【Battery Products】Characteristics and comparative analysis of cylindrical, square shell, soft pack battery cells and battery packs

 1.Overview of three types of batteries

1. Cylindrical battery cell

The cylindrical battery cell was first invented by Sony Corporation of Japan in 1992. It has a long history of development and a high degree of standardization. Common models include 18650, 21700, 46800, etc. It adopts a mature winding process, with a high degree of automation, high production efficiency, good consistency and relatively low cost. For example, the 21700 battery used in Tesla Model 3 and Model Y, and the 46800 battery that is about to be mass-produced, once made the cylindrical battery cell popular in the market. However, there are also some problems with cylindrical batteries. Due to the cylindrical shape, the space utilization rate is low, and in the case of modularization, a large space waste will be caused. At the same time, the radial thermal conductivity is not good, the number of windings of the battery cannot be too many, and the single cell capacity is small. When used in electric vehicles, a large number of single cells are required to form battery modules and battery packs, and the connection loss and BMS management complexity are greatly increased. In addition, the weight is large. Compared with some other shapes of batteries, the weight at the same capacity may be large, which may not be suitable for some applications with high lightweight requirements.

2.Square shell battery

Square shell batteries are widely used in China. Their structure is relatively simple. Unlike cylindrical batteries, they use high-strength stainless steel as the shell and accessories such as explosion-proof safety valves. Therefore, the overall accessories are light and have a relatively high energy density. The number of cells naturally decreases, so the requirements for the BMS battery management system are lower. However, square shell batteries also have some disadvantages. Since they can be customized according to the size of the product, there are thousands of models on the market, and because there are too many models, it is difficult to unify the process. The level of production automation is not high, and the differences between monomers are large. In large-scale applications, there is a problem that the system life is much lower than the monomer life. In addition, the shell is heavy, the energy density is limited, and there are too many sizes, making it difficult to unify the process.

3. Soft-pack battery cells

The packaging material of soft-pack batteries is aluminum-plastic composite film, and the key technology is difficult. It has obvious advantages, good safety performance, and is structurally packaged with aluminum-plastic film, which is less likely to explode than hard-shell packaging; high energy density, soft-pack batteries are 40% lighter than steel-shell lithium batteries of the same capacity, and 20% lighter than aluminum-shell lithium batteries; small internal resistance, the minimum internal resistance of domestic soft-pack batteries can be below 35m, greatly reducing the battery's self-consumption; flexible design, can be customized according to customer needs, can be made thinner. But soft-pack batteries also face some problems. Standardization and high cost, the existing models are very limited, and the cost of developing new models is too high. At this stage, soft-pack lithium batteries can only meet a small part of the market demand. Aluminum-plastic film is heavily dependent on imports. At present, the high-end aluminum-plastic film used in domestic soft-pack lithium batteries is still dependent on imports, and the cost is very high. The consistency is poor, the reliability is low, and it is easy to leak.

2. Comparison of battery group characteristics

 

1. Group security

Since the capacity of a single cylindrical cell is small, thermal runaway is relatively easy to control, and fuses can be welded on the cell to reduce the safety risk of overcurrent. For example, in some small electronic devices, this feature of cylindrical cells enables them to better control risks when abnormal situations occur. Soft-pack cells are thinner and have good heat dissipation. They are not prone to thermal runaway at high temperatures, and the aluminum-plastic film breaks first in the event of thermal runaway, which plays a certain early warning role. In contrast, square-shell cells have poor heat dissipation performance and large capacity, and are difficult to control once thermal runaway occurs. According to statistics, under the same conditions, the probability of thermal runaway for cylindrical cells is about X%, for soft-pack cells is X%, and for square-shell cells is X%.

2.Grouping efficiency

The grouping efficiency of square shell batteries is relatively high, and the grouping efficiency of square shell batteries is generally greater than 70%. The grouping efficiency of cylindrical batteries is greater than 65%, and the grouping efficiency of soft-pack batteries is greater than 60%. Although square shell batteries have an advantage when comparing the grouping efficiency of batteries alone, due to the higher energy of soft-pack batteries and cylindrical batteries themselves, the energy of the three types of batteries after grouping is basically the same at the system level. This means that in practical applications, the type of battery cannot be selected based solely on the grouping efficiency, but other factors need to be considered comprehensively.

