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The cathode material is one of the key core materials of lithium ion batteries, and its performance directly affects the performance indexes of lithium ion batteries. Currently, the cathode materials of lithium ion batteries in the market include lithium cobalt acid, lithium manganese acid, lithium iron phosphate and ternary materials. Compared with other raw materials for lithium-ion batteries, cathode materials are more diverse in variety and more complex in production process. Therefore, the risk of quality failure is greater, so the quality management requirements are higher
1. The cathode material is mixed with metal foreign matter
When the anode material of iron (Fe), copper (Cu) and chromium (Cr), nickel (Ni), zinc (zinc), silver (Ag), and other metal impurity, battery into phase voltage after reaching the REDOX potential of metal elements, the metal will be first in the anode oxidation to the cathode reduction, when the cathode in metal elemental accumulated to a certain degree, The hard edges of the deposited metal Pierce the diaphragm, causing the battery to self-discharge. Self-discharge can cause fatal effects on lithium ion batteries, so it is particularly important to prevent the introduction of metal foreign bodies from the source.
There are many production processes of positive electrode materials, and every link in the manufacturing process will have the risk of introducing metal foreign bodies, which puts forward higher requirements for the equipment automation degree and on-site quality management level of material suppliers. However, due to cost constraints, material suppliers tend to have a low degree of equipment automation, more breakpoints in production and manufacturing processes, and an increase in 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 bodies from the five aspects of human, machine, material, method and ring.
First of all, from the personnel control, employees should not carry metal foreign matter into the workshop, do not wear jewelry, enter the workshop should wear work clothes, work shoes, wear gloves, avoid contact with metal foreign matter and then contact with powder. The supervision and inspection mechanism should be established to cultivate the quality consciousness of employees, so that they consciously comply with and maintain the workshop environment.
Production equipment is the main link of foreign body introduction, such as the equipment parts and tools in contact with the material rust, inherent material wear phenomenon; The equipment parts and tools that are not directly in contact with the material will float into the material due to the airflow in the workshop after dust adhesion. According to the degree of influence, different treatment methods can be adopted, such as painting, replacing with non-metallic material coating (plastic, ceramic), and wrapping bare metal parts. Managers should also formulate corresponding rules and regulations, make clear provisions on how to manage metal foreign bodies, develop a point inspection list, and require employees to check regularly to prevent problems.
Raw materials are the direct source of metal foreign matter in cathode materials. The content of metal foreign matter should be regulated for the purchased raw materials. After entering the factory, the content should be strictly inspected to ensure that the content is within the prescribed range. If the content of metal foreign matter in the raw material exceeds the standard, it is difficult to remove it in the subsequent process.
In order to remove the metal foreign bodies, electromagnetic iron process has become a production of the anode materials, electromagnetic iron removal machine is widely used, but the device of non-magnetic metals such as copper, zinc, etc, so the workshop should avoid the use of copper and zinc components, such as must be used also as far as possible don't direct contact with powder or bare in the air. In addition, the installation position, installation number and parameter Settings of the electromagnetic iron remover also have a certain impact on the iron removal effect.
In order to ensure the workshop environment, to achieve positive pressure in the workshop, the establishment of double doors, air shower doors to avoid dust from the outside into the workshop pollution material is also a very necessary measure, at the same time workshop equipment, steel structure should avoid rust, the ground should also be painted and regular magnetic removal.
2.The moisture content of the cathode material exceeds the standard
Most of the cathode materials are micron or nanometer particles, which are easy to absorb moisture in the air, especially the ternary materials with high N I content. When preparing the positive paste, if the water content of the positive material is high, the solubility of P V DF will be reduced after N M P absorbs water in the process of slurry stirring, leading to the jelly shape of the paste gel and affecting the processing performance. After the battery is made, its capacity, internal resistance, circulation and rate will be affected, so the moisture of the cathode material and the metal foreign matter should be as the key control items.
The higher the degree of automation of production line equipment, the shorter the exposure time of powder in the air, and the less water is introduced. Promote material suppliers to improve the degree of equipment automation, such as the realization of the whole pipeline transportation, monitoring the dew point of the pipeline, the installation of the manipulator to realize automatic charging and cutting contribute greatly to the prevention of moisture introduction. However, some material suppliers are limited by plant design or cost pressure, equipment automation is not high, manufacturing process breakpoints should strictly control the exposure time of powder, powder in the transfer process is best to use a nitrogen filled bucket.
Temperature and humidity in the production workshop is also a key control index. Theoretically, the lower the dew point, the better. Most material suppliers will focus on the moisture control after the sintering process. They believe that the sintering temperature of about 100 degrees can remove most of the moisture in the powder. As long as the moisture introduction from the sintering process to the packaging stage is strictly controlled, the moisture of the material can be guaranteed not to exceed the standard. Of course, this does not mean that there is no need to control the moisture before the sintering process, because if the moisture is introduced too much before the sintering process, the sintering efficiency and the microstructure of the material will be affected. In addition, the way of packaging is also very important, most material suppliers use aluminum bag vacuum packaging, it seems that this way is still the most cost-effective.
Of course, with different material designs, there will be great differences in water absorption, such as differences in coating materials and specific surface area, which will affect its water absorption. Some suppliers prevent the introduction of water in the manufacturing process, but the material itself is easy to absorb water, after the sheet is made of water is extremely difficult to dry out, which causes problems for battery manufacturers. Therefore, the problem of water absorption should be taken into account when developing new materials to develop materials with higher universality, which is of great benefit to both supply and demand.
3. The batch consistency of cathode material is poor
For battery manufacturers, the less different and more consistent the cathode material between batches, the more stable the finished battery performance. As we all know, one of the main disadvantages of lithium iron phosphate cathode materials is poor batch stability. In pulping, the viscosity and solid content of each batch of slurry are unstable due to large batch fluctuation, which brings trouble to users and requires constant adjustment of the process to adapt.
Improving the automation degree of production equipment is the main means to improve the batch stability of lithium iron phosphate material. However, at present, the equipment automation degree of domestic lithium iron phosphate material suppliers is generally low, the technical level and quality management ability is not high, and the materials provided have different degrees of batch instability. From the user's point of view, if the batch differences cannot be eliminated, we hope that the larger the weight of a batch, the better, provided of course that the same batch of materials are uniform and stable. Therefore, in order to meet this requirement, iron lithium material suppliers often add a step of mixing process after the finished product is made, that is, several batches of materials are evenly mixed. The larger the volume of the mixing kettle, the more materials are contained, and the larger the quantity of a batch is mixed. Lithium iron material particle size, specific surface area, such as moisture, pH indicators will affect the made into slurry viscosity, but often these indicators have strictly controlled within a certain range, but still there will be a batch of pulp viscosity differences, in order to prevent the abnormal, when batch use tend to simulate production formula preparation in advance before put into use some test paste viscosity, After meeting the requirements, the battery manufacturer will be put into use. However, if the battery manufacturer has to test before each production, the production efficiency will be greatly reduced. Therefore, the battery manufacturer will forward the work to the material supplier and require the material supplier to complete the test and meet the requirements before shipping. Of course, with the progress of technology and the improvement of process capacity of material suppliers, the distribution of physical property indicators is becoming smaller and smaller, so the step of viscosity testing before delivery can be eliminated. In addition to the measures mentioned above to improve consistency, we should use quality tools to minimize this batch instability and prevent quality problems.