Capacity is the first property of the battery, lithium battery cells low capacity is also a frequent problem encountered in samples, mass production, how to immediately analyze the causes of low capacity problems encountered, today to introduce to you what are the causes of low capacity lithium battery cells?
The matching of materials, especially between the cathode and the electrolyte, has a significant impact on the capacity of the cell. For a new cathode or new electrolyte, if repeated tests reveal a lithium precipitation low capacity every time the cell is tested, then it is very likely that the materials themselves are not matched. The mismatch could be due to the SEI film formed during formation not being dense enough, too thick or unstable, or the PC in the electrolyte making the graphite layer peel off, or the design of the cell not being able to adapt to large charge/discharge rates due to excessive surface density compaction.
Diaphragms are also an influential factor that can cause low capacity.We have found that hand wound diaphragms produce wrinkles in the longitudinal direction in the middle of each layer, where the lithium is not sufficiently embedded in the negative electrode and thus affects the cell capacity by around 3%. Although the other two models use semi-automatic winding when the diaphragm wrinkling is much less and the impact on capacity is only 1%, it is not a basis for discontinuing the use of the diaphragm.
Inadequate capacity design margins can also result in low capacity. Due to the impact of positive and negative electrode coating, the error of the capacity divider and the impact of the adhesive on capacity, it is important to allow for a certain amount of capacity margin when designing. When designing the capacity margin, it is possible to leave a surplus after calculating the capacity of the core with all processes exactly in the middle line, or to calculate the surplus after all factors affecting the capacity have occurred at the lower limit. For new materials, an accurate assessment of the gram play of the cathode in that system is important. The partial capacity multiplier, the charge cut-off current, the charge/discharge multiplier, the type of electrolyte, etc., all affect the cathode gram play. If the design value of the positive gram performance is artificially high in order to achieve the target capacity, this also equates to an inadequate design capacity. There is nothing wrong with the interface of the cell, nor is there anything wrong with the overall process data, but the capacity of the cell is low. Therefore, new materials must be evaluated for accurate cathode grammage, as not the same cathode will have the same grammage as any cathode or electrolyte.
Excess negative electrode can also affect the performance of the positive electrode to a certain extent, thus affecting the capacity of the cell. Negative overload is not "as long as there is no lithium precipitation". If the negative overload is increased to the lower limit of the non-lithium precipitation overload, there will be a 1% to 2% increase in the positive gram performance, but even if it is increased, the negative overload is still sufficient to ensure that the capacity output is as high as possible. When the negative electrode excess is too high, the positive electrode will play a lower role because more irreversible lithium is needed for chemistry, but of course the probability of this happening is almost none.
When the liquid injection volume is lower, the corresponding liquid retention volume will also be lower. When the liquid retention volume of the cell is low, then the effect of lithium ion embedding and de-embedding in the positive and negative electrodes will be affected, thus triggering low capacity. Although there will be less pressure on costs and processes with a lower injection volume, the premise of lowering the injection volume must be that it does not affect the performance of the cell. Of course, lowering the fill level will only increase the probability of low capacitance due to insufficient liquid retention in the cell, but is not an inevitable consequence. At the same time, the more difficult it is to absorb liquid, the more excess electrolyte there should be to ensure better contact with the electrode during electrolyte wetting. Insufficient cell retention will result in the positive and negative electrodes being dry and a thin layer of lithium precipitation on top of the negative electrode, which can be a factor in low capacitance due to poor retention.
A lightly coated positive or negative electrode can directly cause a low capacity core. When the positive electrode is lightly coated, the interface of the fully charged core will not be abnormal. The negative electrode, as the recipient of lithium ions, must provide a greater number of embedded lithium positions than the number of lithium sources provided by the positive electrode, otherwise excess lithium will precipitate on the surface of the negative electrode, resulting in a thin layer of more uniform lithium precipitation. As mentioned before, because the negative electrode weight cannot be taken directly from the baking weight of the cores, so one can make another experiment to find the proportion of negative electrode weight gain in order to deduce the coating weight through the baking weight of the negative electrode cores. If the negative electrode of a low capacity core has a thin layer of lithium precipitation, the possibility of insufficient negative electrode is high. In addition, the cathode or negative electrode coating cathode side can also cause low capacity, and the negative electrode single side coating is mainly light, because even if the positive electrode coating heavy, although the gram play will be reduced, but the total capacity will not be reduced but may even increase. If the negative electrode is coated in the wrong place, a direct comparison of the relative weight ratios of the single and double sides after baking, as long as the data is similar to the A side is 6% lighter than the B side coating, can basically determine the problem, of course, if the problem of low capacity is very serious, it is necessary to further reverse the actual surface density of the A/B side. If the problem of low capacitance is serious, it is necessary to further infer the actual density of the A/B side. Rolling destroys the structure of the material, which in turn affects the capacity. The molecular or atomic structure of a material is the fundamental reason why it has properties such as capacity, voltage, etc. When the density of the positive electrode rolls exceeds the process value, the positive electrode will be very bright when the core is dismantled. If the positive electrode compaction is too large, the positive electrode piece is easy to break after winding, which will also cause low capacity. However, as the positive electrode compaction will cause the pole piece to break as soon as it is folded, the positive electrode roller press itself requires a lot of pressure, so the frequency of encountering positive electrode compaction is much lower than negative electrode compaction. When the negative electrode is compacted, a strip or block of lithium precipitation will form on the surface of the negative electrode, and the amount of liquid retained in the core will be significantly reduced.
Low capacity can also be caused by excessive water content. Low capacitance is possible when the water content of the electrode before filling, the dew point of the glove box before filling, the water content of the electrolyte exceeds the standard, or when moisture is introduced into the de-aerated second seal. Trace amounts of water are required for the formation of the core, but when the water exceeds a certain value, the excess water will damage the SEI film and consume the lithium salts in the electrolyte, thus reducing the capacity of the core. The water content exceeds the standard of the cell full charge negative course a small piece of dark brown.
Post time: Aug-16-2022