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, a solid electrolyte interface film (SEI film). The quality of the SEI film will directly affect the cycle life, stability, and Electrochemical properties such as self-discharge and safety meet the requirements of "maintenance-free" sealing for secondary batteries. However, SEI films formed by different chemical formation processes are different, and the impact on battery performance is also very different.
The traditional low-current pre-charging method is conducive to the formation of a stable SEI film, but long-term low-current charging will lead to an increase in the resistance of the formed SEI film, which will affect the rate discharge performance of lithium-ion batteries, and the long process time will affect production efficiency. Different lithium battery systems have different formation processes. This paper analyzes the lithium iron phosphate(LFP) battery system as the object.

The formation process of LPF battery is usually selected as follows:

• Charging current 0.05C~0.2C, cut-off voltage 3.6~3.7V, charging cut-off current 0.025C~0.05C, after standing for a period of time (10-20min), discharge at 0.1~0.2C to 2.5V, stand for a period of time (20 -60min). Under different charging and discharging mechanisms, different charging currents affect the formation and quality of SEI, and the resting time and charging cut-off current affect the formation process time of the battery.

The battery formation process of LFP battery needs to choose a suitable cut-off voltage. From the perspective of material crystal structure, when the charging voltage is greater than 3.7V, the lattice structure of lithium iron phosphate may be damaged, thereby affecting the cycle performance of the battery.

Part of the internal resistance experiment and the pole piece SEM observation results also prove the correctness of the following conclusions:

• 1. Appropriately reducing the formation voltage and formation time can effectively reduce the generation of lithium precipitation on the surface of the negative electrode, so that a negative electrode sheet with a smoother surface can be obtained. This is because when the formation voltage is high, the rate of gas production inside the battery is so fast that the gas inside the battery cannot be discharged in time and deposits on the surface of the separator, which affects the contact balance between the separator and the negative electrode. During the deintercalation process of lithium ions, the contact imbalance between the two causes excessive intercalation of lithium ions in some areas, causing the surface of the negative electrode to be rough, and finally affecting the performance of the battery.
• 2. After 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 rough surface of the negative electrode and the formation of white spots. Because the white spots are lithium compounds and have poor conductivity, the internal resistance of the battery is relatively large.
• 3. Appropriately reducing the formation voltage in the design of the formation process can increase the initial charge and discharge capacity of the battery and improve the cycle performance of the battery. Excessive formation voltage will easily cause lithium and its compounds to deposit on the surface of the negative electrode, which will increase the irreversible capacity of the battery and will inevitably affect the capacity of the battery. Due to the presence of lithium and its compounds, the capacity of the battery will decay more and more during the charge-discharge cycle. Fast, affect the battery cycle life.

XWELL can provide the equipments for LFP battery formation:

• Cylindrical cell formation and grading machine
• Prismatic cell formation and grading machine