Needless to say, the importance of cycle performance on lithium-ion batteries; in addition, from a macro perspective, longer cycle life means less resource consumption. Therefore, the factors that affect the cycle performance of lithium ion batteries are issues that everyone related to the lithium battery industry has to consider. The following text lists several factors that may affect the battery cycle performance for your reference. Types of materials: The choice of materials is the first factor that affects the performance of lithium-ion batteries. If the material with poor cycle performance is selected, the process will be reasonable and the cycle will be perfect, and the cycle of the cell will inevitably be unable to be guaranteed; if you choose a better material, even if there are some problems in the subsequent manufacture, the cycle performance may not be bad. Too outrageous (Lithium cobaltate is only about 135.5mAh / g and the lithium is discharged. Although 1C dives more than 100 times, it is 0.5C and 500 times more than 90%; after the battery is disassembled, the negative electrode has black graphite particles ), The cycle performance is normal). From a material point of view, the cycle performance of a full battery is determined by the poorer of the cycle performance after the positive electrode and the electrolyte are matched, and the cycle performance after the negative electrode and the electrolyte are matched. The cycle performance of the material is poor. On the one hand, the crystal structure may change too fast during the cycle to continue the lithium insertion and delithiation. On the one hand, the active material and the corresponding electrolyte cannot generate a dense and uniform SEI film. A side reaction with the electrolyte prematurely causes the electrolyte to be consumed too quickly and affects the circulation. In the design of the cell, if one pole confirms the selection of materials with poor cycle performance, the other pole does not need to select materials with better cycle performance, which is wasteful. Positive and negative electrode compaction: The positive and negative electrode compaction is too high, although it can improve the energy density of the cell, but it will also reduce the cycling performance of the material to a certain extent. From the theoretical analysis, the greater the compaction, the greater the damage to the structure of the material, and the structure of the material is the basis for ensuring that the lithium ion battery can be recycled; in addition, the battery cells with higher positive and negative electrode compaction are difficult to guarantee. The high liquid retention capacity is the basis for the battery cell to complete the normal cycle or more cycles. Moisture: Excessive moisture will cause side reactions with the positive and negative active materials, destroy their structure and affect the circulation. At the same time, too much moisture is not conducive to the formation of SEI film. But while trace amounts of water are difficult to remove, trace amounts of water can also ensure the performance of the battery cell to a certain extent. Unfortunately, Wen Wu's personal experience in this area is almost zero, and he can't say much. If you are interested, you can search the forum for information on this topic, there are still many. Coating film density: A single variable considering the effect of film density on circulation is almost an impossible task. The inconsistency of the film density will either bring about a difference in capacity, or a difference in the number of cell windings or lamination layers. For batteries of the same model, same capacity and same material, reducing the membrane density is equivalent to adding one or more layers of winding or lamination. The correspondingly increased separator can absorb more electrolyte to ensure circulation. Considering that the thinner film density can increase the rate performance of the cell, the baking and dewatering of the pole piece and the bare cell will also be easier, of course, the error when coating the film thickness is too thin may be more difficult to control, active substances The large particles may also have a negative effect on coating and rolling. More layers means more foils and separators, which in turn means higher costs and lower energy density. Therefore, the evaluation also needs to be balanced. Excessive negative electrode: In addition to the need to consider the effect of the first irreversible capacity and the density deviation of the coating film, the impact on the cycle performance is also a consideration. For the lithium cobaltate plus graphite system, it is more common for the negative electrode graphite to become the short side in the circulation process. If the excess of the negative electrode is insufficient, the cell may not release lithium before cycling, but the structure of the positive electrode changes little after hundreds of cycles, but the structure of the negative electrode is seriously damaged and cannot fully receive the lithium ions provided by the positive electrode to precipitate lithium, resulting in excessive capacity. Early drop. The amount of electrolyte: There are three main reasons for the lack of electrolyte to affect the circulation. First, the amount of liquid injection is insufficient. Second, although the amount of liquid injection is sufficient, the aging time is insufficient or the positive and negative electrodes are not immersed due to excessive compaction. Fully, the third is that the electrolyte inside the circulating cell is consumed. Insufficient liquid injection and insufficient liquid retention Wen Wen wrote "Effects of Electrolyte Loss on Cell Performance" before, and will not repeat them here. For the third point, the microscopic performance of the matching between the positive and negative electrodes, especially the negative electrode and the electrolyte, is the formation of dense and stable SEI, while the performance visible in the right eye is the rate of electrolyte consumption during the cycle. The incomplete SEI film cannot effectively prevent the negative reaction between the negative electrode and the electrolyte and consume the electrolyte. On the one hand, the SEI film will be regenerated as the cycle progresses to consume the reversible lithium source and electrolyte . Whether it is a battery cell that has been circulated hundreds or even thousands of times or a battery cell that has been diving for dozens of times, if the electrolyte is sufficient before the cycle and the electrolyte has been consumed after the cycle, it is possible to increase the amount of electrolyte retained. To a certain extent, improve its cycle performance. Objective conditions of the test: charge and discharge rate during test, cut-off voltage, charge cut-off current, over-charge and over-discharge during test, test room temperature, sudden interruption during test, contact resistance between test point and cell, etc. Factors will more or less affect the cycle performance test results. In addition, different materials have different sensitivities to the above objective factors. It is sufficient for daily work to unify test standards and understand the commonality and characteristics of important materials. Summary: Just like the principle of the wooden barrel, among the many factors that affect the cycle performance of the cell, the final decisive factor is the shortest board among many factors. At the same time, there are interactions among these influencing factors. Under the same material and manufacturing capacity, the higher the cycle, the lower the energy density. It is the most important task to find the junction that just meets the customer's needs and try to ensure the consistency of the battery core. .
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