Lithium-ion battery performance is closely related to the quality of the cathode material, metal foreign matter, moisture exceeding the standard, poor batch consistency, etc. are positive effects of the cathode material The form of failure. This paper clarifies the serious hazards of these failure modes on the performance of the battery, and explains how to avoid the occurrence of these failures from the perspective of quality management, in order to further prevent the occurrence of quality problems and improve the lithium ion battery. Quality is a strong guarantee.
The positive electrode material is one of the key core materials for lithium-ion batteries, and its performance directly affects the performance of lithium-ion batteries.The cathode materials for lithium ion batteries that have been marketed at present include lithium cobaltate, lithium manganate, lithium iron phosphate and ternary materials. Compared with other raw materials for lithium-ion batteries, the varieties of cathode materials are more diverse, the production process is more complicated, and the risk of quality failure is greater, so the requirements for quality management are higher. This article discusses the common failure modes of lithium ion battery cathode materials and the corresponding preventive measures from the perspective of material users.
I. Metal material in the cathode material
When When there are metal impurities such as iron, copper, chromium, nickel, zinc, silver, etc. in the positive electrode material, the voltage in the battery formation stage reaches the reduction potential of these metal elements, and these metals are first oxidized in the positive electrode. Then, when the anode is reduced, when the metal element at the negative electrode accumulates to a certain extent, the hard corners of the deposited metal will pierce the separator, causing the battery to self-discharge.
Self-discharge will have a fatal effect on lithium-ion batteries, so it is very important to prevent the introduction of metal foreign matter from the source. There are many production processes for cathode materials, and every step in the manufacturing process will be There is a risk of introduction of metal foreign bodies, which puts higher demands on the equipment supplier's equipment automation level and on-site quality management level. However, material suppliers often have lower equipment automation due to cost constraints, and manufacturing process breakpoints are more Many, uncontrollable risks increase. Therefore, in order to ensure stable battery performance and prevent self-discharge, battery manufacturers must promote material suppliers to prevent metal from the following five aspects: human, machine, material, law, and ring. Foreign matter is introduced.
First, from the beginning of personnel control, employees should be prohibited from carrying metal foreign objects into the workshop, jewellery is prohibited, and work clothes, work shoes, and gloves should be worn in the workshop. To avoid contact with metal foreign matter and then contact with powder. It is necessary to establish a supervision and inspection mechanism to cultivate employees' quality awareness so that they can consciously abide by and maintain the workshop environment.
Production equipment is the main link of foreign matter introduction, such as rust, inherent material wear, etc. of equipment parts and tools that come into contact with materials Phenomenon; equipment parts and tools that are not in direct contact with the material. After the dust adheres, it floats into the material due to the airflow in the workshop. Depending on the degree of influence, different treatment methods can be adopted, such as painting, replacing with non-metallic coatings (plastics, ceramics), and wrapping bare metal parts. Managers should also formulate corresponding rules and regulations, clearly define how to manage metal foreign bodies, 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 cathode material, and the metal foreign matter content of the purchased raw materials shall be specified. Strict test 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 metal foreign objects, electromagnetic iron removal has become a necessary process for the production of positive electrode materials. Electromagnetic iron removers are commonly used, but the equipment does not work for non-magnetic metal materials such as copper and zinc, so the workshop should avoid copper. For the use of zinc parts, if necessary, do not directly contact or expose the powder to the air. In addition, the installation position, number of installations and parameter settings of the electromagnetic iron remover have certain influence on the iron removal effect.
In order to ensure the workshop environment, to achieve positive pressure in the workshop, establish double doors and air shower doors to avoid external dust from flowing into the workshop. [u>Contamination materials are also very The necessary measures, at the same time, workshop equipment, steel structure should avoid rust, the ground should also be painted and periodically demagnetized.
Second, cathode material Excessive moisture
The positive electrode materials are mostly micron or nano-sized particles, which easily absorb moisture in the air, especially ternary materials with high Ni content. .In the preparation of the positive electrode slurry, if the positive electrode material has a high moisture content, the NMP absorbs water during the slurry agitation process, which causes the PVDF solubility to decrease, resulting in the slurry gel being frozen and affecting the processing performance. After the battery is fabricated, its capacity, internal resistance, circulation and rate will be affected. Therefore, the moisture of the positive electrode material is the same as the metal foreign matter.
