Why does Tesla use 18650 cylindrical and lithium-ion batteries?

Let's start with a conceptual clarification: stacked batteries versus polymer batteries versus bagged batteries

The question referred to "laminated (polymer) lithium-ion batteries." 18650 pack builder Please note that laminated batteries and polymer batteries are two different concepts. The so-called polymer battery refers to a battery containing a polymer or colloidal electrolyte, while the electrolyte of a traditional lithium-ion battery is dissolved in a liquid electrolyte. The so-called laminated battery is that the internal electrode sheet and diaphragm are stacked layer upon layer, corresponding to the winding cell (the 18650 battery is the winding battery). Polymer batteries generally use a laminated structure, so they can also be called laminated batteries. However, a laminated lithium-ion battery may not be a polymer lithium-ion battery, as it can also use a liquid electrolyte. In fact, most laminated power batteries (Nissan Leaf, GE Volt, etc.) For electric cars it's liquid electrolyte. The reason for this confusion is probably because laminated batteries and polymer batteries are generally packaged with aluminum-plastic films, which we also call "bag-like" batteries. However, the inside of a bagged battery can also be a coiled battery cell.

Polymer batteries don't seem to be used in electric vehicles at the moment, battery manufacturing machine and I suspect the main reason may be that colloidal electrolytes don't conduct electricity very well, so they have high internal resistance.

Why does Tesla use 18650 batteries?

I'm asking questions.

What are the advantages of Tesla's Battery Management system (BMS) over other electric vehicles?

Several reasons are mentioned in this article.

Consistency

The 18650 battery is the earliest, most mature and most stable lithium-ion battery, li ion battery construction which is widely used in electronic products, with about several billion units shipped each year. Over the years, Japanese manufacturers have accumulated a lot of experience in the production process of 18650 batteries, making the consistency and safety of the produced 18650 batteries reach a very high level.

In an exchange with an American battery professor, he half-jokingly said that if you buy the same batch of batteries directly from a Japanese manufacturer, then use your battery charging and discharging tester at the same time for testing. If you find different discharge curves for different channels, you should question the consistency of your device, not the battery.

Stacked lithium-ion batteries, by contrast, are far from mature (see

Why does the electric vehicle technology industry not promote enterprise standardized batteries?

Even in terms of size, size, pole position and so on are not uniform, the battery manufacturer is inexperienced, and the consistency of the battery can not reach the standard of the 18650 battery. If the consistency of the battery cannot meet the requirements, the management of a large number of batteries in series and parallel will be a nightmare.

To sum up, the 18650 battery cell capacity is small (generally not more than 3 Ah), the number of cells required will be larger (Roadster has 6831), but the consistency is very good; The capacity of stacked batteries can be made larger (30 Ah or even 40 Ah), and the number of cells can be reduced, but the consistency is poor.

Tesla is not a battery company, it does not have a lot of accumulated battery technology, it is difficult to devote a lot of energy and resources to working with battery suppliers to improve the manufacturing of stacked batteries. So when it came to developing the sports car and Model S, Tesla's only option was to buy batteries from the market and develop its own battery system. Developing a battery system that manages more than 6,000 cells with good consistency should be technically easier than developing a battery system that manages more than 200 cells with poor consistency. Even as the number of individual batteries increases, it is easier to manage them if their performance is reliable.

In contrast to the Nissan Leaf, another successful pure electric car, it uses a laminated lithium-ion battery. This is because Nissan and NEC have cooperated for many years and accumulated a lot in battery technology, so they should have considerable skills in quality control. The LEAF's batteries come from AESC, a joint venture between Nissan and NEC.

It is a very interesting thing to compare automobile manufacturers in the United States, Japan, and different countries and regions at the medium level in the development and utilization of electric vehicles and battery manufacturers.

Cost

Tesla's 18,650 batteries allow it to build on previous production lines from Japanese manufacturers such as Panasonic. As the competition for 18650 batteries in consumer electronics gets tougher, I think companies like Panasonic would love to work with Tesla to upgrade their product lines and produce power batteries.