3. Grouping flexibility

Cylindrical cells are the smallest in size and can adapt to the space requirements of different PACKs, with the best flexibility. Whether in a small space or a complex shape, cylindrical cells can find a suitable placement. Soft-pack cells are mostly long and small in size, and can also adapt to most PACK space requirements through different placements. Square-shell cells have larger capacity and size, a single placement form, and poor grouping flexibility. For example, in some special-shaped devices, cylindrical and soft-pack cells can better meet space requirements.

4. Grouping complexity

Cylindrical cells have small capacity, many group parts, and complex systems. Since a large number of cells are required to form a battery pack, the connection loss and management complexity are greatly increased. Soft-pack cells have poor strength and require structural support. The system is more complex than square-shell batteries but better than cylindrical cells. Square-shell cells have large capacity, high strength, and are the simplest to group. In actual production, the complexity of grouping directly affects production costs and production efficiency.

5. Grouping cost

The assembly cost of the three types of cells is proportional to the complexity of the assembly. Cylindrical cells have the highest assembly cost due to the large number of assembly parts and complex systems. Pouch cells require structural support and are second to none. Square-shell cells are the simplest to assemble and have the lowest assembly cost. Cost is an important consideration when designing and producing battery packs.

6. Thermal management efficiency

Soft-pack batteries have a small thickness, a large heat dissipation surface, and the highest thermal management efficiency. This allows soft-pack batteries to dissipate heat better in high-temperature environments and extend the battery life. Cylindrical batteries have a small capacity and a large heat dissipation surface, but a small effective contact heat transfer area and a medium thermal management efficiency. Square shell batteries have a small heat dissipation surface and a low thermal management efficiency. In some application scenarios with high thermal management requirements, soft-pack batteries have obvious advantages.

3.Advantages and challenges of battery packs with different cells

1.Cylindrical battery cell grouping

1. Group characteristics

High production efficiency, mature technology, and low production cost. The production process of cylindrical cells is relatively simple, with a high degree of automation, and can be quickly mass-produced. For example, the cylindrical cell production lines of some large battery manufacturers can produce hundreds of cells per minute, greatly improving production efficiency. At the same time, due to mature technology, the production cost is relatively low.

 

The assembly components are complex, and there are not many domestic manufacturers specializing in cylindrical assembly of power batteries. Since the capacity of cylindrical cells is small, a large number of cells and complex connecting components are required when assembling them, which makes the assembly system complicated. Moreover, there are relatively few domestic manufacturers specializing in cylindrical assembly of power batteries, which also brings certain challenges to the application of cylindrical cells.

2. Technology and Challenges

Thermal runaway verification is difficult, and thermal runaway of single cells needs to be controlled to prevent heat spread. According to relevant research, when a 21700 cylindrical cell experiences thermal runaway, 11-13L of gas will be ejected, generating about 27 kilojoules of heat per gram, which is about 5 times that of TNT. This makes thermal runaway verification very difficult, and effective measures need to be taken to control thermal runaway of single cells and prevent heat spread.

 

The size and energy density of the battery cells increase, the thermal runaway temperature rise rate is fast, the ejection volume is large, the space utilization rate is high, and it is difficult to balance the mass flow, heat flow, and current. With the development of technology, the size and energy density of cylindrical battery cells continue to increase. For example, cylindrical batteries with specifications such as 4695, 46125, and 46135 are currently on the market, close to 50Ah, and the energy density has reached about 300Wh/kg. However, this also brings a series of problems, the thermal runaway temperature rise rate is fast, the ejection volume per unit time is large, the space utilization rate is higher, and it is more difficult to balance the mass flow, heat flow, and current.

 

Fast charging and thermal management issues are prominent, the temperature field uniformity requirements are high, and the thermal management balance and heat distribution are great challenges. With the increasing demand for fast charging of electric vehicles, the thermal management problem of cylindrical batteries has become more prominent. Many companies are upgrading to 4C fast charging, which means that the heat generated by the battery per unit time and the heat that needs to be dissipated will increase exponentially. In a limited space, it is a very big challenge to achieve high uniformity requirements for different temperature points in the temperature field, and thermal management balance and heat distribution under the condition of increasing volume utilization.

 

The side impact strength is low and it is difficult to improve the group stiffness. The square battery pack itself has stronger structural strength than the cylindrical battery pack, but the side impact strength of the cylindrical battery pack is low. How to improve the stiffness of the cylindrical battery pack is a big challenge in the future. Especially when the available space of the battery pack is getting smaller and smaller, it is more difficult to effectively improve the stiffness.

 

The maintainability of the group is poor, and the CTx solution is mainly glued, which makes disassembly difficult. At present, the CTx solution for cylindrical battery cells is mainly glued, which makes subsequent disassembly very difficult and the maintainability of the group is poor.

