When iron (Fe), copper (Cu), chromium (Cr), nickel (Ni), zinc (Zn), silver (Ag) and other metal impurities exist in the cathode material, the voltage of the battery formation stage reaches the redox potential of these metal elements, these metals will be oxidized in the cathode and then reduced at the negative electrode, and when the metal monomers at the negative electrode accumulate to a certain extent, the hard corners of their deposited metals will pierce the diaphragm, causing self-discharge [2]. This causes self-discharge of the battery [2]. Self-discharge can be fatal to lithium-ion batteries, so it is particularly important to prevent the introduction of metal foreign matter from the source.
The production process of cathode materials is more complicated, and every link in the manufacturing process will have the risk of metal foreign matter introduction, which puts forward higher requirements on the automation degree of equipment and on-site quality management level of material suppliers. However, material suppliers often have a lower degree of automation of their equipment due to cost constraints, with more breakpoints in the manufacturing process and increased uncontrollable risks. Therefore, in order to ensure stable battery performance and prevent self-discharge from occurring, battery manufacturers must promote material suppliers to prevent the introduction of metal foreign objects from five aspects: people, machines, raw materials, methods and environment.
First of all, starting from personnel control, employees should be prohibited from carrying metal foreign objects into the workshop, prohibit the wearing of jewelry, enter the workshop should be wearing work clothes, work shoes, gloves, to avoid contact with metal foreign objects before contacting the powder. Supervision and inspection mechanism should be established to cultivate employees' quality consciousness, so that they consciously abide by and maintain the workshop environment.
Production equipment is the main link of the introduction of foreign matter, such as rust, inherent material wear and tear of equipment parts and tools in contact with the material; equipment parts and tools not in direct contact with the material, the dust adheres to the workshop air flow due to the role of floating into the material. According to the degree of impact, different treatment methods can be taken, such as painting, replacement with non-metallic coating (plastic, ceramic), bare metal parts for wrapping and so on. Managers should also formulate appropriate rules and regulations to clearly define how to manage metal foreign matter, develop a point check list, and require employees to check regularly to prevent problems before they occur.
Raw materials are the direct source of metal foreign matter in positive electrode materials, and the content of metal foreign matter should be stipulated in the purchased raw materials, which should be strictly inspected after entering the factory to ensure that the content is within the stipulated range. If the content of metal foreign matter in raw materials exceeds the standard, it is difficult to remove it in subsequent processes.
In order to remove metal foreign matter, electromagnetic iron removal has become a necessary process for the production of positive electrode materials, electromagnetic iron removal machine is commonly used, but the equipment does not work on non-magnetic metal substances such as copper, zinc, etc., so the workshop should avoid the use of copper, zinc parts, such as the use of must try not to be in direct contact with the powder or exposed in the air. In addition, the electromagnetic iron removal machine installation location, installation of the number of parameters set to remove iron effect also has a certain impact.
In order to ensure the workshop environment, realize the positive pressure of the workshop, the establishment of double doors, air shower door to avoid outside dust into the workshop pollutants material is also a very necessary measure, while the workshop equipment, steel structure should be avoided rust, the ground should also be painted and regularly demagnetized.
