Lithium-Ion Battery Formation Guide: How Pre-Charging Builds the SEI

Lithium‑ion battery formation is a core electrochemical process after cell assembly, serving as a precise interface engineering step rather than a simple first charging operation. After electrolyte filling and sufficient post‑injection wetting/resting, cells undergo controlled pre‑charging to activate electrode active materials and build a stable SEI (Solid Electrolyte Interphase) on the anode surface.

The quality of the formation process directly determines key cell indicators including first‑cycle Coulombic efficiency (ICE), internal impedance, gas generation, cell thickness consistency, rate performance and long‑term cycling stability. For battery R&D and production engineers, optimizing formation parameters is the key to reducing batch inconsistency and extending battery service life.

1. Core Definition: Battery Formation & Pre‑Charging

Battery formationrefers to the controlled initial charge or charge‑discharge process implemented after electrolyte filling and complete post‑injection wetting/resting. It is fundamentally different from conventional battery charging, focusing on regulating electrode‑electrolyte interfacial reactions.

During pre‑charging, lithium ions detach from the cathode, migrate through the electrolyte to the anode, and trigger irreversible reduction reactions of electrolyte solvents, lithium salts and additives at low anode potential. The reaction deposits form a dense passivation film on the anode surface, which is the SEI layer.

Critical industry correction: Pre‑charging and formation cannot replace electrolyte wettingIncomplete pre‑wetting will result in uneven ionic resistance and non‑uniform SEI growth, and the formation process can only reveal these defects without addressing them.

2. SEI Film: Composition, Characteristics and Qualification Standards

The SEI film is a nano‑scale passivation layer formed on the anode surface during the first lithiation process, with no fixed universal composition. Its components mainly include inorganic substances such as LiF, Li₂CO₃, Li₂O and organic lithium compounds decomposed from electrolyte additives.

The final SEI structure is jointly determined by electrolyte chemistry, electrode materials, formation current, temperature and cutoff voltage.

2.1 This Properties of High ‑QuAlibabaty SEI

A qualified SEI achieves a precise functional balance, which is the core of battery stability:

• Ionic transport: Allows Li⁺ transport through the interphase with sufficiently low resistance for normal cell operation
• Electron suppression: Strongly suppresses electron transport to limit continuous electrolyte decomposition and side reactions
• Mechanical stability: Resists volume expansion and contraction during cycling to avoid cracking and repeated regeneration
• Low impedance: Avoids excessive film thickness that increases cell internal resistance

The core goal of formation is not to form the thickest SEI, but a stable, low‑resistance, and fully passivating interfacial layer.

3. Root Cause of First‑Cycle Capacity Loss

SEI formation is a major contributor to the first‑cycle irreversible lithium loss, together with other parasitic and material‑related irreversible reactions. As a result, practical first‑cycle Coulombic efficiency is typically below 100%. Part of the active lithium is consumed in electrolyte reduction and film‑forming reactions and cannot return fully to the cathode during discharge.

There is an inevitable process trade‑off: Insufficient SEI passivation leaves ongoing electrolyte corrosion; excessive irreversible reactions lead to severe lithium loss and higher impedance. A mature formation process precisely balances this contradiction.

4. Five Core Controllable Parameters of SEI Formation

Based on CANRD’s industrial formation framework, formation current, mechanical pressure, temperature, electrolyte chemistry, cutoff voltage/SOC are five coupled core variables. Parameter optimization must be systematic, and fixed universal formulas are not applicable to all battery systems.

4.1 Formation Current: Control Reaction Polarization Rate

Formation current directly governs the electrochemical reaction rate and interfacial polarization degree. Higher formation current increases polarization and can shift the apparent film‑forming reaction window, thereby changing the resulting SEI formation behavior. Low current reduces polarization and enables closer observation of early interfacial events, but it extends processing time.

There is no standardized universal C‑rate. Engineers need to determine the optimal current via experimental verification based on cell chemistry, electrode design and electrolyte formulas. dQ/dV differential capacity analysisis an efficient tool to identify film‑forming reaction peaks and evaluate current‑induced polarization shifts.

4.2 Formation Pressure: Optimize Interfacial Uniformity

Mechanical constraint pressure is a key control parameter for pouch cell formation. Appropriate external pressure flattens electrode stacks, tightens interlayer contact, and uniformizes current distribution across the electrode surface, effectively improving SEI consistency and cell thickness stability.

Pressure follows the "optimal interval" rule rather than "higher is better". Excessive pressure may alter electrolyte distribution, reduce effective pore accessibility or deform the electrode stack, depending on cell structure and electrode design. Insufficient pressure leads to cell swelling, enlarged interlayer gaps and inconsistent interfacial reactions.

4.3 Formation Temperature: Balance Reaction Kinetics and SEI Stability

Increasing temperature generally lowers electrolyte viscosity, increases ionic conductivity and changes the transport and reaction kinetics of film‑forming species. However, temperature also changes the rates of desired and undesired interfacial reactions. Therefore, the optimum formation temperature must be determined experimentally for the specific cell chemistry and process.

Crucially, 45±3℃ is not a universal standard temperature. Different laboratory and pouch‑cell processes can adopt widely different temperature windows according to materials, electrolyte and production objectives.

