The importance of cycle performance to lithium-ion batteries is self-evident; 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 involved in the lithium battery industry must consider. The following is a list of several factors that may affect battery cycle performance for your reference.
1. Material types
The choice of materials is the first factor affecting the performance of lithium-ion batteries. The materials with poor cycle performance are selected, the process is reasonable, and the production is perfect, and the cycle of the battery cannot be guaranteed; even if the better materials are selected, the cycle performance may not be poor even if there are some problems in the subsequent production. Too ridiculous (once lithium cobaltate exerts only about 135.5mAh / g and lithium-discharged cells, although 1C dives more than a hundred times, 0.5C, 500 times more than 90%; the negative electrode has black graphite particles after the cell is disassembled Battery, cycle performance is normal). From a material point of view, the cycle performance of a full cell is determined by the poorer of the cycle performance after the positive electrode matches the electrolyte and the cycle after the negative electrode matches the electrolyte. The material has poor cycle performance. On the one hand, it may be that the crystal structure changes too quickly during the cycle to complete the lithium insertion and delithiation; on the other hand, it may be because the active material and the corresponding electrolyte cannot generate a dense and uniform SEI film, which causes the active material. Premature side reactions with the electrolyte cause the electrolyte to be consumed too quickly and affect the cycle. In the design of the battery cell, if one pole confirms that a material with poor cycle performance is selected, the other pole does not need to choose a material with better cycle performance, which is wasteful.
2.Positive and negative electrode compaction
The positive and negative electrode compaction is too high, although the energy density of the battery can be increased, but the cycle performance of the material will be reduced 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, it is difficult to ensure that the positive and negative electrode cells with high compaction can be more compact. High liquid holding capacity, which is the basis for the battery to complete a normal cycle or more cycles.
Too much moisture will cause side reactions with the positive and negative electrode active materials, destroy its structure and affect the circulation. At the same time, too much moisture is not conducive to the formation of the SEI film. However, while the trace amount of water is difficult to remove, the trace amount of water can also guarantee the performance of the battery 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 a lot.
4. Coating film density
Considering the effect of membrane density on cycling in a single variable is almost an impossible task. Inconsistent film density either results in a difference in capacity or a difference in the number of cell windings or laminated layers. For cells of the same type, same capacity and material, reducing the film density is equivalent to increasing the number of layers of one or more layers of winding or lamination. The corresponding increase in the separator can absorb more electrolyte to ensure circulation. Considering that a thinner film density can increase the rate performance of the battery cell, the baking and dewatering of the pole piece and the bare battery cell will also be easier. Of course, the error when coating the thinner film density may be more difficult to control. Large particles may also have a negative impact on coating and rolling. More layers means more foils and separators, which in turn means higher cost and lower energy density. Therefore, the assessment also needs to be balanced.
The reason for the excess of the negative electrode is to consider the influence of the irreversible capacity for the first time and the density deviation of the coating film, as well as the influence on the cycle performance. For lithium cobaltate plus graphite systems, it is more common for negative graphite to become a short board in the cycle. If the negative electrode is not excessive enough, the cell may not precipitate lithium before the cycle, but the structure of the positive electrode changes slightly after hundreds of cycles, but the structure of the negative electrode is severely damaged, and the lithium ions provided by the positive electrode cannot be fully received to cause lithium precipitation, resulting in excessive capacity. Drop early.
6.Amount of electrolyte
There are three main reasons for the insufficient electrolyte volume to affect the circulation. One is the insufficient liquid injection amount. The other is that the liquid injection amount is sufficient but the aging time is insufficient or the positive and negative electrodes are not sufficiently immersed due to high compaction. With the circulation of the electrolyte inside the cell is consumed. Insufficient injection volume and insufficient retention volume Wen Wu wrote previously "Effects of lack of electrolyte on cell performance" and will not repeat them here. For the third point, the microscopic performance of the matching of the positive and negative electrodes, especially the negative electrode and the electrolyte, is the formation of dense and stable SEI, while the performance seen by the right eye is the consumption rate of the electrolyte during the cycle. An incomplete SEI film cannot effectively prevent the negative electrode from reacting with the electrolyte to consume the electrolyte. On the one hand, the defective part of the SEI film will regenerate the SEI film as the cycle progresses, thereby consuming the reversible lithium source and the electrolyte. . Regardless of whether the battery is circulated hundreds or even thousands of times, or for dozens of batteries that have been diving, if the electrolyte is sufficient before the cycle and the electrolyte has been consumed after the cycle, it is likely to increase the electrolyte retention. To a greater extent its cycle performance.