Analysis of factors affecting the cycle life of lithium ion batteries
Source: China new energy network china-nengyuan.com
With the development of science and technology and the mature of technology, the application of lithium ion battery is becoming more and more extensive. Lithium ion battery has the advantages of high monomer voltage, relatively light weight and friendly environment. But after several cycles of charging and discharging cycle, the performance of battery will decrease. The faster the battery capacity attenuates under the same condition, the quality of the battery is relatively poor. The cycling performance of lithium ion batteries is an important indicator of its quality, and many of the standards for lithium ion batteries have a cycle life.
The charging and discharging cycle of lithium ion battery is a complex physical and chemical reaction process, and the influence factors of its cycle life are many aspects. On the one hand, it is related to the characteristics of the battery itself, such as design, manufacturing process and material performance degradation. On the other hand, the battery is affected by external influences, such as the use environment and charging and discharging system. The factors that affect the cycle life of the lithium ion battery are analyzed.
A brief introduction to the structure and principle of 1 lithium ion batteries
Li ion battery is mainly composed of positive and negative pole materials, electrolyte, separator, collector and battery case. The positive and negative pole materials consist of two different lithium ion intercalation compounds. When charging, the lithium ion is inlaid from the positive pole to embed the negative electrode through the electrolyte through the electrolyte, and the discharge is opposite. In the first charge and discharge process of lithium ion batteries, a layer of passivation film can be formed on the interface between the anode and the electrolyte. It plays a role in the diaphragm between the electrode and the electrolyte, is an excellent conductor insulator is electronic lithium ion, lithium ions can pass through the passivation layer free insertion and extraction, has the characteristics of solid electrolyte, so the passivation film called "electrolyte" (solidel ectrolyte interface), referred to as SEI film. The charge and discharge electrode reaction of the lithium ion battery is as follows:
2 Influence of design and manufacturing process
In the process of battery design, the selection of materials is the most important factor. The performance characteristics of different materials are different, and the performance of the battery is also disparities. The cycle performance of the positive and negative material matching is good, and the cycle life of the battery is long. In terms of ingredients, attention should be paid to the amount of positive and negative materials. Generally speaking, during the assembly process, the negative capacity is usually more than that of the cathode. If there is no excess, the lithium will be precipitated in the negative electrode during the charging process, thus forming the lithium dendrite, which will affect the safety. The negative electrode has too much excess positive electrode, and the positive pole may excessively remove lithium, resulting in the collapse of the structure.
The electrolyte is also a very important factor in the effect of the reversible capacity of the battery. The process of stripping and intercalation of lithium ion is always a process of interaction with electrolyte. This interaction has an important influence on the interface state and internal structure of electrode materials. In the process of interaction with positive and negative pole materials, the electrolyte will lose. In addition, when the battery is formed to form SEI film and precharge, it will also consume part of the electrolyte. Therefore, the type of electrolyte and the volume of liquid injection also affect the battery life.
The manufacturing process of lithium ion batteries mainly includes positive and negative ingredients, coating, filmmaking, winding, shell entry, injection, sealing and chemical formation. In the process of battery production, the process of each step is required to be very strict. No control of any of the processes may affect the performance of the battery cycle.
In the process of positive and negative ingredients, we should pay attention to the addition of the binder, the stirring speed, the concentration of the slurry, the temperature and humidity, and ensure that the material can be dispersed evenly.
4 Influence of battery environment
The use environment of lithium ion batteries is also very important to the impact of its cycle life. Among them, the temperature of the environment is a very important factor. Low or high ambient temperature will affect the cycle life of lithium batteries.
Chen Jitao and so on studied the charge and discharge performance of C/LiCoO2 Li ion battery at - 20 C. The results show that the discharge performance of the battery becomes worse at low temperature, and the discharge capacity of 0.2C is only 77% of the normal temperature capacity, and the capacity of 1C discharge is only 4% of the capacity of 0.2C discharge. At low temperature, the time of constant pressure charging is increased, and the charging performance is also obviously deteriorated.
The main reasons for the decrease of discharge capacity of lithium ion battery at low temperature include: the electrolyte conductivity is worse, the wetting and / or permeability of the diaphragm become worse, the migration speed of lithium ion slows down, and the charge transfer rate at electrode / electrolyte interface slows down. In addition, the impedance of the SEI film will increase at low temperature, which slows the lithium ion through the interface of the electrode / electrolyte. The reason for the increase of the impedance of the SEI film is that it is easier to remove the lithium ion from the negative electrode at low temperature, and it is more difficult to embed. When charging, lithium will appear and react with the electrolyte, forming a new SEI film, which will cover the original SEI film and increase the impedance of the battery, which will lead to the decrease of battery capacity.
