Analysis of manufacturing processes of lithium batteries with different packaging methods: square, cylindrical, and soft pack

Automotive Technician

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In the new energy era, lithium batteries are the core power and energy storage unit, and their importance is self-evident. Among the many characteristics of lithium batteries, the external form of packaging shape actually contains complex technical considerations and process logic. The three mainstream packaging shapes of square, cylindrical, and soft pack each correspond to a unique process, like three keys, opening the door to different application scenarios. This article will deeply analyze the technical routes and process secrets behind these three lithium battery packaging shapes.

1. Prismatic lithium battery: the craftsmanship behind its sturdiness and squareness

1. Structural and design advantages

The square lithium battery is known for its regular shape, which has significant advantages in space utilization. Its flat structure can be arranged closely, which is suitable for scenarios with high requirements for space layout, such as battery modules for electric vehicles. From the perspective of structural design, the square shell provides a stable support frame for the electrodes, diaphragms and other components inside the battery, which helps to improve the overall strength and stability of the battery.

2. Manufacturing process

1. Shell manufacturing: The shell of a square lithium battery is usually made of metal, such as aluminum alloy or stainless steel. The manufacturing process involves stamping, stretching and other processes, and the metal sheet is processed into a shell with a specific shape and size through a mold. The stamping process requires extremely high mold precision to ensure the dimensional consistency and surface flatness of the shell, which is crucial for the subsequent assembly of the internal components of the battery. The stretching process further shapes the depth and shape of the shell so that the shell can accommodate the battery core.

2. Cell assembly: The cell is the core part of the square lithium battery, and its assembly process is rigorous and complicated. After the positive and negative electrodes and separators are stacked or wound into a cell in sequence, they are carefully placed in the formed shell. The stacking process can improve the energy density and charge and discharge performance of the battery because it can reduce the resistance and mechanical stress inside the cell. The winding process is relatively more suitable for large-scale production and has higher production efficiency. After the cell assembly is completed, the electrolyte is injected. The injection amount and uniformity of the electrolyte directly affect the performance and life of the battery.

3. Sealing and welding: To ensure the sealing and safety of the battery, laser welding or resistance welding is used to seal the outer shell and the top cover. Laser welding has the advantages of high energy density, fast welding speed, narrow and beautiful welds, and can effectively prevent electrolyte leakage and external air from entering. Parameter control during the welding process is extremely critical, such as laser power, welding speed, pulse frequency, etc. Any slight deviation may lead to welding quality problems and affect the overall performance of the battery.

3. Application scenarios and challenges

Prismatic lithium batteries are widely used in electric vehicles, energy storage systems and other fields. In electric vehicles, their compact structure can better fit the vehicle chassis space and improve the vehicle's range and handling performance. However, the manufacturing process of prismatic lithium batteries is relatively complex, costly, and has strict requirements on production equipment and process control. In addition, prismatic batteries have certain challenges in heat dissipation and need to be equipped with an efficient thermal management system to ensure stable performance of the battery under different operating conditions.

2. Cylindrical lithium battery: high efficiency in a round shape

1. Unique structural features

Cylindrical lithium batteries are unique for their rounded shape. Common cylindrical lithium batteries, such as 18650 and 21700, have standardized sizes. This standardized design facilitates large-scale production and interchangeable use, and has inherent advantages in the field of consumer electronics. The cylindrical structure makes the heat dissipation path inside the battery more uniform, which is beneficial to the heat dissipation of the battery during the charging and discharging process, and improves the safety and stability of the battery.

(II) Manufacturing process analysis

1. Shell molding: The shell of a cylindrical lithium battery is generally made of metal material, mostly using a seamless steel tube stretching process. The metal tube is stretched in a specific mold to form a cylindrical shell with a certain wall thickness and length. During the stretching process, the material properties of the tube, the stretching speed, and the lubrication conditions of the mold need to be precisely controlled to ensure the dimensional accuracy and surface quality of the shell. Compared with the manufacturing of square battery shells, the cylindrical shell manufacturing process is relatively simple and has higher production efficiency.

2. Cell winding: Cylindrical lithium battery cells usually adopt winding process. After the positive and negative electrodes and separators are stacked alternately, they are wound into cylindrical cells by a winding machine. During the winding process, the control of winding tension is very critical. Too much or too little tension will affect the internal structure and performance of the cell. At the same time, the speed and accuracy of winding are also directly related to the consistency and yield rate of the battery.

3. Sealing and assembly: After the battery cell is wound, the sealing and assembly process is carried out. In the sealing process, resistance welding is usually used to seal the top cover and the outer shell. Resistance welding generates high temperature at the welding part by applying high current instantly, achieving metal fusion. The assembly process also includes assembling components such as the battery cell, electrolyte, and protection circuit into the outer shell to form a complete battery.

(III) Application scenarios and limitations

Cylindrical lithium batteries dominate in consumer electronics such as laptops and mobile power supplies. Their standardized size and good heat dissipation make product design and manufacturing more convenient. However, in the field of electric vehicles, the application of cylindrical lithium batteries faces some challenges. Since the capacity of a single cylindrical battery is relatively small, a large number of batteries need to be connected in series and parallel to meet the high energy requirements of electric vehicles, which increases the complexity and cost of the battery management system. In addition, the space utilization of cylindrical batteries in the battery module is relatively low, which affects the energy density of the entire battery system to a certain extent.

