Exploring the Application of Powder Resistivity and Compaction Density in Quality Monitoring

The electrochemical performance of lithium-ion batteries is closely related to the performance of positive and negative electrode powder materials. Among them, the presence of metal foreign matter impurities in positive electrode materials, excessive moisture, and poor batch consistency may cause lithium-ion batteries to fail or even cause safety problems. For battery companies, incoming inspection is a key link in the battery manufacturing process. In incoming inspection, the smaller the difference between batches of positive electrode powder materials, the better the consistency and the more stable the finished battery.

In batch stability control of cathode materials, particle size, tap density, compacted density, specific surface area, moisture, pH value, resistivity are strictly monitored as standard monitoring indicators. In this paper, the powder resistivity&compaction density meter of IEST is used to test the long-term stability of powder resistivity and compaction density, and provide a monitoring method for the performance consistency of powder materials.

1.Index And Monitoring Method

1.1Different batches of the same NCM material are packaged separately, and one package is unpacked every day for 3 sets of repetitive tests, for a total of 30 days.

1.2Use PRCD3100 (IEST) to test the resistivity and compaction density of the material, and the test equipment is shown in Figure 2.

Test Parameters: pressure range 12-20MPa, interval 2MPa, hold pressure for 10s; 80-180MPa, interval 50MPa, hold pressure for 10s.

Figure 2. (a) Appearance of PRCD3100; (b) Structure of PRCD3100

2.Test Results

2.1Precautions Before Testing

There are many factors affecting the resistivity and compacted density stability test, including human, machine, material, method, environment and other links, the proficiency of personnel operation methods, control of key details, stability of test equipment, state of samples to be tested, rationality of test principle selection, and stability of the test environment are the keys to the stability test of resistivity and compacted density factor. In order to ensure the reliability of the overall test data, the key modules of the equipment need to be systematically tested and calibrated before the experimental test, which mainly includes calibration of the pressure module, thickness module and resistance module, to ensure the accuracy of the overall test of the equipment.

The actual test is carried out at 25°C±2°C temperature-controlled environment; in terms of sample selection, ensure that the samples are in the same state, and there is no abnormal agglomeration or agglomeration. Different batches are packaged separately, with a total of 30 packages, to minimize the impact of the sample on the environment over time, in case of moisture absorption, the actual test process unpacks one pack per day and repeats 3 sets of repeatability tests. Since the PRCD3100 is a dual-function device of resistivity and compaction density, the two parameters are output from the same set of parameters. After the test is completed, the data of 30 days will be systematically summarized and analyzed, including the summary mean, coefficient of variation, range, standard deviation (sigma) and control limits of plus or minus three sigma of the mean under different pressure points.

2.2 Resistivity Test Results

Resistivity is a physical quantity that characterizes the resistance characteristics of materials. The determination of resistivity of lithium-ion battery powder can be used in the evaluation of material modification, process condition evaluation and quality monitoring, with the development of the lithium-ion battery industry, the variety of materials is more diversified, the production process is more complicated, and the risk of material failure is greater. Therefore, the requirements for its quality management are also higher. This experiment is based on the long-term stability monitoring of the resistivity of powder materials, and explores the application of powder resistivity detection in quality monitoring.

Table 1.Summary Table of 30-day Resistivity Test Data Analysis

For example, table 1 is a summary table for the analysis of 30-day resistivity test data of NCM samples. From the test data, the resistivity of the powder samples decreases with the increase of pressure, and from the analysis results of its coefficient of variation COV, the COV of the 30-day test data at a low pressure of 12MPa is 4.13%, and at a high pressure of 180MPa it is 2.28%. 

The overall test repeatability is at a good level. But strictly speaking, the repetitive COV is too large under low pressure, this is mainly because in the actual resistivity measurement process, the powder filling is not tight under low pressure conditions, the void ratio between powder particles is large, and the consistency of repeatability testing is relatively poor. As the pressure increases, the powder particles slide and rearrange to form a tight packing state, the void ratio between the particles decreases, and the repeatability test consistency becomes better. At the same time, high pressure can reduce part of the contact resistance in the test process, thereby improving the repeatability and consistency of the overall test. This is one of the reasons why it is recommended to use high pressure conditions for testing in actual tests.

