The importance of cycle performance for lithium batteries need not be said. In a macro perspective, longer cycle life means less resource consumption. Therefore, the factors affecting the cycle performance of lithium-ion batteries are everyone involved in the lithium battery industry. Have to consider the issue of large-capacity lithium batteries.
1.Moisture
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. Can guarantee the performance of the battery to a certain extent.
2.Positive and negative electrode compaction
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, it is difficult for a battery with a high positive and negative electrode compaction to ensure a high liquid retention capacity, which is the number of times that the battery completes a normal cycle 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 over-charge and over-discharge 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 battery cell will all be more or less. It affects the cycle performance test results slightly. In addition, different materials have different sensitivities to the above-mentioned objective factors. Unified test standards and understanding of commonality and characteristics of important materials should be sufficient for daily work.
4.Excessive anode
The reason for the excess of the negative electrode is to consider the impact 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" in the cycle process. "One side is more common. If the negative electrode is not enough, the cell may not precipitate lithium before cycling, but the positive electrode structure changes slightly 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, resulting in premature decline in capacity of large-capacity lithium batteries.
5, coating film density
It is almost an impossible task to consider the effect of membrane density on the cycle of a single variable. Inconsistent membrane density will either bring about 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 Rate performance, pole pieces and bare cells will be easier to remove water, of course, too thin film density coating error may be more difficult to control, large particles in the active material may also affect the coating, roll Pressure has a negative impact. More layers means more foils and separators, which in turn means higher cost and lower energy density. Therefore, it is also necessary to consider the balance when evaluating.
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 more reasonable and the production is perfect. 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 material's cycle performance is poor. On the one hand, the crystal structure may change 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 form 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 is confirmed to use a material with poor cycle performance, the other pole does not need to choose a material with better cycle performance, which wastes a large-capacity lithium battery.
7.Amount of electrolyte
There are three main reasons for the insufficient electrolyte volume to affect the circulation. The first is the insufficient injection volume. The second is that although the injection volume is sufficient but the aging time is not sufficient or the positive and negative electrodes have insufficient immersion due to high compaction. As the electrolyte inside the battery cell is consumed. Third, the microcosmic performance 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 the incomplete SEI. On the one hand, the membrane cannot effectively prevent the negative electrode from reacting with the electrolyte to consume the electrolyte. On the other hand, the defective part of the SEI membrane will regenerate the SEI membrane as the cycle progresses, thereby consuming the reversible lithium source and the electrolyte. Regardless of whether the battery is circulated hundreds or even thousands of times or for dozens of cells that have been diving, if the electrolyte before the cycle is sufficient and the electrolyte has been consumed after the cycle, it may be possible to increase the electrolyte retention. To a great extent improve its cycling performance.