18650 lithium battery screening process and control points.

18650 lithium battery screening process and control points.

Due to differences in battery raw materials, production processes, and other performance factors such as battery capacity, voltage, and internal resistance, the performance of battery packs cannot reach the level of individual batteries, and their service life is much shorter than that of individual batteries, which affects the use of electric vehicles. The battery screening process includes activation, formation, aging, sorting, and capacity separation processes.

The activation process involves placing the battery cells in a constant temperature environment for a period of time after coating them, allowing the electrolyte to fully infiltrate the electrodes and separators to prevent lithium deposition due to uneven electrolyte infiltration. The activation process requires controlling the temperature and duration of the storage environment. The formation process is to charge the lithium ion battery for the first time. This process promotes the formation of a layer of "solid electrolyte interface facial mask" (SEI film) on the electrode surface. It refers to the interface facial mask formed during the first charge of the lithium ion battery due to the oxidation and reduction decomposition of the electrolyte and deposition on the surface of the electrode material. It has the characteristics of ion conduction and electronic insulation. Li+can be freely embedded and removed through this film. Because of the characteristics of its solid electrolyte, it is called "solid electrolyte interface facial mask". SEI film is a guarantee condition for the long-term stable operation of lithium-ion batteries, which has a crucial impact on their capacity, rate, cycling, safety performance, etc. Choosing appropriate formation current, formation voltage, formation temperature, etc. plays a very important role in optimizing and improving battery performance. Therefore, this process is very important. In practical operation, it is necessary to prevent battery reverse connection, overcharging, and poor contact.

The aging process involves placing a battery cell in a constant temperature environment under a certain state of charge for a period of time, and filtering the data based on the voltage before and after testing to eliminate cells with large or abnormal voltage drops. When micro short circuits or severe side reactions occur inside a typical battery cell, the voltage drop of the cell will be significant, indicating poor performance and potential safety risks. Therefore, voltage drop is one of the important reference indicators for battery cell screening. Due to the long aging time at room temperature, in order to shorten the production cycle, the vast majority of lithium-ion battery manufacturers adopt high-temperature or high charge state aging methods. The key points of process control need to closely monitor whether there are any abnormal situations during the storage process, and take timely action to ensure safety. As a general high-temperature aging room is a closed space, it is not easy to be detected in the initial stage. Once a fire occurs, the consequences are unimaginable.

The sorting process is to select cells with different internal resistance levels based on the AC internal resistance of the cells, eliminate cells with abnormal internal resistance, and easily select suitable cells for series or parallel combination. Therefore, internal resistance is also an important reference indicator for cell screening. The capacity division process involves selecting cells according to different capacity levels after measuring their capacity, and removing cells with unqualified capacity. Due to the significant relationship between capacity and temperature, the more accurate the environmental temperature control during capacity division, the closer the capacity is to the true value, and the more accurate the level allocation. Therefore, temperature control is the key to capacity division control. The shipped battery cells must be qualified cells that have undergone voltage drop, internal resistance, and capacity screening.