According to Battery.com.cn, the importance of cycle performance to lithium-ion batteries is not much to say. Macroscopically speaking, longer cycle life means less resource consumption. Therefore, the factors affecting the cycle performance of lithium-ion batteries have to be considered by everyone involved in the lithium-ion industry.
1. Moisture
Too much water will react with positive and negative active substances, destroy their structure, and then affect the cycle. At the same time, too much water is not conducive to the formation of SEI film. However, while trace water is difficult to remove, trace water can also ensure the performance of the core to a certain extent.
2. Positive and negative pole compaction
Excessive compaction of positive and negative electrodes can improve the energy density of the core, but it can also reduce the cycling performance of the material 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, it is difficult for the high compaction of positive and negative electrodes to guarantee a higher liquid retention. The liquid holding capacity is the basis for the normal cycle or more cycles of the core.
3. The Objective Conditions of Testing
External factors such as charge-discharge ratio, cut-off voltage, charge-cut-off current, over-charge and over-discharge in test, temperature of test room, sudden interruption in test process, contact resistance between test point and core will more or less affect the test results of cycle performance. In addition, the sensitivity of different materials to the above objective factors varies, and the test standard is unified. Understanding the commonalities and the characteristics of important materials should be enough for daily use.
4. Negative excess
In addition to the influence of the first irreversible capacity and the density deviation of the coating film, the influence on the cycling performance is also a consideration. For the lithium cobalt acid-graphite system, the negative graphite becomes the "short plate" in the cycling process, which is more common. If the negative is excessive, the core may not precipitate lithium before the cycling, but after several hundred cycles, the positive pole may not be precipitated. The structure of the cathode has little change, but the structure of the negative electrode has been destroyed seriously, and the lithium ion supplied by the cathode can not be fully received, thus lithium can be precipitated, resulting in premature capacity decline.
5. Coating film density
Considering the effect of membrane density on the cycle is almost impossible for a single variable. The inconsistency of membrane density either brings about the difference of capacity or the difference of the number of coiling or laminating layers of cores. For the same type of cores with the same capacity and material, reducing membrane density is equivalent to increasing the number of coiling or laminating layers of one or more layers, and the corresponding increase of membrane can absorb more electrolytes. In order to ensure circulation, considering that thinner film density can increase the rate performance of the core, the baking and dewatering of the electrodes and bare cores will be easier. Of course, the error of too thin film density coating may be more difficult to control. Large particles in the active substances may also have a negative impact on coating and rolling. More layers mean more foil and diaphragm, which means higher. Costs and lower energy densities require balanced consideration when assessing.
6. Material Types
Material selection is the first factor affecting the performance of lithium-ion batteries. Selecting materials with poor cycling performance will inevitably not guarantee the core cycle if the technology is reasonable and the manufacture is perfect. Selecting better materials, even if there are some problems in the subsequent manufacture, the cycling performance may not be too bad. From the material point of view, the cycling performance of a full battery is: The poor cycling performance of materials is determined by the cycling performance of cathode matched with electrolyte and that of negative matched with electrolyte. On the one hand, the change of crystal structure during the cycling process may be too fast to continue lithium intercalation and lithium removal. On the other hand, the dense and uniform SEI can not be produced by active material and corresponding electrolyte. Membranes cause premature side reactions between active substances and electrolyte, which lead to excessive consumption of electrolyte and affect the cycle. In the design of the core, 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, which is wasteful.
7. Electrolyte Volume
The insufficient electrolyte volume has three main reasons for the influence on circulation. One is insufficient injection volume, the other is insufficient aging time or insufficient immersion of positive and negative electrodes due to excessive compaction, and the third is that the electrolyte inside the circulating core is consumed. Thirdly, the matching of positive and negative electrodes, especially negative electrodes, with electrolyte shows the formation of compact and stable SEI, while the right eye can see that the incomplete SEI film can not effectively prevent the side reaction between negative electrodes and electrolyte, thus consuming electrolyte. On the one hand, the defective part of SEI film will follow the cycle. The SEI film is regenerated to consume reversible lithium source and electrolyte. If the electrolyte before the cycle is sufficient and the electrolyte after the cycle is exhausted, it is possible to increase the retention of electrolyte to a certain extent to improve its cycle performance, whether for the cores with hundreds or thousands of cycles or for the cores with dozens of diving cycles.
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