Factors affecting the cycle performance of lithium-ion batteries

Jun 27, 2019   Pageview:72

Everyone in the lithium battery industry has to consider the lithium-ion battery cycle performance, while a longer cycle life means less resource consumption. So what exactly determines the performance of the lithium-ion battery cycle?

 

Material Type: The choice of materials is the first factor affecting the performance of lithium-ion batteries. The material with poor cycle performance is selected, the process is reasonable, and the refinement is made. The cycle of the battery core is also inevitable; the better material is selected, and even if there are some problems in subsequent fabrication, the cycle performance may not be bad. Too ridiculous (one lithium cobaltate gram is only 135.5 mah / g and lithium battery, 1 c although more than 100 times diving but 0.5 degrees Celsius, 500 times more than 90%; after the core is disassembled, the negative electrode has black graphite The cell of the particle has normal cycle performance). From a material point of view, the cycle performance of a full battery is determined by the poor performance of the cycle performance after the positive electrode is matched with the electrolyte, 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 changes too fast during the cycle, so that lithium insertion can not be completed. On the one hand, the active substance and the corresponding electrolyte cannot form a dense and uniform SEI film to cause active substances. A side reaction occurs prematurely with the electrolyte to cause the electrolyte to be consumed too quickly to affect the cycle. In the design of the battery core, 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 and waste.

 

Positive and negative compaction: The positive and negative compaction is too high. Although the energy density of the cell can be increased, the cycle performance of the material is also reduced to some extent. From the theory to analyze, the greater the compaction, the greater the structural damage to the material, and the structure of the material is the basis for ensuring the recycling of lithium-ion batteries. In addition, it is difficult to guarantee the higher voltage of the positive and negative electrodes. High liquid holding capacity, while the liquid holding capacity is the basis for the cell to complete a normal cycle or more cycles.

 

Moisture: Excessive moisture will react with the positive and negative active materials, destroying its structure and affecting the cycle. At the same time, too much moisture is not conducive to the formation of SEI film. However, trace amounts of water can also ensure the performance of the cell to a certain extent while the trace amount of water is difficult to remove. Unfortunately, my personal experience in this area is almost zero and I can not say much. Everyone is interested in searching the forum for information on this topic, or a lot.

 

Coating Film Density: A single variable considering the effect of film density on the cycle is almost an impossible task. Inconsistent film densities either result in a difference in capacity or a difference in the number of cell windings or laminations. For batteries of the same type and capacity of the same material, reducing the film density is equivalent to increasing the number of layers or layers of winding or lamination, and the corresponding diaphragm can absorb more electrolyte to ensure circulation. Considering that a thinner film density can increase the rate performance of the cell, it is easier to bake and remove the water from the pole piece and the bare cell. Of course, the error in coating film density that is too thin may be more difficult to control, in the active material. Large particles can also have a negative impact on coating and rolling. More layers mean more foil and separator, which means higher cost and lower energy density, so when evaluating Need to balance considerations.

 

Excessive anode: The reason for the excess of the anode is that in addition to the influence of the first irreversible capacity and the deviation of the coating film density, the effect on the cycle performance is also a consideration. For the lithium cobaltate plus graphite system, the negative graphite is more common on the "short board" side of the cycle. If the anode is not excessive enough, the electric core may not analyze lithium before circulation, but after several hundred cycles, the structure of the positive electrode changes little but the structure of the negative electrode is severely damaged, and the lithium ion provided by the positive electrode cannot be completely received, thereby causing lithium. Early decline.

 

The amount of electrolyte: There are three main reasons for the lack of electrolyte solution. First, the amount of liquid injection is insufficient. Second, although the amount of liquid injection is sufficient, the aging time is not enough or the positive and negative electrodes are not soaked due to high compaction. Sufficient, the third is that the internal electrolyte of the circulating cell is consumed.Insufficient fluid injection and insufficient liquid retention, I have previously written, "the lack of electrolytic fluid on the performance of the core," so no longer elaborate. For the third point, the microscopic representation of the matching of the positive and negative electrodes, especially the negative electrode and the electrolyte, is the formation of a dense and stable SEI, while the performance visible to the right eye is both the rate of consumption of the electrolyte during the cycle. On the one hand, the incomplete SEI film can not effectively prevent the negative electrode from reacting with the electrolyte to consume the electrolyte. On the other hand, in the defective portion of the SEI film, the SEI film is regenerated as the cycle progresses, thereby consuming the reversible lithium source and electrolyte. . Whether it is a cell with hundreds or even thousands of cycles or a battery with dozens of dips, 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. To the extent that it improves its cycle performance.

 

Objective conditions of the test: charge and discharge rate during the test, cut-off voltage, charge cut-off current, overcharge and over discharge during the test, test room temperature, sudden interruption during the test, contact with the cell and internal resistance of the cell, etc. Factors will affect the cycle performance test results more or less. In addition, different materials are sensitive to the above objective factors. Uniform testing standards and understanding of the characteristics of commonality and important materials should be sufficient for daily work.

 

Summary: Like the principle of wooden barrels, among the many factors affecting the cycle performance of batteries, the final decisive factor is the shortest of many factors. At the same time, there are interactions between these factors. Under the same material and manufacturing capacity, the higher the cycle, the lower the energy density, the more the joints that just meet the customer's needs, and the consistency of the electric core is the most important task.

 

The page contains the contents of the machine translation.

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