Compaction Density Measurement of Powder Materials-Pressurizing, Unpressurizing & Bouncing

1. Preface

Powder materials’ resistivity and compaction density are important indicators for material monitoring in the current lithium industry, and their determination is usually done under different quantitative pressures. The process of powder compression is a complex physical phenomenon involving inter-particle interactions, displacements, deformations, and ultimately the formation of a close-packed state. In the initial stage of pressure, the powder particles are in a loose stacking state, with large pores between the particles; the external force particles gradually move closer, separate, slide and rotate, making the particles rearranged to form a tight stacking structure, resulting in a rapid reduction in the volume of the powder, the density increases rapidly. With the increase of pressure, the contact point between the particles began to occur at the elastic deformation, the gap between the particles was further compressed, but the particles themselves did not occur permanent deformation, the density of the powder continues to increase, but the rate of increase gradually slowed down. When the pressure continues to increase, the contact stress between the particles exceeds the material’s yield limit or strength limit, the particles begin to undergo plastic deformation or brittle broken. Plastic deformation results in a permanent contact surface between the particles, accompanied by cold welding and strong mechanical engagement. Brittle fragmentation leads to a decrease in particle size and further filling of the inter-particle voids. At this stage, the density of the powder increases significantly and gradually approaches the theoretical density of the material. In addition to pressurizing the material during actual testing, the pressure release, or depressurization, phase after pressurization is also a key concern for industry material monitoring. The powder is transformed into a porous body during the pressing process, and based on the change in the density of the powder with the current pressure, it is possible to determine the change in the stresses in the porous matrix as a function of its strain. This can reveal, among other things, compaction during pressing, strain hardening and fracture characteristics of brittle powder particles. This paper evaluates the differences in the determination of the compaction density during the pressurization and depressurization of three types of powder materials, NCM, LCO, and LFP, respectively, in conjunction with the PRCD/PCD series of equipment from IEST, to provide a new way of thinking for the evaluation of the mechanical properties of materials.

2. Experimental Program

Combined with IEST PRCD/PCD series equipment (Figure 1), the compaction density tests were conducted under unloading conditions of NCM, LCO, and LFP materials to evaluate the changes of pressurized, unloaded, and rebound compaction densities, respectively.

Figure 1.Schematic diagram of PRCD series powder resistance & compaction density tester

2. Testing and Analysis

Compaction density measurement of lithium-ion battery cathode material powder is one of the important monitoring indexes in the current powder quality control and material development process. 5 samples with particle sizes as in Table 1 were selected for the compaction density measurement in the unpressurized mode. Unloading mode is a reciprocal testing process of pressurization and depressurization of the material. During the pressurization process, the powder material is compressed, the thickness is gradually reduced and formed into a piece, and the compacted density is gradually increased, which is accompanied by the flow rearrangement and elastic-plastic deformation of the powder particles. When the pressure reaches the set maximum pressure/pressure and completes the holding pressure, the program control carries on the pressure unloading regulation, the pressure/pressure applied to the powder sample end is unloaded to a relatively small state, the pressure/pressure applied to the powder sample end is smaller, the existence of internal stress between the particles in the flake powder material and particles there will be a certain rebound effect, the greater the internal stress rebound is more pronounced, the thickness of the overall sample will be greater, the corresponding compaction density will be reduced, and the overall sample thickness will be bigger, the corresponding compaction density will be reduced. Corresponding compaction density will be reduced, as shown in Figure 2 for the sample compaction density with the pressure change diagram, which ① for the pressurized pressure stage, ② for the unpressurized pressure stage, from the curve, the pressurized stage compaction density is obviously greater than the unpressurized stage of the compaction density, this is due to the powder samples by the pressurization process is accompanied by the particles and particles of the internal void emptying and internal stress changes in the process, when the pressure loaded on the sample end is released, the particles and particles of the internal stress between the particles and particles, the compaction density of the sample is significantly higher than that of the compaction density. When the pressure loaded on the sample end is released, the internal stress between particles and particles and the existence of its own rebound effect, the thickness of the sample after unpressurization rebound becomes larger, and the final compaction density under the same mold size becomes smaller. The size of the rebound amount after unpressurization is closely related to the particle size ratio of the powder, the morphology of the particles, compressive strength and internal stress changes, defining the difference between the compaction density of the pressurized portion and that of the unpressurized portion as the compaction density of the rebound of the material after pressurization and unpressurization.

