Battery Cell Manufacturing Process

In order to engineer a battery pack it is important to understand the fundamental building blocks, including the battery cell manufacturing process. This will allow you to understand some of the limitations of the cells and differences between batches of cells. Or at least understand where these may arise.

So What are the Major Steps in Battery Pack manufacturing, Lets have a look at the Overall Major steps below:

Image Sources: Electrode Manufacturing, Cell Assembly, Cell Finishing, Module Production, Pack Production.

Lets Start with the First Three Parts: Electrode Manufacturing, Cell Assembly and Cell Finishing

1. Electrode Manufacturing

Lets Take a look at steps in Electrode Manufacturing

Step 1 – Mixing

The anode and cathode materials are mixed just prior to being delivered to the coating machine. This mixing process takes time to ensure the homogeneity of the slurry.

Cathode: active material (eg NMC622), polymer binder (e.g. PVdF), solvent (e.g. NMP) and conductive additives (e.g. carbon) are batch mixed.

Anode: active material (eg graphite or graphite + silicon), conductive material (eg carbon black), and polymer binder (eg carboxymethyl cellulose, CMC)

N-Methyl-2-pyrrolidone (NMP): this is a toxic substance, widely used in the plastics industry as it is nonvolatile and able to dissolve a wide range of materials. NMP residual will be a Quality Control test downstream (Gas Chromatography-Mass Spectrometry can be used to test sample) as that will affect cell performance reactively.

Challenges

• Homogeneity of the mix
• Ensuring low ppm H₂O – suppliers will be asked to guarantee a low ppm
• No breakup of the particles
• Particle and pore size distribution will be targeted with suppliers or via in process Milling
  • Some developments concentrate on how to produce dual layers (to form a quasi-heterogeneous bi-layer) to aid electrolyte soaking. The calendaring process can achieve this to a degree.
• Moving from a batch mixing process to continuous mixing
• Ensuring no alien particulates are in the mix
  • Magnetic filters often used to remove metal particles, this will only work down to a certain size

Step 2 – Coating

The anode and cathodes are coated separately in a continuous coating process. The cathode (metal oxide for a lithium ion cell) is coated onto an aluminium electrode. The polymer binder adheres anode and cathode coatings to the copper and aluminium electrodes respectively.

Challenges

• Controlling thickness and thickness over time
• Foil surface oxidation homogeneity can add variance to the challenge
• Controlling foil moisture levels
• Besides thickness, tensiometer peel tests and even indentation resistance (hardness test) might be used to further qualify the coating integrity

Step 3 – Drying

Immediately after coating the electrodes are dried. This is done with infrared heating on a continuous process. The solvents are recovered from this process.

Challenges

• Centre to edge homogeneity of drying process
• Recovering solvent
• Avoiding cracking

Step 4 – Calendering

This is a rolling of the electrodes to a controlled thickness and porosity.

Challenges

• Controlling uniform thickness
• Avoiding cracking

A look at the Sodium Ion Cell Manufacturing Process, but perhaps more usefully a look at the differences compared to the lithium based cell manufacturing processes.

2. Cell Assembly

Lets Take a look at steps in Cell Assembly below

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 tags 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
• Check humidity
  • an important test that should be done before assembly and before the filling stage, is checking the humidity of the separator

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

3. Cell Finishing

Lets Take a look at steps in Cell Finishing below

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.

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

Energy Required to Make a Cell

The cell manufacturing process requires 50 to 180kWh/kWh.

Note: this number does not include the energy required to mine, refine or process the raw materials before they go into the cell manufacturing plant.

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

What does 1 GWh of cells look like? How many cells? How much energy is that?

How much lithium metal is in 1 GWh of Lithium-ion cells?

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.

References

1. Yangtao Liu, Ruihan Zhang, Jun Wang, Yan Wang, Current and future lithium-ion battery manufacturing, iScience, Volume 24, Issue 4, 2021
2. Kumari Konda, Sahana B. Moodakare, P. Logesh Kumar, Manjusha Battabyal, Jyoti R. Seth, Vinay A. Juvekar, Raghavan Gopalan, Comprehensive effort on electrode slurry preparation for better electrochemical performance of LiFePO4 battery, Journal of Power Sources, Volume 480, 2020
3. Alex Cushing, Tianyue Zheng, Kenneth Higa and Gao Liu, Viscosity Analysis of Battery Electrode Slurry, Polymers, 2021, 13, 4033
4. Fabian Duffner, Lukas Mauler, Marc Wentker, Jens Leker, Martin Winter, Large-scale automotive battery cell manufacturing: Analyzing strategic and operational effects on manufacturing costs, International Journal of Production Economics, Volume 232, 2021
5. Lithium-Ion Battery Cell Production Process, RWTH Aachen University