Figure 3. 30-day resistivity test data 12MPa and 180Mpa pressure overall fluctuation summary chart

Sigma is a measurement unit of standard deviation, represented by the Greek letter (σ). In the quality monitoring of lithium-ion battery incoming materials, Sigma is a key indicator. Usually the standard specification is 2~3σ of the long-term data monitoring average.

Figure 3 is a summary chart of the overall fluctuation of the 30-day resistivity test data under the pressure of 12MPa and 180Mpa. Combined with the analysis data in Table 1, it can be clearly seen that the 30-day data of the material all meet the range of Mean±3σ, in actual quality monitoring, resistivity can be used as a key indicator. Combined with the real situation of resistivity tests of different batches Of samples in the early stage, systematically analyze the fluctuation changes of its coefficient of variation, mean value and sigma, etc. Determine 2~3σ as the standard quality monitoring specification, which can be combined with this specification and actual application scenarios to carry out standardized monitoring later, such as batch stability process evaluation, incoming material consistency monitoring, etc.

2.3 Compaction Density Test Results

With the development of lithium-ion batteries, to pursue the development of higher-performance lithium-ion batteries, more and more companies have started the research and development and production of high-capacity, high-voltage and high-pressure dense materials to meet the needs of more consumers.

The evaluation of the compaction density in the industry mostly stays at the electrode end. Experiments have proved that the compaction density is closely related to the specific capacity, efficiency, internal resistance and battery cycle performance. To a certain extent, the higher the compaction density, the higher the specific capacity of the battery, therefore, the effective evaluation of the compacted density is very important for the optimization and improvement of the battery, compared with the evaluation of compaction density at the pole piece end, rapid evaluation of the density from the powder material end can also be used as an effective means of material evaluation and screening, and it is also of great significance in quality monitoring.

Table 2. 30-day compacted density test data analysis summary form

For example, Table 2 is an overall summary table of the 30-day compacted density data of NCM samples, from the data point of view, the compaction density increases with the increase of the pressure, the repeatability COV of the overall data is less than 0.5%, and the repeatability test range is less than 0.06g/cm3, overall, the overall test stability and repeatability of the data are good.

Further systematically analyzed the ±3σ range of the compacted density, and combined with the 30-day data for a systematic summary of the compacted density fluctuations. As shown in Figure 4, the test data fluctuation diagrams under 30-day compaction density of 12MPa and 180MPa all meet the specification range of ±3σ. But compared with the data fluctuation under 12MPa, the data fluctuation under 180MPa is smaller, this has something to do with the state of the voids in the powder under different pressures and the rearrangement of the particles with the flow of pressure. In addition, the overall fluctuation of the compaction density is also closely related to the control of key points in the test, such as sample state, equipment stability, test mold stability and other factors, in the actual test process, it is also necessary to reasonably control these key factors, and minimize the influencing factors as much as possible.

In the actual quality monitoring, similar to the application of resistivity, in the early stage of research and development, a large number of parameters related to compaction density can be collected for different sample test requirements and test scenarios, and key factors can be controlled, then systematically analyze the sigma specification range or strict range monitoring range of the compacted density of the material, and then carry out compacted density measurement for new batches of samples or incoming materials, so as to achieve the purpose of quality monitoring.

Figure 4. Summary of the overall fluctuation of the 30-day compacted density test data at 12MPa and 180Mpa pressure

Summarize

In this paper, the PRCD3100 is used to test the resistivity and compaction density of the same material for 30 days. Through systematic analysis of the data, it demonstrates the application scenarios in the direction of quality monitoring, provides a method for sample batch stability monitoring, and helps stabilize the production of lithium battery materials and optimize new processes.