Lithium-Ion Battery Formation Process: SEI, Gas Generation and Pouch Cell Swelling Troubleshooting

canrd July 30, 2026 72

Introduction

Lithium-ion battery formation is the mandatory initialization procedure implemented after electrolyte filling and sufficient cell resting. All frontline cell R&D engineers, production supervisors and quality inspectors frequently encounter formation-related failures including pouch swelling, elevated internal resistance, low initial coulombic efficiency and inconsistent batch performance.

1. Rapid Reference Table for Abnormal Formation Defects

Observed Cell Defect	Priority Inspection Items
Excessive gas yield during formation charging	Raw material moisture content, electrolyte solvent ratio, formation current ramp, fixture heating temperature
Visible pouch swelling before degassing	Normal first-charge side reactions, insufficient airbag reserved volume, uneven electrode contact gap
Continuous thickening after secondary sealing	Unstable incomplete SEI layer, residual trapped gas, moisture contamination inside cell
Low initial coulombic efficiency	Excessive irreversible lithium consumption, high graphite specific surface area, low-voltage sustained side reactions
Sharply increased internal resistance post formation	Inadequate electrode pre-wetting, insufficient clamping pressure, over-thick SEI, sluggish charge transfer reaction
Wide capacity & OCV deviation within one batch	Uneven fixture pressure, inconsistent electrolyte injection, temperature difference between formation channels
Rapid OCV drop after aging storage	Severe interface instability, internal micro-shortage, persistent electrolyte reduction reactions

2. What Is Lithium-Ion Battery Formation?

Formation refers to the activation process of cathode and anode active materials after electrolyte injection and full cell resting, realized through standardized charge-discharge cycles. The core product of formation is the Solid Electrolyte Interphase (SEI) film grown on anode particle surfaces.

Formation is the initialization step for lithium-ion cells, and the formation of SEI film is its most critical core content. The SEI film forms on graphite anode surfaces during the first charge when lithium ions deintercalate from cathode materials and migrate to the anode.

Diagram illustrating lithium-ion battery formation, showing lithium-ion migration and SEI film formation on the graphite anode during the first charge.

Advantages of qualified SEI films

Stabilize electrode active substances and block continuous irreversible reduction reactions between electrolyte and active particles, directly determining cell capacity, rate performance, cycle life and high/low temperature performance.

Drawbacks brought by SEI film

Consumes part of active lithium inside the cell, requiring extra cathode material loading to compensate irreversible lithium loss during the first charge;
Increases electrode-electrolyte interfacial resistance and brings obvious voltage hysteresis during charge and discharge.

Note: Thin passivation film can also form on cathode surfaces, but its electrochemical influence is far weaker than anode SEI, so all subsequent SEI mentioned in this article refers to the anode interphase film.

3. Why Do Lithium Cells Generate Gas During Formation?

Gas is an inevitable byproduct of irreversible electrolyte reduction reactions in the first charge. This chapter adopts the test data from Canrd 1300mAh LCO/graphite pouch cell experiment:

Test cell configuration: 1300mAh capacity, 4.2V lithium cobalt oxide cathode, synthetic graphite anode, PP separator, 1mol/L LiPF6 EC:DMC:EMC=1:1:1 electrolyte, test method: water displacement gas measurement GC-MS gas component analysis, charging rate 0.02C with gradient cutoff voltages.

Gas generation at different formation voltages during lithium-ion battery formation, showing the relationship between formation voltage and SEI film formation.

3.1 Total gas volume change rule

2.0V–2.5V: Extremely low gas output, only trace water & lithium salt hydrolysis reactions occur, no obvious SEI film-forming reaction;
3.0V–3.5V: Gas generation reaches the peak value, EC cyclic carbonate decomposes violently, which is the core voltage window for SEI generation;
Above 3.5V: Complete SEI coverage suppresses EC reduction, total gas volume drops rapidly;
Above 3.8V: Linear carbonate DMC and EMC become the main decomposition objects, generating useless alkane gas with no contribution to SEI protection.

Gas composition at different formation voltages during lithium-ion battery formation, showing changes in H₂, CO₂, C₂H₄, CH₄, C₂H₆, C₃H₅, and CO during SEI film formation.

