3.7V 18650 lifepo4 battery

　　What are the process design points of 18650 cylindrical battery?

　　Key points of process design for 18650 cylindrical battery. 1. Liquid injection volume: Generally speaking, the liquid injection volume of ternary series 18650 2.6-3.2Ah cylindrical batteries is between 5-5.5g. The specific liquid injection volume depends on the physical parameters of the positive and negative electrode materials (specific surface area, morphology, particle size distribution), winding tightness,

　　Key points of process design for 18650 cylindrical battery.

　　1. Injection volume:

　　Generally speaking, the liquid injection volume of the ternary system 18650 2.6-3.2Ah cylindrical battery is between 5-5.5g. The specific amount of liquid injection depends on the physical parameters of the positive and negative electrode materials (specific surface area, shape, particle size distribution), winding tightness, surface density, compaction density, etc.

　　If the amount of liquid injected is insufficient, the interior will not be completely infiltrated, the internal resistance will be too large, and the number of cycles will be reduced. Severe cases will lead to lithium precipitation and lead to danger; if the liquid injection volume is too large, it will cause insufficient internal space (large internal pressure), rapid capacity decay, and additional costs. Let me tell you here that the rapid capacity fading caused by the large amount of liquid injection is due to the negative electrode, especially when the temperature is high and during the charging process, the rich free electrolyte will react with the lithium carbon compound of the more active negative electrode and consume effective substances.

　　Generally, manufacturers determine the amount of liquid injection by immersing the wound battery core in the electrolyte, calculating the difference in weight before and after, and then leaving 0.2-0.4g in excess, and finally the optimal liquid injection amount. Of course, although this method is simple and feasible, it is not rigorous enough. The best way is to do electrochemical performance experiments with gradient liquid injection volumes, and finally take into account the rate, high and low temperature, cycle, etc.

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

　　The principles of manganese, lithium, iron and lithium are basically similar. I won't go into details.

　　2. Tightness:

　　The calculation of the tightness is the sum of the bottom area of the positive electrode, negative electrode, separator and needle gap divided by the inner bottom area of the cylindrical battery. Generally 88%-93%.

　　Similarly, the specific tightness depends on the use of the battery and the performance of the final battery you require. If the tightness is too low, space will be wasted, cost will be increased, the electrolyte will be difficult to infiltrate (liquid consumption), and the battery cell will shake, etc.; if the tightness is too high, the space will be insufficient due to the expansion of the subsequent battery cell, which will affect the electrochemical performance and internal pressure. Large CID is easy to disconnect, etc.

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

　　It should be noted that, taking into account different materials (such as negative electrode physical parameters), different design parameters (such as compaction density), etc., the subsequent battery expansion coefficients are also different. When we design, we need to consider the actual situation comprehensively.

　　Three, the number of ears:

　　The addition of the number of tabs can only increase the AC internal resistance of the battery to a limited extent, and has no direct relationship with the DC internal resistance in the final use process. Moreover, increasing the number of tabs will increase the difficulty of battery process design, increase cost and end-use risk. Therefore, whether it is feasible to increase the number of tabs one-sidedly in order to increase the final battery capacity or rate performance, we must be cautious in the design.

　　The reason can be roughly explained: adding the number of tabs means adding machines, materials and labor to the blanking and tab welding process, and the cost will naturally increase; adding the number of tabs will also affect the tension of the pole piece, resulting in uneven tension and new Increase the difficulty and risk of winding; Improper handling 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, which increases the probability of lithium precipitation.

　　The general design is that one lug for the positive and negative poles of the capacity battery is enough; more lugs can be considered for the rate battery, but the DC internal resistance ultimately determines the performance.

　　For example: normally there is one lug for the positive and negative poles of the battery without PTC, the AC internal resistance is 30m, and the DC internal resistance is 50m for 1C discharge; now a new tab is added for the negative pole, the AC internal resistance is 20m, but the 1C discharge DC internal resistance 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 increasing the number of tabs, depending on the requirements.