Degassing in Lithium-Ion Pouch Cell Manufacturing

Degassing is the step where gas trapped inside a pouch cell after formation is pulled out, and the pouch is closed again. Pouch cells are made with an extra pocket of foil, often called a gas bag or airbag, which collects the gas during formation so that the main cell area stays free of it.

The process sits straight after activation (formation) and the rest period that follows. Once gas is removed and the new seal is made, the cell goes on to trimming of the spare pouch, folding of the edges, electrical checks and final inspection. It is the point where the cell stops being an open, temporary package and becomes a sealed final product.

Why this stage matters

During the first charge, a small amount of electrolyte reacts at the electrode surfaces and releases gas. Some gas is normal. Left inside the active area, it separates the electrode layers, creates gaps in ion flow and makes the cell thick and uneven. Local gaps can lead to lithium plating and early capacity loss.

Degassing also fixes the final sealed condition. The cell is sealed under vacuum, so the pouch hugs the jelly roll closely, and later swelling shows up clearly as a fault rather than being hidden by free space. A poor reseal is dangerous in another way: it can allow air and moisture to leak in over time, or electrolyte to leak out. Both lower cell life and safety. For this reason degassing quality is judged on two things together, how completely the gas is removed and how reliable the new seal is.

How the process works

1. Receive the formed cell. Cells arrive from the formation area after their rest time and the gas bag looks inflated.
2. Position and clamp. The cell is held in a fixture, with the gas bag aligned with the piercing or cutting tool and the sealing bars.
3. Close the chamber and pull vacuum. Many machines work inside a vacuum chamber so the opening and gas removal take place in a controlled atmosphere, often in the dry room.
4. Open the gas bag. A needle or blade makes a small opening, and the gas escapes into the vacuum line.
5. Hold for the dwell time. The vacuum is kept for a set time so that gas trapped between the layers moves toward the opening.
6. Squeeze or press. Rollers or press plates close the cell from the cell body toward the gas bag, pushing remaining gas out and keeping the electrolyte inside.
7. Make the final seal. Heated sealing bars join the two laminate layers at a line close to the cell body, closing off the opened bag.
8. Cut off the gas bag. The excess pouch is trimmed away, leaving the sealed cell. The seal is inspected visually or by camera.

Key process parameters

Typical values depend on cell design, materials and equipment supplier. The settings are written in the cell maker's specification and equipment recipe, and they should not be changed without approval.

Typical defects, causes and corrective actions

• Incomplete gas removal. Causes: vacuum too weak, dwell too short, blocked vacuum line. Action: check pump and filters, confirm recipe, increase time only through approved change.
• Improper vacuum conditions. Causes: leaking chamber door seal, worn gasket, pump problem. Action: leak-rate test of the chamber and routine seal replacement.
• Seal damage at resealing. Causes: electrolyte or particles in the seal zone, wrong temperature, uneven bar pressure. Action: clean the bars, verify temperature and pressure, check for contamination, and review the seal inspection data.
• Excessive pouch deformation. Causes: too much pressing force, misaligned fixture. Action: reduce force within the recipe window and realign tooling.
• Residual gas leading to swelling. Causes: gas generated after degassing, incomplete removal, moisture in the cell. Action: look at formation data, baking results and degassing records together.
• Electrolyte loss or splash. Causes: too aggressive vacuum or squeezing. Action: soften the profile, check weight loss across the step.

Quality checks and measurements

• Machine log review for vacuum, time and force on every cell.
• Visual and camera check of the final seal: width, wrinkles, bubbles, electrolyte stains.
• Cell weight before and after, if used, to detect excessive electrolyte loss.
• Thickness measurement to confirm that the cell is flat and that no gas pocket remains.
• Periodic seal peel or burst tests on sample cells, following the plant's method.
• Leak checking by hold and recheck of the cell later in the flow, for example a thickness or weight check after storage.

Safety and handling notes

• The gas from formation may contain flammable components, along with electrolyte vapour. The vacuum exhaust must go to the plant's approved extraction or treatment system, never into the room.
• Electrolyte is irritating to skin, eyes and lungs, and contact with moisture can create harmful acidic products. Wear gloves, eye protection and follow the spill kit procedure.
• Work in the dry room under its humidity rules, and keep doors closed so the dew point holds.
• Sealing bars are hot. Never touch them, and use the guards and light curtains provided.
• Cells are charged, so avoid shorting the tabs with tools. Do not handle swollen cells with force, and put leaking cells in the designated container.
• Apply lockout and tagout before clearing jams in the chamber or pump system.

Interview questions

1. Why is gas produced in formation? Electrolyte and additives react at the electrode surfaces while the SEI forms, releasing gases.
2. What is the gas bag for? It collects the gas away from the active area so it can be removed and cut off.
3. Why is the cell sealed under vacuum? It pulls the pouch tight to the jelly roll and removes remaining gas.
4. What happens if electrolyte reaches the seal area? It can weaken the seal and lead to leakage, so the seal area must stay clean.
5. Why monitor dew point during degassing? The cell is open at this time, and moisture entering reacts with electrolyte and harms the cell.
6. How can you tell degassing was inadequate? By unusual thickness, a soft or bulging pouch, or swelling found in later checks.

FAQs

Is degassing done once or more than once?

Usually once after formation, but some processes use a second degassing after aging, depending on the design.

Can a cell be degassed without a vacuum chamber?

Some equipment designs do it differently, but vacuum-based removal is common because it keeps outside air away from the electrolyte.

Does degassing remove all gas for the whole life of the cell?

No. It removes the gas formed during formation. Small amounts may still form over the cell's life, which is why swelling is tracked.

Related reading

• Activation (ACT / Formation) in Lithium-Ion Pouch Cell Manufacturing
• Sealing Defects in Li-ion Pouch Cell Manufacturing (Most Critical Stage)
• Vacuum Baking & Electrolyte Injection in Lithium-Ion Pouch Cell Manufacturing
• Cutting & Finishing Defects in Li-ion Pouch Cell Manufacturing