Winding / Stacking in Lithium-Ion Pouch Cell Manufacturing

Winding and stacking are the two ways of assembling the electrode layers into the working core of the cell. In both, a negative electrode (anode), a separator and a positive electrode (cathode) are brought together in a fixed order, so that the electrodes face each other across the separator but never touch. In winding, long strips are rolled together into a flat coil, called a jelly roll. In stacking, individual cut sheets are piled one over another, or placed between a folded separator ribbon.

This step comes after electrode slitting and cutting, when the electrode rolls have been turned into strips or sheets with tabs, and before the core is inserted into the formed laminate pocket. It is the first time all the active parts meet, so the geometry built here fixes much of the cell's capacity, resistance and safety margin.

Why this stage matters

The separator is the only barrier between the two electrodes. If layers shift, if a burr or a metal particle sits between them, or if a separator edge is pulled back, an internal short circuit can result, immediately or after some time in service. Alignment also affects performance. The anode is normally made slightly larger than the cathode, and the separator slightly larger again, so every cathode area has an anode opposite it. If the cathode extends beyond the anode, lithium can deposit as metal on the edge during charging, which reduces capacity and creates a safety concern.

Winding is fast and well suited to continuous strips, while stacking gives flat, even layers and is often used for larger or high-power pouch cells. Both demand the same basics: clean material, accurate positions and steady tension.

How the process works

The two methods differ in detail, so the main steps are given for each.

Winding

1. Anode strip, cathode strip and two separator layers are loaded on unwind shafts with their tension set.
2. The strips are guided through edge-position control so they stay on centre.
3. Strips are fed into the winding needle or mandrel, and winding starts with the separator.
4. Winding continues at a controlled speed until the required length or turns have been reached.
5. Strips are cut, the end is fixed with tape, and the roll is flattened and pressed to its final thickness.
6. The roll goes to inspection and then to the next station.

Stacking

1. Cut anode and cathode sheets are picked up from magazines by a vision-guided picker.
2. A separator is laid down, either as a long ribbon folded in a zigzag or as separate sheets.
3. Anode and cathode sheets are placed alternately between the separator layers in the correct order.
4. The process repeats until the required number of layers is complete.
5. The stack is fixed with tape and pressed so it holds shape.
6. The stack is inspected and sent on for tab welding and packing.

Key process parameters

Typical settings depend on cell design, materials and equipment supplier. The cell maker's specification defines the limits for overhang, tension and thickness.

Typical defects, causes and corrective actions

Quality checks and measurements

• Insulation (short-circuit) test on the finished roll or stack, applying a defined voltage between anode and cathode to detect touching layers.
• Vision measurement of overhang and separator margin, inline or on samples.
• X-ray inspection of sampled units to see internal layer alignment without cutting.
• Thickness and weight of each assembly, compared with the design range.
• Layer count verification through counters and sample checks.
• Visual inspection of the end tape, edges and surface for wrinkles or contamination.

Safety and handling notes

• Moving rolls and pickers can trap fingers. Use guards, interlocks and lockout before clearing jams.
• Work in the clean, controlled area with gowns, gloves and caps, so hair, fibres and dust do not enter between layers.
• Electrodes contain active material that is a fine powder. Avoid creating dust, and use extraction.
• A pressed assembly can short if it is dropped or if metal tools touch exposed foil. Use non-conductive tools and designated trays.
• Separator and electrode materials should be stored dry. Follow humidity limits for storage and the time allowed before baking.

Interview questions

1. What is a jelly roll?

It is the wound assembly of anode, separator and cathode, rolled into a compact flat coil.

2. Why must the anode be larger than the cathode?

So every part of the cathode has an anode area facing it, which prevents lithium plating at the edges.

3. Compare winding and stacking briefly.

Winding is faster and continuous but has curved ends. Stacking gives flat layers with good space use but needs more placement steps.

4. What is telescoping?

Layers slide sideways relative to each other in a wound roll, so the roll edge is no longer even.

5. How do you detect a short between layers at this stage?

By an insulation test between the electrodes, and by X-ray or visual checks for particles and shifts.

6. Why is tension control important in winding?

Low tension gives loose, wrinkled layers. High tension can stretch or damage the foil and separator.

FAQs

Do all pouch cells use stacking?

No. Both wound and stacked pouch cells are made; the choice depends on cell size, performance target and equipment.

Why is the separator wider than the electrodes?

The extra width keeps electrode edges apart even if small shifts occur.

Is this step done in the dry room?

It depends on the plant layout. Many plants assemble in a clean, humidity-controlled area, and then bake the cell before filling.

Related reading

• Electrode Slitting in Lithium-Ion Pouch Cell Manufacturing
• Jelly Roll (JR) Handling Defects in Li-ion Pouch Cell Manufacturing
• Tab Related Defects in Li-ion Pouch Cell Manufacturing
• Electrode Calendering in Lithium-Ion Pouch Cell Manufacturing