Overall, lithium battery production includes electrode manufacturing, battery assembly, and final processes such as electrolyte injection, sealing, formation, and aging. Each of these three stages can be further divided into several key processes, each of which significantly impacts the final battery performance.
The electrode manufacturing process can be further divided into five steps: slurry preparation, slurry coating, electrode rolling, electrode slitting, and electrode drying. The battery assembly process, depending on the battery specifications and model, is broadly divided into winding, casing, and welding processes. The electrolyte injection process after assembly includes electrolyte injection and sealing. Finally, there are three steps: formation, aging, and capacity testing. After battery manufacturing, the battery undergoes initial pre-activation and stabilization, which is the final formation-aging-capacity testing process.
I. Transformation
The concept of pre-formation refers to subjecting the manufactured lithium battery to a small-current charge and discharge cycle. After the lithium battery is manufactured, it undergoes a small-current charge and discharge process. The purpose of pre-charging is twofold:
1. After the battery is manufactured, the electrode material is not in its optimal state, or its physical properties are unsuitable (e.g., the particles are too large, the contact is not tight, etc.), or the phase itself is incorrect (e.g., some metal oxide anodes with alloy mechanisms). It is necessary to perform the first charge and discharge to activate it.
2. During the first charge of a lithium battery, Li+ ions are released from the positive electrode active material and, after passing through the electrolyte-separator-electrolyte cycle, embed into the interlayer of the graphite material at the negative electrode. During this process, electrons migrate from the positive electrode to the negative electrode along the peripheral circuitry. At this point, because the lithium ions have a lower embedding potential in the graphite negative electrode, the electrons react with the electrolyte first to form an SEI film and some gas. For more information on the uses and formation mechanism of SEI, see the article: What is SEI? It has such a significant impact on lithium batteries!
During this process, some gas is produced along with a small amount of electrolyte consumption. Some battery manufacturers perform venting and electrolyte replenishment after this process, especially for LTO batteries, where a large amount of gas can cause the battery bulge to exceed 10% in thickness. For graphite anodes, the amount of gas produced is less, so venting is unnecessary. This is because the SEI film that forms during the first charge hinders further reactions between electrons and electrolyte, preventing further gas production. This is the source of the irreversible capacity of graphite-based batteries; although it causes irreversible capacity loss, it also contributes to the battery's stability.
II. Aging
Aging generally refers to the storage period after the battery has been assembled and filled with electrolyte, followed by its first charge and formation. It can be done at room temperature or at high temperature, both of which aim to stabilize the properties and composition of the SEI film formed after the initial charge and formation, ensuring the stability of the battery's electrochemical performance. The main purposes of aging are threefold:
1. After the pre-formation process, a certain amount of SEI film will form on the graphite anode inside the battery. However, this film has a dense structure and small pores. Aging the battery at high temperature will help the SEI structure to recombine and form a loose and porous film.
2. After formation, the battery voltage is in an unstable stage, and its voltage is slightly higher than the actual voltage. The purpose of aging is to make its voltage more accurate and stable.
3. Placing the battery at high or normal temperature for a period of time can ensure that the electrolyte can fully wet the electrode plates, which is beneficial to the stability of battery performance.
The formation and aging process of batteries is essential. In actual production, the battery charging and discharging process is selected according to the battery's material system and structural system. However, battery formation must be carried out under low current conditions. After these two key processes, the stabilized batteries are then subjected to capacity testing, and after packaging and other processes, they are ready for shipment.
