Explain the aging mechanism of lithium iron phosphate battery in detail

1. Putting the lithium iron phosphate battery at high temperature or room temperature for a period of time can ensure that the electrolyte can sufficiently infiltrate the pole pieces, which is conducive to the stability of the performance of the lithium iron phosphate battery; 2. After the lithium iron phosphate battery undergoes a pre-formation process The graphite negative electrode inside the lithium iron phosphate battery will form a certain amount of SEI film, but the film structure is tight and the pores are small. The aging of the lithium iron phosphate battery at high temperature will help the SEI structure reorganization to form a loose porous film . 3. After the formation, the voltage of the lithium iron phosphate battery is in an unstable stage. After the active material in the positive and negative electrode materials is aged, it can promote the acceleration of some side-uses, such as gas production, electrolyte analysis, etc., so that the lithium iron phosphate The electrochemical performance of lithium batteries quickly stabilized. 4. Eliminate unqualified lithium iron phosphate batteries with serious self-discharge, which is convenient for screening lithium iron phosphate batteries with high consistency. Among them, the aging process to screen internal micro-short circuit cells is an important purpose. The open circuit voltage of the lithium iron phosphate battery will drop during storage, but the amplitude will not be very large. If the open circuit voltage drops too fast or the amplitude is too large, it is an abnormal phenomenon. The self-discharge of lithium iron phosphate batteries can be divided into physical self-discharge and chemical self-discharge according to different reaction types. Considering the influence of self-discharge on the lithium iron phosphate battery, self-discharge can be divided into two types: self-discharge in which loss of capacity can be reversibly compensated and self-discharge in which permanent capacity loss is lost. Generally speaking, the energy loss caused by physical self-discharge is recoverable, while the energy loss caused by chemical self-discharge is basically irreversible. The self-discharge of the lithium iron phosphate battery comes from two aspects: (1) The self-discharge caused by the chemical system itself; this part is mainly caused by the side reaction inside the lithium iron phosphate battery, including the changes in the surface film of the positive and negative materials in detail. The potential change caused by the thermodynamic instability of the electrode; the dissolution and precipitation of metal impurities; (2) The internal micro-short circuit of the lithium iron phosphate battery caused by the separator between the positive and negative electrodes leads to the self-discharge of the lithium iron phosphate battery. When the lithium iron phosphate battery is aging, the change of K value (voltage drop) is the formation and stabilization process of the SEI film on the surface of the electrode material. If the voltage drop is too large, it means that there is a micro short circuit inside, which can determine the phosphoric acid Lithium iron batteries are unqualified products. The K value is a physical quantity used to describe the self-discharge rate of the cell. Its calculation method is the open circuit voltage difference of two detections divided by the time interval between two voltage detections △t, the formula is: Ku003d(OCV2-OCV1)/△t . Particles or trace metal residues on the pole piece, tiny defects on the diaphragm, and dust introduced during the assembly process of the cell, etc., will cause the internal micro short circuit of the cell. Regarding the micro-short circuit cell, the screening cannot be completed only by the capacity and the primary voltage, so K value detection must be introduced: by accurately calculating the voltage drop rate to determine whether the cell has a micro-short circuit, as shown in Figure 1. There are two basic principles for the internal short circuit of a lithium iron phosphate battery caused by metal foreign bodies, as shown in Figure 2. Larger metal particles pierce the diaphragm straightly, causing a short circuit between the positive and negative electrodes, which is a physical short circuit. In addition, when metal foreign matter is mixed into the positive electrode, the potential of the positive electrode rises after charging. The metal foreign matter dissolves at high potential and diffuses through the electrolyte. Then the metal dissolved at the low potential of the negative electrode precipitates and accumulates on the surface of the negative electrode. Short circuit, which is a chemical dissolution short circuit. Fe, Cu, Zn, Al, Sn, SUS, etc. are the most common metal foreign objects on the site of lithium iron phosphate battery factory. Faced with such complicated metal foreign bodies, measures are often taken at the manufacturing site to prevent foreign bodies from being mixed into lithium iron phosphate battery products, as shown in Figure 3. For example, the electrode slurry uses electromagnetic iron removal equipment to remove metal impurities such as Fe, the pole piece slitting or die-cutting process uses a brush to remove the cutting burrs, and the pole ears or coating edges are taped to protect the process (welding ) Use a dust collector to absorb foreign matter, etc. In the process test, the lithium iron phosphate battery detects the internal short circuit unqualified products through the withstand voltage detection before the liquid injection; the aging process detects the unqualified products through the voltage drop of the lithium iron phosphate battery ΔV. The voltage drop K value is a function of time t, state of charge and temperature T. Therefore, the aging process must have three process parameters: (1) the charge state of the aging lithium iron phosphate battery, (2) the aging storage temperature, and (3) the aging time. Under certain temperature conditions, the relationship between K and time is shown in Figure 4. When the temperature is pressed, K decreases with the extension of the standing time. This just means that the self-discharge rate of the lithium iron phosphate battery will decrease with time, but the size of the self-discharge within a certain period of time is certain, which does not substantially improve the self-discharge. Under the condition of a certain storage time, the K value increases with the increase of temperature. As the temperature increases, the activity of the system increases, the reaction rate increases, the loss of active lithium is accelerated, and some side reactions even occur. The dissolution of metal impurities in the positive electrode and the precipitation in the negative electrode will also accelerate as the temperature rises. Because the internal micro short circuit of the lithium iron phosphate battery takes a long time to manifest. Therefore, high temperature aging can speed up the process of selecting unqualified products, saving time and processing costs. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete.