From：CTECHI GROUP Limited Release time：2019-11-28
Overview：The importance of cycle performance for lithium-ion batteries is self-evident. From a macro perspective, longer cycle life means less resource consumption.
7 factors affecting the cycling performance of lithium-ion batteries
The importance of cycle performance for lithium-ion batteries is self-evident. From a macro perspective, longer cycle life means less resource consumption. Therefore, factors affecting the cycle performance of lithium-ion batteries are related to the lithium battery industry. People have to think about it.
Excessive moisture will cause side reactions with the positive and negative electrode active materials, destroy its structure and affect the circulation. At the same time, excessive moisture is not conducive to the formation of the SEI film. However, while the trace amount of water is difficult to remove, the trace amount of water is also difficult to remove. Can guarantee the performance of the battery to a certain extent.
2, positive and negative 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 cycling performance of the material to a certain extent. From the theoretical analysis, the greater the compaction, the greater the structural damage to the material, and The structure of the material is the basis for ensuring that the lithium-ion battery can be recycled. In addition, cells with higher positive and negative electrode compaction are difficult to ensure a higher liquid retention capacity, which is the number of times that the battery core completes normal cycles or more. The basis of the cycle.
3. Objective conditions for testing
The external factors such as the charge and discharge ratio during the test, the cut-off voltage, the charge cut-off current, the overcharge and overdischarge during the test, the temperature of the test room, the sudden interruption during the test, and the internal resistance of the contact point between the test point and the cell will all be more It affects the cycle performance test results slightly. 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.
The reason for the excess of the negative electrode needs to consider the effect of the first-time irreversible capacity and the coating film density deviation, as well as the impact on the cycle performance. For the lithium cobaltate plus graphite system, the negative electrode graphite becomes a "short board" during the cycle. "One side is more common. If the negative electrode is not enough, the cell may not precipitate lithium before cycling, but the structure of the positive electrode changes little after hundreds of cycles, but the negative electrode structure is severely damaged and cannot fully receive the lithium ions provided by the positive electrode. Lithium, causing premature reduction in capacity.
5. Coating film density
It is almost an impossible task to consider the influence of film density on the cycle with a single variable. Inconsistent film density will either cause a difference in capacity, or a difference in the number of cells wound or laminated layers. For batteries, reducing the film density is equivalent to adding one or more layers of winding or lamination. The corresponding increase in the separator can absorb more electrolyte to ensure circulation. Considering that a thinner film density can increase the battery The rate performance, baking and water removal of pole pieces and bare cells will also be easier. Of course, the error when coating with too thin film density may be more difficult to control, and large particles in the active material may also affect the coating and roll. Pressure has a negative impact. More layers means more foils and separators, which in turn means higher cost and lower energy density. Therefore, a balanced consideration is also needed in the evaluation.
6. Material types
The selection of materials is the first factor that affects the performance of lithium-ion batteries. Materials with poor cycle performance are selected. The process is reasonable and the production is perfect, and the cycle of the battery cannot be guaranteed; even if a better material is selected, There are some problems with the production, and the cycle performance may not be too bad. From a material point of view, the cycle performance of a full battery is the cycle performance after the positive electrode matches the electrolyte, and the cycle after the negative electrode matches the electrolyte. The performance is determined by the poorer one. The cycle performance of the material is poor. On the one hand, it may be that the crystal structure changes too quickly during the cycle to continue to complete lithium insertion and delithiation. On the one hand, it may be due to the activity. Substances and corresponding electrolytes cannot produce a dense and uniform SEI film, which causes premature side reactions between the active material and the electrolyte, which consumes the electrolyte too quickly and affects the cycle. In the design of the battery cell, if one pole confirms that a material with poor cycle performance is selected, the other pole does not need to choose a material with better cycle performance, which is wasteful.
7. Amount of electrolyte
Insufficient electrolyte volume has three main effects on circulation
I. Insufficient injection volume;
2. Although the injection volume is sufficient, the aging time is not enough or the positive and negative electrodes are not sufficiently immersed due to excessive compaction. Third, the electrolyte inside the circulating cell is consumed.
3. The microscopic performance of the matching of the positive and negative electrodes, especially the negative electrode and the electrolyte, is the formation of dense and stable SEI, while the performance seen by the right eye is both the consumption rate of the electrolyte during the cycle and an incomplete SEI film. On the one hand, it is impossible to effectively prevent the negative electrode from reacting with the electrolyte solution to consume the electrolyte solution. On the other hand, the SEI film defects will regenerate the SEI film as the cycle progresses, thereby consuming the reversible lithium source and electrolyte solution. Regardless of whether the battery is circulated hundreds or even thousands of times, or for dozens of batteries that have been diving, if the electrolyte is sufficient before the cycle and the electrolyte has been consumed after the cycle, it is likely to increase the electrolyte retention. To a greater extent its cycle performance.