1. Metal foreign matter mixed in the positive electrode material
When there are metal impurities such as iron, copper, chromium, nickel, zinc, and silver in the positive electrode material, after the voltage in the formation stage of the battery reaches the redox potential of these metal elements, these metals will be oxidized at the positive electrode and then reduced at the negative electrode. When the metal element at the negative electrode accumulates to a certain extent, the hard edges and corners of the deposited metal will pierce the separator, causing the battery to self-discharge.
Self-discharge can have a fatal impact on lithium-ion batteries, so it is particularly important to prevent the introduction of metal foreign matter from the source. There are many production processes for cathode materials, and every link in the manufacturing process will have the risk of introducing metal foreign matter, which puts forward higher requirements on the degree of equipment automation and on-site quality management level of material suppliers. However, due to cost constraints, material suppliers often have a low degree of automation of their equipment, many production and manufacturing process breakpoints, and increased uncontrollable risks. Therefore, in order to ensure stable battery performance and prevent self-discharge, battery manufacturers must promote material suppliers to prevent the introduction of metal foreign matter from five aspects: human, machine, material, method and environment.
First of all, starting from personnel control, employees should be prohibited from bringing foreign metal objects into the workshop, and it is forbidden to wear jewelry. When entering the workshop, they should wear overalls, work shoes, and gloves to avoid contact with metal foreign objects before contacting powder materials. It is necessary to establish a supervision and inspection mechanism, cultivate the quality awareness of employees, and make them consciously abide by and maintain the workshop environment.
Production equipment is the main link for the introduction of foreign objects. For example, the equipment parts and tools in contact with the material are rusted and the inherent material is worn; the equipment parts and tools that are not in direct contact with the material will float to the material due to the air flow in the workshop after the dust adheres. middle. Depending on the degree of impact, different treatment methods can be adopted, such as painting, replacing with non-metallic coating (plastic, ceramic), wrapping bare metal parts, etc. Managers should also formulate corresponding rules and regulations, clearly stipulate how to manage foreign metal objects, formulate checklists, and require employees to check regularly to prevent problems before they occur.
The raw material is the direct source of metal foreign matter in the positive electrode material. The purchased raw material should be subject to regulations on the content of metal foreign matter. After entering the factory, it should be strictly inspected to ensure that its content is within the specified range. If the metal foreign matter content of the raw material exceeds the standard, it is difficult to remove it in the subsequent process.
In order to remove foreign metal objects, electromagnetic iron removal has become a necessary process for the production of positive electrode materials. Electromagnetic iron removal machines are widely used, but this equipment does not work on non-magnetic metal substances such as copper and zinc. Therefore, copper and zinc should be avoided in the workshop. The use of parts, if necessary, should also try not to directly contact the powder or expose it to the air. In addition, the installation position, number of installations, and parameter settings of the electromagnetic iron remover also have a certain influence on the iron removal effect.
In order to ensure the workshop environment and realize the positive pressure of the workshop, it is also necessary to establish double doors and air shower doors to prevent external dust from flowing into the workshop contaminated materials. At the same time, the workshop equipment and steel structure should avoid rusting, and the ground should also be painted and regularly removed. magnetic.
2. The moisture content of the positive electrode material exceeds the standard
Most of the cathode materials are micron or nanoscale particles, which can easily absorb moisture in the air, especially ternary materials with high Ni content. When preparing positive electrode slurry, if the water content of the positive electrode material is high, the solubility of PVDF will decrease after NMP absorbs water during the slurry stirring process, resulting in a jelly-like slurry gel, which affects the processing performance. After the battery is made, its capacity, internal resistance, cycle and rate will be affected, so the moisture of the positive electrode material should be the same as the metal foreign matter as a key control item.
The higher the automation of the production line equipment, the shorter the time the powder is exposed to the air, and the less moisture is introduced. Promoting material suppliers to improve the degree of automation of equipment, such as the realization of whole-process pipeline transportation, monitoring of pipeline dew point, and the installation of manipulators to realize automatic material loading and unloading make great contributions to preventing the introduction of moisture. However, some material suppliers are limited by plant design or cost pressure, and the degree of automation of equipment is not high, and when there are many breakpoints in the manufacturing process, it is necessary to strictly control the exposure time of powder materials. The temperature and humidity of the production workshop is also a key control indicator. In theory, the lower the dew point, the better. Most material suppliers will focus on moisture control after the sintering process. They believe that the sintering temperature of about 1000 degrees can remove most of the moisture in the powder. As long as the introduction of moisture from the sintering process to the packaging stage is strictly controlled, the material can basically be guaranteed. Moisture does not exceed the standard. Of course, this does not mean that there is no need to control moisture before the sintering process, because if too much moisture is introduced in the previous process, the sintering efficiency and the microscopic morphology of the material will be affected. In addition, the packaging method is also very important. Most material suppliers use the aluminum-plastic bag vacuum packaging method. At present, this method is still the most economical and effective.
