Fraunhofer Research Institution for Battery Cell Production FFB
Fraunhofer Research Institution for Battery Cell Production FFB

Skill and Scale up: The production process of a lithium-ion battery

How is a battery cell manufactured?

Electromobility has become an integral part of our sustainable daily lives. Lithium-ion batteries are at the heart of this transformation, powering not only vehicles but also many of our electronic devices. Their production varies depending on the type of battery, but the key steps and processes are essentially similar. These can be broken down into electrode production, assembly and cell finishing: We will explain this in more detail in the fifth part of our "SkillandScaleUp" information campaign. 

 

In 1983, Professor Akira Yoshino filed a patent application for the lithium-ion battery as we know it today. Since then, it has worked on much the same principle. The challenge in manufacturing modern batteries is to maximize energy density, minimize manufacturing costs, and extend battery life. There are many steps involved in making a battery cell. It all starts with electrode manufacturing.

 

© Studio Wiegel
The process begins with the production of the electrode paste. Demineralized water, an aqueous binder solution and conductive carbon black are mixed with the active material. The materials are continuously mixed in the extruder using an extruder screw.
© Studio Wiegel
In the coating system, a carrier film is coated with the moist electrode paste and then dried in a drying oven.
© Studio Wiegel
After drying, the coated film is subjected to a precise quality check and wound.

Electrode Production  

The process begins with the production of the electrode paste, also known as "slurry". This paste varies by formulation and requires homogeneous mixing without impurities or air bubbles. Demineralized water, an aqueous binder solution and conductive carbon black are mixed with the active material. The materials are continuously mixed in the extruder using an extruder screw. When this step is complete, the electrode paste emerges from the end plate of the extruder. The result of the mixing process is the electrode paste, which is applied in a very thin layer to a carrier film in the next step, "coating and drying". The electrode foil then passes through the drying chamber, where the solvent evaporates to a defined proportion and the slurry is bonded to the foil. The solvent is collected, recovered and can be reused. 

The drying process is very energy intensive and comparable to a convection oven. The maximum conveyor speed is 80 m/min and the maximum temperature in the process zone is 160°C. Rotating rollers then compact the coating and can influence the porosity. The foil is wound in a "roll-to-roll" process. The rolled electrode foil (the mother coil) is then slit lengthwise into several smaller strips called daughter coils. Very high speeds of 80-150 m/min can be achieved. The final step, vacuum drying, takes more time. It can take 6-24 hours to transfer a dry electrode foil to the next production process, assembly. However, this step is essential because the residual solvent content and moisture in the film must evaporate in the vacuum oven. This is because any moisture can have a negative impact on the quality of the battery cell later on.  

This means that the electrode production lays the foundation for the later quality of the battery cell. Therefore, it is important to detect possible impurities or defects throughout the process to avoid problems in subsequent steps.

© Fraunhofer FFB
The battery goes through many individual stages before it is finished. It all starts with electrode production, followed by assembly until the battery cell is finally charged and discharged for the first time during cell finalization.

Cell Assembly  

While the electrode production is the same for all cell formats, the assembly process differs depending on the cell type. For this reason, each cell format requires its own assembly line. This process step, however, places high demands on cleanliness and dryness to ensure the smooth, high-quality and safe production of battery cells. For example, the so-called clean and dry rooms must maintain a dry room climate, resulting in very low humidity at a dew point of minus 60 degrees Celsius - the temperature at which moisture begins to condense. This is drier than the desert.  

In assembly, the first step is to unwind the dried daughter coils and feed them to the appropriate separation station. Separation means that the individual sheets are then punched out (mechanical separation) or lasered (thermal separation) in order to be stacked into a cell stack or wound into a round cell in the next process step. This means that the anode is followed by the separator, the cathode, and so on. The most common methods to date are single sheet stacking, i.e. the individual electrodes are stacked alternately, or Z-folding. Here, the separator is clamped in the middle and the separated electrode sheets are inserted alternately from the left and right into a separator pocket.  

In order to function, each battery cell requires cathode and anode contact tabs. For this purpose, the current tabs of the electrode stack are welded to the contact tabs of the cells during the subsequent "contacting" process. The battery cells must then be packaged. Pouch cells are packaged in an aluminum composite film (pouch film) and sealed gas-tight in three places. One side is left open for electrolyte filling. This is done with a very precise metering lance. Once the cell is filled, it is welded at the last missing point. The cell is now ready to be electrically charged and discharged for the first time.

Cell finishing  

The final production step requires more endurance: first, the cells are stored at an elevated temperature (between 30 and 50 degrees Celsius), either in individual chambers or in temperature-controlled high-bay warehouses, so that the electrolyte can be better distributed throughout the cell. During the subsequent "forming" process, the battery cell is charged and discharged for the first time in a controlled manner. The goal is to form a homogeneous "solid electrolyte interphase" (SEI). This passivation layer forms when a voltage is first applied and is essential for the successful operation of the battery cell. It plays a key role in determining the electrochemical performance and lifetime of a Li-ion battery as it is subjected to high stress during each charge and discharge cycle. However, this process, forming, can take up to 24 hours. It has a significant impact on the quality of the cell. The battery cell then undergoes a series of end-of-line tests to check the quality and performance of the battery cell and to detect internal short circuits, for example. Only after these steps have been completed is the battery cell ready for use in a variety of applications.

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