Cell Assembly

There are 6 steps in the cell assembly process, but there is variations depending on the cell format:

1. Slitting
2. Final drying
3. Cutting
4. Winding or Stacking
5. Terminal welding
6. Canning or Enclosing

Step 5 – Slitting

The electrodes up to this point will be in standard widths up to 1.5m. This stage runs along the length of the electrodes and cuts them down in width to match one of the final dimensions required for the cell.

It is really important that no burrs are created on the edges of the electrodes during this process as they can cause damage to the separator and a possible short-circuit at a later date.

Challenges

• Avoiding burrs on edges
  • Insulation tape may be applied here to mitigate foil to foil shorts from burrs
• Ensuring no loose metallic particles contaminate coatings

Step 6 – Final Drying

The electrodes are dried again to remove all solvent content and to reduce free water ppm prior to the final processes before assembling the cell.

Step 7 – Cutting

The final shape of the electrode including tabs for the electrodes are cut. At this point you will have electrodes that are exactly the correct shape for the final cell assembly.

Challenges

• Avoiding burrs on edges
• Ensuring no loose metallic particles contaminate coatings

Step 8 – Winding or Stacking

In a cylindrical cell the anode, cathode and separator are wound into a spiral. For pouch cells the electrodes stacked: anode, separator, cathode, separator, anode, separator etc.

Some prismatic cells have stacked electrodes and some have a flat wound jelly roll.

Challenges

• Alignment of layers
• Avoid punctures of separator
• Separator folding
  • lots of countermeasures applied over time like separator envelope welding not all manufacturers countermeasure in this way
  • one option is Lamination of the separator to the electrodes
• Check humidity
  • an important test that should be done before assembly and before the filling stage, is checking the humidity of the separator

Stacked vs Wound Cells

When we talk about Stacked vs Wound cells we are considering the Anode-Separator-Cathode stack and how that is assembled within a battery cell.

Pouch and Prismatic cells can be stacked or wound, but what is the advantage and disadvantage of each approach?

Step 9 – Terminal Welding

The anodes are connected to the negative terminal and the cathodes to the positive terminal. The process and robustness of this joint are important to understand as welding the cell to busbars can damage the internal welds.

Challenges

• Trimming of tabs and avoiding any burrs or particles being left behind
• Gathering all of the foils and presenting them to the welder
• Aligning gathered electrode foils with tab
• Weld position alignment, whether that is Laser Alignment, spot weld or ultrasonic horn and anvil alignment
  • Wear of electrodes / horn / anvil
  • Consistent energy burst, energy oscillation, changes in materials or even surfaces
• Ensuring no sputter contaminates cell
• Ensuring good consistent electrical connections

Step 10 – Canning or Enclosing

The electrodes either as a roll or pack of stacked layers are loaded into the can or pouch. Depending on the cell format will change how this canning or enclosing process is completed.

Challenges

• Ensuring no debris in can
• Ensuring no damage to jelly roll or stack
• Pouch
  • Pouch Cup formation – avoid die inclusions
  • Stack placement in pouch – alignment
  • 2nd Pouch (over) cup – avoid die inclusions
  • Heat Sealing integrity time, applied energy – avoid heater bar contamination affecting seal integrity
  • Open cell seal handling prior to injection
  • Pouch Taping – a line of tape applied on inner side of cup corresponding to placement of stack
• Isolation Testing
  • HiPot testing or Capacitance tests are done at this stage to establish the integrity of the cell, this should find:
    • debris / metallic foreign particles
    • folds in the separator
    • holes in the separator
    • burrs on current collectors

See also: Cell Manufacturing Equipment, Cylindrical Cell Joining Methods