The higher the automation of the line equipment, the shorter the exposure time of the powder in the air and the less moisture is introduced. Promote material suppliers to improve the degree of equipment automation, such as to achieve full pipeline transportation, monitoring pipeline dew point, installation of mechanical tools, automatic feeding, cutting to contribute to the prevention of moisture introduction. However, some material suppliers are limited by the design of the plant or the cost pressure. The degree of automation of the equipment is not high. When the number of breakpoints in the manufacturing process is large, the exposure time of the powder should be strictly controlled. The powder in the transfer process is preferably filled with a cylinder filled with nitrogen. The temperature and humidity of the production workshop is also a key control indicator. In theory, the lower the dew point, the more favorable it is. Most material suppliers 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 moisture introduction at the stage of the sintering process is strictly controlled, it can basically ensure that the moisture of the material does not 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 in the previous process, the sintering efficiency and the microscopic morphology of the material are affected. In addition, the packaging method is also very important, most material suppliers use aluminum plastic bags to vacuum packaging, it seems that this method is still the most cost-effective.
Of course, the material design is different, and the water absorption will be greatly different. For example, the difference in coating material and the difference in specific surface area will affect the water absorption. Although some material suppliers prevent the introduction of moisture during the manufacturing process, the material itself has the property of being easily absorbed, and the moisture is extremely difficult to be baked after the pole piece is formed, which causes trouble for the battery manufacturer. Therefore, the development of new materials should take into account the problem of water absorption and the development of more universal materials, which is of great benefit to both the supply and demand sides.
Three, poor positive consistency of batch material
for For battery manufacturers, the smaller the difference between the batches of positive electrode materials and the better the consistency, the more stable the performance of the finished battery. Everyone knows that one of the main disadvantages of lithium iron phosphate cathode materials is the poor batch stability. In the pulping process, the viscosity and solid content of each batch of slurry are often unstable due to large batch fluctuations. It is a hassle, and it is necessary to constantly adjust the process to adapt. Increasing the automation of production equipment is the main means to improve the stability of batches of lithium iron phosphate materials. However, at present, the domestic equipment of lithium iron phosphate materials is generally low in automation, and the technical level and quality management ability are not high. There are varying degrees of batch instability issues.
From the perspective of the user, if the batch difference cannot be eliminated, we hope that the larger the weight of a batch, the better.Of course, the premise is that the materials of the same batch are even and stable. Therefore, in order to meet this requirement, iron-lithium material suppliers often add a one-step mixing process after the finished product is finished, that is, several batches of materials are uniformly mixed. The larger the volume of the mixing kettle, the more materials are contained, and the mixture is mixed. The larger the amount of one batch.
The particle size, specific surface area, moisture, pH and other indicators of iron-lithium materials will affect the viscosity of the slurry, but often these indicators are strict Within a certain range, there may still be a large difference in the viscosity of the batch slurry. In order to prevent abnormalities during batch use, some of the slurry test viscosities are prepared in advance by simulating the production formula before being put into use, and then put into use after meeting the requirements. However, if the battery manufacturer tests it before each production, it will greatly reduce the production efficiency, so that the work is put forward to the material supplier, and the material supplier is required to complete the test and then deliver the product. Of course, with the advancement of technology, the material supplier's process capability is improved, the distribution of physical property indicators is getting smaller and smaller, and the step of testing the viscosity before shipment can be omitted.
In addition to the above-mentioned measures to improve consistency, we should also use quality tools to minimize this batch instability and prevent quality problems. happened. Mainly from the following aspects.
(1) Establishment of the operating procedures. The inherent quality of the product is designed and manufactured. Therefore, how the operator operates is particularly important for controlling product quality, and detailed and specific operational standards should be established.
(2) Identification of CTQ. Identify key indicators and key processes that affect product quality, and monitor these key control indicators and develop appropriate emergency response measures. The orthophosphoric acid railway line is the mainstream of the current preparation of lithium iron phosphate. The 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 consistency of the particle size of the finished product will be affected.This in turn affects the batch consistency of the material.
(3) Use of SPC. SPC time-critical monitoring of key characteristic parameters of key processes, analysis of abnormal points, finding out the causes of instability, taking effective corrective measures and preventive measures to prevent defective products from flowing to the client.
Four other bad conditions
When pulping, The positive electrode material is mixed with solvent, glue and conductive agent in a certain proportion in a pulping tank, and then discharged through a pipeline. A filter screen is installed at the discharge port, and the purpose is to intercept large particles and foreign matter in the positive electrode material to ensure coating. The quality of the cover. If the positive electrode material contains large particles, the filter will be clogged. If the composition of the large particles is still the positive electrode material itself, it only affects the production efficiency and will not affect the battery performance, and the loss can be smaller. But if the composition of such large particles is uncertain, it is other metal foreign matter,The slurry that has already been produced will be completely scrapped, and the losses will be enormous. This kind of abnormality should be caused by the quality management inside the material supplier. Most of the positive electrode materials are produced through the screening process, whether the screen is damaged, whether it is inspected and replaced in time, if the screen is damaged or not. Measures, whether to check for large particles at the time of factory inspection, etc., still need to be improved.