Material: The choice of material is the first element that affects the performance of lithium-ion batteries. Choose a material with poor cycle performance, no matter how reasonable the process is, and no matter how perfect the production 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 subsequent production, the cycle performance may not be bad Too outrageous (Lithium cobalt oxide is only about 135.5mAh/g and lithium-deposited battery cells, 1C although more than 100 dives, but 0.5C, 500 times more than 90%; after a battery is disassembled, the negative electrode has black graphite particles The battery cell, the cycle performance is normal).
From the point of view of materials, the cycle performance of a full battery is determined by the poorer of the cycle performance after the positive electrode is matched with the electrolyte, and the cycle performance after the negative electrode is matched with 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 the lithium insertion and removal. On the other hand, it may be that the active material and the corresponding electrolyte cannot form a dense and uniform SEI film, which causes the active material. Premature side reactions with the electrolyte will cause the electrolyte to be consumed too quickly and affect the circulation.
When designing the battery cell, if one pole is confirmed to choose a material with poor cycle performance, the other pole does not need to choose a material with better cycle performance to avoid waste.
Positive and negative electrode compaction: The positive and negative electrode compaction is too high, although it can increase the energy density of the battery, but it will also reduce the cycle performance of the material to a certain extent. From a theoretical analysis, the greater 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 to ensure that the cells with higher positive and negative electrodes are more compact. High liquid retention capacity, and liquid retention capacity is the basis for the battery cell to complete the normal cycle or more cycles.
Moisture: Excessive moisture will cause side reactions with the positive and negative active materials, destroy its structure and affect circulation. At the same time, too much moisture is not conducive to the formation of SEI film. But while the trace amount of water is difficult to remove, the trace amount of water can also guarantee the performance of the battery to a certain extent.
Coating film density: It is almost impossible to consider the influence of film density on the cycle of a single variable. Inconsistent film density will either bring about a difference in capacity, or a difference in the number of battery core windings or laminated layers. For batteries of the same model, capacity, and material, reducing the film density is equivalent to increasing the number of one or more winding or lamination layers, and the correspondingly increased diaphragm can absorb more electrolyte to ensure circulation.
Excessive negative electrode: In addition to the influence of the first irreversible capacity and the deviation of the coating film density, the reason for the excessive negative electrode is also a consideration on the cycle performance. For lithium cobalt oxide plus graphite systems, it is more common for negative graphite to become the "short board" in the cycle. If the negative electrode is excessively insufficient, the battery cell may not release 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, thereby causing the capacity to be excessive. Drop early.
Electrolyte volume: There are three main reasons for the lack of electrolyte volume to affect the cycle. One is insufficient injection volume, and the other is that although the injection volume is sufficient, the aging time is not enough or the positive and negative electrodes are not immersed due to high compaction. Sufficiently, the third is that the electrolyte inside the battery cell is consumed with the circulation.
The objective conditions of the test: charge-discharge rate, cut-off voltage, charge cut-off current during the test, 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 cell, etc. Factors will more or less affect the cycle performance test results. In addition, different materials have different sensitivity to the above objective factors. A unified test standard and an understanding of common and important material characteristics should be enough for daily work.
Summary: Just like the wooden barrel principle, among the many factors that affect the cycle performance of the battery, the final decisive factor is the shortest among many factors. At the same time, these influencing factors also have interactive effects. Under the same material and manufacturing capacity, the higher the cycle, the lower the energy density. The most important task is to find the integration point that just meets the needs of customers and try to ensure the consistency of the battery cells. .