Seven factors affect the cycle performance of lithium-ion batteries

The battery we are most concerned about is the lithium-ion battery, because our mobile phones, pads, and laptops are powered by lithium-ion batteries, and its battery life has always been a key research direction for enterprises. The importance of cycle performance to lithium-ion batteries is self-evident. From a macro perspective, a longer cycle life means less resource consumption. Therefore, the factors that affect the cycle performance of lithium-ion batteries are issues that everyone involved in the lithium battery industry has to consider.

1. Moisture

Too much water will cause side reactions with the positive and negative active materials, destroy their structure and thus affect the cycle. At the same time, too much water is not conducive to the formation of SEI film. However, while trace amounts of water are difficult to remove, trace amounts of water can also guarantee the performance of the battery cell to a certain extent.

2. Positive and negative electrode compaction

Although excessive compaction of the positive and negative electrodes can increase the energy density of the battery cell, it will also reduce the material's cycle performance to a certain extent. From a theoretical point of view, the greater the compaction, the greater the damage to the material structure, and the material structure is the basis for ensuring that lithium-ion batteries can be recycled; in addition, batteries with high compaction of the positive and negative electrodes are difficult to ensure a high liquid retention capacity, and the liquid retention capacity is the basis for the battery cell to complete normal cycles or more cycles.

3. Objective conditions of the test

External factors such as the charge and discharge rate, cut-off voltage, charge cut-off current, overcharge and over-discharge during the test, test room temperature, sudden interruption during the test, contact internal resistance between the test point and the battery cell, etc. will more or less affect the cycle performance test results. In addition, different materials have different sensitivities to the above objective factors. Unifying the test standards and understanding the common and important material characteristics should be sufficient for daily work.

4. Excessive negative electrode

In addition to considering the impact of the first irreversible capacity and the coating film density deviation, the impact on the cycle performance is also a consideration for the reason for the excess negative electrode. For the lithium cobalt oxide plus graphite system, it is common for the negative electrode graphite to become the "short board" in the cycle process. If the negative electrode is insufficiently excessive, the battery cell may not deposit lithium before the cycle, but after hundreds of cycles, the positive electrode structure changes little, but the negative electrode structure is severely damaged and cannot fully receive the lithium ions provided by the positive electrode, thereby depositing lithium, causing the capacity to drop prematurely.

5. Coating film density

It is almost an impossible task to consider the impact of membrane density on the cycle as a single variable. Inconsistent membrane density either leads to differences in capacity or differences in the number of winding or stacking layers of the battery cell. For batteries of the same model, capacity and material, reducing the membrane density is equivalent to adding one or more layers of winding or stacking. The corresponding increased diaphragm can absorb more electrolyte to ensure circulation. Considering that a thinner membrane density can increase the rate performance of the battery cell, it will be easier to bake and remove water from the pole pieces and bare cells. Of course, a membrane density that is too thin may make it more difficult to control the error during coating, and large particles in the active material may also have a negative impact on coating and rolling. More layers means more foil and diaphragms, which means higher costs and lower energy density. Therefore, a balanced consideration is also needed during evaluation.

6. Material Type

The choice of materials is the first factor that affects the performance of lithium-ion batteries. If materials with poor cycle performance are selected, no matter how reasonable the process is or how perfect the manufacturing is, the cycle of the battery cannot be guaranteed; if better materials are selected, even if there are some problems in the subsequent manufacturing, the cycle performance may not be too bad. From the material point of view, the cycle performance of a full battery is determined by the cycle performance after the positive electrode and the electrolyte are matched, and the cycle performance after the negative electrode and the electrolyte are matched. The poor cycle performance of the material may be due to the fact that the crystal structure changes too quickly during the cycle and cannot continue to complete the lithium insertion and delithiation. On the other hand, it may be due to the fact that the active material and the corresponding electrolyte cannot form a dense and uniform SEI film, causing the active material and the electrolyte to react prematurely, causing the electrolyte to be consumed too quickly and affecting 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, which is a waste.

7. Electrolyte volume

There are three main reasons why insufficient electrolyte affects the cycle. First, the amount of liquid injection is insufficient. Second, although the amount of liquid injection is sufficient, the aging time is not enough or the positive and negative electrodes are not fully immersed due to excessive compaction. Third, the electrolyte inside the battery cell is consumed as it cycles. Third, the microscopic performance of the matching of the positive and negative electrodes, especially the negative electrode and the electrolyte, is the formation of a 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, the incomplete SEI film cannot effectively prevent the negative electrode from reacting with the electrolyte and consuming the electrolyte. On the other hand, the defective parts of the SEI film will regenerate the SEI film as the cycle progresses, thereby consuming the reversible lithium source and electrolyte. Whether it is a battery cell that has been cycled hundreds or even thousands of times or a battery cell that has been drained dozens of times, if the electrolyte is sufficient before the cycle and the electrolyte has been consumed after the cycle, increasing the amount of electrolyte is likely to improve its cycle performance to a certain extent.