 

2. Square Shell Cell Grouping

1. Group advantage

There are many suppliers and low procurement costs. There are relatively many suppliers of square shell batteries in the market, which makes the procurement cost relatively low. Competition among different suppliers also helps to reduce prices.

 

High space utilization, large single cell volume and capacity, and high energy density. The single cell volume and capacity of square shell batteries are large, and more energy can be stored in a limited space. For example, in some electric vehicles, the energy density of square shell batteries can reach 210-230Wh/kg, which is higher than that of soft pack batteries and cylindrical batteries.

Simple grouping and high strength. The structure of the square shell battery is relatively simple, and does not require too many complex components when grouping. At the same time, the square shell battery has high strength and can withstand certain external impacts.

2. Assembly equipment advantages

The square shell battery technology has advantages in energy density, safety and heat dissipation performance. The square shell battery technology uses aluminum alloy, stainless steel and other raw materials as the shell, which protects the battery cell better than the soft pack battery. At the same time, the heat dissipation performance of the square shell battery cell is relatively good, which can effectively reduce the temperature of the battery and improve safety.

Focusing on automation and intelligence, the production efficiency and consistency are high. The square shell battery module assembly equipment focuses on automation and intelligence, and uses advanced robots and automation equipment to achieve precise assembly of battery components, greatly improving production efficiency and consistency. For example, Guangdong Guoyu Technology's square shell battery module assembly equipment can achieve high-precision assembly to ensure the quality and stable performance of each battery module.

The cell matching technology is used to improve the performance and life of the battery components. Through precise electrical performance testing, the battery chips are assembled into modules according to performance matching to ensure the consistency of each module during the charging and discharging process. This cell matching technology not only improves the overall performance of the battery components, but also extends the service life of the battery components.

 

Strictly control safety and take measures to detect hidden dangers in time. The square shell battery module assembly equipment also has strict control in terms of safety. Through advanced safety detection devices, the production line can detect potential safety hazards in time and take corresponding measures during the production process to ensure the safety and stability of battery components.

3. Soft-pack battery cell grouping

1. Group analysis

There are various grouping paths, such as soft blade, short blade and long blade, large module and other solutions. There are various grouping paths for soft-pack cells, and different solutions can be selected according to different needs. For example, the soft blade solution is suitable for small electronic devices, the short blade and long blade solutions are suitable for medium-sized electronic devices, and the large module solution is suitable for large equipment such as electric vehicles.

 

Farasis Energy's SPS solution is uniquely designed, reducing components, lowering costs, improving volume utilization and heat dissipation efficiency, and configuring high-nickel content cells to prevent thermal runaway from spreading. Farasis Energy's SPS (Super Pouch Solution) solution is uniquely designed, improving the performance and competitiveness of soft-pack cells by reducing components, lowering costs, improving volume utilization and heat dissipation efficiency. At the same time, configuring high-nickel content cells prevents thermal runaway from spreading and improves safety.

 

The cell design has changed, with high charge and discharge rates and the use of semi-solid electrolytes to improve intrinsic safety and compatibility with a variety of material systems. The cell design of soft-pack cells is also constantly changing, with features such as high charge and discharge rates and the use of semi-solid electrolytes, which improves intrinsic safety and is compatible with a variety of material systems. For example, some soft-pack cells use high-nickel ternary materials, and the energy density can reach 240-250Wh/kg, which is higher than square cells and cylindrical cells.

 

Compatible with different models by replacing the battery cell and adjusting the stacking height. Soft-pack batteries can be compatible with the needs of different models by replacing the battery cell and adjusting the stacking height. This makes soft-pack batteries have a wider application prospect in the field of electric vehicles.

2. Group characteristics

The number of battery cells in a group is small, and the system group efficiency is about 66-74%. Since the single cell energy density of the soft-pack battery is relatively high, the number of battery cells required for grouping is small. For example, in some electric vehicles, the system group efficiency of the soft-pack battery can reach 66-74%, which is higher than that of cylindrical batteries and square shell batteries.

 

There is room for improvement in the energy density of single cells, but the module design requirements are high and safety is difficult to control. The energy density of single cells of soft-pack batteries still has room for improvement, but the module design requirements are high and safety is difficult to control. For example, the strength of soft-pack batteries is poor and structural support parts are required, which increases the complexity of the system. At the same time, soft-pack batteries are prone to bulging or cracking during thermal runaway, and effective measures need to be taken to improve safety.

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