Seven factors affecting the life of lithium-ion batteries

The importance of cycle performance to lithium-ion batteries is self-evident; in addition, from a macro perspective, a 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 has to consider. The following article lists several factors that may affect the cycle performance of batteries for your reference.

1. Material type : The choice of materials is the first factor affecting the performance of lithium-ion batteries. If materials with poor cycle performance are selected, no matter how reasonable the process is and how perfect the manufacturing is, the cycle of the battery cell cannot be guaranteed; if better materials are selected, even if there are some problems in the subsequent manufacturing, the cycle performance may not be too bad (the primary cobalt oxide lithium gram performance is only about 135.5mAh/g and the battery cell with lithium deposition, although 1C dives more than 100 times, it is more than 90% at 0.5C and 500 times; the battery cell with black graphite particles on the negative electrode after the primary battery cell is disassembled has normal cycle performance). From the perspective of materials, the cycle performance of a full battery is determined by the cycle performance after the positive electrode and the electrolyte are matched, and the cycle performance after the negative electrode and the electrolyte are matched, whichever is worse. The poor cycle performance of the material may be due to the rapid change of the crystal structure during the cycle process, which makes it impossible to continue to insert and remove lithium, and it may be due to the inability of the active material and the corresponding electrolyte to form a dense and uniform SEI film, which causes the active material and the electrolyte to react prematurely, causing the electrolyte to be consumed too quickly and thus affecting the cycle. When designing a battery cell, if one electrode is confirmed to use a material with poor cycle performance, there is no need to use a material with better cycle performance for the other electrode, which would be wasteful.

2. Compacting of positive and negative electrodes : Although the compaction of positive and negative electrodes is too high, it can increase the energy density of the battery cell, but it will also reduce the material's cycle performance to a certain extent. From a theoretical analysis, the greater the compaction, the greater the damage to the material structure, and the material structure is the basis for ensuring that lithium-ion batteries can be recycled; in addition, batteries with high compaction of positive and negative electrodes are difficult to ensure a high liquid retention, and the liquid retention is the basis for the battery cell to complete normal cycles or more cycles.

3. Water : Excessive water will react with the positive and negative active materials, destroy their structure and affect the cycle. At the same time, excessive water is not conducive to the formation of SEI film. However, while trace amounts of water are difficult to remove, trace amounts of water can also guarantee the performance of the battery to a certain extent. Unfortunately, Wenwu has almost no personal experience in this area and can't say much. If you are interested, you can search the forum for information on this topic, there is still a lot.

4. Coating film density : It is almost an impossible task to consider the impact of membrane density on the cycle as a single variable. Inconsistent membrane density either leads to differences in capacity or differences in the number of winding or stacking layers of the battery cell. For batteries of the same model, capacity and material, reducing the membrane density is equivalent to adding one or more layers of winding or stacking. The corresponding increased diaphragm can absorb more electrolyte to ensure circulation. Considering that a thinner membrane density can increase the rate performance of the battery cell, it will also be easier to bake and remove water from the pole pieces and bare cells. Of course, the error during coating of a film density that is too thin may be more difficult to control, and large particles in the active material may also have a negative impact on coating and rolling. More layers mean more foils and diaphragms, which in turn means higher costs and lower energy density. Therefore, a balanced consideration is also required during the evaluation.

5. Excessive negative electrode : In addition to the impact of the first irreversible capacity and the coating film density deviation, the impact on the cycle performance is also a consideration for the excessive negative electrode. For the lithium cobalt oxide plus graphite system, it is common for the negative electrode graphite to become the "short board" in the cycle process. If the excess negative electrode is insufficient, the battery cell may not precipitate lithium before the cycle, but after hundreds of cycles, the positive electrode structure changes little, but the negative electrode structure is severely damaged and cannot fully receive the lithium ions provided by the positive electrode, thereby precipitating lithium, causing the capacity to drop prematurely.

6. Electrolyte quantity : There are three main reasons why insufficient electrolyte quantity affects the cycle. First, the amount of liquid injection is insufficient. Second, although the amount of liquid injection is sufficient, the aging time is insufficient or the positive and negative electrodes are not fully immersed due to excessive compaction. Third, the electrolyte inside the battery cell is consumed as the cycle progresses. Insufficient liquid injection and insufficient liquid retention. Wenwu has previously written "The Impact of Electrolyte Lack on Battery Performance" so I will not repeat it 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 a dense and stable SEI, and the performance visible to the right eye is the consumption rate of the electrolyte during the cycle. On the one hand, the incomplete SEI film cannot effectively prevent the negative electrode from reacting with the electrolyte and consuming the electrolyte. On the other hand, the defective parts of the SEI film will regenerate the SEI film as the cycle progresses, thereby consuming the reversible lithium source and electrolyte. Whether it is a battery cell that has been cycled hundreds or even thousands of times or a battery cell that has been drained after dozens of cycles, if there is sufficient electrolyte before the cycle and the electrolyte has been consumed after the cycle, increasing the amount of electrolyte retained is likely to improve its cycle performance to a certain extent.

7. Objective test conditions : external factors such as charge and discharge rate, cut-off voltage, charge cut-off current, overcharge and overdischarge during the test, test room temperature, sudden interruption during the test, contact resistance between the test point and the battery cell, etc., will more or less affect the cycle performance test results. In addition, different materials have different sensitivities to the above objective factors. Unified test standards and understanding of common and important material characteristics should be sufficient for daily work.

Summary: Just like the barrel principle, among the many factors that affect the cycle performance of the battery cell, the final decisive factor is the shortest board among many factors. At the same time, these influencing factors also have mutual influences. Under the same material and manufacturing capacity, the higher the cycle, the lower the energy density. Finding the combination point that just meets customer needs and ensuring the consistency of battery cell manufacturing as much as possible is the most important task.