The manufacturing process of li ion batteries is complex with numerous procedures. Mistakes in any link will affect the performance of cells of li ion batteries or bring safety issues. Therefore, qualified cells with excellent performance and safety can only be produced by strictly controlling each manufacturing process. Taking lithium ion 18650 as an example, this article will introduce the manufacturing process of the battery in detail, and explain the key points of control of each manufacturing process and its impact.
(1) Blending process
The most important process control point is the humidity control of the environment. Moisture is a key indicator that needs to be strictly controlled during the production of lithium ion 18650. Therefore, factors such as high environmental humidity and the use of water-based binders will increase the moisture content during the preparation of the pole piece.
(2) Slitting process
It is necessary to focus on controlling the size of the burr on the edge of the pole piece. There should be no obvious material loss, otherwise, the pole piece will pierce the diaphragm due to the material loss or the large burr, which will cause a short circuit. In the production process, it is necessary to prevent false welding, partical welding, welding leakage or falling material from the pole piece. Also, insulation adhesive partial paste or leakage paste should be avoided.
(3) Winding process
The winding process is an important process for forming lithium ion 18650. The strip-shaped positive and negative pole pieces and the separator are rolled into a cylindrical core by a winding machine. The key point of this process control is that the negative pole piece must completely cover the positive pole piece, and the separator must completely cover the negative pole piece, which requires high process accuracy.
After the core-winding is completed, it is necessary to put it into the steel shell and connect the negative electrode lug with the steel shell through bottom welding. At this time, the whole steel shell is the negative electrode of the battery, and the roll core in the steel shell is fixed by the roller groove. The process of entering the shell and the roller groove should not damage the core. The height of the roller groove must be strictly controlled, the core will be destroyed if it is too low, and the core will easily loosen if it is too high. Finally, the rolled core should be put into the shell in the baking box to brake. The next process can be continued if the moisture content of the core reaches the standard. Baking is a crucial step in strictly controlling the moisture content of the core.
(1) Liquid injection
It refers to the coil core that has qualified moisture requirements after baking by injecting the electrolyte into the liquid injection machine. After liquid injection, the four main materials of li ion batteries are all applied to the cell. The key of the liquid injection process is to precisely control the amount of liquid injected, control humidity and temperature, and be waterproof. Also, the electrolyte should be able to better infiltrate into the positive and negative pole pieces.
The amount of electrolyte is directly related to the safety performance and capacity of the battery. High injection volume will lead to large internal gas production of the battery, and the safety valve of the cylindrical battery tends to open prematurely. Small injection volume will lead to low battery capacity and lithium precipitation, which is more likely to produce thermal runaway and even cause an explosion.
The cover plate is welded to the positive electrode lug, and the whole cover plate is the positive electrode of the battery. The welding control point is to prevent false welding, partial welding and poor appearance of the cap.
The sealing process is to seal the steel shell and the cover plate to isolate the whole coil core from the external environment. The whole coil core is a closed electrochemical system. The sealing process is the last important process in cell manufacturing. The stability of the voltage forming technology determines whether the battery's sealing is intact and reliable. The completion of the sealing process means that the manufacture of a cell has been completed.
(4) Cleaning process
The purpose is to remove the residual electrolyte on the surface of the battery steel shell and prevent the electrolyte from corroding the steel shell. However, the coating process is to ensure that the positive and negative cells are separated to prevent short circuits in the external circuit. At the same time, it is necessary to make the battery have a good appearance. Therefore, screening the poor appearance of the battery is important in these two processes.
(1) Activation process
After the cell is covered with a film, it needs to be left in a constant temperature environment for a period to allow the electrolyte to fully infiltrate into the pole piece and separator to prevent lithium precipitation due to uneven electrolyte infiltration. The shelf environment temperature and shelving time should be controlled during the activation process.
(2) Formation process
This is the first charge of the lithium ion 18650. This step promotes the formation of a "solid electrolyte interface film" on the surface of the electrode. SEI film is a guarantee condition for long-term stable operation of li ion batteries, and it has a vital influence on capacity, rate, cycle, and safety performance. It is very important to choose appropriate formation current, formation voltage and formation temperature to optimize and improve battery performance. Therefore, this process is very important. In actual operation, it is necessary to prevent the battery from reverse connection, overcharge and poor contact.
(3) Aging process
A cell under a certain state of charge should be left in a certain temperature environment for a while. Cells with large or abnormal voltage drops should be filtered through the data before and after the test. In general, when a small short circuit occurs inside the cell or a severe side reaction exists, the cell voltage drop will be relatively large, which reflects the poor performance of the cell. And there will be safety risks. Therefore, the voltage drop is one of the important reference indexes for cell screening. Since the normal temperature aging takes a long time, most li ion battery manufacturers adopt high temperature or high charge aging methods to shorten the production cycle. The key points of process control should closely monitor whether there are any abnormal situation occurs during the shelving process, and timely actions should be taken to ensure safety.
(4) Sorting process
Different internal resistance ranges of the cell can be sorted based on the size of the AC internal resistance. Cells with abnormal internal resistance should be eliminated to make it easy to select suitable cells in series or parallel. Therefore, internal resistance is also one of the important reference indexes for cell screening. The capacity division process is to determine the capacity of the cells based on different capacity gears, and the cells with unqualified capacity should be eliminated. Due to the greater relationship between capacity and temperature, the more precise the environmental temperature control during the division, the closer the capacity is to the real value, and the more accurate the gear allocation. Therefore, temperature control is the key point of volume control. The cell shipped must be screened and qualified by voltage drop, internal resistance and capacity.