The secret of Uncovering the battery: How to increase the battery

Recently, Tesla's 100kWh model has passed the assessment of the EU certification body RDW. This means that the Model S/X 100D will be available soon! The theoretical range of cruising range will reach 613km (based on the NEDC standard).

According to EU regulations, models sold in EU member states must be certified by their authorized agencies. RDW is a Dutch company commissioned by Tesla to obtain a license to sell in the EU. Let's explore how this 100kWh is done?

Elon Musk once said that Tesla's battery life (electricity) is increasing at a rate of 5% per year. From the iterative situation of the current battery pack, this goal is basically achieved. In addition to the 60kWh as an entry-level configuration, 70kWh and 85kWh have been upgraded to 75kWh and 90kWh respectively.

Soon after, 100kWh and 120kWh battery packs will also enter the option list. Currently, 60kWh still exists as a 配置 version to promote Tesla's sales. The real story is 70kWh and 85kWh, how to increase the power of 5kWh each.

One thing is certain, that is, the battery pack structure has not changed during the battery pack increase. The number of internal battery modules has not changed. Let's take a brief look at the internal structure of the Tesla battery pack.

There are 14 battery packs inside 60kWh, each battery pack contains 384 batteries, a total of 5376 batteries; 85kWh consists of 16 battery packs, each battery pack contains 444 batteries, a total of 7102 batteries Core composition.

The 70kWh that was added later was actually a 75kWh battery pack, which was limited by software. The extra 5kWh was originally offered to the owner as an optional package worth $3,000. As long as the OTA software is updated, the 70D can be changed to 75D.

So the question is, how did the 75kWh battery pack come from? On this issue, Tesla official did not make a technical explanation. According to the author's judgment, the 75kWh is actually a 85kWh battery pack, which is reduced by 2 battery packs. In the 85kWh battery, the capacity of each battery pack is 5.3kWh, and 14 such battery packs are 74.2kWh.

This is the relationship between 70kWh, 75kWh, and 85kWh. As for 60kWh, this is just a configuration to reduce the barrier to entry. So, how did 90kWh come from?

From 85kWh to 90kWh, 5kWh is added. Is there a battery pack added? In the 85kWh battery pack structure, it is no longer possible to superimpose the battery pack. The only possibility is to replace the new battery. Of course, it still uses the 18650 model battery, but the chemical material has been adjusted to increase the energy density.

In this process, Tesla added a small amount of silicon to the graphite anode of the cell, which increased the energy density of the cell.

The addition of silicon to the anode has been recognized as a way to increase energy density in the battery field. In order to avoid excessive stacking of the battery pack, the quality of the battery pack is too large, and Tesla can only focus on the development of high energy density batteries. However, for ternary lithium-ion batteries, it is far from simple to increase the energy density through silicon.

The basic principle is that after the silicon is added to the graphite anode, the absorption capacity of the anode for lithium ions is enhanced because the structure of the silicon atom can accommodate more lithium ions than the graphite. In a single charge and discharge cycle, the more lithium ions in the anode, the greater the energy density.

However, after fully absorbing lithium ions, the volume of silicon expands by 300%, which is much larger than the expansion rate of 7% after graphite absorbs lithium ions. This repeated volume change causes the solid state electrode to become "soft" and easily collapse. As a result, the cycle life of the battery is reduced.

Another factor is the formation of the SEI film of the Lithium Battery electrolyte due to the expansion/expansion characteristics of the silicon anode due to charge and discharge. This film is formed during the initial cycle of the lithium battery and has a protective effect on the anode material to prevent the material structure from collapsing.

For the above reasons, the use of silicon as the anode, although the energy density can be significantly improved, but also with side effects, eventually leading to shortened battery life. Therefore, Tesla's solution is to gradually add a small amount of silicon to the graphite anode to find a balance point between energy density and cycle life.

As we all know, the 18650 battery used by Tesla is produced by Panasonic. As the cooperation between the two sides deepens, Tesla is also developing new cylindrical batteries. After the Model 3 is officially put into production, the new 21700 battery will replace the 18650 and become a new battery.

The 21700 battery is still a ternary lithium battery and the cathode material is nickel cobalt lithium aluminate (NCA). This cylindrical ternary battery is currently the most energy-efficient power battery solution. Compared with square-shaped batteries, such batteries have high energy density, but have poor stability and require better BMS (Battery Management System) support.

Tesla's earliest Roadster used Panasonic's NCR18650A battery with a rated voltage of 3.6V and a capacity of 3.1Ah. The previous 85kWh battery pack uses the NCR18650B battery with a rated voltage of 3.6V and a capacity of 3.1Ah.

The 90kWh battery model is not known, but it should not be directly supplied by Panasonic, but developed jointly by Tesla and Panasonic, specially designed for Tesla models. At present, among the 18650 batteries produced by Panasonic, the NCR18650G is the highest capacity model, reaching 3.6Ah. According to this calculation, 7102 batteries in the 85kWh battery pack are replaced by G-type batteries, which is exactly 90kWh.

Therefore, there is a possibility that in the 90kWh battery pack, the battery cell is of the NCR18650G type; and in the 85kWh battery pack, the battery cell is of the NCR18650B type. In short, in the case where the number of cells is constant (the battery pack structure is unchanged), the power of 90 kWh can be ensured only by increasing the capacity of a single cell to 3.6 Ah.

To achieve 100kWh, there are two options: one is to superimpose two battery packs, which can get 100kWh according to the capacity of 5.3kWh per battery pack; the second is to replace the batteries with higher energy density. The author believes that the latter is the best and most likely one.

Because 90kWh is based on the 85kWh battery pack structure. This structure has been finalized under the 18650 battery specification, and the cost of changing its design structure is very high. In fact, there is no more space in the battery pack to stack more battery packs.

If you increase the battery pack, not only the quality of the battery pack will increase, but also the cooling cycle system of the battery pack. Therefore, improving the capacity of the battery is the most economical and feasible solution.

Imagine that in a 100 kWh battery pack, the capacity of a single battery cell should be increased to 3.9 Ah without changing the battery pack structure, so that it is possible to achieve a capacity of 100 kWh. Therefore, the author suspects that Tesla has developed a 3.9Ah 18650 battery with Panasonic. This credit can only be attributed to the silicon in the anode.