What is battery cycling? What affects the cycle time of lithium-ion batteries?

Over the course of a lithium-ion battery's life, its actual usable capacity gradually decreases compared to its factory rated capacity, a phenomenon known as capacity degradation. Any undesirable side effect that causes an imbalance of lithium ions can lead to a decline in battery performance,Battery recycling and this change is irreversible and accumulates over many cycles.

There are a number of factors that affect the cycle time of lithium-ion batteries. Charging and discharging a battery once is called a cycle, and cycle life is an important indicator of battery performance.cylindrical cell assembly machine The root cause of the cycle life of lithium-ion batteries is a reduction in the number of lithium ions involved in energy transfer.

In some cases, the lithium content of the battery is not reduced, but rather the "activated" lithium ions are trapped in places or the transport channels are blocked, and are unable to freely participate in the charging and discharging process. Therefore, several factors affect the cycle time of lithium-ion batteries, including:

1. lithium metal plating: for lithium cobaltate and graphite systems, the anode graphite becomes the "short plate" side of the battery cycle process is more common. If the negative electrode is insufficient,battery making machine the battery may not be plated with lithium prior to cycling, but after hundreds of cycles, the anode structure is not changed much, but the negative electrode structure is severely damaged and is unable to fully receive the lithium ions provided by the positive electrode, resulting in a premature drop in capacity. Plating of lithium metal usually occurs on the surface of the negative electrode. When lithium ions migrate to the surface of the negative electrode, part of the lithium ions do not enter the negative electrode active material to form a stable compound, but instead gain electrons and deposit on the surface of the negative electrode to become lithium metal, which no longer participates in the subsequent battery cycling process, leading to a decrease in capacity. For example, when overcharging or insufficient anode material, the negative electrode cannot accommodate lithium ions migrating from the positive electrode, resulting in plating of lithium metal; in high-speed charging, too many lithium ions reach the negative electrode in a short period of time, resulting in channel blockage and plating.

2. Decomposition of cathode material: lithium metal oxide of cathode material will be decomposed continuously during long-term use, generating some electrochemical inert substances and some combustible gases, which will disrupt the capacity balance between electrodes and lead to irreversible loss of capacity.

3. SEI film on the electrode surface: in the first cycle, the electrolyte will form a solid electrolyte (SEI) film on the electrode surface, which will consume lithium ions in the process of formation, and the SEI film is unstable. During battery cycling, it will continue to fracture, exposing a new negative electrode surface and reacting with the electrolyte to form a new SEI film. This leads to a continuous loss of lithium ions and electrolyte, resulting in a decrease in battery capacity. In addition, the SEI membrane lithium ion diffusion channel may be blocked, which will also lead to a reduction in battery capacity.

4. Electrolyte Loss: During the continuous process of battery cycle, the battery electrolyte will continue to decompose and evaporate, resulting in a decrease in the total amount of electrolyte, which cannot fully penetrate into the anode and cathode materials. And the charging and discharging reactions are incomplete, leading to a decrease in the actual use capacity. In addition, if there is a certain amount of water in the electrolyte, the water will react with LiFP6 to produce LiF and HF, which in turn will damage the SEI membrane and produce more LiF, leading to the deposition of LiF and the continuous depletion of active lithium ions, resulting in a reduction of the battery cycle life.

5. Diaphragm blockage or damage: In the process of lithium-ion battery cycling, the gradual drying and failure of the diaphragm is also one of the reasons for the capacity decline. Due to the drying of the diaphragm, the ohmic resistance of the battery increases, leading to blockage of the charge and discharge channels and incomplete charging and discharging. The capacity of the battery cannot be restored to the initial state, greatly reducing the capacity and service life of the battery.

6. Dislodging of cathode and anode materials: The active substances of anode and cathode are fixed on the substrate by adhesive. In the process of long-term use, due to the failure of the binder and the battery due to mechanical vibration and other reasons, the active substances of the positive and negative electrodes are constantly shed into the electrolyte, resulting in a continuous reduction of the active substances that can participate in the electrochemical reaction, and a continuous decrease in the cycle life of the battery.

7. Material type: when you choose a material with poor cycle performance, even if the process is reasonable and perfect, the battery cycle is bound to be unable to be guaranteed; but when you choose a good material, even if there are some problems in the future, the cycle performance will not be too bad. From the material's point of view, the cycle performance of the battery depends on the worse of the two, i.e., the matching of the cycle performance of the positive electrode and the electrolyte, or the matching of the battery cycle performance of the negative electrode and the electrolyte. The poorer cycling performance of the material may, on the one hand, be due to the continuation of the completion of lithium embedding in the process of too rapid a change in the crystal structure; on the other hand, it may be due to the inability of the active substance and its corresponding electrolyte to produce a dense and homogeneous SEI film, and the premature side effects of the active substance and the electrolyte, which overconsumed the electrolyte, thus affecting the battery cycling. In battery design, if it is confirmed that a material with poorer cycling performance is selected for one pole, it is not necessary to select a material with better cycling performance for the other pole.

8. Positive and negative electrode compaction: If the positive and negative electrodes are too highly compacted, although the energy density of the battery can be increased, the cycling performance of the material will also be reduced to a certain extent. Theoretically, the greater the compaction, the greater the damage to the material structure, which is the basis for ensuring the battery cycle. In addition, batteries with higher compaction of positive and negative electrodes can hardly guarantee higher liquid retention rate, which is the basis for batteries to complete normal cycles or more cycles.

9. Coated Film Density: Considering the effect of film density on battery cycling, it is nearly impossible to use a single variable. Inconsistent film densities result in differences in the capacity of the cell or the number of windings or laminations. For cells of the same type, capacity, and material, decreasing film density is equivalent to increasing the number of windings or laminations by one or more layers, and the corresponding increase in diaphragm can absorb more electrolyte to ensure cell cycling. Considering that higher film densities improve the cell's multiplier performance, it will also be easier to bake poles and bare cells in addition to baking them in water. Of course, when the density of the film is too thin, coating errors may be more difficult to control, and larger particles in the active substance may also negatively affect the coating, and rolling. More layers mean more foils and diaphragms, which in turn means higher costs and lower energy density. Therefore, the assessment also needs to be balanced.

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