Quality Management of Lithium Battery Cathode Materials
The performance of lithium-ion batteries is closely related to the quality of the cathode materials.
The article discusses several failure modes of cathode materials that significantly impact the performance of lithium-ion batteries, such as the presence of metal impurities, excessive moisture, and poor batch consistency. It elucidates the serious harm these failure modes can cause to battery performance and explains how to prevent these failures from occurring from a quality management perspective. This provides strong assurance for preventing quality issues and improving the quality of lithium-ion batteries.
As widely known, cathode materials are one of the key core materials of lithium-ion batteries, and their performance directly affects various performance indicators of lithium-ion batteries.
Currently commercialized cathode materials for lithium-ion batteries include lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, and ternary materials, among others.
Compared to other raw materials for lithium-ion batteries, the variety of cathode materials is more diversified. Besides, the production processes are more complex, which increases the risk of quality failures. Therefore, the requirements for quality management are higher.
This article discusses common failure modes of cathode materials for lithium-ion batteries and the corresponding preventive measures from the perspective of material users.
1. Presence of Metal Impurities in Cathode Materials
When there are metal impurities such as iron (Fe), copper (Cu), chromium (Cr), nickel (Ni), zinc (Zn), silver (Ag), etc., in cathode materials, during the battery formation stage, when the voltage reaches the oxidation-reduction potential of these metal elements, they will undergo oxidation in the cathode before being reduced in the anode.
When the accumulated metal substance at the anode reaches a certain level, its sharp edges can penetrate the separator, leading to self-discharge of the battery.
Self-discharge has a detrimental effect on lithium-ion batteries, making it particularly important to prevent the introduction of metal impurities at the source.
Cathode Material Production Involves Multiple Processes, and at each stage of manufacturing, there is a risk of introducing metal impurities. This places higher demands on the level of automation of equipment and on-site quality management for material suppliers.
However, due to cost limitations, material suppliers often have lower levels of equipment automation, and there are many breakpoints in the production process, increasing uncontrollable risks.
Therefore, in order to ensure stable battery performance and prevent self-discharge, battery manufacturers must encourage material suppliers to prevent the introduction of metal impurities from five major aspects: personnel, machinery, materials, methods, and the environment.
Measures to Prevent the Introduction of Metal Impurities
* Personnel Management
Starting with personnel control, employees should be prohibited from bringing metal impurities into the workshop.
They should refrain from wearing jewelry and must wear work uniforms, work shoes, and gloves when entering the workshop to avoid contact with metal impurities before handling the powder materials.
It is essential to establish a supervision and inspection mechanism, cultivate employees’ awareness of quality, and make them consciously abide by and maintain the workshop environment.
* Production Equipment Management and Inspection
Production equipment is a key area where impurities can be introduced.
This includes equipment components and tools that come into contact with materials exhibiting rust or inherent material wear. Additionally, equipment components and tools not in direct contact with materials may accumulate dust, which, due to workshop airflow, can float into the materials.
Depending on the severity of the issue, different approaches can be taken, such as painting, replacing with non-metallic coatings (plastic, ceramic), or encasing exposed metal components.
Managers should also establish corresponding rules and regulations specifying how to manage metal impurities, create inspection checklists, and require employees to perform regular inspections to prevent potential issues.
* Raw Material Quality Inspection
Raw materials are the direct source of metal impurities in positive electrode materials. Specifications for metal impurity content should be established for purchased raw materials. And strict inspections should be conducted upon arrival to ensure that the content falls within the specified range.
If the metal impurity content of raw materials exceeds the standard, it becomes challenging to remove them in subsequent processes.
* Use of Electromagnetic Iron Removers
To remove metal impurities, electromagnetic iron removers has become an essential process in the production of positive electrode materials. Electromagnetic iron removers are widely used.
However, this equipment does not work on non-magnetic metals such as copper and zinc. Therefore, workshops should avoid the use of copper and zinc components. And if necessary, they should try to avoid direct contact with or exposure to the air with the powder materials.
In addition, the installation location, number of installations, and parameter settings of electromagnetic iron removers also have some impact on their effectiveness.
* Workshop Environmental Assurance
To ensure the workshop environment and maintain positive pressure in the workshop, establishing double doors and air shower doors to prevent external dust from entering the workshop and contaminating materials is a necessary measure.
At the same time, workshop equipment and steel structures should be protected from rust, and the floor should be painted and demagnetized regularly.
2. Excessive Moisture in Positive Electrode Materials
Positive electrode materials are mostly composed of micrometer or nanometer-sized particles, making them highly susceptible to absorbing moisture from the air, especially in the case of ternary materials with high nickel (Ni) content.
When preparing the positive electrode slurry, if the positive electrode material has high moisture content, the absorption of water by NMP during the slurry mixing process can lead to a reduction in PVDF solubility, causing the slurry to gel into a jelly-like substance, which affects processing performance.
After making the battery, its capacity, internal resistance, cycling performance, and rate capability are all affected. Therefore, moisture content in positive electrode materials, like metal impurities, should be a key focus for control.
Ways to Control Moisture in Positive Electrode Materials for Material Suppliers
* Increase Automation of Equipment
The higher the level of automation of production line equipment, the shorter the exposure time of the powder to air, resulting in less moisture intake.
Encouraging material suppliers to improve the level of automation of equipment, such as implementing full pipeline transportation, monitoring pipeline dew points, and installing robots for automatic loading and unloading, contributes significantly to preventing moisture intake.
