The formation process of lithium batteries is a crucial link in the production process of lithium batteries and has a vital impact on the performance of lithium batteries. Its functions are mainly reflected in the following aspects:

Activate the active substances of the battery

Formation is the first charging process of a lithium battery after liquid injection. This process can activate the active substances in the battery and activate the lithium battery. Through formation, both the positive and negative electrode materials are fully activated, and the materials themselves can normally intercalate and deintercalate lithium ions, that is, the material structure is not damaged. At the same time, the materials can be connected to the electrochemical system, and the electrolyte can wet every particle, providing a transport channel for lithium ions, allowing the lithium-ion battery to complete the transformation from "a pile of materials" to a stable "electrochemical system".

Form a solid electrolyte interface film (SEI film)

During the formation process, lithium salts undergo side reactions with the electrolyte, generating a solid electrolyte interface (SEI) film on the negative electrode side of the lithium battery. The SEI film has the properties of a solid electrolyte and is an electronic insulator, but it is an excellent conductor of Li⁺, and Li⁺ can pass through the SEI film freely. The key components of the SEI film are substances such as Li₂CO₃, LiF, LiOH, ROCO₂Li, and ROLi. It can effectively prevent the reaction between the solvent and the active material of the negative electrode, allowing lithium ions to undergo deintercalation and intercalation, thereby protecting the active material of the electrode and preventing the electrolyte from continuously undergoing irreversible chemical reactions with the active material. Ensure that lithium-ion batteries can be charged and discharged multiple times and maintain good electrochemical performance.

Affect the electrochemical performance of the battery

The quality of the SEI film is highly correlated with the formation process. The formation process directly affects the film-forming quality of the SEI film and determines the electrical performance of the battery cell. Different formation process conditions, such as formation current, formation temperature, and applied pressure, can affect the structure and properties of the SEI film, thereby influencing the battery's cycle life, initial capacity loss, rate performance, self-discharge, and safety performance.

Formation current: A high current density leads to a fast formation rate of crystal nuclei, which can result in a loose structure of the SEI film and an unstable adhesion to the negative electrode surface. On the contrary, at a low current density, the formation rate of crystal nuclei is slow, and the structure of the SEI film is denser. However, the loosely structured SEI film can impregnate more electrolyte, thereby making the ionic conductivity of the SEI film formed at high current densities greater than that of the SEI film formed at low current densities.

Formation temperature: On the one hand, temperature affects the formation rate and composition of the SEI film. On the other hand, at high temperatures, some components of the SEI film will decompose, causing the SEI film to rupture, which will further consume lithium to form a new SEI film. General experiments show that the most suitable temperature for formation is 20 to 35 degrees Celsius. However, at present, most lithium-ion battery manufacturers choose a temperature slightly higher than the optimal one (30 to 60 degrees Celsius) for formation to improve the battery's cycle performance and storage performance.

External pressure: During the formation process, gas is produced. If the gas is not promptly discharged, it will increase the transmission distance of lithium ions, enhance impedance, and reduce the battery's charging capacity. If an appropriate rolling pressure is applied during charging, it can help eliminate gas, which not only increases the battery formation capacity but also significantly improves the battery's rate and cycle performance.

Remove the residual moisture

The formation process can reduce the moisture content inside the battery through electrolytic decomposition, minimizing the adverse impact of moisture on battery performance.