The EV Battery Manufacturing Process: Step by Step

As part of my technical consulting work, I’ve had the opportunity to work on every aspect of EV battery manufacturing and design.

The battery is the most expensive part in an electric car, so a reliable manufacturing process is important to prevent costly defects. Electric vehicle batteries are also in high demand, which puts pressure on manufacturers to maximize production without compromising quality. As a result, robot automation is almost everywhere during battery manufacturing.

The production process of electric batteries includes many steps. Before going over each step, let’s review the structure of battery cells.

• The Basics: The Cell Structure
• The Manufacturing Steps
  • 1. Mixing of the Slurry Preparation
  • 2. Coating & Calendering
  • 3. Slitting of the Sheets
  • 4. Identification for Traceability
  • 5. Stacking
  • 6. Foil-to-Tab Welding
  • 7. Filling, Degassing & Sealing
  • 8. Forming, Inspection & Grouping
  • 9. Bonding of Module & Pack Components
  • 10. Tab-to-Busbar Laser Welding
  • 11. Installation & Application of Last Components

The Basics: The Cell Structure

The most important component in an EV battery is the cell. Cells are where the energy of the battery is stored. While different chemistries exist, lithium-ion batteries are the most common on the market because they offer the best compromise between energy density and cost.

Three types of EV battery cells are manufactured: cylindrical cells, prismatic cells, and pouch cells. Cylindrical cells are by far the most popular, but prismatic cells are also important. 

All types of cells include the following components, which are assembled in similar ways.

EV Battery Manufacturing Steps

1. Mixing of the Slurry Preparation

A coating called the slurry is applied on current collectors to help produce and conduct electricity. The slurry is a homogeneous paste that is mixed under vacuum to make sure that there are no air bubbles or moisture in it. 

Here’s what it contains:

• Binding agents to bring all the ingredients together;
• Active materials to produce electrical energy;
• Conductive materials to increase electrical conductivity.

The slurry does not contain the same materials for the anode and the cathode. In lithium-ion batteries for example, it contains lithium-metal oxide for the cathode and graphite for the anode.

Before mixing, the properties of raw materials used in the slurry are measured and validated before mixing.

2. Coating & Calendering

Once the slurry is ready, electrode sheets are coated with it in a machine called the coater. Here are the different operations performed during this process:

• Pouring: The mixture is poured onto the sheets
• Scrapping: Excess slurry (or paste) is scrapped off the sheets
• Drying: The remaining paste is dried
• Calendering: The sheet is compressed using a rolling press to achieve the right porosity and thickness

3. Slitting of the Sheets

The electrode sheets are cut so that they have the exact shape needed. The sheets not only need to fit into the casing, but they need to meet specific form factors. 

• For cylindrical cells, sheets are long and narrow to be shaped into a jelly roll. 
• For prismatic cells, sheets are rectangular to fit into the box-like structure. 
• For pouch cells, the rectangular shape must align with the edges of the pouch.

Laser cutting is the technology typically used for high-volume production. In smaller production lines or R&D labs, die cutters are also used.

4. Identification for Traceability

Laser marking is used to permanently identify the anode and cathode sheets with 2D codes. This will be used to:

• Optimize performance by grouping cells that have the same electrical and mechanical properties;
• Know which cells are in battery modules and packs when they are scanned;
• Track where cells are in the supply chain if a production problem is identified.

5. Stacking

The electrode and separator sheets form what is known as a stack. The stack is a series of sheets that are alternated (anode, separator, cathode, separator, etc.)  The stack will later be inserted in the cell casing, whether it’s a cylindrical case, and prismatic case, or a pouch. 

Different methods are used to create these stacks:

• Single Sheet Stacking: Sheets are placed one by one on the stack.
• Winding: The sheets are placed on rolls and a “stack holder” rotates to wind the sheets on it. For cylindrical cells, the stack holder has a cylindrical shape.
• Z-Folding with Single Electrode: Anodes and cathodes are placed alternately on a separator sheet roll.
• Z-Folding: Sheet rolls are folded together by pulling the sheets in a left-right movement.

6. Foil-to-Tab Welding

A metal strip is welded to the anode stack of copper foils, and another to the cathode stack of aluminum foils. Both ultrasonic bonding and laser welding can be used to make these connections. These metal strips (or tabs) are then connected to the cell terminals and will later be joined to a busbar to create an electrical circuit.

7. Filling, Degassing & Sealing

Electrolyte is poured into the casing and is absorbed by the separator sheets. The choice of the electrolyte chemistry is important for the battery’s performance. For example, additives can be used to affect factors like viscosity and improve conductivity.

After the electrolyte is poured and absorbed, air bubbles are removed in a vacuum chamber. This step is called degassing, and it ensures that the electrolyte is uniformly distributed.

The casing is then sealed hermetically using methods such as crimping, laser welding, ultrasonic bonding, or heat sealing.

8. Forming, Inspection & Grouping

Forming involves the initial charging and testing of battery cells. During this step, cells are connected and undergo multiple charge and discharge cycles (with resting in between) that help set the cells’ electrochemical properties.

The final step of cell manufacturing (before module and pack assembly) is cell inspection. 

After forming, cells undergo final testing to verify their electrical properties like capacity, voltage, and internal resistance. Mechanical properties are also validated with vision cameras. Cells that do not meet requirements are rejected.

Cells with similar properties are grouped together. They will be assembled in the same modules to ensure the high performance of the module as a whole.

9. Bonding of Module & Pack Components

During the assembly of battery modules and packs, sealants and adhesives are applied using a dispenser. Adhesives are used to join components together while providing additional properties like thermal conductivity, electrical conductivity, or insulation.

They are applied on battery housings, cells casings, and other components like cooling tubes. The curing time is controlled with active methods like UV light curing to prevent slowdowns.

To maximize the strength of the adhesive bonds, laser surface preparation is used before adhesives and sealants are applied. This process removes all contaminants and can modify the surface roughness if needed.

10. Tab-to-Busbar Laser Welding

11. Integration of Last Components

When the battery pack is assembled, other components that are essential for the good functioning of the battery need to be integrated.

• Battery Management System: The battery management system (BMS) is a computer that manages all the functions of the battery and communicates with other powertrain components such as the charger, inverter, and vehicle control unit. The BMS has its own independent electrical circuit. Numerous sensors are installed at key locations in the battery pack to monitor cells and heat, and a cable harness connects these sensors to the BMS.
• Hydraulic Connections: Various thermal and hydraulic connections are used to control the flow and distribution of liquid coolant in the battery. The battery’s thermal management system includes tubes, pumps, valves, and other components that need to be installed and connected to the battery management system.
• Fireproof Protection: Endothermic coatings are applied on cells and modules to absorb heat and help regulate the battery’s temperature. They act as flame retardants by absorbing heat and blocking flames in the case of a thermal runaway. They also help improve the performance, safety, and longevity of the battery.
• Final Validation: A series of tests validate that everything is properly connected, that voltages are correct, that there are no leaks in the cooling system, and that the battery case is watertight.

Laser Solutions for EV Battery Production

Laserax provides laser solutions for EV battery manufacturing. You can learn more about our solutions here. Or, contact us if you want to discuss one of the following:

• Laser welding of tab-to-busbar connections
• Laser cleaning before ultrasonic bonding or welding
• Laser surface preparation for thermal and structural adhesives
• Laser marking of electrodes and cell casings
• Laser cleaning of housings before liquid gasketing

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Header image source: Munro Live