The last phase of a battery cell’s production is called cell finishing. The goal of the intricate process is to maximize the battery cell’s quality, safety, and performance through several steps. Depending on the kind of battery cell being made, the procedure may differ, but it usually entails the following steps:
Pre-Treatment
- Several optional technologies can be applied after electrolyte filling to enhance electrode wetting and prevent inactive areas from forming. One of these technologies is soaking at a high temperature.
- In this procedure, the cells are placed in a rack or chamber system and heated to a temperature between 30°C and 50°C.
- Elevated temperature causes the electrolyte to become less viscous, which enhances the liquid electrolyte’s contact and facilitates the electrolyte’s entry into the electrode’s mesopores.
- This guarantees that the electrolyte will enter the electrode and separator pores more quickly.
Formation
- The term “formation” describes the battery cell’s initial charging and discharging procedures.
- The cells are contacted by contact pins after being inserted into formation racks in unique product carriers.
- Next, the cells are charged or discharged in accordance with current and voltage profiles that have been carefully defined.
- Lithium ions are deposited in the graphite crystal structure on the anode side during the formation process. This creates the Solid Electrolyte Interface (SEI), a layer that separates the electrode and electrolyte.
- The parameters that affect cell performance during formation differ based on the manufacturer of the cell. The formation, which embodies the fundamental expertise of a cell manufacturer, is dependent upon both the cell concept and the cell chemistry.
- During the forming process, pouch cells are typically pressurized by specialized product carriers.
Degassing
- Much gas is produced during the first charging cycle, particularly with larger cells.
- These gases that were created during formation are gathered in a dead space, also known as a “gas pocket,” inside the pouch cell. A vacuum is applied, and the gas pocket is punctured during the degassing procedure. Exhausted gases are removed. Next, a vacuum is used to seal the space between the gas pocket and the cell. After being isolated, the gas pocket is disposed of as hazardous waste. It is optional to perform a final folding and, if required, gluing of the sealing edges to minimize the pouch cell’s outer dimensions.
- The temporary stopper is taken out of the prismatic cell, and the cell is vacuum degassed as well. This process step may also involve a second filling if necessary. The cell is eventually sealed.
- The extracted gases must undergo aftertreatment (e.g., generation, RTO) prior to being fed into the exhaust air system, depending on regulations pertaining to environmental protection and occupational health and safety.
- After degassing, the gas ash is not separated, especially in the case of smaller cells with lower gas generation and depending on the manufacturer.
Aging
- Aging is a component of the End-of-Line (EoL) test and one of the steps in determining cell quality.
- The cells are kept in what are known as aging racks and/or towers for this reason.
- There is a difference between aging at room temperature (RT) and aging at high temperatures (HT).
- Over a predetermined amount of time (up to three weeks), regular open circuit voltage measurements are used to track the characteristics of the cell.
- The cell is fine and ready for shipment if there isn’t any noticeable change in its properties over the course of time.
- The exact cell manufacturer and the cell chemistry employed have a significant impact on how long the aging process takes.
- This stage of the process does not pressurize pouch cells, in contrast to formation.
EoL Testing
- The cells are examined in an End-of-Life (EoL) test stand prior to being removed from the plant.
- The cells are fed to the test station after being taken out of the product carriers in the aging racks.
- A variety of tests, such as pulse tests, internal resistance measurements (DC), optical inspections, OCV tests, capacity tests, and leak tests, are carried out depending on the manufacturer. The cells are then released to their shipping state of charge.
- Many cell manufacturers grade their cells based on performance data (grading) after testing.
- The cells can be packaged and shipped after the testing is finished and every test has been passed.
- The cells are often stacked in a cardboard box and covered with a plastic cover for transportation.
Conclusion
A crucial step in the production of batteries, cell finishing guarantees the performance, safety, and dependability of battery cells. It entails a sequence of actions that maximize the cell’s functionality, safety, and quality, from formation and preparation to quality control and packing. When cell finishing is done well, battery cells are guaranteed to fulfill all necessary requirements and provide reliable, high-caliber performance for the duration of their lives.
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