiency, and mature technology, and will be the mainstream method of pole piece cutting in the next few years.
Summarize:
The internal resistance of the wound battery is large, and the internal resistance can be greatly reduced through structural improvements. For example, the full-tab structure can achieve an internal resistance level similar to that of the laminated structure, but it requires higher equipment capabilities and quality control.
Laminated batteries have a flat structure, low internal resistance, and high space utilization. They have shined in the field of power batteries. Battery companies represented by LG and BYD adhere to the lamination route.
An important factor restricting the development of lamination structures is that lamination efficiency is too slow. However, with the continuous improvement of lamination equipment and processes, the development of cutting and lamination integration, thermal lamination technology, and rolling technology will help significantly improve lamination efficiency.
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