3.7v 2200mah 18650 lithium battery.Lithium battery technology; 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 shell: soft pack, square aluminum shell and cylinder. Since domestic new energy vehicles became popular in 2015, power lithium-ion batteries have also ushered in a rare golden period. Of course, this is directly related to national policies and subsidies.

　　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 shell: soft pack, square aluminum shell and cylinder. Since domestic new energy vehicles became popular in 2015, power lithium-ion batteries have 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 pack, square aluminum case or cylindrical battery, is most suitable for powering cars?

　　In fact, each has its own advantages, and each family's situation is different, so it is a matter of opinion, and discussion is fruitless and meaningless.

　　1. Injection volume:

　　Generally speaking, the liquid injection volume of ternary system 186502.6-3.2Ah cylindrical battery is between 5-5.5g. The specific amount of liquid injection depends on the physical parameters (specific surface area, morphology, particle size distribution), winding tightness, surface density, compaction density, etc. of the positive and negative electrode materials.

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

　　Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%

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　　If the amount of liquid injected is insufficient, it will result in incomplete infiltration of the interior, high internal resistance, and low number of cycles. Seriously, it will lead to lithium precipitation and cause danger; if the injection volume is too large, it will cause insufficient internal space (high internal pressure), rapid capacity decay, new costs, etc. Let me talk about it here. The reason why the capacity of the negative electrode will decay quickly due to the large amount of liquid injection is because, especially when the temperature is high and during the charging process, the rich free electrolyte will react with the more active negative electrode lithium carbon compound, consuming effective substances.

　　The method generally used by manufacturers to determine the liquid injection amount is to immerse the wound battery core in the electrolyte, calculate the weight difference before and after, and then add 0.2-0.4g as a surplus. The final amount is the optimal liquid injection amount. Of course, although this method is simple and feasible, it is not rigorous enough. The best method is to conduct electrochemical performance experiments with gradient injection amounts, and ultimately take into account rate, high and low temperature, cycle, etc.

　　It is more scientific to determine the appropriate amount of liquid injection based on the specific use of the battery.

　　The principles of manganese, lithium, iron and lithium are basically similar. No need to go into details.

　　2. Tightness:

　　Low temperature and high energy density 18650 3350mAh-40℃ 0.5C discharge capacity ≥60%

　　Charging temperature: 0~45℃ Discharge temperature: -40~+55℃ Specific energy: 240Wh/kg -40℃ Discharge capacity retention rate: 0.5C Discharge capacity ≥ 60%

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　　The tightness is calculated by dividing the sum of the bottom areas of the positive electrode, negative electrode, separator and needle gaps by the inner bottom area of the cylindrical battery. Generally it is 88%-93%.

　　Similarly, the specific tightness depends on the use of the battery and the various performance requirements of the final battery you require. If the tightness is too low, it will cause a waste of space, new costs, difficulty in infiltrating the electrolyte (liquid consumption), shaking of the battery cells, etc.; if the tightness is too high, it will cause insufficient space due to subsequent expansion of the battery cells, affecting electrochemical performance and internal pressure. Large CID is easy to disconnect, etc.

　　Generally speaking, the tightness of rate batteries is lower, generally below 91%; the tightness of capacity batteries is higher, and some can even exceed 95%. The reason can be thought about, it is very simple.

　　It should be noted that considering different materials (such as negative electrode physical properties), different design parameters (such as compaction density), etc., subsequent battery expansion coefficients are also different. We must take practical and comprehensive considerations into account when designing.

　　3. Number of poles:

　　Increasing the number of poles can only increase the AC internal resistance of the battery to a limited extent, and has no direct relationship with the DC internal resistance during final use. And the additional number of poles will increase the difficulty of battery process design, increase costs and end-use risks. Therefore, whether it is feasible to unilaterally increase the number of tabs in order to increase the final battery capacity or rate performance, one must be careful when designing.

　　The reason can be roughly explained: the new number of pole tabs means that new machines, materials and labor are added to the blanking and tab welding processes, and the cost will naturally increase; the new number of pole tabs will also affect the tension of the pole pieces, resulting in uneven tension. Increases the difficulty and risk of winding; improper encapsulation of the solder joints at the tabs will pierce the insulation; the current density at the tabs is the highest and the electrode potential is the lowest, increasing the probability of lithium precipitation.

　　The general design is that one tab for the positive and negative poles of the capacity battery is enough; for the rate battery, you can consider adding more tabs, but the final determinant of performance is the DC internal resistance.

　　For example: Normally, the positive and negative poles of a battery without PTC have one tab each, the AC internal resistance is 30m, and the DC internal resistance of 1C discharge is 50m; now there is a new tab in the negative pole, the AC internal resistance is 20m, but the DC internal resistance of 1C discharge The resistance may be 48m. Therefore, what we ultimately look at is the DC internal resistance caused by the concentration polarization and electrochemical polarization of the battery.

　　In a word, there are advantages and disadvantages to adding more poles, depending on the requirements.

　　4. Center needle:

　　Some cylindrical battery manufacturers will finally place a stainless steel rolling pin in the center of the rolling pin.

　　Basic parameters (Lishen winding core as an example):

　　center pin

　　Material

　　Lengthmm

　　Outer diameter mm

　　Inner diameter mm

　　Mass g

　　strength

　　price

　　Stainless steel

　　59.3

　　2.5

　　2.0

　　0.33

　　extremely high

　　0.07 yuan

　　Its uses are:

　　1. Internal support purpose:

　　During the use of the battery, the deintercalation reaction of lithium ions will occur many times, and the internal materials will also age over time. This will cause the entire roll core to deform when the battery is cycled for a certain number of times. Due to the cavity inside the roll core, it will be unable to prevent the stress of the roll core from deforming internally. At least, the deformation of the battery will cause the electrochemical performance to change. If it is poor or severe, an internal short circuit may occur, which may lead to safety threats.

　　As shown in the picture above, after the battery has been cycled for a certain number of times, there is a collapse phenomenon inside the core, and the overall battery core is slightly deformed. This will not happen with batteries with center pins.

　　2. Respond to battery injection test:

　　Lithium-ion test UL1642 includes a spray test (Projectile Test), which requires that when the battery is heated to high temperature, no cells or parts of the battery can penetrate the screen or the entire cell or battery protrudes from the screen, that is, no foreign matter can be sprayed out over a large distance. . There is a central needle that can add a channel for gas ejection, so that the core will not eject. The picture below shows a cylindrical battery with a central needle. After burning, no material is ejected.

　　There are pros and cons to central acupuncture, and whether it depends on the specific requirements and process control. In my opinion, it's not necessary.

　　5. Pole piece correspondence:

　　There are four corresponding pole pieces: 1. Positive electrode powder corresponds to negative electrode powder; 2. Positive electrode powder corresponds to negative electrode foil; 3. Positive electrode foil corresponds to negative electrode powder; 4. Positive electrode foil corresponds to negative electrode foil.

　　Among them, 2, we understand very well that such a design will cause lithium precipitation in the negative electrode foil, and the consequences will be serious, so it is not advisable.

　　In fact, 3 is equally serious and must be eliminated in our design!

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