Analysis of Low Capacitance of Cells—Thinking
Hearing that there is a low capacitance of the battery, the first reaction should be to confirm whether the low capacitance problem is true. To put it simply, it is first necessary to confirm whether the capacity dividing process is set incorrectly (for example, whether the discharge current is set too large, or whether the charging time is set short); if there is no problem with the setting of the capacity dividing step, it is necessary to replace the test point and then adjust the battery cells. Carry out the re-distribution, and at the same time recite in my heart, "After the second partition, you must not be low-minded anymore." Of course, for mass production and even samples, the probability of batch low capacity caused by the error of the sub-capacity cabinet is very low. Generally, there is a real problem with the battery cells. If the low capacitance is still low after the retest, it can be confirmed that the low capacitance problem really exists (at the same time, it is best to keep an eye on the retest: fully charge the 3pcs retest battery for later use).
After confirming the existence of low capacitance, it is necessary to further confirm the frequency and severity of low capacitance, and grasp the actual situation of low capacitance as a whole. The samples are often a batch, not much to say; but the mass production model has two situations: "this model has always been low capacity" and "this model is occasionally low capacity". For the former, the analysis should take the design, material selection angle and stubborn problems encountered in mass production for a long time as the entry point and priority direction (for example, is this material matching unverified? Low-capacity anomalies but have been procrastinating and unresolved); for the latter, production line operations and process changes need to be prioritized (for example, is this batch of negative electrodes crushed? Is it because the production line is shortened for output aging time? Is the process changed from before and this change has the risk of causing low capacitance). After the frequency is confirmed, it is necessary to confirm the relatively unimportant severity, that is, the proportion of low-capacitance cells and the proportion of the capacity lower than the required value. To a greater extent, the confirmation of the severity is to provide a basis for the possible relaxation of the capacity specification and the determination of the number of out-of-stocks. For the analysis of the problem itself, the significance is not as important as the confirmation frequency, but it is still essential.
After grasping the actual situation of low tolerance as a whole, it is necessary to start the analysis. For experts with a high level who have encountered the same problem, removing the 3pcs battery should be able to roughly determine the actual cause of the low capacity. But for ordinary people, one is that it is difficult for us to have similar abilities and accumulation, and the other is that taking photos of three battery cells cannot fully explain the problem to superiors and colleagues (even if your conclusion is correct). Therefore, a more systematic approach is required. Before the analysis of the system, you can disassemble the low-capacitance cell that was re-tested and fully charged to see the interface. If there is no problem, it is likely to be the cause of the light coating of the positive electrode or insufficient design margin; if there is a problem with the interface , it may be a problem in all aspects of the process or design (this is not nonsense).
Analysis begins. First of all, you need at least 8pcs cells with low capacity + 8pcs cells with qualified capacity. The low-capacitance cells are then randomly divided into two groups, the low-capacity group A and the low-capacity group B, and the qualified cells are randomly divided into two groups, the qualified group A and the qualified group B. Then discharge the two group A cells to a static voltage of about 3.0V (Wenwu is used to discharging 0.5C to 3.0V and then 0.2C to 2.5V; of course, the objects are lithium cobalt oxide and ternary + graphite negative electrode); Solve the low-capacity and qualified cells, vacuum bake the positive electrode sheet at a temperature above 85 °C for 24 hours (the specific baking parameters have not been verified by DOE, but it can be determined that the given parameters can be analyzed), and then weigh low The weight difference between the positive electrode and the qualified positive electrode; if the weight of the low-capacity electrode is significantly lower than the qualified positive electrode or lower than the process range, it can basically be judged that the low-capacity is caused by the light coating of the positive electrode. There are two points that need to be added for weighing the weight of the pole piece after baking: First, although the first irreversible lithium source of the positive electrode will cause the positive electrode to lose a little weight, the weight of the total irreversible lithium source only accounts for about 5% of the positive electrode lithium source, accounting for The weight of the positive electrode is less than 0.5%, even if the irreversible lithium source caused by lithium precipitation is added, the lightness of the positive electrode will not be less than 1% of the total weight of the electrode; it is impossible for the electrolyte during the baking process. Fully baked, but the actual residual weight is also limited relative to the pole piece weight. In general, the difference between the weight of the pole piece after baking the positive electrode and the actual weight of the pole piece before winding will not exceed 2%. Moreover, compared with the weight of the qualified positive electrode with capacity and the weight of the low-capacity electrode piece, this method is quite credible (in addition, I have heard that something can be wiped on the electrode piece to help dry the electrolyte, the details of which are I don’t understand the principle of civil and military affairs, I hope that friends who know it will give me advice). The second is that the same method is not suitable for the negative electrode, because the negative electrode will increase a lot of weight when it is formed, but the weight gain ratio of the negative electrode after the conversion can be given through experiments, and then the weight of the negative electrode sheet can be reversed to determine whether the low capacity is caused by the excess negative electrode; However, Wenwu has not done similar experiments, and interested friends can test it by themselves.
