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.

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), 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.

Challenges

  • Homogeneity of the mix
  • No breakup of the particles
  • Moving from a batch mixing process to continuous mixing

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

  • Control thickness

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

  • 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

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
  • Ensuring no loose metallic particles contaminate coatings

Step 6 – Final Drying

The electrodes are dried again to remove all solvent content 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
cell manufacturing schematic

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

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

  • 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. 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

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

  • Wetting of all layers within the jelly roll or stack with electrolyte

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.

Challenges

  • Checking cell is sealed
  • 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

Step 14 Final Control Checks

Alongside the charge/discharge cycle data, a visual inspection is often carried out to check the integrity of the cell post cycling and prior to dispatch. 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

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