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 will inevitably not be guaranteed; if a better material is selected, even if there are some problems in the subsequent manufacturing, the cycle performance may not be better. big gap. From the material point of view, the cycle performance of a full battery is determined by the poorer one of the cycle performance after matching the positive electrode and the electrolyte, and the cycle performance after matching the negative electrode and the electrolyte. The cycle performance of the material is poor. On the one hand, it may be that the crystal structure changes too fast during the cycle to continue to complete lithium intercalation and delithiation. On the other hand, it may be that the active material and the corresponding electrolyte cannot form a dense and uniform SEI film. Premature side reactions with the electrolyte cause the electrolyte to be consumed too quickly and affect the cycle. When designing the battery, if one pole is confirmed to use a material with poor cycle performance, the other pole does not need to choose a material with better cycle performance.
1. Positive and negative compaction
If the positive and negative electrodes are compacted too high, although it can increase the energy density of the cell, it will also reduce the cycle performance of the material to a certain extent. From a theoretical analysis, compaction
The larger it is, the greater the damage to the structure of the material is, 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 a high liquid retention capacity for cells with higher positive and negative electrodes compaction, while The liquid retention is the basis for the battery to complete a normal cycle or more cycles.
2. Moisture
Excessive moisture will cause side reactions with positive and negative active materials, destroy their structure and affect circulation, and excessive moisture is also not conducive to the formation of SEI film. However, while traces of water are difficult to remove, traces of water can also guarantee the performance of the battery to a certain extent. It's a pity that Wenwu has almost zero personal experience in this area, so he can't say much. If you are interested, you can search the forum for information on this topic, there are quite a few.
3. Coating film density
Considering the effect of membrane density on cycling as a single variable is an almost impossible task. The inconsistency of the film density will either bring about the difference in capacity, or the difference in the number of core winding or laminated layers. For batteries of the same model, capacity and material, reducing the film density is equivalent to increasing one or more layers of winding or stacking layers, and the corresponding increased separator can absorb more electrolyte to ensure circulation. Considering that a thinner film density can increase the rate performance of the cell, and it will be easier to bake and remove water from the pole piece and the bare cell. Of course, the error when the film density is too thin may be more difficult to control. In the active material Large particles of large particles may also cause negative effects on coating and rolling
Influence, more layers means more foils and separators, which in turn means higher cost and lower energy density. Therefore, a balanced consideration is also required in the evaluation.
Negative excess
The reason for the excess of the negative electrode is to consider the influence of the first irreversible capacity
In addition to the deviation of coating film density, the impact on cycle performance is also a consideration. For the lithium cobalt oxide plus graphite system, it is more common for the negative electrode graphite to become the "short board" in the cycle process. If the negative electrode is excessive and insufficient, the cell may not decompose lithium before cycling, but the structure of the positive electrode changes little after hundreds of cycles, but the structure of the negative electrode is severely damaged and cannot fully receive the lithium ions provided by the positive electrode.
Lithium is precipitated, resulting in a premature decline in capacity.
4. Electrolyte volume
There are three main reasons for the impact of insufficient electrolyte volume on circulation. One is insufficient liquid injection; The electrolyte inside the cell is consumed as the cycle progresses. Insufficient liquid injection and insufficient liquid retention Wen Wu wrote "The Influence of Electrolyte Lacking on the Performance of Cells" before, so I won't repeat it here. Regarding the third point, the microcosmic performance of the matching between the positive and negative electrodes, especially the negative electrode and the electrolyte, is the formation of 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, an incomplete SEI film cannot effectively prevent the side reaction between the negative electrode and the electrolyte to consume the electrolyte.
The defective part of the membrane will regenerate the SEI membrane as the cycle progresses to consume the reversible lithium source and electrolyte. Whether it is a battery that has been cycled hundreds or even thousands of times or a battery that has dived dozens of times, if the electrolyte is sufficient before the cycle and the electrolyte has been consumed after the cycle, then increasing the electrolyte reserve is likely to be sufficient. Improve its cycle performance to a certain extent.
5. Objective conditions of the test
External factors such as charge-discharge rate, cut-off voltage, charge cut-off current, overcharge and over-discharge during the test, temperature of the test room, sudden interruption during the test, contact internal resistance between the test point and the battery cell during the test, etc. Or less affect the cycle performance test results. In addition, different materials have different sensitivities to the above-mentioned objective factors. Unifying the testing standards and understanding the commonality and characteristics of important materials should be sufficient for daily work.
Just like the wooden 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, there is also an interaction among these influencing factors. Under the same material and manufacturing capacity, the higher the cycle, the lower the energy density. Finding the combination point that just meets the customer's needs and ensuring the consistency of the cell manufacturing is the most important task.