3.2 Gas component classification by voltage interval

Low voltage (<2.5V): Main gases H₂, CO₂, and CO are produced by side reactions involving trace moisture hydrolysis;
Medium voltage (3.0V~3.5V): Main gas CO₂, C₂H₄; ethylene is the characteristic product of EC decomposition and effective SEI generation;
High voltage (>3.8 V): Main gases are CH₄, C₂H₄, C₂H₆, C₃H₈, and CO—alkane byproducts resulting from the decomposition of linear carbonates.

3.3 Core chemical reactions generating gas

Main reversible cell reactions

Cathode delithiation: LiCoO2 → Li1-xCoO2 + xLi+ + x e-

Anode lithiation: 6C + xLi⁺ + xe⁻ = LiₓC₆

Full-cell overall reaction:LiCoO2 6C = Li1-xCoO2 LixC6

Low-voltage impurity side reactions (below 2.5V)

H2O + e- = OH- + 1/2H2(g)
OH-Li = LiOH(s)
LiOH Li e- = Li2O(s) 1/2H2(g)
LiPF6 = LiF PF5
PF5 H2O = 2HF PF3O
2HF Li2CO3 = LiF H2CO3
H2CO3 = H2O CO2(g)

EC solvent film-forming & gas-producing reaction

EC e- = EC·(EC free radical)
2EC· 2Li = CH2=CH2(g) (CH2OCO2Li)2(s)
EC 2Li 2e- = CH3OLi(s) CO(g)

DMC & EMC linear carbonate side reactions

DMC e-Li = CH3OCO2Li(s)CH3·
CH3· 1/2H2 = CH4(g)
EMC e-Li = CH3OCO2Li(s) C2H5·
C2H5· 1/2H2 = C2H6(g)

All gas products generated in formation include H₂, CO, CO₂, C₂H₄, CH₄, C₂H₆, so temporary airbags must be reserved for all pouch cells to store gas generated during the first charge.

4. Why Does a Pouch Cell Swell After Formation?

Pouch swelling is the most intuitive failure phenomenon in formation workshops, which can be divided into four root cause categories:

Normal first-charge side reaction swelling

Within the 3.0–3.5V optimal SEI window, EC decomposition will inevitably produce gas. If the airbag volume is designed according to the benchmark gas yield, the thickness will recover to normal after complete degassing.
Excessive abnormal side reaction swelling

When raw material moisture exceeds the standard, electrolyte formula is mismatched or formation cutoff voltage is lower than 3.0V, solvent continuous decomposition will generate far more gas than the control group.
Swelling caused by insufficient degassing

Inadequate vacuum degree or short holding time in the degassing station leads to residual gas trapped inside the cell stack, forming mild bulges after secondary sealing.
Long-term swelling after sealing

The SEI film formed in formation is discontinuous and incomplete. Electrolyte keeps contacting graphite particles, triggering sustained decomposition and slow gas accumulation during storage.

Symptom comparison table for swelling judgment

Swelling performance	Corresponding root cause	Recommended verification method
Swell during formation, stable after degassing	Nominal SEI gas generation	Compare airbag volume design with benchmark gas data
Gas volume far higher than standard batches	Moisture pollution or defective electrolyte	GC-MS gas composition test raw material moisture detection
Thin bulge remains after secondary sealing	Incomplete vacuum degassing	Adjust degassing vacuum and holding time
Cell thickens gradually after long-term storage	Discontinuous protective SEI layer	EIS batch screening accelerated aging test
Uniform thickness rise with little gas detected	Graphite lithium intercalation expansion	Check formation target SOC and fixture pressure

5. How Does the SEI Film Grow During the First Charge?

Illustration of the SEI film formation mechanism in lithium-ion batteries, showing electron transport, solvated lithium ions, and SEI growth on graphite particles during the first charge. Illustration of the SEI film growth mechanism during lithium-ion battery formation, showing the initial and later stages of SEI development and the electron tunneling effect on the graphite anode.

5.1 Four-step dynamic SEI generation mechanism

Electron transmission: Electrons flow from copper current collector through conductive agent and aggregate at Point A inside graphite particles;
Solvated lithium ion diffusion: Lithium ions wrapped by solvent molecules diffuse from cathode to Point B on the surface of existing SEI;
Electron tunneling transmission: SEI is an electronic insulator, electrons can only pass through thin interphase via quantum tunneling effect to reach outer reaction Point B;
Interfacial reduction precipitation: Tunneled electrons react with lithium salt, solvent and film-forming additives to precipitate new inorganic & organic lithium compounds, continuously thickening SEI until graphite surface is fully covered.

