The difference between cylindrical battery, pouch lithium battery and square battery

With the continuous development of the lithium battery industry, at present, there are three main packaging forms of mainstream lithium-ion batteries, namely cylinder, square and soft package. Different packaging structures mean different characteristics.

1. Cylindrical battery: Cylindrical batteries are mainly steel shell cylindrical lithium iron phosphate batteries. This kind of battery is characterized by high capacity, high output voltage, good charge and discharge cycle performance, stable output voltage, high current discharge, and battery life. Stable chemical properties, safe use, wide operating temperature range, and environmental friendliness. Widely used in solar lamps, lawn lamps, backup energy, power tools, toy models.

The earliest cylindrical lithium-ion battery was the 18650 lithium-ion battery invented by Japan's SONY company in 1992. Because the 18650 cylindrical lithium-ion battery has a long history, the market penetration rate is very high, and the use of cylindrical lithium-ion batteries is quite mature. Advanced winding process, high degree of automation, stable product quality and relatively low cost. There are many types of cylindrical lithium-ion batteries, such as 14650, 17490, 18650, 21700, 26650 and so on. Cylindrical lithium-ion batteries are more popular among lithium-ion battery companies in Japan and South Korea. There are also companies of considerable scale producing cylindrical lithium-ion batteries in my country.

The structure of a typical cylindrical battery includes: positive electrode cover, safety valve, pTC element, current cut-off mechanism, gasket, positive electrode, negative electrode, separator, and casing.

2. Soft-pack lithium battery: Soft-pack lithium-ion battery is a liquid lithium-ion battery covered with a polymer shell. The biggest difference from other batteries is the soft packaging material (aluminum-plastic composite film), which is also a soft-pack lithium-ion battery. The most critical and technically difficult material. The packaging material and structure of soft-pack lithium batteries give them many advantages.

1. Good safety performance: the electrolyte of the soft-pack lithium battery has less leakage, and the soft-pack lithium battery will burst when there is a safety hazard, unlike the hard-shell battery, which will explode if the internal pressure is too high;

2. Light weight: the weight of the soft-packed lithium battery is 40% lighter than the steel-cased square battery of the same capacity, and 20% lighter than the aluminum-cased square battery;

3. Large battery capacity: the soft pack saves 20% of volume, which is 50% higher than steel shell batteries of the same specification and size, and 20-30% higher than aluminum shell batteries;

4. Good cycle performance: the cycle life of the soft pack lithium battery is longer, and the attenuation of 100 cycles is 4% to 7% less than that of the aluminum case;

5. Small internal resistance: The internal resistance of the soft-pack lithium battery is smaller than that of lithium-ion batteries, and the domestic minimum can be below 35mΩ, which greatly reduces the self-consumption of the battery;

6. Flexible design: the shape can be customized according to customer needs, and can be made thinner. The ordinary aluminum shell can only be 4mm, and the soft case can be 0.5mm.

In view of the advantages of soft-pack lithium batteries, industry experts predict that with the development of battery routes, the penetration rate of soft-pack lithium batteries in the new energy vehicle market will continue to increase. In the future, the proportion of soft-pack lithium batteries in various types of batteries is expected to exceed 50%.

3. Square batteries: Square lithium-ion batteries usually refer to square batteries with aluminum or steel shells. The penetration rate of square batteries is very high in China. With the rise of lithium batteries for automobile power in recent years, there is a contradiction between the mileage of automobiles and battery capacity. It has become increasingly prominent that domestic power lithium battery manufacturers mostly use aluminum shell square batteries with high battery energy density, because the structure of square batteries is relatively simple, unlike cylindrical batteries that use high-strength stainless steel as the shell and have explosion-proof safety valves. And other accessories, so the overall accessories should be light in weight and relatively high in energy density. There are two different processes for prismatic batteries: winding and stacking.

Comparative analysis of technical characteristics

1. Battery shape: Square lithium-ion batteries can be of any size, while soft-pack lithium batteries can be made thinner, which is incomparable to cylindrical batteries.

2. Rate characteristics: The process limitation of welding multi-pole tabs for cylindrical lithium-ion batteries, so the rate characteristics are slightly worse than the square multi-pole method.

3. Discharge platform: The same positive electrode material, negative electrode material, and electrolyte are used, so the discharge platform is theoretically the same, but the internal resistance of the square battery is slightly dominant, so the discharge platform is slightly higher.

4. Product quality: The process of cylindrical lithium-ion batteries is very mature, and the probability of common secondary slitting defects of the pole pieces is low, and the winding process is more mature and automated than the lamination process. The lamination process is still using a semi-manual method. , so there is an adverse effect on the quality of the battery.

5. Tab welding: Cylindrical lithium-ion battery tabs are easier to weld than square lithium-ion batteries, and square batteries are prone to false welding, which affects battery quality.

6. Grouping of pACKs: Circular batteries are relatively easier to use, so the pACK method is simple and has a good heat dissipation effect. When pACKing square batteries, the heat dissipation problem must be solved.

7. Structural features: The chemical activity at the corners of the square battery is poor, and the performance of the battery will decline significantly after long-term use.

In short, regardless of cylindrical, square or soft-pack lithium batteries, the reason why they can develop rapidly at present is because they have been well used in their respective application fields.