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Formation is an important process in the production process of lithium batteries. During formation, a passivation layer is formed on the surface of the negative electrode, that is, the solid electrolyte interface facial mask (SEI film). The quality of the SEI film affects the electrochemical performance of the battery, such as the cycle life, stability, self discharge, safety, etc., which meets the requirements of "maintenance free" secondary battery sealing. Different formation processes form different SEI films, The impact on battery performance is also very different.

The traditional low current pre charging method is conducive to the formation of stable SEI film, but the low current charging for a long time will lead to the increase of the impedance of the formed SEI film, which will affect the rate discharge performance of the lithium ion battery. The long process time will affect the production efficiency. Different lithium battery systems have different formation processes. This paper analyzes the lithium iron phosphate battery system.

The formation process of lithium iron phosphate system is generally selected as follows:

The charging current is 0.05C~0.2C, the cut-off voltage is 3.6~3.7V, and the charging cut-off current is 0.025C~0.05C. After standing for a period of time (10-20min), 0.1~0.2C is discharged to 2.5V, and standing for a period of time (20-60min). Under different charging and discharging motor systems, the different charging current affects the formation and quality of SEI, and the standing time and charging cut-off current affect the formation process time of the battery.

The battery formation process of lithium iron phosphate system needs to select an appropriate cut-off voltage. From the crystal structure of the material, when the charging voltage is greater than 3.7V, the lattice structure of lithium iron phosphate may be damaged, thus affecting the cycle performance of the battery. Some internal resistance experiments and electrode slice SEM observation results also prove that the following conclusions are correct:

1. Properly reducing the formation voltage and time can effectively reduce the generation of lithium precipitation on the negative electrode surface, so that a more smooth surface of the negative electrode can be obtained. This is because when the formation voltage is high, the internal gas production rate of the battery is fast, so that the internal gas of the battery cannot be discharged in time and deposited on the surface of the diaphragm, affecting the contact balance between the diaphragm and the negative electrode. In the process of lithium ion deinterlacing, due to the influence of the contact imbalance between the two, lithium ions are excessively embedded in some areas, resulting in the unsmooth surface of the negative electrode, and finally affecting the battery performance.

2. Through the internal resistance test of the battery after formation, it is found that the internal resistance of the battery can be reduced by appropriately reducing the formation voltage and the formation time. The high internal resistance caused by the high formation voltage is also related to the unsmooth surface of the negative electrode and the formation of white spots. Because the white spots are lithium compounds with poor conductivity, the internal resistance of the battery is too large.

3. Properly reducing the formation voltage in the formation process design can improve the first charge and discharge capacity of the battery and improve the cycle performance of the battery. Excessive formation voltage is easy to cause lithium and its compounds to deposit on the negative electrode surface, which increases the irreversible capacity of the battery, and will inevitably affect the capacity of the battery. Due to the existence of lithium and its compounds, the capacity of the battery will decline faster and faster in the charge discharge cycle, affecting the cycle life of the battery.

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