Formation is a key link of lithium battery. It will form a protective film on the negative pole of the battery, which is called SEI film. The quality of SEI film is directly related to the electrochemical characteristics of the battery, such as cycle life, stability, self discharge, safety, so as to achieve the goal of "maintenance free".

The traditional low current pre charging mode is conducive to the formation of SEI films, but long-term low current charging will increase the impedance of the films, thereby reducing their magnification, and will also reduce the working efficiency of the battery. Different preparation methods were used for different lithium ion battery systems.

Typically, lithium iron phosphate system with the following steps is selected:

The charging current is 0.05 C~0.2 C, the turn off voltage is 3.6~3.7 V, the power supply is cut off 0.025~0.05 C, after 10~20 minutes of storage, the discharge is 0.1~0.2 ℃~2.5 V, and the storage time is 20~60 minutes. Under different charging and discharging mechanisms, different charging and discharging currents have a certain impact on the formation and quality of SEI, while the static time and charging turn off current have a certain impact on the conversion process of the battery.

In the battery process of lithium iron phosphate system, appropriate turn off voltage should be selected. In terms of crystal structure, if it exceeds 3.7 V, the lattice structure will be damaged to some extent, and then its cycle characteristics will be affected. Part of the internal resistance experiment and the observation of the electrode slice SEM also confirmed that the following conclusions are correct:

1. By appropriately reducing the formation voltage and the formation time, lithium evolution on the negative electrode surface can be effectively reduced, thereby obtaining a smoother negative electrode sheet. This is due to the rapid generation of gas in the battery under high switching voltage, which causes the gas in the battery to not be discharged to the separator in time, thus affecting the contact balance between the film and the cathode. During de embedding, due to the contact imbalance between electrodes, lithium ions in some areas are embedded too much, resulting in uneven electrode surface, which leads to the decline of battery performance.

2. The internal resistance of the battery after formation is tested. The results show that the internal resistance of the battery can be reduced by appropriately reducing the formation voltage and shortening the formation time. The high internal resistance caused by high formation voltage is also related to the uneven electrode surface and white spots. This is because the white spots are made of lithium compounds, and the difference in their conductivity leads to the high internal resistance of the battery.

3. In the chemical process design, appropriately reducing the chemical formation voltage can improve the initial charge and discharge capacity of the battery and improve the cycle characteristics of the battery. Excessive formation voltage will easily cause lithium and lithium compounds to deposit on the negative electrode, increasing the irreversible capacity, which will have a certain impact on the capacity of the battery.

Shenzhen Topband Battery Co., Ltd.，established in 2006, is a wholly-owned subsidiary of Shenzhen Topband Co., Ltd. (002139.SZ). We focus on lithium iron phosphate batteries,with independent research and development in cells and BMS. With the ability to integrate the entire industry chain, we are committed to providing customers with one-stop lithium battery solutions and services. Our quality products are widely used in energy storage and light power.