Why does the maximum capacity of lithium batteries decrease as the number of uses increases?

It will be divided into internal and external factors: 1. Internal factors (1) In terms of electrodes, repeated charging and discharging reduce the active surface area of u200bu200bthe electrode, increase the current density, and increase the polarization; the structure of the active material changes; the electrical contact of the active particles changes. The electrode material (including the current collector) is corroded; the current commonly used battery negative electrode is graphite, the positive electrode is LiCoO2, LiFePO4 and LiMn2O4, etc. The electrolyte will form a layer of SEI (solid electrolyte) film on the electrode surface at the beginning of the battery discharge. , Its components are mainly ROCO2Li (EC and PC cyclic carbonate reduction products), ROCO2Li and ROLi (DEC and DMC and other chain carbonate reduction products), Li2CO3 (residual water and ROCO2Li reaction product), if LiPF6 is used, The remaining HF will be combined with the ROCO2Li in the SEI, so that LiF and ROLi are the most important components in the SEI. SEI is a Li+ conductor. The volume of the carbon electrode changes very little when lithium is released. However, even if it is small, the internal stress will cause the negative electrode to be broken, and the new carbon surface will be exposed and react with the solvent to form a new SEI film. This causes the loss of lithium ions and electrolyte. At the same time, the expansion of the active material of the positive electrode material exceeds a certain level, which will also cause permanent structural contact loss that cannot be repaired. In this way, the continuous loss of the positive electrode and the negative electrode causes the continuous decline of the capacity; The increased SEI film will cause the resistance layer frame of the interface, increase the polarization potential of the electrochemical reaction, and cause the battery performance to decline in the electrode. With the progress of the charge and discharge reaction, the performance of the binder will gradually decrease, and the bonding strength Reduce, make the electrode material fall off; copper foil and aluminum foil are commonly used negative and positive current collectors, both of which are prone to corrosion, the corrosion products are concentrated on the surface of the current collector to form a film, adding internal resistance, copper ions can also form dendrites, Penetrate the diaphragm and make the battery fail. (2) In terms of electrolyte solution, the analysis of electrolyte or conductive salt causes its conductivity to decrease, and the analyte causes interface passivation; lithium battery liquid electrolyte is generally composed of solutes (such as LiPF6, LiBF4, LiClO4 and other lithium salts), solvents and special additives constitute. The electrolyte has good ionic conductivity and electronic insulation, and plays the role of transporting ions to conduct current between the positive and negative electrodes. During the first charge and discharge, overcharge and overdischarge of lithium batteries, and after long-term cycles, the electrolyte will be degraded and accompanied by the appearance of gas. The composition of the gas is more complicated and cannot be eliminated in the battery through a certain reaction. With the increase of battery charge and discharge times. Due to the oxidative corrosion of the electrode material, a part of the electrolyte will be consumed, resulting in a lack of electrolyte, and the pole pieces cannot be completely moisturized to the electrolyte, resulting in incomplete electrochemical reaction, making the battery capacity less than the design requirements. (3) The diaphragm is blocked or damaged, and the battery is short-circuited. The purpose of the diaphragm is to separate the positive and negative electrodes of the battery to prevent direct short-circuiting of the two poles. During the cycle of lithium batteries, the gradual drying up of the separator and failure is an important reason for the early performance decline of the battery. This is mainly due to the increase in the resistance of the solution due to the drying of the electrolyte in the diaphragm, the deterioration of the electrochemical stability and mechanical properties of the diaphragm, and the deterioration of the electrolyte wettability during repeated charging. Due to the drying of the diaphragm, the ohmic internal resistance of the battery increases, resulting in incomplete discharge. The battery is repeatedly overcharged with large capacity, and the battery capacity cannot be restored to the initial state, which greatly reduces the discharge capacity and service life of the battery. 2. External factors (1) Rapid charge and discharge during rapid charge, the current density is too large, the negative electrode is severely polarized, and the deposition of lithium will be more distinct, making the copper foil at the boundary between the copper foil and the carbon-based active material brittle and easy Cracks appear. The spontaneous winding of the battery core is limited by the fixed space, and the copper foil cannot be stretched freely and there is pressure. Under the application of pressure, the original cracks spread and grow, and the copper foil breaks due to insufficient expansion space. (2) When the temperature is clearly higher than room temperature, the thermal stability of the organic electrolyte becomes the first issue to be considered. This all includes the thermal stability of the organic electrolyte itself and the thermal stability of the electrode and the electro-mechanical electrolyte. . It is generally believed that the impact of the positive electrode/organic electrolyte reaction on the safety of the kern ion battery is an important factor. Because the reaction kinetics of the positive electrode and the electrolyte are very fast, it controls the heat resistance of the entire battery. If the ambient temperature of the battery is sufficient to cause the positive electrode electrolyte to react, it will cause the battery to become thermally out of control, and even catch fire or explode. The magnitude of the discharge current directly affects the discharge capacity of the lithium battery. During high-current discharge, there is not only a serious electrolyte interface polarization, but also the active body, that is, the intercalation of ions and the diffusion polarization in the electrode. (3) Long-term deep charge and discharge point discharge should be transferred. From the internal structure, one will cause excessive volatilization of the electrolyte, and second, the excessive reaction of the negative electrode of the lithium battery will cause the dielectric film to change, resulting in a decrease in the deintercalation capacity and the formation of capacity Permanent loss; charging is critical from voltage stability and the rise of the grid voltage in the middle of the night. The charger that has stopped charging will continue to charge after the voltage rises, causing the battery to overcharge and lead to the change in the structure of the positive electrode material. The capacity loss is analyzed. Oxygen reacts violently with electrolyte and burns and explodes; electrolyte organic solvent/electrolyte lithium salt analysis; negative electrode lithium evolution over discharge may cause the negative electrode copper collector to dissolve and the positive electrode to form copper dendrites. (4) Vibration and collision are generally the reasons for the process, such as whether the welding is firm or not, whether the internal circuit is designed properly, and so on. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous post: What are the different technical characteristics of square, cylindrical and soft packages