Li Lianxing and other lithium batteries were tested at 300 charge and discharge cycles at 60 and normal temperatures, respectively. In the initial stage, the battery showed high discharge capacity at 60 C. But with the cycle of the battery capacity decreases quickly, reduce the cycle stability, even late battery bulging phenomenon. Charge and discharge temperature of lithium ion battery recycling is not stable, high temperature lead battery electrode electrochemical polarization and increased gas production, caused by the bulging phenomenon, at the same time, the charge transfer resistance increased, reducing the ion transport dynamics performance.
At present, lithium ion batteries mostly use LiPF6 as electrolyte. Because of the impure or micro water catalyzed decomposition of conductive salts, the electrolyte contains HF. HF will react with the main components of the SEI membrane, ROLi, ROCO2Li and so on, and the LiF is deposited on the surface of the negative electrode. The SEI membrane containing LiF hinders the migration of lithium ions. At the same time, the produced high impedance material will insulate the graphite particles. With the high temperature charge and discharge, the performance of the negative electrode will gradually deteriorate and eventually lead to the failure of the battery.
The equipment used for lithium ion batteries may be tested under conditions of vibration, shock, collision and other conditions in the case of transportation or normal work. Some lithium batteries are charged and discharged when communicating with the system and receive data information at a certain frequency. The frequency of the vibration of the device may interfere with the frequency of the battery, thus causing the chip data to go wrong or triggering the protection circuit action. Under strong vibration or impact, the poles, external connections, terminals and solder joints of lithium ion batteries may break or fall off, and active substances on the battery pole will also spoil, which will affect the battery life and even dangerous situations.
Influence of charge discharge system in the 5 cycle process
The use of lithium ion battery is the process of charging and discharging, the size of charge and discharge, the choice of charge and discharge cut-off voltage and the charging and discharging system. The performance of the battery will be reduced by blindly increasing the working current of the battery, increasing the charging cut-off voltage and reducing the discharge cut-off voltage.
The charge discharge cut-off voltage of the lithium ion batteries with different electrochemical systems is different. Over charge over the charging cut-off voltage is considered to occur during the charge of lithium ion batteries. K.Maher will charge LiCoO2 battery voltage from 4.2V to 4.9V are set, and the electrode material after the experiment were analyzed by X - ray diffraction and Raman spectrum experiment, show that graphite anode and lithium cobalt oxide cathode had structural changes, by testing different cut-off electrode voltage after charging different SOC entropy curve, also also found that the electrode materials have structure change. When the lithium ion battery is overcharged, the excess lithium ions will be deposited or embedded on the negative electrode from the positive electrode. The active lithium will react with the solvent, releasing heat and increasing the battery temperature. The anode is heated and released to release the oxygen to make the electrolyte easy to decompose and produce a large amount of heat. Over discharge is formed when the discharge voltage of a lithium battery is lower than the cut-off voltage of the discharge. In the process of overdischarge, the lithium ion is excessively removed from the negative electrode, and it will be more difficult to recharge the next time. Yu Zhongbao et al. When the battery with MCMB negative pole and LiCoO2 cathode is discharged to 0V, the copper foil collector is seriously corroded. The negative SEI film is damaged, and the performance of the re formed SEI film is poor. The negative electrode impedance increases and the polarization increases. The discharge capacity and charge discharge efficiency of the battery are greatly reduced during the cycle of over discharge.
The discharge of 18650 type lithium ion battery at different ratio was studied by Li Yan and so on. The results show that the capacity attenuation of the battery increases almost proportionately with the increase of charge discharge ratio. The capacity attenuation of LiCoO2/ graphite lithium ion battery with high rate cycle is serious. Through analysis, it is concluded that the basic reason for the serious attenuation of capacity is the change of the structure of cathode material and the increase of Li+ on the surface of negative surface, resulting in the reduction of the number of diffused channels and the blockage of diffusion channels. In the case of large current discharge, it is necessary to insert and remove the positive and negative poles quickly, and the reaction speed is very fast. Tang Zhiyuan and et al. Through experimental analysis, it is considered that because the battery needs to release large capacity in a relatively short time when the battery is discharged at a large current, and the electrode reaction is fast and intense, some lithium ions are not enough to deintercalate or pass through the anode material, and the discharge process is over. In addition, the battery's polar ear may fuse under the condition of large current, and the device elements may be damaged.
6 Conclusion
The analysis shows that there are many factors that affect the cycle life of lithium ion batteries in the process of design and use. The application of lithium ion batteries is becoming more and more widely. The demand for lithium batteries is higher in quantity and quality. The cycle life directly affects the use time and quality of lithium ion batteries, so it is very necessary for the producers to study the influence factors. Only in the process of research and development and production process will affect the cycle life of all factors can be grasped, enterprises can occupy the active position in the fierce market competition. In the process of use, consumers should pay attention to the characteristics of lithium ion batteries and use the batteries correctly according to the instructions of the instructions.