5.2 Layered inorganic-organic compositional gradient of SEI

Two types of reduction reactions coexist during formation, creating a non-uniform layered structure along SEI thickness direction:

Early formation stage (near graphite substrate): High electron density on bare graphite surface triggers two-electron reactions, producing inorganic components LiF, Li₂O, Li₂CO₃, LiOH as the main inner layer substances with high stability and low ion resistance;
Late formation stage (electrolyte side): Electrons need to tunnel through pre-formed SEI, electron concentration decreases sharply, single-electron radical reactions dominate to generate flexible organic lithium salts (ROCO₂Li, ROLi) forming the outer layer.

Before 3.5V, alkyl lithium carbonate and Li₂CO₃ are the main SEI components; after exceeding 3.5V, alkyl lithium carbonate and alkoxylithium become dominant substances.

Illustration of the SEI film compositional gradient during lithium-ion battery formation, showing two-electron and one-electron reactions that create inorganic-rich inner and organic-rich outer SEI layers.

Two-electron reactions tend to generate inorganic lithium salt components, while one-electron reactions mostly produce organic lithium salts. This rule is verified by PC solvent ES additive electrolyte system test.

5.3 Why SEI cannot grow infinitely

The core limiting factor is electron tunneling distance. Electrons can only pass through ultra-thin insulating media. Once SEI thickness exceeds the critical tunneling limit, almost no electrons can reach the outer film surface to drive reduction reactions, and SEI growth naturally stops.

Special exception: Silicon-carbon anode particles produce severe volume expansion during cycling, repeatedly tearing the SEI barrier, which will trigger unlimited repeated growth of interphase film and continuous rise of internal resistance.

5.4 Anisotropic SEI thickness on graphite crystal planes

Argon ion sputtering XPS depth profiling test on highly oriented graphite shows obvious thickness difference:

End plane (main lithium intercalation channel): SEI thickness ~30nm, requiring thick protective film to resist solvent erosion during frequent ion embedding;
Base plane (low lithium activity): SEI thickness only ~2nm, with negligible side reactions.

6. Why Is Initial Coulombic Efficiency Low After Formation?

Low first-cycle efficiency originates from irreversible lithium loss in SEI forming reactions. Lithium ions extracted from cathode participate in irreversible reduction and are fixed inside solid inorganic/organic SEI precipitates, unable to return to cathode during discharge, forming permanent irreversible capacity loss.

Moderate lithium consumption is necessary to build protective SEI. Excessively low ICE usually corresponds to the following process defects:

Excessively high graphite specific surface area provides massive side reaction sites;
Formation cutoff voltage lower than 3.0V leads to incomplete SEI and sustained solvent decomposition in later cycles;
Excessive trace moisture in electrolyte and separations accelerates hydrolysis side reactions;
Over-high formation current density causes uncontrolled violent reduction reactions.

Optimization direction: Calibrate cutoff voltage within the system-specific 3.0–3.5V window, strictly control raw material moisture index, and reduce the current of Step 1 SEI forming stage.

7. Why Does High Internal Resistance Appear Post-Formation?

Four major root causes of increased cell resistance after formation:

Over-thick dense SEI film: Excessive formation current or temperature accelerates rapid interphase precipitation, forming an ion-blocking barrier and raising RSEI;
Inadequate pre-formation wetting: Dry electrode pores cannot be fully filled by electrolyte, increasing bulk solution resistance Rs;
Insufficient formation fixture pressure: Uneven cathode-anode gaps break the continuous conductive network and expand charge transfer resistance Rct;
Over-abundant organic outer SEI: Long low-current holding time thickens the flexible organic outer layer and slows solid-phase lithium diffusion.

Engineers can distinguish different resistance abnormalities via EIS spectrum testing.

EIS impedance analysis diagram of a lithium-ion battery anode showing solution resistance, SEI film resistance, charge transfer resistance, and lithium-ion diffusion impedance.

Illustration of electron conduction and lithium-ion transport mechanisms in a lithium-ion battery, showing charge transfer through the cathode, graphite anode, conductive network, and electrolyte.

