802540 lipo battery.Cylindrical lithium battery 21700 from a technical perspective

Compared with soft-pack and square lithium batteries, 18650 cylindrical lithium batteries are the earliest commercialized, the most automated production, and currently the lowest cost power battery. Tesla has been holding on for many years and has basically maintained a three-part world share with soft packs and prismatic batteries.

Compared with soft-pack and square lithium batteries, 18650 cylindrical lithium batteries are the earliest commercialized, the most automated production, and currently the lowest cost power battery. Tesla has been holding on for many years and has basically maintained a three-part world share with soft packs and prismatic 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. Unless otherwise stated below, the cylindrical battery refers specifically to 18650.

Cylindrical battery structure

If you pay attention to power batteries, you will find that the cylindrical 18560 battery is the battery 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 safety valve and PTC are shown in the figure below.

The design of each manufacturer's single overcurrent protection device is different. According to 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 avoid further thermal runaway. happened.

The fuse device is basically 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 to work normally.

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 main battery cell forms. This enables a high degree of consistency and correspondingly improved yields. Data shows that the yield rate of major foreign manufacturers such as Samsung and Panasonic can reach 98%, while Chinese manufacturers can also exceed 90%.

advantage

1) As mentioned earlier, the monomer has good consistency;

2) 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;

3) The technology is mature and the cost is low, but at the same time, the space for cost optimization has been almost exhausted;

4) The energy of a single body is small, and the form is easy to control when an accident occurs, but this is also becoming the reason for its replacement (as the popular saying goes, what makes you will destroy you, what doesn’t kill you will also make you) You are strong, the same applies to everything)

Disadvantages: 1) In the context of electric vehicles, 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, compared with the other two types of batteries, the system level cost is Cylindrical batteries are on the high side;

2) 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, it was possible to mass-produce Qualified power batteries are only cylindrical batteries. The safety and thermal management requirements of the battery are the driving force for the development of its powerful electronic control system.

3) The room for increase in energy density is already very small. According to the news in 2016, Chaowei increased the cell capacity to 4050mAh and the cell specific energy to 306Wh/kg. There has been no higher record since then. In a given space, only materials can be used, which is admittedly a difficult road to follow.

Tesila’s Model 3 battery system hits 21,700

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%.

21700 related manufacturers and models

At present, there are only a handful of companies in the world that can mass-produce 21700 batteries. In addition to the mass production of 21700 batteries jointly developed by Panasonic and Tesla, Samsung SDI has also previously displayed related 21700 products. It is understood that this product has not yet been mass-produced.

Domestic 21700 manufacturers, news materials show that manufacturers working on 21700 include BAK, Lishen, Yiwei Lithium Energy, Tianpeng Power, Far East Foster, Mengshi New Energy, etc.

With the announcement of two products of Nanjing Jinlong and BAIC, on August 16, the Ministry of Industry and Information Technology released the announcement of new vehicle products in the "Announcement of Road Motor Vehicle Manufacturers and Products" (Batch 299). Among them, two pure electric van transport vehicles (models NJL5040XXYBEV25 and BJ5040XXYCJ06EV) produced by Nanjing Jinlong Bus Manufacturing Co., Ltd. and BAIC (Changzhou) Automobile Co., Ltd. are the first to be equipped with 21700 ternary lithium-ion batteries.

From 18650 to 21700, what do you think about the increase in capacity?

Ever since I saw this picture on the Internet, I always wanted to find an opportunity to include it in the article. There is no other reason. The summary of specific energy improvement in the picture can be said to be very comprehensive. Following the idea in the picture, if the specific energy of the battery core is increased chemically, the future of 18650 is not bright.

Challenges faced by the development of 18650 lithium-ion batteries. Faced with the current situation where people are constantly improving specific energy, if the appearance size remains unchanged and the energy density must be increased, 18650 faces many challenges:

1) The supply chain of new materials such as NCA and silicon carbon is still immature, the cost is high, and the supply is difficult to stabilize. For example, the higher energy density material 811 has a long way to go in terms of stability and process control compared to mass production. The result is short-term The 18650 of the 811 is much more expensive, but its performance is much worse;

2) New material manufacturing processes have high environmental requirements, high investment in fixed assets, and huge energy consumption;

3) The capacity of single battery is low, and the technical requirements and cost of PACK grouping are relatively high;

4) 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;

5) When high energy density and high-rate charging are required at the same time, the design space is very small. 18650 uses 523+ graphite system. According to the new national standard, 1C has reached the design limit of 2.4AH.

Larger diameter cylindrical lithium-ion batteries will become an inevitable trend. The data is shown in the figure below. A comparison of larger battery cells and 18650 in terms of tab design and winding curvature shows that large-size battery cells show obvious advantages.

To sum up, the size is increased from 18650 to 21700, and the benefits are as follows:

1) When the energy density is appropriately increased, conventional materials can be selected, which have stable performance and high cost performance;

2) The multi-pole mechanism can be designed appropriately to reduce internal resistance;

3) Under the same energy density, graphite with fast charging characteristics can be selected to improve fast charging performance;

4) Appropriately increasing the diameter and height can obtain more effective volume.

5) The capacity of the single cell increases, the proportion of auxiliary components decreases, and the Pack cost is reduced.

In the newly promulgated national standard "GB/T34013-2017 Product Specifications and Dimensions of Power Batteries for Electric Vehicles" issued by the Ministry of Industry and Information Technology, only 18650 and 32650 in the original draft for comments have been modified into 4 specifications including 21700, which are also counted. A major benefit of 21700.

The path to increasing capacity conflicts with the early view that small units equate to high security.