Of course, different material designs will have great differences in water absorption. For example, differences in coating materials and differences in specific surface area will affect its water absorption. Although some material suppliers prevent the introduction of moisture in the manufacturing process, the material itself has the characteristics of easy water absorption, and it is extremely difficult to bake out the moisture after the pole piece is made, which causes trouble to the battery manufacturer. Therefore, the issue of water absorption should be taken into account when developing new materials, and the development of materials with higher universality is of great benefit to both supply and demand.
3. The batch consistency of cathode material is poor
For battery manufacturers, the smaller the batch-to-batch variation and the better the consistency of the cathode material, the more stable the performance of the finished battery can be. We all know that one of the main shortcomings of lithium iron phosphate cathode materials is the poor batch stability. During pulping, the viscosity and solid content of each batch of slurry are often unstable due to large batch fluctuations, which brings users a lot of trouble. There is trouble, and the process needs to be constantly adjusted to adapt. Improving the automation degree of production equipment is the main means to improve the batch stability of lithium iron phosphate materials. However, at present, the equipment automation degree of domestic lithium iron phosphate material suppliers is generally low, and the technical level and quality management ability are not high. There are varying degrees of batch instability.
From the user's point of view, if batch differences cannot be eliminated, we hope that the larger the weight of a batch, the better. Of course, the premise is that the materials in the same batch are uniform and stable. Therefore, in order to meet this requirement, iron and lithium material suppliers often add a mixing process after the finished product is made, that is, to mix several batches of materials evenly. The larger the amount of a batch.
The particle size, specific surface area, moisture, pH value and other indicators of iron-lithium materials will affect the viscosity of the prepared slurry, but these indicators are often strictly controlled within a certain range, but there will still be large differences in batch slurry viscosity. In order to prevent abnormality during batch use, some slurry is often prepared in advance to simulate the production formula before being put into use to test the viscosity, and then put into use after meeting the requirements. Therefore, this work is forwarded to the material supplier, and the material supplier is required to complete the test and meet the requirements before shipping. Of course, with the advancement of technology and the improvement of the process capability of material suppliers, the dispersion of physical property indicators is getting smaller and smaller, and the step of testing the viscosity before delivery can be omitted.
In addition to the above-mentioned measures to improve consistency, we should use quality tools to minimize this batch instability and prevent quality problems. Mainly from the following aspects.
(1) The establishment of operating procedures. The inherent quality of a product is designed and manufactured. Therefore, how the operator operates is particularly important for controlling product quality, and detailed and specific operating standards should be established.
(2) Identification of CTQ. Identify key indicators and key processes that affect product quality, conduct special monitoring on these key control indicators, and formulate corresponding emergency response measures. The orthophosphoric acid railway line is currently the mainstream for the preparation of lithium iron phosphate. Its processes include batching, ball milling, sintering, pulverization, packaging, etc. The ball milling process should be managed as a key process, because the consistency of the primary particle size after ball milling is not controlled. Well, the particle size consistency of the finished product is affected, which in turn affects the batch consistency of the material.
(3) The use of SPC. The key characteristic parameters of key processes are monitored by SPC from time to time, and abnormal points are analyzed to find out the cause of instability, take effective corrective and preventive measures, and prevent defective products from flowing to the client.
4. Other adverse situations
During pulping, the positive electrode material is evenly mixed with solvent, glue and conductive agent in a certain proportion in the pulping tank and then discharged through the pipeline. A filter screen is installed at the discharge port to intercept large particles and foreign matter in the positive electrode material. , to ensure the quality of the coating. If the positive electrode material contains large particles, the filter will be blocked. If the large particles are still the positive electrode material itself, it will only affect the production efficiency and will not affect the battery performance. Such losses can be smaller. However, if the composition of such large particles is uncertain, and it is other metal foreign matter, the slurry that has been made will be scrapped, and the loss will be huge. If this abnormality occurs, it should be a problem with the internal quality management of the material supplier. Most of the production of cathode materials has a screening process, whether the screen is damaged, whether it is checked and replaced in time, and if the screen is damaged, whether it is foolproof Measures, whether to carry out the detection of large particles during the ex-factory inspection, etc., still need to be improved.