Formation is a critical step in the lithium-ion battery manufacturing process, during which a passivation layer—known as the Solid Electrolyte Interphase (SEI) film—forms on the surface of the negative electrode. The quality of the SEI film directly influences electrochemical performance characteristics such as cycle life, stability, self-discharge, and safety, while also enabling the battery to meet "maintenance-free" sealing requirements. However, different formation processes yield SEI films with varying properties, leading to significant differences in battery performance.

While traditional low-current pre-charging facilitates the formation of a stable SEI film, prolonged low-current charging increases the film's impedance, thereby impairing the battery's rate discharge performance; furthermore, the extended processing time reduces production efficiency. As different lithium-ion battery chemistries require distinct formation processes, this article focuses its analysis on the lithium iron phosphate (LFP) battery system.

For Lithium Iron Phosphate (LFP) systems, the formation process typically involves the following parameters: a charging current of 0.05C–0.2C, a cut-off voltage of 3.6–3.7V, and a charge cut-off current of 0.025C–0.05C; following a rest period (10–20 min), the cell is discharged at 0.1–0.2C to 2.5V, followed by another rest period (20–60 min). Variations in charging current influence the formation and quality of the Solid Electrolyte Interphase (SEI), while the rest duration and charge cut-off current affect the total time required for the formation process.

Selecting an appropriate cut-off voltage is crucial for the formation process of LFP batteries. From a material crystal structure perspective, charging voltages exceeding 3.7V can damage the LFP crystal lattice, thereby impairing the battery's cycle performance. Internal resistance measurements and SEM observations of the electrode plates support the following conclusions:

1. Moderately lowering the formation voltage and reducing the formation time can effectively minimize lithium plating on the anode surface, resulting in a smoother anode electrode. High formation voltages accelerate internal gas generation; if gas cannot be vented promptly, it accumulates on the separator surface, disrupting the contact interface between the separator and the anode. During lithium-ion intercalation and de-intercalation, this uneven contact leads to localized excessive lithium intercalation, causing surface roughness and ultimately degrading battery performance.

2. Internal resistance testing of formed cells reveals that moderately lowering the formation voltage and reducing the formation time can decrease internal resistance. The high internal resistance associated with high formation voltages is linked to anode surface roughness and the formation of white spots; these spots consist of lithium compounds with poor conductivity, thereby increasing the battery's internal resistance.

3. In the design of the formation process, moderately lowering the formation voltage can enhance the initial charge/discharge capacity and improve cycle performance. Excessively high formation voltages tend to cause the deposition of lithium and lithium compounds on the anode surface, increasing irreversible capacity and inevitably reducing overall capacity. The presence of lithium and these compounds accelerates capacity fade during charge/discharge cycling, thereby shortening the battery's cycle life.