Compared to pouch and prismatic lithium batteries, the 18650 cylindrical lithium battery is the earliest commercialized, most automated in production, and currently the lowest-cost type of power battery. With Tesla's long-standing presence, it has essentially maintained a three-way competition with pouch and prismatic batteries in the market.
Since Tesla announced that the Model 3 would use the 21700, the cylindrical battery family has gained a new star member. This article will explore some key technical aspects of cylindrical batteries. Unless otherwise specified, "cylindrical battery" in the following text refers specifically to the 18650.
Cylindrical battery structure
A closer look at power batteries reveals that the cylindrical 18560 battery is the most researched and technically discussed type. A single cell mainly consists of a positive electrode, negative electrode, separator, positive and negative current collectors, safety valve, overcurrent protection device, insulation components, and casing. Early casings were mostly made of steel, but currently aluminum casings are predominant. The safety valve and PTC are shown in the following diagram.
Individual overcurrent protection devices vary in design from manufacturer to manufacturer, and prices differ depending on safety requirements; customization is available. Common safety devices mainly fall into two categories: PTC positive temperature coefficient resistors and fuses.
When a PTC experiences excessive current, the resistor heats up, and the accumulated temperature further increases the PTC resistance. Once the temperature exceeds a certain threshold, the resistance increases sharply, effectively isolating the faulty cell from the overall circuit and preventing further thermal runaway.
A fuse is essentially a fuse wire. When it encounters an excessive current, the fuse wire melts and the circuit is broken.
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 faulty battery cell in order to function properly.
Cylindrical cell characteristics
Cylindrical cells, especially the 18650, have the highest level of automation in production among the three main cell types due to their structural characteristics and standardized model design. This enables high consistency and consequently improves the yield rate. Data shows that major international manufacturers such as Samsung and Panasonic can achieve a yield rate of 98%, while Chinese manufacturers can exceed 90%.
advantage
1) As mentioned above, the monomer consistency is good;
2) The single cell itself has good mechanical properties. Compared with square and pouch cells, the closed cylinder can achieve the highest bending strength at approximately the same size.
3) The technology is mature and the cost is low, but at the same time, the room for cost optimization has been almost exhausted;
4) Its low energy content makes it easier to control in the event of an accident, but this is also becoming a reason for its replacement (as the popular saying goes, what makes you will also destroy you, and what doesn't kill you will make you stronger; the same principle 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. Compared with the other two types of batteries, the system-level cost of cylindrical batteries is relatively high, both in terms of structure and management system.
2) Under uneven temperature conditions, the probability of significant changes in the characteristics of a large number of battery cells increases. Of course, Tesla's initial choice of 18650 cells was likely a reluctant one, because 10 years ago, the only qualified power batteries that could be mass-produced were cylindrical batteries. The safety and thermal management requirements of the batteries, however, became the driving force behind the development of its powerful electronic control system.
3) The room for improvement in energy density is already very small. In 2016, news broke that AWDC had achieved a single-cell capacity of 4050mAh and a cell specific energy of 306Wh/kg. Since then, no higher records have been seen. Within the given space, the only option is to relentlessly pursue advanced materials, which is widely recognized as a difficult path.
Tesila's Model 3 battery system sparked a surge in demand for 21,700 units.
Tesla's Model 3 has fully adopted 21700 ternary lithium batteries, ushering in a new era of capacity increases for cylindrical batteries. The 21700 battery system in the Tesla Model 3 has an energy density of around 300Wh/kg, which is more than 20% higher than the 18650 batteries used in the previous Model S, with a 35% increase in single-cell capacity and a reduction of about 9% in system cost.
21700 related manufacturers and models
Currently, there are only a handful of companies in the world capable of mass-producing 21700 batteries. In addition to the 21700 batteries jointly developed by Panasonic and Tesla, Samsung SDI has also previously showcased related 21700 products, but it is understood that these products have not yet entered mass production.
According to news reports, domestic manufacturers producing 21700 series batteries include BAK, Lishen, EVE Energy, Tianpeng Power, Far East Foster, and Mengshi New Energy.
Following the public announcement of two products from Nanjing Golden Dragon and BAIC, on August 16, the Ministry of Industry and Information Technology released the "Announcement of Road Motor Vehicle Manufacturers and Products" (Batch 299), listing new vehicle products. Among them, two pure electric vans (models NJL5040XXYBEV25 and BJ5040XXYCJ06EV) produced by Nanjing Golden Dragon Bus Manufacturing Co., Ltd. and BAIC (Changzhou) Automobile Co., Ltd. were the first to be equipped with 21700 ternary lithium-ion batteries.
How should we view the increase in capacity from 18650 to 21700?
Ever since I saw this image online, I've wanted to include it in an article. The reason is simple: the summary of improving specific energy in the image is quite comprehensive. Following the line of thought in the image, if we try to improve the specific energy of the cell through chemical methods, the future of the 18650 is not bright.
The challenges facing the development of 18650 lithium-ion batteries: Given the current trend of continuously increasing energy density, maintaining the same physical dimensions while simultaneously improving energy density presents numerous challenges for the 18650 battery.
1) The supply chain for new materials such as NCA and silicon carbide is still immature, with high costs and unstable supply. For example, the higher energy density material 811 itself is still far from mass production in terms of stability and process control. As a result, in the short term, 18650 811 is much more expensive, but its performance is much worse.
2) New material manufacturing processes have high environmental requirements, high fixed asset investment, and huge energy consumption;
3) Low single-cell capacity, high requirements and costs for PACK assembly technology;
4) A single cell can only adapt to positive single-tab or negative bitab structures, and this has a significant impact on energy density;
5) When high energy density and high charging rate are required at the same time, the design space is very small. The 18650 uses the 523+ graphite system. According to the new national standard, 1C to achieve 2.4AH has reached the design limit.
Larger diameter cylindrical lithium-ion batteries will become an inevitable trend. As shown in the figure below, the larger cells have obvious advantages compared to the 18650 cells in terms of tab design and winding curvature.
In summary, the benefits of increasing the size from 18650 to 21700 are as follows:
1) When the energy density is appropriately increased, conventional materials can be selected, which offer stable performance and high cost-effectiveness;
2) A multi-pole mechanism can be appropriately designed to reduce internal resistance;
3) For the same energy density, graphite with fast-charging properties can be selected to improve fast-charging performance;
4) Appropriately increasing the diameter and height can yield more effective volume.
5) Increased single-cell capacity and reduced proportion of auxiliary components reduce pack costs.
