Factors affecting the formation process and performance of 23A battery

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

The traditional small current pre-charging method helps to form a stable SEI film, but long-term small current charging will increase the impedance of the formed SEI film, thereby affecting the rate discharge performance of lithium-ion batteries. The long process time affects production efficiency. Different lithium battery systems have different formation processes. This article analyzes the lithium iron phosphate battery system.

The formation process of lithium iron phosphate system 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), 0.1~0.2C discharge 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 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 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. Some internal resistance experiments and electrode SEM observation results also prove the correctness of the following conclusions:

1. Appropriately reducing the formation voltage and shortening the formation time can effectively reduce the generation of lithium precipitation on the negative electrode surface, thereby obtaining a negative electrode with a smoother surface. This is because when the formation voltage is high, the gas production rate inside the battery is fast, so that the gas inside the battery cannot be discharged in time, and deposits on the surface of the diaphragm, affecting the contact balance between the diaphragm and the negative electrode. In the process of lithium ion deintercalation, the lithium ions are over-embedded in some areas due to the imbalance of contact between the two, causing the surface of the negative electrode to be rough, and finally affecting the battery performance.

2. After the battery internal resistance test after formation, it was found that the internal resistance of the battery can be reduced by appropriately reducing the formation voltage and reducing the formation time. The high internal resistance caused by 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, which have poor conductivity and the internal resistance of the battery is relatively large.

3. Appropriately reducing the formation voltage in the formation process design can increase the initial charge and discharge capacity of the battery and improve the cycle performance of the battery. Excessive formation voltage can easily cause lithium and its compounds to deposit on the surface of the negative electrode, increase the irreversible capacity of the battery, and inevitably affect the capacity of the battery. Due to the presence of lithium and its compounds, the capacity of the battery decays faster and faster during the charge and discharge cycle, affecting the battery cycle life.

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