Analysis of Factors Affecting the Determination of Materials Compaction Density – Pressurization Method

1. Preface

Lithium-ion battery industry is one of the important directions affecting the development of the national economy at present, in the design and research process of lithium-ion battery, materials compaction density is one of the key indicators affecting the performance of the battery, and the level of compaction density is closely related to the parameters such as the particle size and distribution of positive and negative electrode powders, which are key materials, and is closely related to the capacity, internal resistance of the battery, and the life of the battery. The study of materials compaction density includes two directions: electrode density and powder density. For the evaluation of powder materials compaction density, the traditional way is to evaluate the electrode density, which needs to be tested through the steps of batching and stirring, coating and roller pressing, baking, weighing and thickness measurement, and calculating, and is further related to the electrical performance of the battery, and Figure 1 shows the relationship between the electrode density and the internal resistance and capacity of the battery. This electrode density evaluation method has a long testing cycle, low testing efficiency, and the production process of the electrode sheet will also cause certain hazards to personnel and the environment. Compared with the traditional method, more researchers will choose an efficient and safe positive and negative electrode powder direct compaction density determination method to realize. The fixed inner diameter of the circular through the mold, combined with stable and efficient pressure mechanism for the powder material, and the use of high-precision thickness measurement system to achieve the measurement of the thickness of the material pressed piece, and thus realize the stable and effective measurement of the powder materials compaction density.

Figure 1. Schematic diagram of the relationship between electrode density and battery internal resistance and capacity

In the current design and manufacturing process of lithium-ion batteries, the evaluation of powder compaction density has become an indicator that many material factories and OEMs focus on. The stability measurement of powder compaction density is particularly important. The measurement of powder materials compaction density is actually the ratio of the total mass of the tablet and the total volume after tableting. In the actual measurement process, people, machines, materials, methods, and environment are all key indicators that affect the measurement. This article mainly combines the compaction density measurement under different pressurization methods to evaluate the impact of differences in measurement methods on test results.

2. Test Methods and Parameters

2.1 Three kinds of materials, NCM, LFP and Graphite, were selected respectively, and were compared and tested in three ways: single-point experiment, variable pressure experiment and unpressurized experiment;

2.2 PRCD3100 (IEST) was used to test the materials compaction density respectively, and the testing equipment is shown in Figure 2. Test parameters: single-point experiment: 50MPa, 100MPa, 150MPa, 200MPa single-point test, respectively, holding pressure 10s; variable-pressure experiment: pressure range 10-200MPa, interval 10MPa, holding pressure 10s; unloading experiment: 10-200MPa, interval 10MPa, unloading to 3MPa, holding pressure 10s.

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

3. Analysis of Results

In the lithium-ion battery research work, the monitoring of the powder materials compaction density has been used as a key index for material evaluation, and has been emphasized in the research and development of material modification, stability evaluation of material production batch difference, incoming material monitoring and other aspects, and the stability of its stability in the actual determination has been widely concerned, and the experiment is mainly conducted by comparing the three test modes of single-point pressurization, variable pressure and pressure unloading, to assess the effect of the pressurization method on the powder materials compaction density determination. This experiment was conducted to evaluate the effect of pressing on the materials compaction density determination by comparing the three test modes of single-point pressing, variable pressing and pressure release. As shown in Figure 3, the pressure changes when pressurized to 200MPa under different pressing modes are shown in the schematic diagram, in which the single-point pressing experiment is to keep the sample stable for 10s after pressurizing it directly to 200MPa, and then analyze the results of the 10ths; the variable pressure experiment is to set up the test parameters of 10-200MPa and 10s of pressure preservation through the software terminal and analyze the results of the data of pressure preservation to 10s at each point of pressure strength; The unpressurized experiment is to set 10-200MPa, unpressurized to 3MPa, holding pressure 10s through the software terminal, and take the data of holding pressure to the 10th s under the corresponding unpressurized pressure of each pressurized pressure point to analyze the results. As shown in the figure, there are obvious differences in the pressure process and time of the samples under different pressurization methods, which will directly affect the filling process of the powder after being pressurized, and then affect the results of the determination of the materials compaction density.

