802540 polymer battery.Let’s take a look at several technical points related to cylindrical batteries

Compared with soft-pack and prismatic lithium batteries, 18650 cylindrical lithium batteries were the earliest commercialized power battery with the highest degree of production automation and the lowest cost at the time. Tesla has been holding on for many years and has basically maintained a three-point national situation with soft packs and prismatic batteries. Since Tesla announced that Model 3 will use 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.

Compared with soft-pack and prismatic lithium batteries, 18650 cylindrical lithium batteries were the earliest commercialized power battery with the highest degree of production automation and the lowest cost at the time. Tesla has been holding on for many years and has basically maintained a three-point national situation with soft packs and prismatic batteries. Since Tesla announced that Model 3 will use 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 reviews. The monomer is mainly composed of a positive electrode, a negative electrode, a separator, a positive electrode and a negative electrode collector, a safety valve, an overcurrent protection device, an insulator and a casing. As for the shell, there were many steel shells in the early days, and aluminum shells were the main ones at that time. The safety valve and PTC are shown in the figure below.


Each manufacturer's plan for single overcurrent protection equipment 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.
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PTC, when excessive current occurs, the resistor heats up, and the temperature accumulation further promotes the rise of the PTC resistance. When the temperature exceeds a threshold, it increases suddenly, which is equivalent to isolating the faulty battery from the entire circuit to prevent further heating. Loss of control occurs.


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 in order to operate normally.


Cylindrical battery characteristics


Cylindrical battery cells, especially 18650, due to their structural characteristics and the standardization of their models, the automation level of cylindrical battery cell production is the highest among the three main battery cell methods. This makes a high degree of consistency possible, and yields are correspondingly improved. 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%.
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Advantages: 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 approximate standards;


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 single energy is small, and the method 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 successful will also destroy you, and what cannot kill you will also destroy you) You are strong, the same applies to everything)


Defects, 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 mechanism or the processing system, relative to the other two types of batteries, the system level The cost of cylindrical batteries is on the high side;


2) Under the condition of uneven temperature environment, the probability of alienation of many battery cell characteristics increases. Of course, the reason why Tesla chose 18650 at the beginning of the planning is believed to be an unavoidable choice, because 10 years ago, it could be mass-produced Qualified power batteries are only cylindrical batteries. The safety and heat treatment requirements of the battery are the driving force for the research and development of its strong 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 specific energy of the cell to 306Wh/kg. Since then, no higher records have been seen. In a given space, as long as you stick to materials, it 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. The energy density of Tesla Model 3's 21700 battery system is around 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 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 by 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 Manufacturing Enterprises and Products" (Batch 299). Among them, two pure electric vans (models NJL5040XXYBEV25 and BJ5040XXYCJ06EV) produced by Nanjing Jinlong Bus Manufacturing Co., Ltd. and BAIC (Changzhou) Automobile Co., Ltd. were the first to be equipped with 21700 ternary lithium-ion batteries.


From 18650 to 21700, how do you view 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 the forward ratio energy in the picture can be said to be very comprehensive. Following the idea in the picture, if the specific energy of the cell is improved chemically, the future of 18650 is not bright.


Challenges faced by the development of 18650 lithium-ion batteries. Faced with the current situation of people constantly improving specific energy, if the shape specifications remain unchanged and the energy density must be improved, 18650 faces many challenges:


1) The supply chain of new materials such as NCA, silicon carbon and other new materials is not yet mature, the cost is high, and the supply is difficult to stabilize. For example, the higher energy density material 811, its own stability and process control are far from mass production. The result is In the short term, 811’s 18650 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) Single cells are mostly used to have a positive single-pole and negative bipolar lug structure, and the impact on energy density is obvious;


5) When high energy density and high rate charging are required at the same time, the planning space is very small. 18650 uses 523+ graphite system. According to the new national standard, 1C has reached the limit of planning to achieve 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 standard cells and 18650 in terms of tab design and winding curvature shows that large standard cells have obvious advantages.


To sum up, the standard advances from 18650 to 21700, and the benefits are as follows:


1) When the energy density is appropriately increased, conventional materials can be selected, which are stable in performance and cost-effective;


2) Multi-pole mechanism design can be properly carried out to reduce internal resistance;


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


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


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


In the newly announced national standard "GB/T34013-2017 Electric Vehicle Power Battery Product Specifications" issued by the Ministry of Industry and Information Technology, only 18650 and 32650 in the original draft for comments have been revised to 4 specifications including 21700, which can be considered A serious positive for 21700.


However, the path to increasing capacity conflicts with the early view that small units are equivalent to high security. There are many sophisticated methods to prevent and control the thermal safety of small lithium-ion batteries (<3Ah) and battery modules (<150Ah), such as adding PTC, introducing current interruption mechanisms or pressure sensors, etc. However, there are still challenges in the safety control of large lithium-ion single batteries (>6Ah) or modules (>200Ah). Compared with small batteries, large-scale power lithium-ion batteries contain higher energy. When thermal safety issues arise, the consequences will be more serious. Because the increase in battery volume results in a reduction in the battery's specific surface area, The small size then reduces the heat dissipation area per unit volume of the battery. The inconsistency in the internal temperature of the battery will also appear as the lithium-ion battery becomes larger and grouped. This temperature difference between the battery cells will increase the risk of thermal runaway of the battery, and then lead to a series of problems in the battery.


It is generally believed that if manufacturers master more advanced safety processing technology and can keep the danger within the controllable range of their own systems, they can offset the danger caused by battery cells. On the other hand, it can also be said that the use of larger-capacity batteries to build systems is theoretically premised on the advancement of risk management technologies in single design and battery pack design.


The risks and benefits brought about by capacity advancement are obvious knowledge in the industry. Whether the main engine manufacturers fully accept it and are willing to transfer a certain amount of security for it is a key part of the further development of 21700. Personally, I feel that this will take a long time. The adjustments and updates of equipment required to produce 21700 are affected by the opinions of equipment suppliers.


Most of the advantages of cylindrical batteries come from their scale. Before 21700 can truly scale up, there are still problems with B-class product disposal. This standard that cannot be downgraded is also a test of the manufacturer's determination and financial strength.


From the perspective of technology accumulation alone, 18650 has been studied by enterprises and academic institutions for many years, and there is a lot of public data and materials, such as heat treatment models, thermal runaway predictions, etc. It can also be regarded as a resource for small businesses whose own R&D capabilities are not yet complete. But 21700 is relatively blank.