3. Soft-pack lithium battery: innovative breakthrough in flexibility

1. The charm of flexible structure

Soft-pack lithium batteries are distinguished from square and cylindrical lithium batteries by their unique soft packaging structure. The outer shell is made of aluminum-plastic composite film, which has both the barrier properties of metal and the flexibility of plastic. The soft-pack structure allows the battery to be customized according to different application requirements, such as ultra-thin and special-shaped, meeting the stringent requirements of battery shape in some special scenarios. At the same time, soft-pack batteries have certain advantages in safety. When the internal pressure of the battery is too high, the aluminum-plastic composite film will bulge and rupture, releasing the pressure and avoiding serious accidents such as explosions.

(II) Detailed explanation of manufacturing process

1. Preparation of aluminum-plastic composite film: Aluminum-plastic composite film is a key material for soft-pack lithium batteries, and its preparation process is complicated. Generally, aluminum foil and plastic film are first compounded with adhesives, and then coated on the surface to improve the film's barrier properties, heat sealing properties and electrolyte corrosion resistance. The formulation of the coating material and the coating process have a significant impact on the performance of the aluminum-plastic composite film, which is directly related to the battery's service life and safety.

2. Cell stacking: Soft-pack lithium batteries are mostly made by stacking cells. Unlike the winding process of cylindrical batteries, the stacking process is to stack the positive and negative electrodes and separators one by one in order to form a cell. This process can increase the energy density of the battery because the stacking structure can reduce the gap inside the cell and make the contact between the electrode material and the electrolyte more complete. At the same time, the stacking process also helps to improve the charge and discharge performance and cycle life of the battery.

3. Heat sealing and packaging: After the battery cell is stacked, it is placed in an aluminum-plastic composite film and sealed by a heat sealing process. Heat sealing temperature, pressure and time are key parameters of the heat sealing process and need to be precisely controlled to ensure a good sealing effect and prevent electrolyte leakage. After packaging is completed, subsequent processes such as electrolyte injection and formation are carried out to enable the battery to form stable electrochemical properties.

(III) Application scenarios and development bottlenecks

Soft-pack lithium batteries are widely used in consumer electronics, wearable devices, and some high-end electric vehicles. In wearable devices, their thin, light, and flexible properties enable them to better fit the human body and provide a comfortable wearing experience. In the field of electric vehicles, the high energy density and customized design capabilities of soft-pack batteries make it possible to reduce weight and optimize space in vehicles. However, the manufacturing process of soft-pack lithium batteries has extremely high requirements for environmental humidity and cleanliness, and quality control during the production process is difficult. In addition, the cost of aluminum-plastic composite film is relatively high, which limits the cost advantage of soft-pack batteries in large-scale applications.

4. Comparison of technical routes and future prospects of three packaging shapes

1. Comparison of technical routes

In terms of manufacturing process complexity, square lithium batteries have the most complex manufacturing process and relatively high costs due to their structural design and sealing process requirements; cylindrical lithium batteries have relatively simple manufacturing processes, high degree of standardization, and are suitable for large-scale production; although the soft-pack lithium battery cell stacking process is relatively simple, the aluminum-plastic composite film preparation and heat sealing process have strict requirements on the environment and equipment, and the overall process difficulty should not be underestimated.

In terms of energy density, soft-pack lithium batteries have higher energy density under the same conditions due to their stacking process and relatively compact structure; square lithium batteries can also achieve higher energy density by optimizing the stacking process and structural design; cylindrical lithium batteries have relatively lower energy density due to their internal structural characteristics, but the energy density is gradually increasing through continuous improvement of electrode materials and processes.

In terms of safety, soft-pack lithium batteries have inherent advantages in pressure release; square lithium batteries can also ensure high safety through a complete thermal management system and safety design; the heat dissipation advantage of cylindrical lithium batteries helps to improve their safety, but in extreme cases such as overcharging and short circuiting, protective measures still need to be strengthened.

2. Future Prospects

With the continuous development of new energy technologies, the three types of lithium batteries will continue to innovate in their respective areas of strength. Prismatic lithium batteries will further optimize processes in the electric vehicle and energy storage fields, improve energy density and safety, and reduce costs; cylindrical lithium batteries are expected to expand their applications in the electric vehicle field through technological upgrades, while consolidating their position in the consumer electronics field; soft-pack lithium batteries will achieve greater breakthroughs in wearable devices, high-end electronic products, and some electric vehicle market segments with their flexibility and customization advantages.

In the future, the development of lithium battery packaging shape technology will focus on improving energy density, enhancing safety, reducing costs, and adapting to diverse application scenarios. Lithium batteries with different packaging shapes will compete and complement each other, and jointly promote the new energy industry to a new height. In this game of shape and technology, the ultimate beneficiaries will be the entire new energy ecosystem and the vast number of consumers.