What are the advantages and disadvantages of 21700 cylindrical lithium-ion battery?

Compared with the soft pack and the square lithium batteries, the 18650 cylindrical lithium batteries are the earliest commercialized, one of the most automated and the lowest cost. There are also many years of support from Tesla, which basically keeps the situation with soft packs and square batteries.

After Tesla announced that the Model3 was adopted by 21700, the cylindrical battery family also had a star member. This article takes a look at several technical points related to cylindrical batteries. Unless otherwise stated below, cylindrical batteries are specifically referred to as 18650.

Cylindrical battery structure

Pay attention to the power battery and you will find that the cylindrical 18560 battery is the most studied battery type. The monomer is mainly composed of a positive electrode, a negative electrode, a separator, a positive electrode negative electrode collector, a safety valve, an over current protection device, an insulating member and a casing. The shell has many steel shells in the early days, and is currently dominated by aluminum shells. The safety valve and PTC are shown in the figure below.

Monomer over-current protection device, the design of each manufacturer is not the same, according to the different security requirements, going to different price, can be customized. General safety devices are mainly PTC positive temperature coefficient of resistance and fusing unit two kinds big.

PTC, when too much current, resistance heating, accumulate more promote PTC resistance temperature increases, when the temperature exceeds a threshold, increase abruptly, equivalent to isolate the fault batteries from the overall circuit, to avoid further thermal runaway.

Fusing device, in principle is a fuse, excessive current, blown fuse and circuit is disconnected.

The difference between two kinds of protection device can be restored. The protection of the latter is a one time, once the failure occurs. The system must be considered to replace the problem batteries can work normally.

Characteristics of cylindrical batteries

Cylindrical batteries, especially 18650, due to its structural features, also because of its models of standardization, cylindrical batteries production automation level, the highest in the three main forms of batteries. This makes the high consistency, yield was improved accordingly. Statistics show, Samsung, Panasonic and other major foreign manufacturer rate can reach 98%, and manufacturers in China can be more than 90%.

Advantages

1) As described earlier, monomer has good consistency;

2) Its mechanical performance is good, monomer and square and soft package battery compared to closed cylinder, approximate size, can obtain the highest bending strength;

3) Mature technology, low cost, but at the same time, the cost optimization of space has consumed almost;

4) Monomer energy is small, accidents, the form is easy to control, but it is also be a reason to be replaced it, how can you say the words of pop, achievement you will destroy you, doesn't kill you will make you strong, content with this!)

Disadvantages

1) in the context of electric cars this, the number of columns monomer battery systems are large, this makes the battery system complexity, both institutional and management system relative to the other two types of batteries, the cost of the system level of cylindrical battery on the high side;

2) under the condition of non-uniform temperature environment, the probability of a large number of batteries characteristics of alienation, of course, Tesla was in the beginning of design to choose 18650, believe that only a helpless choice, because ten years ago, to mass production of qualified power battery, cylindrical battery only. The battery safety and thermal management requirements, is it a powerful power electronic control system of the research.

3) The rising space of energy density is already very small. In 2016 news, Chaowei achieved a single capacity of 4050mAh and a cell specific energy of 306Wh/kg. Since then, no higher records have been seen. In the established space, only the dead material is recognized as a difficult road.

Tesila's Model3 battery system makes 21700 popular

Tesla Model3 enabled all 21700 ternary lithium batteries, opened a cylindrical battery capacity of a new phase of ascension. Tesla Model3 type 21700 battery systems, the energy density is controlled in 300wh/kg, than the original modes used18650 battery energy density increase by more than 20%, single capacity 20% increased, system costs by 9% or so.

Current international can batch production 21700 batteries a handful of enterprise, in addition to Panasonic and Tesla joint research and development of 21700 battery production, Samsung SDI has previously display at the 21700 products, according to the product is not for mass production.

Domestic 21,700 manufacturers, the news materials show that the manufacturers of the 21700 have BAK, lishen, Yiwei Lithium, Tianpeng Power, Far East Foster, Lions New Energy and so on.

With the announcement of two products of Nanjing Jinlong and Beiqi, on August 16, the Ministry of Industry and Information Technology issued a new product announcement for the “Road Motor Vehicle Manufacturing Enterprise and Product Announcement” (No. 299). Among them, two pure electric vans (models NJL5040XXYBEV25 and BJ5040XXYCJ06EV) produced by Nanjing Jinlong Bus Manufacturing Co., Ltd. and Beiqi (Changzhou) Automobile Co., Ltd. were the first to carry 21,700 ternary lithium-ion batteries.

From 18650 to 21700, how about let it?

18650Lithium ion batteries challenge for the development, facing the current people to improve the present situation of the specific energy, if maintaining the overall dimensions, and to improve the energy density, 18650 faces many challenges:

1) New materials such as NCA, silicon carbon material supply chain is not yet mature, high costs, supply stability, higher energy density materials, 811, for example, stability and production process control distance itself for a gap, the result is 811 of 18650 much more expensive in the short term, the performance is poor;

2) New material process on the environment demand is high, the fixed assets investment is high, energy consumption;

3) Monomer battery capacity is low, PACK group technology requirements and higher cost;

4) Monomer batteries up to adapt to the list of positive and negative, bipolar structure, and a more significant effect on energy density;

5) high energy density and high rate of charging requirements at the same time, the design space is very small, 18650 with 523 + graphite system, according to the new national standard, 1 c do 2.4 AH has reached the design limit.

Larger diameter cylindrical lithium-ion battery will become the inevitable trend, with the data shown in the figure below, greater size batteries and 18650 in a ear design and winding, comparing the two angles of curvature large size batteries shows obvious advantages.

Summary, size increased from 18650 to 21700, the benefits are as follows:

1) Increasing the energy density cases can choose the regular material, stable performance, cost-effective;

2) Can be appropriate for a multipolar ear mechanism design, reduce the internal resistance;

3) Under the same energy density, can choose quick charge properties of graphite, to improve the quick charge performance;

4) Appropriate increase the diameter and height can get more effective volume.

5) Monomer batteries capacity increase, auxiliary component ratio reduce, reduce the Pack cost.

In the new national standard "GB/T34013-2017 electric vehicle power battery product size" issued by the Ministry of Industry and Information Technology, only 18650 and 32650 in the original consultation draft were revised into four specifications including 21700, also counted as a major benefit of the 21700.

The page contains the contents of the machine translation.