Why choose cylindrical cells, what are the advantages?

Since Tesla proposed 4680 cells in September 2020, the large cylindrical battery has once again become a hot topic in the industry. In addition to Tesla, China’s head battery factories have also begun to join the batch industrialization investment of large cylindrical batteries, such as EVE, BYD, etc. Based on the popularity of cylindrical cells, this article focuses on analyzing the advantages of cylindrical cells (special note: there is no intention to exclude other types cell). In this article, cylindrical cells have the following three major advantages in practical applications for reference:

The flexibility of small capacity
Due to the size and process limitations of cylindrical cells, the capacity of a single cylindrical cell is usually small, such as 18650 NCM cell 2.6Ah, 32700 LFP cell 6Ah, 33140 LFP cell 15Ah, 34190 LFP cell 20Ah, etc..

Aside from electric vehicles and large-scale energy storage markets, in other fields, such as two-wheeled vehicles, tricycles, balance vehicles, power tools, AGVs, portable energy storage, home energy storage, etc., the capacity of single-module batteries is usually not very large. , in practical applications, large-capacity prismatic cells positioned in automobiles and large-scale energy storage do not necessarily match market demand, including size, capacity, etc. The small-capacity cylindrical cells can meet the capacity needs of a certain market in parallel or series, for example, the two types of batteries 24V60Ah and 48V30Ah used in AGV are common needs.

Three types of cylindrical cell, such as 32700, 33140 and 34190, can be connected in parallel or series to form the required 60Ah or 30Ah modules, if a smaller capacity can obviously be achieved.

Meanwhile, it can have more flexible operation possibilities in the space structure to meet the design requirements of different models.

The cost advantage of cylindrical cells
1) Among all types of lithium-ion batteries, cylindrical cells have the highest degree of process standardization and are the first to be commercialized, and their assembly efficiency is significantly higher than that of prismatic cells and pouch cells. With the gradual improvement of automation, the winding process of cylindrical cells currently reaches 200 PPM, even if the efficiency of large cylinders (such as 33140, 34190, 4680, etc.) is slightly lower than that of traditional 18650/21700 cylindrical cells, but it is also much higher than the efficiency of prismatic cell and pouch cell (usually around 10PPM).
2) The reason for the small capacity of the cylindrical cell is that the strip-shaped continuous electrode sheet after coating is cut into small strip-shaped electrode sheets. The miniaturization of the electrode sheet can improve the utilization rate of the electrode sheet. Usually under the same screening criteria and automation level, the material utilization rate of cylindrical cells is 3-5% higher than that of prismatic cells.

The heat dissipation effect of cylindrical cells has obvious advantages
We know that most of the fires of lithium-ion batteries are caused by “thermal runaway”, and the spread of thermal runaway is also the main concern of the safety performance of the battery system. The cylindrical structure is the most likely solution to thermal runaway, while the prismatic and pouch structures are almost unsolvable.

1) The low capacity of a single cylindrical cell leads to a small amount of energy released by thermal runaway. Meanwhile, with the gradual improvement of material safety, the heat release caused by thermal runaway is reduced, which significantly increases the safety of the battery system.
2) The arc-shaped surface structure of the cylindrical battery limits the heat transfer process between the cells to a certain extent, and has a certain positive effect on the spread of thermal runaway, prismatic cell and pouch cell are almost unsolvable in this regard. In the packing of cylindrical cells, the contact between the cells is the “line” contact on the arc surface, while the prismatic and pouch pack are “surface” contact. The “line” contact significantly reduces the heat transfer efficiency.

Meanwhile, there is still some space between the cells, and even if the insulating medium is not filled, the air reduces the heat transfer. In addition, with the promotion of “all tab technology” for cylindrical cells, the heat transfer on the curved surface will also be reduced.