Compared with soft-pack and square lithium-ion batteries, the 18650 cylindrical lithium-ion battery is the earliest commercialized, most automated, and currently the lowest-cost lithium battery cell. And with Tesla’s support for many years, it has basically maintained a three-point world share with soft packs and square batteries. Since Tesla announced that Model 3 will adopt 21700, the cylindrical battery family has also gained a star member. This article will take a look at several technical points related to cylindrical batteries.
1. Cylindrical battery structure
If you pay attention to lithium batteries, you will find that cylindrical 18560 batteries are the type that has been studied the most and has the most technical discussions. The monomer is mainly composed of a positive electrode, a negative electrode, a diaphragm, a positive and negative collector, a safety valve, an overcurrent protection device, insulators and a casing. As for the shell, there were many steel shells in the early days, but now aluminum shells are the main ones.
The design of the single overcurrent protection device is different for each battery brand. According to the different safety requirements, the price will be different and can be customized. General safety devices mainly include PTC (Positive Temperature Coefficient) resistors and fuse devices.
PTC: When excessive current occurs, the resistor heats up, and temperature accumulation further promotes the rise in PTC resistance. When the temperature exceeds a threshold, it increases suddenly, which is equivalent to isolating the faulty cell from the overall circuit to prevent further thermal runaway. happened.
Fuse device: In principle, it is a fuse. When encountering excessive current, the fuse blows and the circuit is disconnected.
The difference between the two protection devices is that the former is recoverable, while the latter's protection is one-time. Once a fault occurs, the system must replace the problematic battery in order to function normally.
2. Characteristics of cylindrical battery cells
Due to its own structural characteristics and the standardization of its models, cylindrical battery cells, especially 18650, have the highest automation level among the three important battery cell forms. This enables a high degree of consistency and correspondingly improved yields.
- As mentioned above, the monomer has good consistency;
- The monomer itself has good mechanical properties. Compared with square and soft-pack batteries, a closed cylinder can obtain the highest bending strength under similar dimensions;
- The technology is mature and the cost is low, but at the same time, the space for cost optimization has been almost exhausted;
- The energy of the single body is small and the form is easy to control in the event of an accident.
- In large-scale battery systems, the number of cylindrical cells in the battery system is very large, which greatly increases the complexity of the battery system. Regardless of the organization or management system, relative to the other two types of batteries, the system-level cost of cylindrical batteries is high.
- Under the condition of uneven temperature environment, the probability of alienation of the characteristics of a large number of battery cells increases. Of course, the reason why Tesla chose 18650 at the beginning of the design is believed to be a helpless choice, because 10 years ago, only cylindrical batteriesit was possible to mass-produce qualified power lithium batteries. The safety and thermal management requirements of the battery are the driving force for the development of its powerful electronic control system.
- The room for increase in energy density is already very small. Since 2016, no higher records have been seen. In a given space, only materials can be used, which is admittedly a difficult road to follow.
3. Tesla's Model 3 21700 battery system
Tesla Model 3 fully uses 21700 ternary lithium batteries, opening a new stage of increasing the capacity of cylindrical batteries. Tesla Model 3's 21700 battery system has an energy density of about 300Wh/kg, which is more than 20% higher than the original 18650 battery used in Model S. The single capacity is increased by 35%, and the system cost is reduced by about 9%.
4. What do you think about the increase in capacity from 18650 to 21700
At present, people are constantly improving the specific energy. If the external dimensions remain unchanged and the energy density needs to be increased, 18650 faces many challenges:
- The supply chain of new materials such as NCA, silicon carbon and other new materials is still immature, the cost is high, and the supply is difficult to stabilize.
- New material manufacturing processes have high environmental requirements, high investment in fixed assets, and huge energy consumption.
- The capacity of single battery is low, and the technical requirements and cost of PACK grouping are relatively high.
- The single battery core is suitable for positive single-pole and negative bipolar lug structures at most, and has a significant impact on energy density.
- When battery high energy density and high-rate charging are required at the same time, the design space is very small. 18650 uses 523+ graphite system, and 1C reaches 2.4AH, which has reached the design limit.
To sum up, the size is increased from 18650 to 21700, and the benefits are as follows:
- When the energy density is appropriately increased, conventional materials can be selected, which have stable performance and high cost performance.
- The multi-pole mechanism can be designed appropriately to reduce internal resistance.
- Under the same energy density, graphite with fast charging characteristics can be selected to improve fast charging performance.
- Appropriately adding diameter and height can obtain more effective volume.
- The capacity of the single cell increases, the proportion of auxiliary components decreases, and the Pack cost is reduced.
From the perspective of technology accumulation alone, 18650 has been studied by companies and academic institutions for many years, and there is a lot of public data and information, such as thermal management models, thermal runaway prediction, etc, while the 21700 is relatively blank.
Regarding the future development of 21700, there are many positives, and the negatives can also be listed casually. But this is all internal comparison of cylindrical batteries. If we consider the competition between prismatic batteries and soft-pack batteries, the big development of 21700 may not come soon.