What Is Battery Formation?

A Complete Guide to the Lithium-Ion Battery Formation Process

Lithium-ion battery formation is a critical step in the post-assembly stage, occurring after the cell has been filled with electrolyte and sealed. Essentially, it involves subjecting the fresh cell to an initial charge-discharge activation process, marking the core transition from a physically assembled unit to a functional battery with electrochemical properties. Although the internal electrodes and electrolyte are in place after assembly, they are not yet truly "activated."

Formation represents the initialization of the lithium-ion cell-an energy conversion process that activates the cell's active materials. It is a highly complex procedure that significantly impacts battery performance. During the initial charge, as lithium ions intercalate into the graphite, electrochemical reactions occur within the cell. Inevitably, a thin passivation layer forms at the interface between the carbon anode and the electrolyte; this layer is known as the Solid Electrolyte Interphase, or SEI film.

The formation of the SEI film has two main consequences: on one hand, it consumes a portion of the battery's limited lithium ions, necessitating the use of extra lithium-containing cathode material to compensate for this initial loss; on the other hand, it increases resistance at the electrode-electrolyte interface, resulting in a certain degree of voltage hysteresis.

Formation is the process of activating the internal positive and negative electrode materials through specific charge-discharge cycles following battery assembly. This step improves the battery's overall performance, including charge-discharge characteristics, self-discharge rates, and storage stability; a battery can only demonstrate its true performance capabilities after undergoing formation.

The primary objective of formation is to generate a Solid Electrolyte Interphase (SEI) film on the surface of the graphite anode. During the initial charge, electrolyte components undergo reductive decomposition, depositing a nanoscale passivation film onto the anode surface. An ideal SEI film possesses two key characteristics: it allows lithium ions to pass freely for intercalation and de-intercalation while blocking the passage of electrons, thereby preventing continuous side reactions between the electrolyte solvent and the anode. If the SEI (Solid Electrolyte Interphase) film is sparse, damaged, or uneven, the electrolyte will be continuously consumed, leading to rapid cell capacity degradation; in severe cases, it can trigger lithium dendrite formation, posing a safety risk of short circuits.

In addition to forming the SEI film, the formation process serves other functions:

1. Activating electrode active materials: It facilitates the initial intercalation of lithium ions into the anode's graphite lattice and the redox activation of the cathode material, opens up lithium-ion diffusion pathways, and enables stable charge-discharge capabilities for both electrodes.
2. Consuming trace residual moisture within the cell: Moisture undergoes electrolytic decomposition, thereby reducing the risk of subsequent gas generation and corrosion.
3. Forming a CEI (Cathode Electrolyte Interphase) film: In certain systems, a CEI film also forms on the cathode surface, protecting the stability of the cathode structure.

Standard industry formation processes typically consist of stages such as pre-charging, charging, and static aging. Process parameters vary depending on the battery chemistry (e.g., NCM/NCA vs. LFP) and form factor (prismatic, pouch, or cylindrical):

1. Low-current pre-charging stage: A low current (typically 0.02–0.1C) is used to charge the cell slowly, controlling the rate of voltage rise. Low current allows the SEI film to grow slowly and uniformly, preventing the vigorous reactions and defects associated with high-current charging, and reducing the likelihood of lithium dendrite formation.

2. Multi-stage constant-current/constant-voltage (CC-CV) charging: After pre-charging, the cell is charged in stages according to process settings, gradually increasing the charge input. Upon reaching the cut-off voltage, the process switches to constant-voltage charging until the current drops to a specific threshold, completing the charge cycle. Cut-off voltages differ significantly between NCM/NCA and LFP batteries, requiring parameter adjustments tailored to the specific material system.

3. Static/Aging treatment: Following charging, the cell undergoes aging at either room temperature or elevated temperatures. Aging in a high-temperature environment (45–60°C) for a set period promotes the further restructuring and densification of the SEI film and allows internal reactions to reach equilibrium; it also accelerates the detection of potential defects, such as micro-short circuits, facilitating subsequent screening and rejection.

4. Some processes include a shallow discharge step before the cells move to the cell grading machine for capacity and internal resistance testing and sorting.

In many production settings, formation and grading are collectively referred to as "formation-grading"; however, they serve distinct functions and should not be conflated.

Formation: Focuses on electrochemical activation, the generation of the SEI (Solid Electrolyte Interphase) protective film, and the establishment of internal cell interfaces; it is essentially the process of "activating the battery."

Grading: Performed after formation, this involves a complete charge-discharge cycle to test actual capacity, internal resistance, and self-discharge rates. Cells are sorted by performance grade and defective units are discarded; it is essentially the process of "testing and screening the battery."

Formation outcomes are determined by a combination of current, temperature, voltage, pressure, and electrolyte formulation. Any deviation in these parameters directly impacts the quality of the SEI film.

1. Formation Current: The initial pre-charge current should not be excessively high. Excessive current accelerates reaction rates, leading to a loose, porous SEI film with poor stability. Conversely, excessively low current extends production time and reduces line efficiency; manufacturers must strike a balance between quality and production capacity. The industry is currently exploring new techniques, such as pulse formation, to shorten processing times while maintaining film quality.

2. Formation Temperature:  The ideal range is typically around 25°C. High temperatures accelerate electrochemical reaction rates, potentially causing abnormal SEI composition and excessive gas generation. Low temperatures result in insufficient reaction kinetics, leading to incomplete SEI film formation and significant residual side-reaction products. While some processes utilize high-temperature formation, this requires comprehensive validation and should not be adopted indiscriminately.

3. Voltage Window:  The initial charging voltage should not be arbitrarily maximized; it must be matched to the specific cathode and anode materials. Excessively high voltage triggers excessive electrolyte decomposition and severe gas generation, while insufficient voltage leads to incomplete SEI formation and inadequate activation.

4. Electrolyte Additives:  Additives are crucial for regulating SEI film composition. Appropriate additives promote the formation of a dense, stable SEI film rich in LiF, thereby improving the battery's cycle performance.

As the production capacity for power batteries continues to expand, traditional low-current, long-duration formation processes tie up significant equipment and factory space, resulting in high production costs. On one hand, the industry is continuously optimizing charging profiles and developing technologies such as pulse formation and staged rapid formation to shorten formation time; on the other, it is integrating temperature control and pressure-assisted processes to ensure SEI film quality while accelerating the process. Furthermore, leveraging big data to collect comprehensive data-including voltage, current, and temperature-throughout the formation process of each individual cell to enable predictive quality assessment represents a key direction for the development of intelligent manufacturing at this stage.

ACEY New Energy is a professional supplier of lithium-ion battery manufacturing and testing equipment, providing reliable solutions for battery cell formation, capacity testing, battery pack assembly, and performance testing. With more than 17 years of experience in the battery equipment industry, we have developed and supplied equipment and production solutions for customers in more than 40 countries.

Our formation and testing equipment is designed to support precise control of key parameters such as charging current, voltage, temperature, and process time, helping battery manufacturers achieve consistent cell performance and stable production quality. We also provide customized equipment and complete battery production line solutions based on different cell types, battery chemistries, production capacities, and process requirements.

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