Figure 3. Schematic diagram of pressure change when pressurized to 200MPa under different pressing methods

Figure 4 shows the comparison of the compaction density test results of NCM, LFP and Graphite at four pressure points of 50MPa, 100MPa, 150MPa and 200MPa under different pressing methods. From the comparison results, it is seen that there are differences in the single-point compaction density, variable-pressure compaction density and unloaded-pressure compaction density under each pressure point, which is mainly due to the results of different pressing methods. From Figure 3, it can be seen that under different pressing methods, there are obvious differences in the way and time of the powder material under pressure during the measurement process, and the powder compression process is extremely complex. At the beginning of the pressing, the porosity between the powder is relatively high, with the pressing, the powder particles will be rearranged and slip, and ultimately form a relatively dense stacking state, and the porosity between the particles decreases; as the pressure continues to increase, the powder particles themselves will undergo elastic deformation, and the porosity between the particles will not change much, but the pore diameter will be reduced; with the further increase in pressure, some of the powder will undergo an irreversible plastic deformation, and the pore diameter will be reduced; with the further increase in pressure, some of the powder will undergo an irreversible plastic deformation, and the pore diameter will be reduced. With the further increase of pressure, some powders will undergo irrecoverable plastic deformation, and the pore size will be further reduced, and at the same time, for some brittle systems, the phenomenon of crushing may also occur, and the pore size reduction will be more obvious^【2】.

Comparing the results of single-point pressing and variable pressure test, there is not much difference under small pressure, and as the pressure increases, the difference gradually appears, which is mainly due to the difference in the force changes of the powder under the two pressing methods. Samsung once studied the use of two-step pressing process in the electrode roller pressing. The first soft roll pressing smoothly reoriented the graphite perpendicular to the pressing force, thereby reducing stress and mechanical damage within the graphite and promoting uniform hole distribution throughout the electrode. The target density of the electrode is adjusted by a subsequent second roll pressing. Electrodes prepared by the two-step roll pressing process exhibited significantly reduced springback and swelling behavior. Therefore, the variable pressure test is equivalent to multiple compaction processes, which are able to reduce the rebound of the particles with higher compaction density^【3】.

Comparison of the unpressurized compaction density results are significantly smaller than the other two pressing methods, which is mainly due to the powder pressing process is subject to elastic deformation and plastic deformation of the dual-factor constraints, when loaded on the powder end of the large pressure into a small pressure, the powder itself will overcome the elastic deformation of the small pressure to rebound, the thickness of the powder compacts will be obvious changes, which will result in the results of the difference.

Compacted density in the actual test and evaluation process, the laboratories need to carry out the sample performance results of the standardization, need to first clarify the test process of the difference between the pressurization methods, to avoid causing the wrong comparison, resulting in a waste of time and cost.

Figure 4. Comparison of the compaction density test results of NCM, LFP, and Graphite under different pressurization methods

Compaction tests on powders can predict the compaction performance of electrodes to know the electrode roll pressing process. Researchers at the Technical University of Braunschweig, Germany, modeled the roll pressing process and revealed the relationship between the coating density ρc and the compaction load qL^【5】.

Among them, ρ_c,max and γ_c can be obtained by fitting experimental data, which represent the maximum compaction density that the coating can achieve and the coating compaction impedance, respectively. These fitting parameters can be obtained by powder compaction experiments, for example, the maximum compaction density ρ_c,max that can be achieved by the coating is the limit value of the powder materials compaction density that basically no longer increases in the variable pressure test experiment. The compaction impedance γ_c can also be obtained by fitting the formula to the results of materialscompaction density under a series of different pressures. In this way, for a specific powder, the compaction density process model can be obtained to know the electrode roll pressure experiment.

4. Summary

The stable and effective evaluation of materials compaction density has become the focus of the current lithium industry, and there are still many differences in the evaluation methods of various laboratories in the industry, so it is necessary to systematically analyze the actual evaluation and comparison process from the testing principle, method differences and influencing factors control and other dimensions, in order to ensure that the results are reasonable and effective. This paper mainly combines the differences in the results of powder materials compaction density determination under different pressurization methods to clarify the influence of pressurization methods on compaction density determination, and to clarify the key indexes of the influence of pressurization methods on compaction density determination.

5. References

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