3.2 Formation

The Formation Procedure is arguably the most critical and complex step within cell finishing. It involves the first controlled charge and discharge cycle(s) applied to the freshly assembled, electrolyte-filled, and pre-treated cell. Its main and most vital purpose is the electrochemical creation of stable, protective layers on the surfaces of both the anode and cathode where they meet the electrolyte. This layer on the anode is known as the Solid Electrolyte Interphase (SEI), while a on the cathode is called the Cathode Electrolyte Interphase (CEI).

A well-formed SEI is essential, as it needs to allow lithium ions to pass through easily but must block electrons to prevent ongoing unwanted reactions with the electrolyte. The quality of this SEI layer significantly influences the key performance characteristics, including its cycle life, safety, rate capability (how fast it can be charged and discharged), and its self-discharge rate.

The formation process is often time-consuming because it typically uses very low C-rates (slow charge/discharge speeds) to ensure the SEI grows in a controlled and effective manner. It is also an energy-intensive step. Additionally, side reactions during SEI/CEI formation produce gases (like ethylene and carbon dioxide), which need to be removed in later degassing steps, especially for larger cells. The exact "formation protocol"—which includes specific C-rates, voltage limits, number of cycles, temperature, and applied pressure—is often a closely guarded secret for cell manufacturers, as it's carefully customized for their cell chemistries and designs.