4.4 Electrolyte Chemistry: Determine Fundamental SEI Properties

Electrolyte composition is the decisive factor for SEI chemical structure and performance. Solvent types, lithium salt concentration, and functional additives (VC, FEC) completely change the electrolyte reduction window and film‑forming mechanism.

Formation parameters optimized for one electrolyte system cannot be directly copied to another. Any adjustment of additives or solvent ratios requires re‑optimization of current, temperature, pressure and cutoff conditions.

4.5 Cutoff Voltage / SOC: Control Reaction Termination Degree

Formation does not need to follow conventional full‑charge cutoffs. The core standard is to complete effective SEI passivation and terminate unnecessary irreversible reactions.

The optimal formation endpoint depends on cathode/anode chemistry and electrolyte reduction characteristics. Combined with dQ/dV analysis, engineers can identify major film‑forming reaction regions and select candidate cutoff voltages or SOC levels. The endpoint should then be validated using ICE, gas generation, impedance and subsequent cycling performance.

5. Auxiliary Formation Process Controls

5.1 Vacuum‑Assisted Gas Management in Specific Formation Systems

Gas generation is an inevitable byproduct of electrolyte reduction during formation. Stagnant gas adheres to the electrode surface, blocking interfacial contact and causing defective SEI.

Some pouch‑cell formation systems integrate staged vacuum with the charging and rest sequence to assist gas management. The reference recipe, which switches among approximately −10, −70 and −80 kPa, should be treated as a process‑specific example rather than an industry‑standard formation profile.

Milder and deeper vacuum stages can be evaluated to balance gas removal and electrolyte retention, but the optimum vacuum level and timing depend on pouch structure, electrolyte amount, equipment configuration and gas‑generation behavior.

5.2 Rest Step Optimization

Intermittent rest steps relieve concentration gradients, stabilize cell temperature and eliminate excessive polarization. They also provide operating windows for vacuum degassing and pressure calibration.

Rest duration requires targeted optimization instead of blind prolongation. Reasonable rest steps assist uniform SEI growth, while redundant rest does not improve film quality and reduces production efficiency.

6. Materials System Difference: Graphite vs Silicon a Carbon Note

Compared with graphite, silicon‑containing anodes generally experience larger dimensional changes and more complex interphase evolution. Therefore, formation current, temperature, pressure, SOC and electrolyte additives should be re‑optimized rather than directly transferred from a graphite recipe.

Direct transfer of a graphite formation recipe may result in suboptimal interphase formation and should be verified experimentally.

7. Common Formation Defects and Troubleshooting

7.1 Low First‑Cycle Coulombic Efficiency

Main causes: Excessive parasitic reactions, mismatched temperature/current parameters, incomplete electrolyte wetting, and incompatible electrolyte‑electrode systems.

7.2 Excessive Gas Generation

Abnormal gas production is mainly triggered by excessive temperature, unqualified electrolyte moisture, mismatched formation voltage interval or poor additive compatibility.

7.3 High Post‑Formation Impedance

Caused by thick/resistive SEI film, non‑uniform interfacial growth, poor electrode contact or incomplete electrolyte infiltration.

7.4 Large Cell‑to‑Cell Inconsistency

Core influencing factors: Unstable fixture pressure, uneven temperature distribution, inaccurate equipment channel calibration and inconsistent wetting time.

8. Systematic Formation Recipe Development Workflow

Reliable formation process relies on systematic verification rather than empirical copying:

1. Support the full cell (electrodes, electrolytes, separators, N/P ratio, electrode loading)
2. Confirm complete electrolyte wetting/resting before formation
3. Screen optimal formation current via dQ/dV and ICE data
4. Define effective film‑forming voltage intervals
5. Cross‑optimize temperature and pressure parameters
6. Calibrate formation cutoff SOC/voltage with electrochemical indicators
7. Verify process robustness under production batch variation

9. FAQ

Q1: What is the core purpose of battery formation?

A central purpose of formation is to establish stable electrode/electrolyte interphases, especially the anode SEI, while initializing the electrochemical state of the cell.

Q2: Does pre‑charging improve electrolyte wetting?

No. Electrolyte infiltration must be completed before formation. Pre‑charging cannot remedy incomplete wetting defects.

Q3: Why is there no universal formation temperature and current?

Formation parameters are highly coupled with cell chemistry, electrolyte formula and electrode structure. Fixed parameters cannot adapt to different battery systems.

Q4: Does higher vacuum and pressure bring better formation quality?

No. Excessive vacuum may cause electrolyte loss, and over‑pressure may alter electrode stack geometry. All auxiliary parameters need precise matching.

Conclusion

Lithium‑ion battery formation is a multi‑factor coupled interface optimization process, not a fixed formula of low current 45℃ clamping deep vacuum. The core engineering goal is to balance irreversible lithium loss and interfacial passivation, forming a durable low‑impedance SEI film.

Professional formation process development must take the full cell material system as the basis, combine electrochemical characterization data for parameter iteration, and avoid blind copying of mature recipes. This systematic approach helps improve formation consistency, initial efficiency and long‑term cycling performance.