Several factors affecting the cycling performance of 18650 batteries.

Several factors affecting the cycling performance of 18650 batteries.

The importance of cycling performance for lithium-ion batteries needs no further elaboration; From a macro perspective, a longer cycle life means less resource consumption. Therefore, the factors that affect the cycling performance of lithium-ion batteries are issues that every person involved in the lithium-ion battery industry must consider. The following list of factors that may affect battery cycling performance is provided for your reference.

Material type: The selection of materials is the first factor that affects the performance of 18650 lithium-ion batteries. Choosing materials with poor cycling performance, even if the process is reasonable and the manufacturing is perfect, the cycling of the battery cells cannot be guaranteed; Choosing good materials, even if there are some issues with subsequent production, the cycling performance may not be too poor (a lithium cobalt oxide battery with only about 135.5mAh/g per cycle and lithium deposition, a 1C battery with over 100 dives but over 90% at 0.5C and 500 cycles; a battery with black graphite particles on the negative electrode after disassembly, and normal cycling performance). From a material perspective, the cycling performance of a full battery is determined by the poorer one between the cycling performance of the positive electrode matched with the electrolyte and the cycling performance of the negative electrode matched with the electrolyte. The poor cycling performance of the material may be due to the rapid changes in crystal structure during the cycling process, which makes it impossible to continue lithium insertion and removal. On the other hand, it may be due to the inability of the active substance and corresponding electrolyte to form a dense and uniform SEI film, resulting in premature side reactions between the active substance and electrolyte, leading to rapid consumption of electrolyte and affecting cycling. When designing battery cells, if one pole confirms the use of materials with poor cycling performance, the other pole does not need to choose materials with better cycling performance, which is wasteful.

Positive and negative electrode compaction: If the positive and negative electrode compaction is too high, although it can increase the energy density of the battery cell, it will also to some extent reduce the cyclic performance of the material. From a theoretical perspective, the greater the compaction, the greater the structural damage to the material, which is the foundation for ensuring the recyclability of lithium-ion batteries; In addition, cells with high positive and negative electrode compaction are difficult to ensure high liquid retention, which is the basis for cells to complete normal or more cycles.

Moisture: Excessive moisture can cause side reactions with positive and negative active substances, damage their structure, and affect circulation. At the same time, excessive 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, Wenwu has almost zero personal experience in this area and cannot say much about it. If you are interested, you can search the forum for information about this topic, there are still many available.

Coating film density: It is almost impossible to consider the impact of film density on cycling as a single variable. Inconsistent membrane density either leads to differences in capacity or differences in the number of layers of cell winding or stacking. For cells of the same model, capacity, and material, reducing the film density is equivalent to adding one or more layers of winding or stacking layers, and the corresponding increased diaphragm can absorb more electrolyte to ensure circulation. Considering that thinner film density can increase the rate performance of battery cells and make it easier to remove water from pole pieces and bare cells during baking, of course, errors in coating with too thin film density may be more difficult to control. Large particles in the active material may also have a negative impact on coating and rolling. More layers mean more foil and separators, which in turn means higher costs and lower energy density. So, a balanced consideration is also needed when evaluating.

Excess negative electrode: In addition to considering the impact of irreversible capacity and coating film density deviation, the impact of excess negative electrode on cycling performance is also a consideration. For the lithium cobalt oxide graphite system, the negative graphite becomes a common "shortcoming" in the cycling process. If the negative electrode is excessive or insufficient, the battery cell may not undergo lithium deposition before cycling. However, after hundreds of cycles, the positive electrode structure changes very little, but the negative electrode structure is severely damaged and cannot fully receive the lithium ions provided by the positive electrode, resulting in lithium deposition and premature capacity decline.

Electrolyte quantity: Insufficient electrolyte quantity has three main reasons for its impact on circulation. Firstly, there is insufficient injection volume. Secondly, although the injection volume is sufficient, the aging time is not enough or the positive and negative electrodes are not fully immersed due to high compaction. Thirdly, as the electrolyte inside the circulating cell is consumed. Insufficient injection and retention of electrolyte, as previously written by Wen Wu on the impact of electrolyte deficiency on battery performance, will not be further elaborated. For the third point, the microscopic manifestation of the compatibility between the positive and negative electrodes, especially the negative electrode, and the electrolyte is the formation of a dense and stable SEI, while the visible manifestation in the right eye is the consumption rate of the electrolyte during the cycling process. An incomplete SEI film cannot effectively prevent the negative electrode from undergoing side reactions with the electrolyte, thereby consuming the electrolyte. On the other hand, in areas where the SEI film is defective, the SEI film will regenerate as the cycle progresses, thereby consuming the reversible lithium source and electrolyte. Whether it is for a battery cell that has been cycled hundreds or even thousands of times or for a battery cell that has dropped several tens of times, if the electrolyte is sufficient before cycling but has already been consumed after cycling, increasing the electrolyte retention is likely to improve its cycling performance to a certain extent.

The objective conditions for testing include external factors such as charging and discharging rate, cut-off voltage, charging cut-off current, overcharging and overdischarging during the testing process, temperature in the testing room, sudden interruption during the testing process, and contact internal resistance between the testing point and the battery cell, all of which can affect the results of cyclic performance testing to varying degrees. In addition, different materials have varying degrees of sensitivity to the objective factors mentioned above. A unified testing standard and understanding of the characteristics of common and important materials should be sufficient for daily work use.

Summary: Like the bucket principle, among the many factors that affect the cycling performance of battery cells, the ultimate determining factor is the shortest board among them. Meanwhile, these influencing factors also have interactive effects. Under the same materials and manufacturing capabilities, higher cycles often mean lower energy density. Finding the perfect combination point to meet customer needs and ensuring consistency in battery cell manufacturing is the most important task.