Cycle life directly affects the service life and quality of lithium-ion batteries. The better the cycle performance, the longer the battery life, which will not only reduce the user’s cost of use, but also, the longer cycle life means less resource consumption.
What are the factors that affect the cycle performance of lithium-ion batteries?
1. Types of materials
The selection 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 the more perfect they are made, the cycle of the battery cells will inevitably not be guaranteed; if better materials are selected, even if the follow-up Make some problems, and the cycle performance may not be too bad.
From a material point of view, the cycle performance of a full battery is determined by the worse 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 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 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. The side reaction occurs prematurely and the electrolyte is consumed too quickly and affects the circulation. During cell design, if one pole confirms to select a material with poor cycle performance, the other pole does not need to select a material with better cycle performance.
2. Positive and negative electrode compaction
Excessive compaction of the positive and negative electrodes can increase the energy density of the cell, 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 structural damage to the material, while the 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 cell with high positive and negative compaction to ensure a high liquid retention, and the liquid retention is the normal cycle or more of the cell. The basis of the cycle.
3. 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 excess negative electrode is also a consideration for the cycle performance. For the lithium cobalt oxide plus graphite system, the negative electrode graphite becomes a “short board” in the cycle process. “One side is more common. If the negative electrode is insufficient, the cell may not precipitate lithium before the cycle, but after hundreds of cycles, the structure of the positive electrode changes little, but the structure of the negative electrode is severely damaged and cannot fully receive the lithium ions provided by the positive electrode. Lithium, causing a premature drop in capacity.
4. The amount of electrolyte
There are three main reasons why the insufficient amount of electrolyte affects the circulation. One is the insufficient amount of liquid injection. The second is that although the amount of liquid injection is sufficient, the aging time is not enough or the immersion liquid of the positive and negative electrodes is insufficient due to excessive compaction. The electrolyte inside the cell is consumed with the cycle. Whether it is a battery that has been cycled hundreds or even thousands of times or a battery that has been diving for dozens of times, if the electrolyte is sufficient before the cycle and the electrolyte has been consumed after the cycle, it is possible to increase the amount of electrolyte retained. improve its cycle performance to a certain extent.
5. Coating film density
It is almost an impossible task to consider the influence of film density on the cycle by a single variable. Inconsistent film densities will either lead to differences in capacity, or differences in the number of cell windings or lamination layers. For the cell, reducing the membrane density is equivalent to increasing the number of one or more winding or lamination layers, and the correspondingly increased separator can absorb more electrolyte to ensure circulation.
Excessive water will cause side reactions with the positive and negative active materials, destroy their structure and affect the circulation. At the same time, too much water is not conducive to the formation of the SEI film. However, while the trace water is difficult to remove, the trace water also The performance of the battery can be guaranteed to a certain extent.
7. Objective conditions of the test
External factors such as charge-discharge rate, cut-off voltage, charge-off current, over-charge and over-discharge during the test, temperature of the test room, sudden interruption during the test, contact resistance between the test point and the cell, etc. In addition, different materials have different sensitivity to the above objective factors, so it should be sufficient for daily work to unify the test standards and understand the characteristics of common and important materials.