Cell Final Assembly and Finishing

There are 4 steps in the final assembly and finishing processes around battery cell manufacture:

  1. Filling
  2. Formation and Sealing
  3. Ageing
  4. Final Control Checks

Step 11 – Filling

The up until now dry cell is now filled with electrolyte. A partial vacuum is created in the cell and a pre-determined quantity of electrolyte is delivered to the cell. The partial vacuum helps the distribution and hence wetting of all layers within the cell.

The electrolyte is dispensed based on a defined volume of liquid. A second quality check is the weight of the cell before and immediately after filling.

filling process

What are the Electrolyte Fill Requirements for a cell versus chemistry, capacity, format, lifetime and other parameters?

The calculation is based on the porosity of the cathode, anode and separator. Added to this is the free volume and then a multiplier to account for losses in the filling process.

Image: Thomas Knoche, Florian Surek, Gunter Reinhart, A process model for the electrolyte filling of lithium-ion batteries, 48th CIRP Conference on MANUFACTURING SYSTEMS – CIRP CMS 2015, Procedia CIRP 41 ( 2016 ) 405 – 410

Challenges

  • Environment ppm control
  • “vacuum” injection pressure integrity
  • The electrolyte needs to be in the very low ppb range for H2O.
    • Higher levels of H2O creates HF not only is a safety hazard, but it also eats the battery from the inside out.
  • Mass flow injection (as opposed to vol flow injection)
  • Traceability finesse of the injection tanks, purge control, downtime in pipework etc
  • Injection and feeder tank residues build up (preventative maintenance control and frequency)
  • Temporary seal integrity checks
  • Wetting of all layers within the jelly roll or stack with electrolyte
    • May require rolling / rotation protocol to enhance wetting

Step 12 – Formation & Sealing

The cell is charged and at this point gases form in the cell. The gases are released before the cell is finally sealed. The formation process along with the ageing process can take up to 3 weeks to complete.

During the formation process a solid-electrolyte interface (SEI) develops. The SEI can prevent the irreversible consumption of electrolyte and protect the anode from overpotential during fast charging.

Note: When the degas is applied can vary. Some apply it after Pre-formation charge others after Formation charge others after Ageing.

Challenges

  • Environmental control in charging bays during formation
  • Time stamp control of applied protocol
  • Degas evacuation pressure level e.g. electrolyte boil off control prior to permanent seal
  • Checking cell is sealed permanently
    • Mass check is often used
  • Running formation cycle without damaging the cell

Step 13 – Ageing

The cells are stored at a controlled temperature for a period of time. This allows the SEI to stabilize.

This step in the process ties up the cells for a length of time, this inventory of cells has a considerable value and hence ties up funds.

Challenges

  • Forming and ageing the cell fast and delivering quality working cells
  • Fire detection in ageing storage system
  • Reducing time for ageing and so reducing inventory of cells
  • Multi barrier Safety environment
    • de oxygenated environment
    • nitrogen Dowse
    • IR Thermography
    • Fibre optic loop sensing

Step 14 Final Control Checks

Alongside the charge/discharge cycle data a number of further finishing steps and checks will be done prior to shipment:

  • Delta OCV rate
  • Cell Trimming
  • Mass check
  • Dimension check
  • Pouch cells
    • Leak check
    • Thickness check
  • Visual Check for Surface, TAB and SEAL anomalies

All data is recorded against the cells unique identification.

This is a first overview of the battery cell manufacturing process. Each step will be analysed in more detail as we build the depth of knowledge.

Home | Manufacturing | Cell Manufacturing | Battery Cell Manufacturing Process | Cell Final Assembly and Finishing