Quality management of positive electrode materials for lithium batteries

Quality management of positive electrode materials for lithium batteries

The performance of lithium-ion batteries is closely related to the quality of positive electrode materials.

This article introduces several failure forms of positive electrode materials that have a significant impact on the performance of lithium-ion batteries, such as mixing with metal foreign objects, excessive moisture, and poor batch consistency. It elucidates the serious harm these failure forms cause to battery performance, and explains how to avoid these failures from a quality management perspective, providing strong guarantees for further preventing quality problems and improving the quality of lithium-ion batteries.

As we all know, the cathode material is one of the key core materials of lithium ion batteries, and its performance directly affects the performance indicators of lithium ion batteries. At present, the marketed cathode materials of lithium ion batteries include lithium cobalate, lithium manganate, Lithium iron phosphate, ternary materials and other products.

Compared to other raw materials for lithium-ion batteries, the variety of positive electrode materials is more diverse, the production process is also more complex, and the risk of quality failure is greater, thus requiring higher quality management requirements. This article discusses the common failure forms and corresponding preventive measures of positive electrode materials for lithium-ion batteries from the perspective of material users.

1. Metal foreign objects mixed in the positive electrode material

When there are iron (Fe), copper (Cu), chromium (Cr), nickel (Ni), zinc (Zn), silver (Ag) and other metal impurities in the cathode material, when the voltage in the formation stage of the battery reaches the oxidation and reduction potential of these metal elements, these metals will be oxidized first in the positive pole and then reduced to the negative pole. When the metal elements at the negative pole accumulate to a certain extent, the hard edges and corners of the deposited metal will pierce the diaphragm, causing self discharge of the battery.

Self discharge can have a fatal impact on lithium-ion batteries, so it is particularly important to prevent the introduction of metal foreign objects from the source.

There are many production processes for positive electrode materials, and there is a risk of metal foreign objects being introduced in every step of the manufacturing process. This puts forward higher requirements for the equipment automation level and on-site quality management level of material suppliers. However, material suppliers often have lower levels of equipment automation due to cost constraints, resulting in 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 objects from five aspects: human, machine, material, method, and environment.

Starting from personnel control, employees should be prohibited from carrying metal foreign objects into the workshop, wearing jewelry, and wearing work clothes, shoes, and gloves when entering the workshop to avoid contact with metal foreign objects before coming into contact with powder. To establish a supervision and inspection mechanism, cultivate employees' quality awareness, and make them consciously comply with and maintain the workshop environment.

Production equipment is the main link for the introduction of foreign objects, such as rust and inherent material wear on equipment components and tools that come into contact with materials; Equipment components and tools that do not come into direct contact with the material, and dust adheres and floats into the material due to the airflow in the workshop. According to the degree of impact, different treatment methods can be adopted, such as painting, replacing with non-metallic material coatings (plastic, ceramic), and wrapping bare metal components. Managers should also establish corresponding rules and regulations to clearly define how to manage metal foreign objects, establish a checklist, and require employees to conduct regular inspections to prevent potential problems.

Raw materials are the direct source of metal foreign objects in positive electrode materials. The purchased raw materials should have regulations on the content of metal foreign objects. After entering the factory, strict inspection should be carried out to ensure that their content is within the specified range. If the content of metal impurities in the raw materials exceeds the standard, it is difficult to remove them in subsequent processes.

In order to remove metal foreign objects, electromagnetic iron removal has become a necessary process in 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, the workshop should avoid the use of copper and zinc components. If necessary, it is also recommended to avoid direct contact with powder or exposure to the air. In addition, the installation position, number of installations, 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 and achieve positive pressure in the workshop, it is also necessary to establish double doors and air shower doors to prevent external dust from entering the workshop and polluting materials. At the same time, workshop equipment and steel structures should avoid rust, and the ground should also be painted and regularly demagnetized.

2. The moisture content of the positive electrode material exceeds the standard

The positive electrode materials are mostly micron or nanoscale particles, which are easy to absorb moisture from the air, especially ternary materials with high Ni content. When preparing the positive electrode paste, if the positive electrode material has high water content, the solubility of PVDF will be reduced after NMP absorbs water during the slurry mixing process, which will cause the paste gel to become jelly, affecting the processing performance. After making a battery, its capacity, internal resistance, circulation, and magnification will be affected, so the moisture content of the positive electrode material, like metal foreign objects, should be a key control project.

