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18650 cylindrical battery design points

As we all know, the mainstream lithium-ion batteries on the market can be divided into three categories according to the type and shape of the casing: soft pack, square aluminum shell and cylinder. Since the beginning of the new domestic energy vehicles in 2015, the power lithium-ion battery has also ushered in a rare golden period, of course, this is directly related to national policies and subsidies. Which of the three types of batteries, soft packs, square aluminum shells and cylinders, is the most suitable for power cars? In fact, each has its own advantages, and each situation is different. Therefore, it is the benevolent seeing the wise and seeing wisdom. The discussion has no result and meaning.

In the following, I will analyze some important design points of the three batteries separately. In this issue, I will talk about the design points of the 18650 cylindrical battery.

First, the amount of liquid injection:

In general, the ternary 18650 2.6-3.2Ah cylindrical battery is injected between 5 and 5.5g. The specific liquid injection amount depends on the physical parameters (specific surface area, morphology, particle size distribution) of the positive and negative materials, winding tightness, areal density, compaction density, and the like.

If the amount of liquid is insufficient, the inside will not be completely infiltrated, the internal resistance will be too large, and the number of cycles will be small. Severe can cause lithium to be dangerous, and if the amount of liquid injection is too large, it will cause insufficient internal space (large internal pressure), rapid capacity decay, and increased cost. Here, the amount of liquid injection caused the capacity to decay quickly because of the negative electrode, especially when the temperature is high and during the charging process, the rich free electrolyte reacts with the more active negative lithium carbon compound, consuming an effective substance.

The general manufacturer determines the amount of liquid injection by immersing the wound core in the electrolyte, calculating the difference between the front and the back, and then making a surplus of 0.2-0.4 g, and finally the optimal liquid injection amount. Of course, this method is simple and feasible, but it is not rigorous. The best method is to conduct electrochemical performance experiments on the gradient injection volume, and finally take into account the magnification, high and low temperature, circulation and so on.

It is also scientific to determine the proper amount of liquid to be used in specific applications.

The principles of manganese lithium and iron lithium are basically similar. Do not repeat them.

Second, the tightness:

The tightness is calculated by dividing the sum of the bottom areas of the positive, negative, diaphragm and needle gaps by the cylindrical inner bottom area. It is generally 88%-93%.

Similarly, the exact tightness depends on the purpose of the battery and the performance of the final battery you require. If the tightness is too low, the space waste cost will increase, the electrolyte will be difficult to infiltrate (liquid consumption), the battery core will shake, etc.; if the tightness is too high, the space caused by the expansion of the subsequent battery core will be insufficient, affecting the electrochemical performance and the internal pressure. CID is easy to disconnect and so on.

In general, the rate of battery tightness is lower, generally below 91%; the capacity battery is more elastic, and some can even exceed 95%. The reason can be considered, it is very simple.

It should be noted that, considering different materials (such as negative physical property parameters) and different design parameters (such as compaction density), the subsequent battery expansion coefficients are also different. We must consider the actual consideration when designing.

Third, the number of ears:

The increase in the number of poles can only increase the AC internal resistance of the battery finitely, and is not directly related to the DC internal resistance during final use. Increasing the number of poles increases the difficulty of designing the battery process, increasing costs and end-use risks. Therefore, in order to increase the final battery capacity or rate performance, it is feasible to increase the number of ears in one piece, and the design must be cautious.

The reason is probably to say: increasing the number of ear means that the whitening and ear welding process increases the machine, material and labor, and the cost naturally increases; increasing the number of ear ears will also affect the tension of the pole piece, resulting in uneven tension, increasing the difficulty of winding and Risk; improper welding of the solder joints at the ear will pierce the partition; the current density at the ear is the largest, and the electrode potential is the lowest, increasing the probability of lithium deposition.

The general design is that the positive and negative poles of the capacity battery are sufficient for each pole; the rate battery can be considered to add more polar, but the final performance is DC internal resistance.

For example: normal without PTC battery positive and negative poles each one, AC internal resistance is 30mΩ, 1C discharge DC internal resistance is 50mΩ; now the negative pole adds a pole, AC internal resistance is 20mΩ, but 1C discharge DC internal resistance May be 48mΩ. Therefore, the final look is the concentration polarization of the battery and the DC internal resistance caused by electrochemical polarization.

In a word, the increase in the number of polar ear has advantages and disadvantages, depending on the requirements.

Fourth, the center needle:

Some cylindrical battery manufacturers will eventually place a stainless steel core in the center of the needle.

Basic parameters (Lishen core as an example):
 
18650 cylindrical battery design points
18650 cylindrical battery design points

1. Internal support:

The lithium ion deintercalation reaction occurs many times during the use of the battery, and the internal material also ages over time. This will cause the entire core to be deformed when the battery is cycled to a certain number of times. Due to the existence of a cavity inside the core, it will not be able to prevent the stress of the core from deforming into the inside, and even if the battery is deformed, the electrochemical performance will be changed. Poor, heavy, may cause an internal short circuit, and then a security threat.

 
18650 cylindrical battery design points
18650 cylindrical battery design points

As shown in the above figure, after the battery has been cycled for a certain number of times, the inside of the core has collapsed, and the overall cell has a slight deformation. This is not the case with a central needle cell.

2. Respond to battery injection test:

    Lithium ion test UL1642 has a Projectile Test, which means that when the battery is heated by high temperature, there is no cell or a part of the battery penetrating the screen or the whole cell or the battery protruding screen, that is, no foreign matter can be ejected. distance. A center pin can add a gas jet channel so that the core does not eject. The following figure shows a cylindrical battery with a center pin. No substance is ejected after burning.

18650 cylindrical battery design points
 
      There are advantages and disadvantages to the center needle, and it is necessary to see the specific requirements and process control. In my opinion, it is not necessary.

Five, the pole piece corresponds to:

There are four types of pole pieces:
1. The positive electrode powder corresponds to the negative electrode powder;
2. The positive electrode powder corresponds to the negative electrode foil;
3. The positive electrode foil corresponds to the negative electrode powder;
4. The positive electrode foil corresponds to the negative electrode foil.

Among them, 2 we understand very well that such a design will cause lithium stripping of the negative electrode foil, and the consequences are very serious, so it is not desirable.

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