Currently, lithium-ion batteries are mainly classified into three categories: prismatic lithium batteries, cylindrical lithium batteries, and pouch lithium batteries. Prismatic and cylindrical batteries primarily use hard casings made of aluminum alloy or stainless steel, while pouch batteries are manufactured using aluminum-plastic film. This is their fundamental difference, as shown in Table 1. The key performance advantages of pouch lithium-ion batteries include:
1. Good safety performance: Soft-pack batteries leak less electrolyte, and in the event of a safety hazard, soft-pack batteries will swell and crack, unlike hard-pack batteries which may explode due to excessive internal pressure;
2. Lightweight: Soft-pack batteries are 40% lighter than steel-cased square batteries of the same capacity and 20% lighter than aluminum-cased square batteries;
3. Large battery capacity: The soft-pack design saves 20% of the volume and has 50% higher capacity than steel-cased batteries of the same size, and 20-30% higher capacity than aluminum-cased batteries;
4. Better cycle performance: Soft-pack batteries have a longer cycle life, with 4% to 7% less degradation after 100 cycles compared to aluminum-cased batteries;
5. Low internal resistance: The internal resistance of pouch batteries is lower than that of lithium-ion batteries, with the lowest being below 35m in China, which greatly reduces the self-discharge of the battery;
6. Flexible design: The shape can be customized according to customer needs, and it can be made thinner. Ordinary aluminum shells can only be 4mm, while soft packs can be 0.5mm.
In terms of performance, pouch batteries have a comprehensive advantage and broad application prospects in the future.
1. Pouch batteries are more suitable for portable applications where space or thickness is a major concern, such as 3C consumer electronics.
2. Although square batteries have high single-cell capacity, they are heavy and large, while pouch batteries have a clear advantage in energy density. Moreover, single-cell batteries are currently developing towards larger capacity and higher rate of return, which will better meet the requirements of mobile power supplies in fields such as new energy vehicles.
3. Although cylindrical batteries have mature manufacturing processes and significant energy density advantages, the small capacity of a single cell means that a battery pack of the same capacity requires more cells. For example, the Tesla Model S battery pack has a capacity of 85kWh, containing approximately 7,000 18650 cylindrical cells. This places very high demands on the Battery Management System (BMS). In contrast, a single pouch battery cell can have the equivalent capacity of more than 10 18650 cylindrical cells, thus requiring less from the BMS. Given the limited BMS capabilities in most parts of China and even globally, large-capacity battery packs will not rely heavily on cylindrical batteries. Furthermore, the high energy density of cylindrical batteries leads to poor safety, and the rigid casing results in high internal pressure, often causing explosions. Pouch batteries, on the other hand, have significant advantages.
In summary, the advantages and disadvantages of pouch batteries are almost intertwined. Because the shape of pouch batteries is flexibly designed and customized according to customer needs, and the cells are also remade, the number of existing pouch batteries of the same model is too small. In addition, the cost of developing a new pouch battery pack is also quite high.