The higher the automation level of the production line equipment, the shorter the exposure time of the powder in the air, and the less water is introduced. Promoting material suppliers to improve equipment automation, such as achieving full pipeline transportation, monitoring pipeline dew points, and installing robotic arms to achieve automatic loading and unloading, contributes greatly to preventing the introduction of moisture. However, some material suppliers are limited by factory design or cost pressures, and when equipment automation is not high and there are many breakpoints in the manufacturing process, it is necessary to strictly control the exposure time of the powder. It is best to use nitrogen filled barrels for the powder during the transfer process.
The temperature and humidity of the production workshop is also a key control indicator, and theoretically, the lower the dew point, the more favorable it is. Most material suppliers focus on moisture control after the sintering process. They believe that a sintering temperature of around 1000 degrees Celsius 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, it can basically ensure that the moisture content of the material 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 microstructure of the material will be affected. In addition, the packaging method is also very important. Most material suppliers use aluminum plastic bags for vacuum packaging, which currently appears to be the most economical and effective method.

Of course, different material designs can also have significant differences in water absorption, such as differences in coating materials and specific surface area, which can affect their water absorption. Although some material suppliers prevent the introduction of moisture during the manufacturing process, the materials themselves have the characteristic of being easy to absorb water, making it extremely difficult to dry out the moisture after being made into electrode plates, which causes trouble for battery manufacturers. Therefore, when developing new materials, consideration should be given to the issue of water absorption and the development of materials with higher universality, which is greatly beneficial for both supply and demand.

3. Poor batch consistency of 3 positive electrode materials

For battery manufacturers, the smaller the difference and better the consistency between batches of positive electrode materials, the more stable the performance of the finished battery can be. As we all know, one of the main disadvantages of Lithium iron phosphate cathode material is the poor batch stability. In the process of pulping, the viscosity and solid content of each batch of slurry are unstable due to large batch fluctuations, which brings trouble to users and requires constant process adjustment 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, the technical level and quality management ability are not high, and the materials provided have batch instability problems of different degrees. From the perspective of users, if batch differences cannot be eliminated, we hope that the larger the weight of a batch, the better, provided that the materials in the same batch are uniform and stable.

So in order to meet this requirement, iron lithium material suppliers often add a mixing process after making the finished product, which is to evenly mix several batches of materials. The larger the volume of the mixing kettle, the more materials it contains, and the larger the amount of mixed batch.

The particle size, specific surface area, moisture, pH value, and other indicators of iron lithium materials can affect the viscosity of the produced slurry. However, these indicators are often strictly controlled within a certain range, and there may still be significant differences in viscosity between batches of slurry. In order to prevent anomalies during batch use, it is often necessary to simulate the production formula and prepare some slurry viscosity tests in advance before putting them into use, and only after meeting the requirements can they be put into use, But if battery manufacturers conduct testing before each production, it will greatly reduce production efficiency, so they will forward this work to the material supplier and require the material supplier to complete the testing and meet the requirements before shipping.

Of course, with the advancement of technology and the improvement of material suppliers' process capabilities, the dispersion of physical properties is becoming smaller and smaller, and the step of testing viscosity before shipment can be omitted. In addition to the measures mentioned above to improve consistency, we should also use quality tools to minimize batch instability and prevent quality issues from occurring. Mainly starting from the following aspects.

(1) Establish operating procedures.

The inherent quality of a product is both designed and manufactured. Therefore, how operators operate is particularly important for controlling product quality, and detailed and specific operating standards should be established.

(2) Identification of CTQ.

Identify key indicators and processes that affect product quality, monitor these key control indicators, and develop corresponding emergency response measures. The orthophosphoric acid railway line is the mainstream of the current preparation of Lithium iron phosphate. Its processes include batching, ball milling, sintering, crushing, packaging, etc. The ball milling process should be managed as a key process, because if the consistency of the primary particle size after ball milling is not well controlled, the consistency of the particle size of the finished product will be affected, which will affect the batch consistency of materials.

(3) The use of SPC.

Conduct SPC real-time monitoring of key characteristic parameters of key processes, analyze abnormal points, identify the causes of instability, take effective corrective and preventive measures, and avoid defective products flowing to the client.

4. Other adverse situations

When making slurry, the positive electrode material is mixed evenly with solvents, adhesives, and conductive agents in a certain proportion in the slurry tank, and then discharged through the pipeline. A filter screen is installed at the outlet to intercept large particles and foreign objects in the positive electrode material and ensure the quality of coating. If the positive electrode material contains large particles, it will cause clogging of the filter screen. If the composition of the large particles is still the positive electrode material itself, it will only affect production efficiency and will not affect battery performance, and such losses can be reduced. But if the composition of these large particles is uncertain and they are other metal foreign objects, the already made slurry will be completely scrapped, resulting in huge losses.

The occurrence of this abnormality should be due to internal quality management issues within the material supplier. Most positive electrode materials are produced through screening processes, and whether the screen is damaged, inspected and replaced in a timely manner. If the screen is damaged, there are no anti leakage measures, and whether large particles are detected during factory inspection still need to be improved.