Vacuum Baking & Electrolyte Injection in Lithium-Ion Pouch Cell Manufacturing

Vacuum baking and electrolyte injection are two linked steps that follow packing. In baking, the packed cell, which still has an open filling side, is heated under vacuum to drive out water held in the electrodes and separator. In injection, a measured dose of electrolyte is put into the pouch, and the cell is then left to let the liquid soak into every pore.

These steps sit between auto packing and formation. Up to now the cell is only a dry stack of coated foils, separator and laminate. After injection it becomes an electrochemical device that can store charge. Anything that goes wrong here, such as leftover moisture or the wrong amount of liquid, will be carried into formation, aging and the cell's whole service life.

Why this stage matters

Common electrolytes use a lithium salt, usually LiPF6, dissolved in organic carbonate solvents. This salt is sensitive to water. Water causes it to break down and form hydrofluoric acid (HF) and other by-products. HF attacks the passivation layers on the electrodes, corrodes metal parts and generates gas. The practical results are higher internal resistance, swelling, shorter cycle life and, in bad cases, a cell that fails capacity tests.

Too little electrolyte leaves dry regions where lithium ions cannot move, so capacity is lost and plating can start. Too much electrolyte increases cost, can leak at the seal and adds to gas volume. Accurate dosing and good wetting are therefore quality items, not only cost items.

How the process works

1. Loading. Packed cells are placed in trays or fixtures and loaded into the vacuum oven. Door interlocks and a leak check on the chamber are part of start-up.
2. Heating under vacuum. The oven brings cells to the temperature set for that design while the pumps reduce chamber pressure. Water vapour leaves the cell through the open side.
3. Hold and purge cycles. The cell is held for the specified time. Some recipes break the vacuum with dry gas and pump down again to help remove moisture.
4. Cooling and transfer. Cells are cooled enough to be handled, then moved to the filling area in dry conditions so they do not take up moisture again.
5. Evacuation and filling. The cell is evacuated, and a metered amount of electrolyte is dispensed through the open edge by the filling head.
6. Temporary sealing. The filling opening is closed with a pre-seal so the liquid stays inside.
7. Soaking (wetting). Cells rest, sometimes with mild warmth or light pressing, so electrolyte spreads through electrodes and separator. They then go to formation.

Key process parameters

Actual values depend on cell design, materials and equipment supplier. As a general guide, baking is often done at an elevated but moderate temperature, commonly somewhere around 70 to 90 degrees Celsius for several hours to a day or more, and the dry room used for filling typically runs at a dew point of about minus 40 degrees Celsius or lower. Always follow the cell maker's specification.

Typical defects, causes and corrective actions

Quality checks and measurements

• Moisture of baked samples by Karl Fischer titration, compared with the limit in the specification.
• Cell weight before and after injection, so the electrolyte mass is known for every cell.
• Oven temperature uniformity checks and regular vacuum-leak tests.
• Dew-point logging in the filling room, with alarms when it drifts.
• Visual check for electrolyte on seal areas and outer surfaces.
• Lot traceability so that a problem found later can be linked to the oven batch and electrolyte lot.

Safety and handling notes

• Electrolyte solvents are flammable and their vapour is irritating. Work with ventilation, away from ignition sources, and keep spill kits ready.
• Lithium salts can release HF on contact with moisture. Skin and eye contact is hazardous. Use the personal protective equipment named in the safety data sheet, and know where the eyewash and first-aid arrangements are.
• Dry-room discipline matters: gowning, closed doors, and no unnecessary opening of cell containers.
• Vacuum ovens are hot and under reduced pressure. Do not open until the chamber is back at normal pressure and cooled, and follow lockout before maintenance.
• Handle filled cells carefully. Dropped, punctured or crushed cells must be isolated according to plant procedure because a short circuit can cause heat or fire.

Interview questions

1. Why are cells baked under vacuum rather than only heated?

Vacuum lowers the pressure so water vapour leaves faster at a safe temperature, which protects the separator and binder.

2. Why is moisture so harmful in a lithium-ion cell?

It reacts with the lithium salt to form HF, which attacks electrode layers and produces gas.

3. How is the amount of electrolyte verified?

By weighing the cell before and after filling and comparing the difference with the target.

4. What is wetting and why is a soak time needed?

Wetting is the penetration of electrolyte into the pores of electrodes and separator. It is slow, so the cell must rest before formation.

5. What can happen if formation is started on poorly wetted cells?

Uneven SEI formation, lithium plating risk in dry zones, lower capacity and higher resistance.

6. Which test measures moisture in electrode samples?

Karl Fischer titration, usually coulometric for low water content.

FAQs

Can baking be extended for safety?

Not freely. Longer or hotter baking can harm separators and binders, so follow the validated recipe.

Why is filling done in a dry room?

Baked components and electrolyte take up moisture quickly from normal air, which would undo the drying.

Is all electrolyte injected at once?

It depends on the design. Some plants use a single dose, others split it into steps with vacuum cycles in between to improve penetration.

Related reading

• Auto Packing Process in Lithium-Ion Pouch Cell Manufacturing
• Electrode Drying in Lithium-Ion Pouch Cell Manufacturing
• Activation (ACT / Formation) in Lithium-Ion Pouch Cell Manufacturing
• Degassing in Lithium-Ion Pouch Cell Manufacturing