Pouch cells vs cylindrical

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

Soft pack battery

Pouch cells vs cylindrical

The flexible lithium battery is a polymer shell on the liquid lithium ion battery. The biggest difference between the flexible packaging material (aluminum plastic composite film) and other batteries is that it is also the most critical and technically difficult material in the flexible lithium battery. The packaging material and structure of the flexible battery make it have many advantages.
(1) Good safety performance: the electrolyte of the soft packed battery leaks less, and the soft packed battery will burst when there is a potential 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 battery is 40% lighter than that of the steel case square battery with the same capacity, and 20% lighter than that of the aluminum case square battery;
(3) Large battery capacity: soft package saves 20% of the volume, which is 50% higher than the steel case battery of the same specification and size, and 20~30% higher than the aluminum case battery;
(4) Good cycle performance: the cycle life of the soft packed battery is longer, and the attenuation of 100 cycles is 4%~7% less than that of the aluminum shell;
(5) Small internal resistance: the internal resistance of the soft packed battery is smaller than that of the lithium battery, which can be less than 35m Ω at least in China, greatly reducing the self consumption of the battery;
(6) Flexible design: the shape can be customized according to customer needs, and it can be made thinner. The ordinary aluminum shell can only reach 4mm, and the soft bag can reach 0.5mm.
In view of the advantages of the flexible battery, industry experts predict that with the development of the battery route, the penetration rate of the flexible battery in the new energy vehicle market will continue to increase, and the proportion of the flexible battery in all types of batteries is expected to exceed 50% in the future.

Cylindrical battery

Pouch cells vs cylindrical

Cylindrical batteries are mainly steel shell cylindrical lithium iron phosphate batteries, which are characterized by high capacity, high output voltage, good charge discharge cycle performance, stable output voltage, large current discharge, stable electrochemical performance, safe use, wide operating temperature range, and environmental friendliness. It is widely used in solar lamps, lawn lamps, backup energy, electric tools and toy models.
The earliest cylindrical lithium battery was the 18650 lithium battery invented by Japanese SONY Company in 1992. Because the 18650 cylindrical lithium battery has a long history, its market penetration rate is very high. The cylindrical lithium battery uses a mature winding process, which has a high degree of automation, stable product quality and relatively low cost. There are many types of cylindrical lithium batteries, such as 14650, 17490, 18650, 21700, 26650, etc. Cylindrical lithium batteries are popular among Japanese and Korean lithium battery enterprises, and there are also enterprises of considerable scale in China that produce cylindrical lithium batteries.
The structure of a typical cylindrical battery includes: positive cover, safety valve, PTC element, current cut-off mechanism, gasket, positive pole, negative pole, isolating membrane and housing.

Prismatic cell

Pouch cells vs cylindrical

Square lithium battery usually refers to aluminum or steel case square battery. The popularity of square battery is very high in China. With the rise of automobile power battery in recent years, the contradiction between vehicle mileage and battery capacity has become increasingly prominent. Domestic power battery manufacturers mostly use aluminum case square battery with high battery energy density because of its simple structure, Unlike cylindrical batteries, which use stainless steel with high strength as the shell and accessories with explosion-proof safety valves, the overall accessories are lighter and have higher relative energy density. Two different processes, winding and lamination, are adopted for square cell.

Comparative analysis of technical characteristics

  1. Battery shape: The square lithium battery can be of any size, while the flexible battery can be thinner than the cylindrical battery.
  2. Multiplication characteristics: the cylindrical lithium battery welding multipole lug is limited by the process, so the multiplication characteristics are slightly worse than the square multipole lug scheme
  3. Discharge platform: the same cathode material, cathode material and electrolyte are used, so theoretically the discharge platform is the same, but the internal resistance of square battery is slightly dominant, so the discharge platform is slightly higher.
  4. Product quality: The cylindrical lithium battery process is very mature, the probability of public secondary slitting defects is low, and the winding process is more mature and automated than the lamination process. At present, the lamination process is still semi manual, so there is a negative impact on the battery quality.
  5. Lug welding: The lug of cylindrical lithium battery is easier to weld than that of square lithium battery, and the square battery is easy to produce faulty soldering, which affects the battery quality.
  6. PACK grouping: round batteries are relatively easy to use, so the PACK scheme is simple and the heat dissipation effect is good. When square batteries are PACKed, the problem of heat dissipation should be solved.
  7. Structural characteristics: The chemical activity energy of square battery is poor at the corners, and the performance of the battery after long-term use decreases significantly.
    In a word, the reason why cylindrical, square and flexible batteries can develop rapidly at present is that they have been well applied in their respective fields of application. Domestic power battery manufacturers are predicting that the square will become the mainstream of power batteries, but the cylindrical power battery application manufacturers represented by Tesla are constantly innovating in technology, the energy density of vehicle batteries is constantly improving, and the battery performance expression is more outstanding.

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