source：Geerady       date： 2021-07-14
The importance of cycle performance to lithium-ion batteries is needless to say; On a macro level, longer cycle life means less resource consumption. Therefore, the factors affecting the performance of lithium-ion battery cycle are the issues that everyone in the lithium-ion industry has to consider. Several factors that may affect the performance of the battery cycle are listed below for your reference.
Material type: the choice of material is the first factor that affects the performance of lithium ion battery. The selection of materials with poor cycle performance, and the process is reasonable and perfect, and the cycle of the cell is inevitably unable to be guaranteed; Choose a better material, even if the subsequent made of some problems, cycling performance may not be bad too far off (a lithium cobalt acid play only about 135.5mAH /g and lithium analysis of the cell, 1C although more than 100 times of diving but 0.5C, 500 times more than 90%; Once the cell is disassembled, the negative electrode has black graphite particles, and the cycle performance is normal. From the point of view of materials, the cycle performance of a full battery is determined by the cycling performance of the positive electrode and the electrolyte, and the cycling performance of the negative electrode and the electrolyte after the matching. Material cycle performance is poorer, on the one hand, is likely to be in circulation in the process of crystal structure change too fast to continue to complete the intercalated-li lithium, on the one hand may be due to the active substance and its corresponding electrolyte can't generate dense and uniform SEI film active material and electrolyte premature side reaction and make the electrolyte excessive consumption will affect cycle. 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 choose materials with good cycle performance, waste. Positive and negative compaction: if the positive and negative compaction is too high, the energy density of the cell can be improved, but the recycling performance of the material will be reduced to a certain extent. Theoretically, the larger the compaction, the greater the damage to the structure of the material, and the structure of the material is the basis to ensure that the lithium ion battery can be recycled. In addition, the higher positive and negative compaction of the cell is difficult to ensure a higher liquid retention, and the liquid retention is the basis of the cell to complete the normal cycle or more cycles.
Moisture: Too much water will react with the positive and negative active substances, destroy their structure and thus affect the circulation. At the same time, too much water is not conducive to the formation of SEI membrane. But in the trace water is difficult to remove at the same time, trace water can also ensure the performance of the cell to a certain extent. Unfortunately, Wenwu's personal experience in this field is almost nil, and he can't say much. Everybody has the interest to be able to search the information about this topic inside the forum, still many.
Coated film density: it is an almost impossible task to consider the effect of film density on circulation in a single variable. Inconsistent film densities either lead to differences in capacity, or differences in the number of layers wound or laminated. For the same type of cell with the same capacity and the same material, reducing the film density is equivalent to increasing the number of winding or laminating layers by one or more layers, and the corresponding increase in the diaphragm can absorb more electrolyte to ensure circulation. Considering more thin film density can increase rate performance of the batteries, the pole piece and naked batteries baked in addition to the water will be easier, too, of course, when the density of thin film coating of error may be more difficult to control, active substances in the larger particles may also cause negative effect to coating, rolling, more layer means more foil and diaphragm, which means higher costs and lower energy density. Therefore, the evaluation also needs to be balanced.
Negative overdose: The reason for negative overdose is not only the impact of the first irreversible capacity and the density deviation of the coating film, but also the impact on the cycling performance. For the lithium cobaltate plus graphite system, the negative graphite is the "short board" side of the cycle process is more common. If the negative electrode is excessive and insufficient, the cell may not digest lithium before the cycle, but after several hundred cycles, the positive electrode structure changes little, but the negative electrode structure is seriously damaged and cannot fully receive the lithium ions provided by the positive electrode to digest lithium, resulting in premature capacity decline.
Electrolyte amount: insufficient electrolyte amount has an impact on the circulation for three main reasons, one is the insufficient amount of liquid injection, the second is that although the amount of liquid injection is sufficient but the aging time is not enough or the positive and negative electrode due to high compaction caused by the leaching is not sufficient, the third is with the circulation of the cell internal electrolyte is consumed. Insufficient liquid injection and insufficient liquid retention Wenwu has written "electrolyte deficiency on the performance of the cell" and therefore no longer repeat. For the third point, the micro performance of the matching between the positive and negative electrodes, especially the negative electrode and the electrolyte, is the formation of the dense and stable SEI, while the visible performance of the right eye is the consumption rate of the electrolyte in the cycle process. On the one hand, the incomplete SEI film cannot effectively prevent the negative electrode from side reaction with the electrolyte and thus consume the electrolyte; on the other hand, the defective SEI film will regenerate with the circulation, thus consuming the reversible lithium source and electrolyte. Whether it is for hundreds or even thousands of cycles of the cell or for dozens of both diving cells, if the electrolyte before the cycle is sufficient and the electrolyte after the cycle has been consumed, it is likely to increase the amount of electrolyte retention can improve its cycling performance to a certain extent.
The objective conditions of the test: external factors such as charge and discharge rate, cut-off voltage, cut-off current, overcharge and over-discharge in the test, temperature of the test room, sudden interruption in the test process, internal resistance of the contact between the test point and the battery core, will more or less affect the test results of the cycle performance. In addition, different materials have different sensitivities to the above objective factors. Unifying testing standards and understanding the characteristics of common and important materials should be sufficient for daily work.
Summary: Like the barrel principle, many factors that affect the performance of the cell cycle, the final decisive factor, is the shortest plate among many factors. At the same time, these influencing factors also have interactive influence. In the same material and production capacity, the higher the cycle, often means the lower the energy density, to find just to meet the customer needs of the joint point, as far as possible to ensure the consistency of the cell made, is the most important task.