Activation (ACT / Formation) in Lithium-Ion Pouch Cell Manufacturing

Activation, also called formation or ACT, is the first controlled charge and discharge of a freshly filled pouch cell. Until this step the cell is only a package of dry-looking electrodes soaked in electrolyte. After activation it becomes a working battery with a stable layer on its anode surface.

In the process line, activation follows vacuum baking, electrolyte injection and the resting period that lets the electrolyte soak into the electrodes. It comes before degassing, final sealing, grading and final quality inspection. Every later electrical test depends on what happens here.

Why this stage matters

During the first charge, a thin film called the Solid Electrolyte Interphase (SEI) grows on the surface of the graphite anode. This film lets lithium ions pass but blocks further decomposition of the electrolyte. If the film is thin, even and stable, the cell ages slowly. If it is patchy or too thick, the cell loses capacity early and may show high resistance.

The first-cycle reactions also consume some lithium and release gas. For this reason formation is the stage where weak cells first reveal themselves: cells with internal shorts, poor electrolyte wetting or contact faults behave differently from good cells on the very first charge. Catching them here is far cheaper than finding them at the customer.

How the process works

1. Load and contact. Rested cells are placed in trays or formation fixtures. Probes touch the tabs, and many lines also clamp the cells flat so the electrode layers stay in close contact while gas forms.
2. Contact and short check. The equipment checks that each channel reads a sensible starting voltage before any current flows. Channels with bad contact are flagged.
3. Initial low-current charge. The first part of the charge is usually done gently, because this is when most of the SEI forms. The current is then raised in steps according to the recipe.
4. Constant-current, constant-voltage charge. The cell is brought up to its specified charge voltage, then held while the current tapers down to a cut-off.
5. Rest. A short pause lets the voltage settle before the next step.
6. Controlled discharge. The cell is discharged to its lower voltage limit, and the delivered capacity is recorded.
7. Optional extra cycle or partial charge. Depending on the recipe, the cell may be cycled again or left at a defined state of charge for the next stage.
8. Data check and sorting. The equipment compares each cell's curve with the recipe limits and marks it pass or hold.

Key process parameters

Typical values vary a lot. The actual numbers depend on cell design, materials and equipment supplier, and the cell maker's specification is the final authority.

Typical defects, causes and corrective actions

• Abnormal voltage rise or fall. Causes: internal micro short, poor probe contact, moisture contamination. Action: confirm contact first, then isolate the cell and review dry-room humidity records.
• Low first-cycle capacity or efficiency. Causes: incomplete electrolyte wetting, too little rest before charging, uneven electrode loading. Action: review wetting time and the earlier coating data.
• Lithium plating. Causes: charge current too high for the temperature, or poor wetting leaving dry patches. Action: lower the early charge rate, check temperature control and wetting.
• Excess gas and swelling. Causes: moisture in electrodes, high temperature, over-voltage. Action: verify baking results and charge limits.
• Capacity spread within a lot. Causes: temperature differences across trays, channel drift, material variation. Action: calibrate channels and map tray temperature.
• High internal resistance after formation. Causes: thick or uneven SEI, weak tab welds. Action: check tab weld records and the formation recipe.

Quality checks and measurements

• Per-channel curves of voltage, current and temperature compared with the recipe.
• First-cycle charge capacity, discharge capacity and their ratio.
• Channel calibration against a reference meter at the interval the plant sets.
• Visual check for swelling, wrinkles or electrolyte traces when cells leave the trays.
• Traceability: each cell's formation result is stored against its barcode.

Safety and handling notes

• Cells coming out of formation hold stored energy. A short circuit across the tabs can cause heating, so use insulated tools and never lay metal objects across cells.
• Electrolyte vapour is irritating, and in contact with moisture it can form harmful acidic compounds. Keep ventilation running and follow the plant's spill procedure.
• Formation areas normally have temperature monitoring, smoke or heat detection and a defined response for a cell that overheats. Know where the isolation and emergency stop points are.
• Before servicing a formation rack, follow lockout and tagout so that no channel can drive current while hands are near the contacts.
• Swollen or leaking cells must be set aside in the designated container, not stacked with good cells.

Interview questions

1. What is formation? It is the first controlled charge and discharge of a new cell, during which the SEI forms on the anode.
2. What is the SEI and why does it matter? It is a thin protective film on the anode. It limits ongoing electrolyte breakdown, so it supports long life and stable capacity.
3. Why is the first charge done at a low rate? Most of the SEI forms then, and a gentle rate gives a more uniform film and less risk of plating.
4. Why are cells often clamped during formation? Clamping keeps electrode layers in contact, helps gas move out of the active area and gives a flatter cell.
5. What could cause gas generation? Normal SEI reactions, plus extra gas from moisture, high temperature or over-voltage.
6. How do you find a bad cell in formation? By comparing its voltage, current and capacity curves with the recipe limits and with neighbouring cells.

FAQs

Is activation the same as formation?

Yes. Different plants use ACT, activation or formation for the same first-cycle step.

Can formation be skipped to save time?

No. Without it the SEI would form uncontrolled in the customer's first cycles, with poor consistency and possible safety problems.

Why does formation take so long?

Gentle currents, rest periods and extra cycles are needed to build a good SEI, and the time depends on cell design and the recipe.

Related reading

• Vacuum Baking & Electrolyte Injection in Lithium-Ion Pouch Cell Manufacturing
• Degassing in Lithium-Ion Pouch Cell Manufacturing
• OCV (Open Circuit Voltage) Testing in Lithium-Ion Pouch Cell Manufacturing
• Electrical Defects in Li-ion Pouch Cell Manufacturing