If it is confirmed that the reason why the positive electrode is too light is the low capacity, it will be lucky, but in fact, the probability of this luck is often just in case. In this case, it depends on the analysis of the low-capacity group B and the qualified group B. Group B cells need to be fully charged, and then disassembled to compare the differences in the negative interface. Low discharge capacity is equivalent to low charge capacity, which is equivalent to full charge of the negative electrode. The interface will be abnormal. In fact, in most cases, as long as low capacity occurs, no matter whether the cell is low capacity or qualified, the interface will have similar abnormalities, but the degree is different. When recording the interface condition of the cell, it is also necessary to record the actual capacity of the corresponding cell at the same time. In the end, it is generally similar to the conclusion that the interface abnormality of the cell with low capacitance and high degree is more serious.
Analysis of Low Capacitance of Cells - Design
The matching of materials, especially the matching of the negative electrode and the electrolyte, has a particularly significant impact on the capacity of the cell. Because Wenwu's theory is poor and it is almost impossible to get an answer from the supplier's mouth similar to "Why does your electrolyte match other's negative electrode will precipitate lithium and low capacity", so although I have encountered several times due to material matching Caused a low-profile situation, but the principle of its occurrence has always been a half-understood, only some of my own guesses. For a new negative electrode or a new electrolyte, if repeating the test finds that every time the cell will precipitate lithium and have a low capacity, there is a high possibility that the materials themselves do not match. 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. Cannot adapt to high rate charge and discharge. Although we can't figure out the theory, if there is a problem, first ask the supplier if there is a good solution suggestion (similar to optimizing the formation process, reducing the charge and discharge current, etc.), if the supplier's suggestion is invalid or not at all Suggest, then change the material.
The diaphragm is also a factor that may cause low capacitance. When Wenwu used a single-layer PP low-cost diaphragm from a domestic factory before (although I hate you, I will not name you), and found that the hand-wound battery diaphragm The longitudinal direction of the middle position of the layer is wrinkled, and the negative electrode in the wrinkled place is not fully inserted into the lithium and does not become golden yellow, which affects the capacity of the cell by about 3%. Although the other two models use semi-automatic winding, the diaphragm is much less wrinkled and has a 1% impact on capacity, but it is not a basis for deactivating the diaphragm.
Insufficient capacity design margin will also cause low capacity. Due to the positive and negative coating, the error of the sub-container and the influence of the glue on the capacity, a certain capacity margin must be left in the design. When designing the capacity margin, you can either calculate the capacity of the cells in all processes that are exactly the center line and then leave the margin, or you can calculate the margin after all the factors that affect the capacity have occurred below the limit. For new materials, accurate evaluation of the gram performance of the cathode under this system is important. The capacity ratio, charge cut-off current, charge and discharge rate, and the type of electrolyte will all affect the performance of the positive electrode. If the design value of the positive electrode 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, new materials must pay attention to evaluating the accurate performance of the positive electrode. Not the same positive electrode has the same performance when matched with any negative electrode and electrolyte.
Excessive negative electrode will also affect the performance of the positive electrode to a certain extent, which in turn affects the capacity of the cell. Excessive negative electrode is not "as long as lithium is not precipitated". Continue to increase the excess negative electrode at the lower limit of the excess negative electrode without lithium precipitation, and the performance of the positive electrode will increase by about 1% to 2% (but the principle is not very clear). Of course, even if it is increased, the larger the design capacity output, the better. If the negative electrode is excessive, as long as it is guaranteed not to precipitate lithium. When the excess of the negative electrode is too high, the gram performance of the positive electrode will decrease again, because more irreversible lithium is required for formation. Of course, the probability of this happening is almost non-existent.
When the liquid injection volume is low, the corresponding liquid retention volume will also decrease. When the liquid retention volume of the battery cell is too small, the effect of lithium ion intercalation and deintercalation in the positive and negative electrodes will be affected, resulting in low capacity. Although the cost and process will have less pressure after reducing the liquid injection amount, the premise of reducing the liquid injection amount must be that it does not affect the performance of the cell. Of course, reducing the liquid injection volume (for example, Wenwu once reduced the liquid injection volume of a model to 2.45g/Ah, and the result was a miserable death) will only increase the probability of low capacity caused by insufficient liquid retention of the battery cell. non-essential results. At the same time, the more difficult the model is to absorb liquid, the excess electrolyte should be more, so as to ensure better contact with the pole piece during the electrolyte infiltration. Since the amount of liquid retention is greatly affected by the material and the hardness of the cell, it is impossible to give a unified standard and can only be based on experiments. When the liquid retention capacity of the battery cell is insufficient, the positive and negative electrode sheets will be relatively dry, and a thin layer of lithium deposition will occur on the negative electrode. This point can be used as a factor for the low capacity caused by the poor liquid retention capacity.