8. What Can EIS Spectra Reflect About Formation Quality?

8.1 Five lithium intercalation steps matched with EIS parameters

Electron transmission inside electrodes & free lithium ion migration in electrolyte (corresponds to Rs solution resistance);
Lithium ion penetration through SEI film (corresponds to RSEI SEI resistance, CSEI interphase capacitance);
Electron-lithium ion charge transfer redox reaction (corresponds to Rct charge transfer resistance, Cdl double-layer capacitance);
Solid-state lithium atom diffusion inside graphite particles (corresponds to Warburg diffusion impedance);
Lithium accumulation changes graphite crystal structure (corresponds to Rb bulk resistance, Cb bulk capacitance, Cint intercalation capacitance).

8.2 EIS application limitations in production

EIS is a non-destructive batch screening tool for comparing differences between cell batches, but it cannot independently confirm absolute SEI thickness, chemical composition or single-variable root causes. Temperature, SOC, cell resting time and contact pressure will all interfere with EIS curve shapes. XPS sputtering and GC-MS are required for accurate surface composition analysis.

9. Key Formation Process Control Variables

The two core factors determining SEI composition are electron density at the reaction interface and reactant concentration in electrolyte. All adjustable parameters that affect these two indicators include formation current, clamping pressure, heating temperature, electrolyte formula and cutoff voltage.

Detailed single-factor test data, pouch cell & coin cell standardized process windows and dQ/dV cutoff potential calibration methods are fully elaborated in the follow-up article: Lithium-Ion Battery Formation Parameters: Current, Pressure, Temperature, SOC and Cutoff Voltage.

10. Standardized Troubleshooting Workflow for Abnormal Formation Batches

Symptom classification: Divide failures into gas/swelling defects, low ICE defects, high resistance defects and poor batch consistency defects;
Control group comparison: Test abnormal cells with identical raw material, cell design and electrolyte formula as qualified control samples;
Multi-dimensional data cross-check: Collect formation voltage curve, dQ/dV differential capacity, total gas yield, post-formation thickness and full EIS spectrum data for comprehensive analysis;
Single-variable verification: Only adjust one formation parameter per batch test to avoid overlapping interference of multiple variables;
Full-process traceability: Link formation records with upstream electrolyte filling and downstream degassing & sealing data to distinguish inherent formation side reactions and post-processing defects.

Frequently Asked Questions

Q1 What is the essential difference between formation charging and conventional charge-discharge?

Formation charging triggers one-time irreversible electrolyte reduction to form protective SEI film, which only occurs on newly filled cells for the first charge. Conventional cycling only realizes reversible lithium intercalation without sustained side reactions.

Q2 Why does SEI formation consume active lithium and reduce initial efficiency?

Lithium ions extracted from cathode participate in irreversible reduction and are fixed inside solid SEI precipitates, unable to return to cathode during discharge, producing irreversible capacity loss.

Q3 Why must pouch cells reserve airbags during formation?

SEI forming side reactions inevitably produce gaseous byproducts. Airbags reserve expansion space to prevent pouch deformation and electrode delamination, and all gas will be removed via vacuum degassing before permanent secondary sealing.

Q4 How does insufficient fixture pressure affect SEI uniformity?

Low pressure creates uneven cathode-anode gaps. Narrow gaps carry high current density to form over-thick SEI, while wide gaps generate discontinuous thin interphase, leading to large cell-to-cell performance deviation.

Q5 Can EIS testing replace XPS to analyze SEI composition?

No. EIS only reflects macroscopic electrochemical impedance response, while XPS realizes direct surface element and composition detection. The two characterization tools are complementary and cannot substitute each other.

Conclusion

Lithium-ion battery formation is targeted electrode-electrolyte interface modification rather than simple pre-charging. All common production failures including swelling, high resistance and low initial efficiency are derived from abnormal SEI growth and voltage-dependent electrolyte decomposition reactions.

Key takeaways for process optimization:

The 3.0–3.5V voltage window corresponds to the most active EC decomposition and SEI generation in LCO/graphite systems, and this range needs re-verification for other battery chemistries;
SEI presents an inorganic inner layer and organic outer layered structure, with quantum electron tunneling effect limiting its unlimited thickening;
EIS can quickly screen batch consistency but must be combined with XPS, GC-MS and other characterization means to accurately judge SEI thickness and composition;
All adjustable formation parameter windows are detailed in the dedicated formation parameter article for process design reference.

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Lithium-Ion Battery Formation Process: SEI, Gas Generation and Pouch Cell Swelling Troubleshooting

Introduction

1. Rapid Reference Table for Abnormal Formation Defects