Moreover, industrial production has scale effects. When the scale of the product reaches an order of magnitude, the cost can be greatly reduced. A Tesla needs thousands of 18650 batteries, so the purchase price of a single battery is low.

Heat dissipation capacity

The stacked battery is thin and large, so it absorbs heat and dissipates heat well, and Nissan's LEAF is bold enough to employ a passive thermal management system (it doesn't!). The heat is carried away by natural convection. The LEAF's single-cell battery looks like this:

A battery module consisting of four cells in two parallel strings is shown as follows:

A battery pack consisting of 48 battery management modules connected in series looks like this:

As you can see, there are no fans, cooling tubes, or other thermal management systems on the battery pack. Looks like a crazy couple.

In contrast, the battery size of 18650 is relatively small, and the internal temperature difference of the single battery will not be too large during normal charging and discharging. However, it is very difficult to keep the temperature of more than 6,000 individual batteries in the range of 5 ° C, but Tesla has done it. How did you do it? Refer to

What are the advantages of Tesla's Battery Management system (BMS) over other electric vehicles?

Let me show you a picture of Tesla's cooling system,

These data pipes are the flow channels of the coolant, and the flow channel crossing is arranged much like the exhaust manifold of the engine, which is the flow resistance of the various pipelines can be roughly balanced in order to develop the enterprise.

In summary, Tesla's thermal management system is significantly more complex due to the adoption of a small capacity 18650 battery. In other words, in terms of heat dissipation, the use of low-capacity 18650 batteries is not the best choice.

Energy density

When it comes to energy density, it is necessary to distinguish between the energy density of a single cell and the energy density of a battery pack.

In terms of the energy distribution density analysis of a single battery, the 18650 battery is higher than the stacked lithium-ion power battery. I can find that the energy information density of 33 Ah lithium ion used by Nissan LEAF through the battery is 157 Wh/kg, and the energy density of the stacked battery used by GM Volt is about 150 Wh/kg. The 18650 battery used in the Roadster typically has an energy utilization density of about 211 Wh/kg. (Note: Neither Roadster nor Volt checked the original battery management data by themselves. I obtained the data model computing power of batteries with the same environmental capacity produced by the same enterprise or battery car manufacturer)

The management system of the 18650 battery is more complex, and the extra weight will make the energy density of the battery pack much lower than that of the single battery. The Roadster's battery pack weighs 450 kg and has an energy density of 118 Whu002Fkg, while the LEAF's battery pack weighs 225 kg and has an energy density of 107 Whu002Fkg. At the battery level, the energy density of the two is already comparable.

The design company of Model S should be much better than Roadster, but I have not collected too much face-to-face research data in this field, and we have no solution for analysis at present.

Security

The advantages of cascaded lithium-ion batteries have been mentioned above, but it also has some disadvantages. As laminated lithium-ion batteries are usually packaged in aluminum-plastic film, the thin thickness and low mechanical strength of the aluminum-plastic film make it difficult for vehicles to survive a collision, so the aluminum-plastic film is easy to break, resulting in safety accidents. That explains why Nissan has added an aluminum case to the four-cell battery pack.

18650 battery is generally steel shell, relatively safe; And as mentioned earlier, with the continuous improvement of the level of 18650 battery production process, safety is also improving.

Tesla has also put a lot of effort into dealing with potential safety issues with its 18,650 batteries. If the temperature of the individual battery is too high, or other abnormal conditions, depending on the severity of the abnormal condition, the battery or its module will be shut down to prevent the spread of the accident. Due to the small capacity of the unit, as long as there is no spread, the severity of the accident will be reduced.

I think Tesla used 18650 batteries at the time, which was really the best choice. However, as Tesla gains experience in the battery field, they should try other battery configurations.

cylindrical lithium-ion batteries clarification

3