Lithium-ion batteries are a type of secondary rechargeable battery that relies on the movement of lithium ions between the positive and negative electrodes to function. During the charging and discharging process, Li+ ions are inserted and removed between the two electrodes: during charging, Li+ ions are removed from the positive electrode and embedded in the negative electrode, which is in a lithium-rich state; during discharging, the opposite occurs. Based on their shape, the common types of batteries in the current new energy vehicle market are: square aluminum shell batteries, pouch batteries, and cylindrical batteries.
The electrode sheets of lithium-ion batteries are the most important components of the battery cell. Slurry coating refers to the preparation process of the slurry material coated on the positive and negative electrodes of the lithium-ion cell. The preparation of the slurry material requires the mixing of positive and negative electrode materials, conductive agents, and binders. The prepared slurry material needs to be uniform and stable. Different manufacturers of lithium batteries have their own slurry coating process recipes, and the order of adding materials, the ratio of materials, and the stirring process during slurry coating have a great impact on the slurry coating effect. After slurry coating, the slurry material needs to undergo tests such as solid content, viscosity, and fineness to ensure that the slurry material meets the requirements.
The prepared positive and negative electrode slurries need to be coated onto aluminum foil or copper foil and dried. This process is called coating. The coating process of pouch batteries is a core process in the manufacturing of lithium-ion batteries, which largely determines the performance of lithium-ion batteries. The coated electrode sheets need to have a smooth surface, uniform color, and no exposed foil, particles, scratches, wrinkles, etc.
The coated electrode sheets need to undergo calendaring, wherein the electrode sheets are pulled between rotating rollers due to the friction generated between the rollers and the electrode sheets. The electrode sheets are compressed and densified. Calendaring is the process of compressing the materials on the positive and negative electrode plates, aiming to increase the compaction density of the positive or negative electrode materials. Suitable compaction density can increase the battery's discharge capacity, reduce internal resistance and polarization loss, extend the battery's cycle life, and improve the utilization of lithium-ion batteries. However, excessive or insufficient compaction density is not conducive to the insertion or removal of lithium ions. Therefore, when calendaring the electrode sheets, the pressing force should not be too large or too small, and should be based on the characteristics of the electrode sheet material.
Due to production capacity and efficiency requirements, the electrode sheets produced during the manufacturing process are relatively large. The rolled electrode sheets need to be cut into the required electrode sheet size, which is the process of slitting and tab welding.
The cut electrode sheets need to be stacked in the order of negative electrode, separator, positive electrode, separator, negative electrode, and separator. This process is called stacking, and the stacked electrode sheets are called battery cells. The stacking methods include Z-fold stacking and oscillating stacking. Some manufacturers use winding technology in this process. Compared to winding technology, the disadvantage of stacking technology is that it requires higher alignment accuracy of the electrode sheets, and currently, stacking machines have lower efficiency and automation. However, battery cells produced using stacking technology have better performance than those produced using winding technology. With the continuous expansion and development of the new energy industry, considering battery safety, production line efficiency, etc., stacking technology is still a long-term trend.
The stacked battery cells need to undergo tab welding, and the welded battery cells are placed in the aluminum-plastic film after being punched with holes and undergo processes such as top and side sealing. This is called encapsulation. In addition to the body of the battery cell, there is also excess aluminum-plastic film, which is called a gas bag. This is because a large amount of gas is generated during the formation of the battery cell, and this gas is removed together with the gas bag during the process.
Electrolyte injection is the process of injecting electrolyte into the encapsulated battery cell. The electrolyte serves as a carrier for the transmission of ions in the battery. By adding specific additives to the electrolyte, the performance of lithium-ion batteries can be improved in terms of safety, high and low-temperature performance, etc.
The battery cells after electrolyte injection need to be charged at a low current, which is equivalent to the activation process of lithium-ion batteries. During the initial charging process, a solid electrolyte interphase (SEI) film is formed on the surface of the negative electrode. The performance of the SEI film directly determines the rate capability and self-discharge performance of the lithium-ion battery, so the quality of the formation process directly determines the quality of the battery. During the formation process, a large amount of gas is generated, which affects the performance of the battery. Therefore, the battery after formation needs to undergo degassing. To ensure consistency in battery performance, lithium-ion batteries also need to undergo tests such as capacity grading, internal resistance, and self-discharge, and the batteries with different performances are grouped together.