Figure 2. Schematic diagram of the variation of sample compaction density with pressure

Table 1.Particle size distribution of the samples

This paper focuses on evaluating the differences in the compaction density of different anode materials in terms of pressurization, depressurization and rebound in conjunction with the depressurization mode. As shown in Table 1, the particle size distributions of the selected samples are tabulated, in which sample No. 1 is clearly polycrystalline hollow sphere in process, sample No. 2 is polycrystalline solid sphere, and sample No. 3 is single-crystalline sphere. As shown in Fig. 3, the schematic diagrams of pressurized (a), unpressurized (b) and rebound compaction density (c), respectively, from the pressurized and unpressurized curves, the pressurized compaction density and the unpressurized compaction density have the same trend of change with the change of compression force and the trend of the difference between different samples is generally the same, and the compaction densities are all presented LCO＞NCM＞LFP; however, the change of the rebound compaction density has a significant difference, in which NCM-1, NCM-2, NCM-3 three compared to the single-crystal sample No. 3, after unpressurization presents a greater compaction density rebound, considering the overall particles and particles of the existence of greater internal stress; NCM-1 presents the smallest rebound, considering the overall material process design end is a hollow sphere, the particles by the large pressure pressure after the pressure, the particles themselves will be non-rebound deformation occurs, the deformation of uninstalled pressure rebound is smaller, NCM-2 The compacted density rebound of the sample is in between. In addition, the rebound compaction density of LCO and LFP also has obvious trend differences between pressurized and unpressurized, which is mainly directly related to the material’s own morphological structure and mechanical property changes.

The main stages of the compaction process include: particle rearrangement, elastic-plastic deformation and crushing. First, under low pressure, particle sliding occurs, leading to particle rearrangement. The second stage involves elastic and plastic deformation of the particles through the contact area, leading to geometrical hardening (i.e. plastic deformation and void closure). Finally, at very high pressures, the resistance to deformation of the material increases rapidly due to strain hardening of the material, the particles break up and the density approaches theoretical values. On the one hand, the structural properties of the material, such as Young’s modulus, hardness, yield stress, fracture strength, and surface properties affect the compaction process, which influences the deformation of the particles, work-hardening, and other processes, respectively. On the other hand, the geometrical characteristics of the powder also influence the behavior between particles during compaction, such as particle size, shape and distribution, as well as the lubrication effect of the additives in the compaction process, but also the average number of particles in contact, the volume fraction of the particles, the orientation and distribution of the contacts, the contact area, and the distance of the centers of mass between the particles.

Figure 3. Schematic diagram of changes in compaction density for pressurization, depressurization and rebound compaction

Through the compaction density comparison of pressurized and unpressurized conditions, the intuitive influence of pressure on the determination of the compaction density of materials can be clarified, and at the same time, the pressure rebound of powder material particles has a very important significance in assessing the changes in the mechanical properties of materials. In the lithium-ion battery R&D, mechanical research has been the focus of scholars and engineers generally, the charge and discharge deformation, stress and strain of the terminal battery level is directly related to the overall safety and life of the battery. Pole piece level is usually rolled, the main purpose is to positive and negative electrode active material and conductive agent, binder and other mixtures evenly pressed into sheet form, to ensure the uniformity, denseness and stability of the electrode sheet, accompanied by a large pressure of rolled pole piece will also be associated with a series of mechanical properties of the changes in the pole piece rolled after the rebound directly affects the subsequent shelling process, and even affects the overall volume of the core energy Density. Compared with the pole piece, the powder material level can judge the change of the mechanical properties of the material earlier, whether it can be directly associated with the change of the pole piece level has been the focus of R&D. For this reason, IEST has combined with the PRCD/PCD series of instruments to evaluate the difference in the compaction density of the table powder materials pressurized and unpressurized, in order to further look for a reliable method of correlation with the level of the pole piece; and at the same time IEST has also been combining with the SPFT series of single particle to construct a single particle mechanical performance test system to test the mechanical performance of the core, which will be the most reliable method. At the same time, IEST is also gradually combining SPFT series of single-particle mechanical property testing system to build a program and method for the evaluation of mechanical properties of single-particle to multi-particle layers.

3. Summary

In this paper, 5 types of cathode powder materials with different particle size distributions were selected for the comparative evaluation of pressurized, unpressurized and rebound compaction density, to clarify the difference of compaction density under pressurized and unpressurized conditions, and at the same time, through the trend of rebound compaction density, the correlation between its rebound and the material process and mechanical properties can be clarified initially, and the actual in the determination of compaction density, in addition to the selection of basic parameters such as the mold size, the amount of sample, the choice of pressure, etc., it is also necessary to further clarify the assessment conditions of pressurization/unpressurization. In the process of compaction density measurement, in addition to the selection of basic parameters such as mold size, sampling volume, pressure selection, etc., it is also necessary to further clarify the conditions of pressurization/unpressurization assessment of compaction density; in addition to this, it is also possible to further assess the change of mechanical rebound properties of materials with the help of compaction density measurement platform.

4. References

[1] B K K A ,  A S A ,  A H N , et al. Internal resistance mapping preparation to optimize electrode thickness and density using symmetric cell for high-performance lithium-ion batteries and capacitors[J]. Journal of Power Sources, 2018, 396:207-212.

[2] Yang Shaobin, Liang Zheng. Lithium-ion Battery Manufacturing Process Principles and Applications.