The light coating of the positive or negative electrode can directly cause the low capacity of the cell. When the positive electrode coating is light, the interface of the fully charged cell will not be abnormal. At this time, the problem needs to be determined by the method of baking and measuring the weight of the positive electrode sheet given by Wen Wu in "Thinking". The negative electrode acts as a lithium ion acceptor, and the number of lithium intercalation sites it can provide must be greater than the number of lithium sources provided by the positive electrode, otherwise excess lithium will be precipitated on the surface of the negative electrode, thereby forming a thin layer of relatively uniform lithium precipitation. As mentioned above, since the weight of the negative pole piece cannot be directly taken from the baking weight of the pole piece removed from the battery cell, one can find the weight gain ratio of the negative pole through another experiment to calculate the baking weight of the negative pole piece removed from the battery cell. The second is to compare the relative weight of the negative electrode of the low-capacity and qualified batteries or the lithium-deposited battery and the non-lithium-deposited battery (it doesn’t matter if the charge is full or not, because the comparison is relative weight), if the relative weight of the qualified battery negative electrode should be heavier At the same time, there is a thin layer of lithium deposition in the negative electrode of the low-capacitance cell, and the possibility of excessive and insufficient negative electrode is very high. In addition, coating the yin and yang sides of the positive electrode or the negative electrode will also cause low capacity, and among them, the negative electrode is mainly coated with light on one side, because even if the positive electrode is coated heavily, although the gram performance will decrease, the total capacity will not decrease. On the contrary, it may even increase. It is very simple to judge the yin and yang sides. One side of the interface is OK and the other side of the lithium deposition can basically be concluded as the yin and yang sides. If the negative electrode process is dislocation coating, directly compare the relative weight ratio of the single and double sides after baking, as long as the ratio similar to the A side is obtained. Data such as 6% lighter coating on the B side can basically determine the problem. Of course, if the low capacitance problem is serious this time, it is necessary to further reverse the actual areal density of the A/B side. (If the head and tail of the negative electrode are aligned and coated, then Wenwu could not find a good way to measure the weight of one side, and I also hope that experienced friends will enlighten me.) Rolling will destroy the structure of the material, thereby affecting the capacity. play. The reason why a material has properties such as capacity, voltage, etc., its molecular or atomic structure is the fundamental reason. When the rolling density of the positive electrode exceeds the process value (the reason may be that the rolling thickness is lower than the lower limit of the process, but the more likely factor is that the coating is heavy and the rolling process is continued), the positive electrode sheet after the cell is disassembled will be bright. If the positive electrode is compacted too much, the positive electrode piece will be easily broken after winding, which will also cause low capacitance. However, since the positive electrode crushing will cause the pole piece to break as soon as it is folded, and the positive electrode rolling itself requires a lot of pressure, the frequency of the positive electrode crushing will be much lower than that of the negative electrode crushing. When the negative electrode is crushed to death, strips or blocks of lithium deposition will form on the surface of the negative electrode (which is quite different from the thin layer of lithium deposition that covers almost the entire surface of the negative electrode when the negative electrode is excessive and insufficient), and the liquid retention of the cell will be Obvious reduction. When the compaction is further increased, the amount of lithium precipitation (the degree to which the color of lithium precipitation is whiter) will also increase while the block area of lithium precipitation increases, and the two colors of platinum on the surface of the battery cell are distinct and dry, which even makes it seem People gag.
Excessive water content can also cause low volume. When the water content of the water pole piece exceeds the standard before the battery is injected, the dew point of the glove box is unqualified, the water content of the electrolyte exceeds the standard, and the second degassing seal introduces moisture, the battery may have a low capacity. A trace amount of water is required when the battery is formed, but when the water exceeds a certain value (the water content of the pole piece before injection is greater than 250ppm, for reference only), the excess water will destroy the SEI film and consume the lithium salt in the electrolyte, thus Reduce cell capacity. Batteries with excessive water content are dark brown in small pieces when they are fully charged and negatively charged.
When analyzing the low capacitance, if you have already made up your mind, you can simply remove a few bad-looking cells to basically confirm the problem. If the low capacitance caused a batch loss, you need to report in detail, or the reason for the low capacitance is something you have never seen before. It is necessary to conduct detailed analysis from the aspects of collecting process data, comparing low-capacity and qualified cells, and proposing improvement plans; especially before confirming a low-capacity situation that you have not seen before, you must repeat serious experiments or the most extreme. At a minimum, process anomaly data collection should be the basis for conclusions drawn. The first few times of careful analysis of the problem can help us develop the idea of analyzing the problem, and then when we encounter the problem again, we will be able to do it with ease.