The analysis of the low capacity design of lithium battery

The reason for low capacity can be divided into two directions of design and process. This article first talks about the design, and then the next part of the process.

The matching of the materials, especially the matching of the negative electrode and the electrolyte, is particularly important for the capacity of the cell. Because the author's theory is poor and it is almost impossible to get an answer from the supplier's mouth, "Your home electrolyte is equipped with someone else's negative electrode is a low-capacity lithium" problem, so although I have encountered several times due to material matching Caused a low-volume situation, but the principle of its occurrence has always been a half-baked, only some of their own guesses. For the new negative electrode or the new electrolyte, if the test is repeated, it is found that each time the cell is decomposed with lithium and low capacity, the possibility that the material itself does not match is very large. The reason for the mismatch may be that the SEI film formed during the formation is not dense enough, too thick or unstable, or it may be that the PC in the electrolyte peels off the graphite layer, and the cell density of the designed surface density is too large to cause the cell to be caused. It cannot adapt to large rate charge and discharge. Although we can't figure out the theory in the middle, if there is a problem, first ask the supplier if there is a good solution (similar to optimizing the process, reducing the charge and discharge current), if the supplier's suggestion is invalid or simply not Suggest, then replace the material.

The diaphragm is also a factor that may cause low volume. I used to use a single-layer PP low-cost diaphragm in a factory in China (although I hate you, but I don't name it), I found that the hand-wound cell diaphragm will be in every In the middle of the layer, the longitudinal direction of the layer is wrinkled, and the place where the negative electrode is embedded in the negative electrode is not golden yellow, which affects the cell capacity of about 3%. Although the other two models use semi-automatic winding, the degree of wrinkling of the diaphragm is much lighter and has a 1% effect on the capacity, but it cannot be the basis for deactivating the diaphragm.

Insufficient capacity design margins can also result in low tolerance. Due to the positive and negative electrode coating, the error of the volumetric cabinet and the influence of the glue on the capacity, a certain capacity margin must be left in the design. When designing the capacity margin, it is possible to reserve the surplus after calculating the capacity of all the cells that are just the center line, or to calculate the margin after all the factors that affect the capacity are generated. For new materials, an accurate assessment of the positive electrode's performance under the system is important. The capacity ratio, charge cut-off current, charge/discharge rate, and type of electrolyte all affect the positive electrode. If the design value is artificially increased in order to achieve the target capacity, it is equivalent to insufficient design capacity. There is no problem with the interface of the battery, and there is no problem with the entire process data, but the battery is low-capacity. Therefore, the new material must pay attention to the evaluation of the accurate positive gram play, not the same positive electrode has the same gram when it is matched with any negative electrode and electrolyte.

The excess of the negative electrode will also affect the positive electrode gram to a certain extent, and thus affect the cell capacity. The excess of the negative electrode is not "as long as no lithium is formed". In the lower limit of the excess of the negative electrode which does not analyze lithium, the excess of the negative electrode is continuously increased, and the positive electrode is also increased by about 1% to 2% (although the original ideal is not well understood). Of course, even if the increase is made, the output is larger as much as possible. If the anode is still excessive, it is only necessary to ensure that no lithium is precipitated. When the excess of the negative electrode is too high, the positive electrode gram will decrease again, because more irreversible lithium is required for the formation, and of course, the probability of occurrence of this case is almost no.

When the amount of liquid injection is low, the corresponding liquid holding amount will also decrease. When the liquid holding amount of the battery core is too small, the effect of lithium ions intercalating and deintercalating in the positive and negative electrodes will be affected, thereby causing low volume. Although the cost and process will have less pressure after reducing the amount of liquid injection, it must be based on the premise of not reducing the performance of the battery. Of course, reducing the amount of liquid injection (for example, I have reduced the injection volume of a model to 2.45g/Ah, and the result is very bad) only increases the probability that the battery will cause low volume due to insufficient liquid retention non-inevitable result. At the same time, the more difficult the type of liquid absorption, the more excess electrolyte should be used to ensure better contact with the pole piece during electrolyte infiltration. Since the liquid holding capacity is greatly affected by the hardness requirements of materials and batteries, it is impossible to give a unified standard and can only be based on experiments. When the battery capacity is insufficient, the positive and negative electrodes will be relatively dry, and a thin lithium chromatogram will be formed on the negative electrode. This can be used as a factor of low volume caused by poor liquid retention.

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