The exterior is complete after the assembly process. Now, it is time to give the battery the electrical properties so that it can play the supposed roles. Let’s look at formation, the third step of battery manufacturing to find out how it’s done.
What is Formation?
After assembly, the battery is activated with electric energy and its safety is checked in the next step: formation. The most part of the process is automated and consisted of repeated aging, charging, and discharging.
* The formation process may differ by manufacturer.
Aging
The first thing to do is let the injected electrolyte permeate into the cathode and anode by leaving the battery at room temperature for about from 30 minutes to 3 hours. The process of putting the battery at a certain temperature and humidity is called aging. In this process, the electrolyte evenly soaks into the inside of the battery, allowing smooth movement of ions between the cathode and anode of the battery.
SEI Layer
After aging, the battery is charged to a certain level. During the charge, all lithium ions move to the anode, prompting a decomposition of electrolyte and forming a thin layer called SEI, or Solid Electrolyte Interphase, on the surface of the anode. SEI allows transportation of only lithium ions, stopping electrons. It improves battery safety by increasing the internal stability and is a critical element for battery performance and life.
Degassing
The charged battery is left at the temperature of 40 – 70 °C for a second aging through which SEI is more stabilized in a regular thickness. If gas forms in the battery during aging and charging, it has to be removed through degassing like in the assembly process.
Testing and Checking
After degassing, aging and charging is repeated twice more. Then, the charging capacity is tested and substandard batteries are winnowed out. Lastly, the batteries are discharged to 0.1 – 1 C (C-rate) before being released.
With the completion of formation, the third process of battery manufacturing, there is only one step left. Next time, we will learn about the pack process, the last part of battery manufacturing.