However, some material suppliers may be limited by factory design or cost pressures, leading to lower levels of equipment automation and more interruptions in the manufacturing process. In such cases, strict control over the exposure time of the powder is necessary, and powder during the transfer process is best stored in nitrogen-filled containers.
* Strictly Control the Temperature and Humidity in the Production Workshop
The temperature and humidity in the production workshop are also a key control parameter, and theoretically, a lower dew point is more favorable.
Most material suppliers pay close attention to moisture control after the sintering process. They believe that a sintering temperature of around 1000 degrees can remove most of the moisture from the powder. As long as moisture introduction is strictly controlled from the sintering process to the packaging stage, it is possible to ensure that the material’s moisture content does not exceed the specified limits.
However, this does not mean that moisture control before the sintering process is unnecessary. Because excessive moisture introduced in earlier processes can affect sintering efficiency and the material’s microstructure.
In addition, the packaging method is also important. And most material suppliers use aluminum-plastic bags with vacuum sealing, which is currently considered the most cost-effective method.
* Consider the Moisture Absorption Characteristics of the Materials
Different material designs can result in significant differences in moisture absorption, such as variations in coating materials and specific surface areas.
While some material suppliers prevent moisture from entering during the manufacturing process, certain materials themselves have a tendency to absorb moisture. After manufacturing the electrode sheets, it becomes extremely difficult to remove moisture, which poses challenges for battery manufacturers.
Therefore, when developing new materials, it is essential to consider moisture absorption characteristics and aim to create more universally applicable materials. This benefits both suppliers and consumers.
3. Poor Consistency Between Batches of Positive Electrode Materials
For battery manufacturers, the smaller the differences between batches of positive electrode materials and the better the consistency, the more stable the performance of the finished batteries.
It is widely known that one of the major drawbacks of lithium iron phosphate positive electrode materials is their poor batch-to-batch consistency. During slurry preparation, significant batch-to-batch variations often lead to unstable viscosity and solid content in each batch of slurry, causing difficulties for users who need to continuously adjust their processes to accommodate these variations.
Improving the automation level of production equipment is the primary means of enhancing the batch stability of lithium iron phosphate materials.
However, the current level of automation in China lithium iron phosphate material suppliers is generally low, and their technological capabilities and quality management are not high. This results in varying degrees of batch instability in the materials they provide.
From the user’s perspective, if batch differences cannot be eliminated, the ideal scenario is to have larger batch sizes, provided that the materials within the same batch are uniformly stable.
Therefore, to meet this requirement, iron-lithium material suppliers often add an additional mixing process after producing the finished product. This involves uniformly mixing materials from several batches. The larger the volume of the mixing vessel, the more material it can hold, resulting in a larger batch output.
Parameters such as particle size, specific surface area, moisture content, and pH value of iron-lithium materials can all affect the viscosity of the slurry. While these parameters are often strictly controlled within certain ranges, variations in slurry viscosity between batches can still occur.
To prevent anomalies during batch usage, a common practice is to prepare some slurry for viscosity testing using a simulated production formula before actual production. Once it meets the requirements, it can be used. However, if battery manufacturers conduct these tests before every production run, it would significantly reduce production efficiency.
Therefore, this testing step is shifted to the material suppliers, who are required to conduct tests and ensure compliance before shipment.
Of course, with technological advancements and improvements in the processing capabilities of material suppliers, the variability of material properties has been significantly reduced. As a result, the step of testing viscosity before shipment can often be eliminated.
In addition to the measures mentioned above for improving consistency, we should also use quality tools to minimize batch-to-batch inconsistencies and prevent quality issues. This can be addressed mainly in the following ways：
(1) Establishment of Operating Procedures
The inherent quality of a product is not only designed but also manufactured. Therefore, how operators perform their tasks is crucial for controlling product quality. Detailed and specific operating standards should be established.
(2) Identification of CTQs (Critical-to-Quality)
Identify the key indicators and key processes that impact product quality. Special monitoring should be applied to these critical control indicators, and corresponding emergency response measures should be developed.
The route for producing lithium iron phosphate is currently the mainstream for preparing lithium iron phosphate materials. The process includes batching, ball milling, sintering, crushing, packaging, and more.
The ball milling process should be managed as a critical process because if the consistency of particle size distribution after ball milling is not well controlled, it can affect the consistency of the final product’s particle size, which in turn impacts batch consistency.
(3) Use of SPC (Statistical Process Control)
Implement real-time monitoring of key characteristic parameters for critical processes using SPC. Analyze outlier data points to identify the causes of instability, take effective corrective and preventive actions, and prevent defective products from reaching customers.
4. Other Quality Issues
During the slurry-making process, the positive electrode material is mixed with solvents, binders, and conductive agents in a certain proportion inside the slurry tank. It is then discharged through a pipeline with a filter installed at the outlet.
The purpose of the filter is to intercept large particles and foreign substances in the positive electrode material to ensure the quality of coating. If the positive electrode material contains large particles, it can lead to filter clogging. If the composition of these large particles is still the positive electrode material itself and only affects production efficiency, it may not impact battery performance significantly, resulting in a relatively small loss.
However, if these large particles contain uncertain components, such as other metallic impurities, all the produced slurry may have to be discarded, leading to significant losses.
The occurrence of such anomalies should be attributed to quality management issues within the material supplier.
Most positive electrode material production processes involve a sieving step. Factors like the condition of the sieve, timely inspection and replacement, measures to prevent sieve damage, and whether large particles are checked during factory inspection need further improvement.