Compaction Density Experiments Of LiCoO2 Powder & Three-dimensional Discrete Element Simulation Of its Force-electric Properties

Figure 1. Schematic diagram of experimental materials and instruments: (a) Four LiCoO2 Powders with a mass of 2 g; (b) Internal diagram of powder compaction detector; (c) External diagram of a PRCD3100; (d) procedure of powder compaction experiment.

2.3 Morphological testing of four materials at 200 MPa:

SEM was used to test the morphology of the four LiCoO2 powder materials under 200 MPa compaction, Figure 2 shows the SEM test results of the four materials, LCO-1, LCO-2, LCO-3 and LCO-4, respectively, from which it can be clearly seen that there are obvious differences in the particle distribution and particle size of the four materials, of which the LCO-1 contains a variety of samples with a range of about 5 μm-30 μm particle size, LCO-2 contains powders in the range of about 5 μm-15 μm, LCO-3 samples contain powders in the range of about 10 μm-45 μm, and LCO-4 consists mainly of small particles with a particle size of about 5 μm. The difference in particle size distribution directly affects the filling effect of the powders during compression and is closely related to the compaction density, electronic conductivity and compression properties between the materials. At the same time, it can be clearly seen in the figure that LiCoO2 secondary particles are fractured and broken by compression, resulting in shear damage.

During the roller pressing process, with the increasing roller pressure, the main morphological changes of the electrode include: ① the thickness of the electrode becomes thinner, the surface roughness decreases, and the surface of the electrode is smoother; ② the contact of each component of the electrode is closer, and the gap compression is reduced; ③ the combination of the collector and the coating is enhanced, and the particles are pressed into the surface of the foil to form a pit; ④ in the case of over-pressurization, the active particles are broken, and the secondary particles are formed into the visible cracks between the particles. The roller pressing process must use appropriate pressure conditions to achieve a reasonable compaction density of the electrode sheet. Overpressure may cause the broken particles to lose the excellent properties of the active particles and become less stable, and may cause the conductive network to break down and reduce the conductivity of the electrode sheet.

Figure 2. Overall and local SEM images of LiCoO2 powder

3. Experimental Analysis

Figure 3 shows the stress-strain curves, compaction density curves, powder plasticity curves, powder thickness and applied force curves, resistivity curves and conductivity curves in the powder compaction process given by the PRCD3100. The compression performance of powder material is related to many factors such as particle shape, particle size and its distribution, the actual powder material under pressure, particle stacking from the initial loose state through the filling effect to further tightly stacked, the filling effect produced by the overall deformation of the powder is the main irreversible deformation; when all the particles for the closest stacking between the particles, the pressure under the action of the particles will be the first elastic deformation will occur, there will be a stress, this deformation is a reversible deformation, the stress is a kind of compaction, the deformation is a kind of reversible deformation. This deformation is a reversible deformation, when the pressure unloading this deformation will occur reversible rebound; when the pressure exceeds the yield strength of the powder material, the particles undergo plastic deformation, which is also irreversible deformation. The actual powder particles compression process is a multi-gravity joint action, the stress is also a comprehensive change in the process.

Figure 3. Powder compaction test data curve

4. Discrete Element Simulation

4.1 Model Construction

Input the real particle size distribution of the powder in the PFC software, respectively, 13418, 50568, 31455 and 61149 spheres were generated to represent four kinds of LiCoO2 powder, fixed constraints were set around the powder to represent the jig tooling, and 10-200MP pressure was applied to the upper surface, and Figure 4 shows the discrete element model.

Figure 4. Discrete elemental model of LiCoO2 powder

4.2 Theoretical Analysis

Combined with the powder stress-strain curves in the experiment in Figure 3, the Edinburgh elastic-plastic adhesion model (EEPA) is applied between the powder particles as a contact model, and the EEPA can be regarded as a deformation of Hooke’s law of the linear elasticity model. Equation (1) gives the stress-strain relationship defined by the EEPA model.

Figure 5. Stress-strain relationships defined by the EEPA model

Figure 6 plots the mechanism of action of the powders in the EEPA model.

Figure 6. Introduction to the EEPA model

4.3 Analysis Of Simulation Results

Figure 7 demonstrates the comparison between the stress-strain curves of discrete element simulation and the experimental curves. The results in the figure show that according to the EEPA model, the curves under the given working conditions can be calculated very well. This indicates that the model agrees well with the experimental process, and the compaction process of particles can be effectively simulated by using the model, which can guide the test of powder compaction conductivity and the roll pressing process of the electrode.

Figure 7. Discrete element simulation diagram


Figure 8 shows the force chain diagram of LCO-4 powder, showing the contact force distribution between particles of the powder at 50 MPa and 200 MPa. Warmer colors and thicker lines indicate higher contact force and better contact. The external load loading method, system size, particle disorder arrangement and particle physical parameters determine the force chain structure, while the force chain structure network determines the stress propagation mode of the system. To a certain extent, the force chain diagram also represents the conduction path of the electric current, and the fracture and reconstruction of the force chain under the influence of external load will lead to the change of the stress transmission path, which will also affect the electron conduction path, and the relationship between the force chain network and the electrical conductivity deserves further study.

Figure 8. Force chain distribution of  LiCoO2 powders

Figure 9 shows the anisotropic contact group configuration, the sphere is divided into 3600 equal surface area regions, the length of the column in each direction indicates the contact strength in that direction, the warmer the color the stronger the strength. As can be seen from the figure, there are obvious anisotropies in each of the powder axial direction after compaction. A large number of studies have shown that the anisotropic characteristics of the battery electrode sheet after roll compaction, such as graphite particles more form a balanced distribution of morphology with the collector; electrode conductivity in the thickness and lateral direction there are several orders of magnitude difference; and pore tortuosity there is also an obvious difference in direction, often thickness direction pore tortuosity is greater than the other direction, especially for the flake or ellipsoidal particles morphology. This contact group configuration is highly correlated with resistivity.

Figure 9. Anisotropic contact grouping

5. Summary

Based on four LiCoO2 powder materials with different particle size distributions, this paper combines IEST PRCD3100 series powder resistance & compaction density tester to test the resistivity and compaction density of the powder under different pressures. At the same time, combined with the experimental results, a discrete element model corresponding to the four LiCoO2 powders is established, giving a reasonable theoretical explanation for the mechanical-electrical changes of LiCoO2 powders during the compaction process, and providing a new idea for the study of the mechanical properties of powder materials.

6. References

[1] Q. Liu, J.G. Wang, B.W. Hu. Progressive Damage Analysis for Spherical Electrode Particles with Different Protective Structures for a Lithium-Ion Battery. Acs Omega.

[2] S.C. Thakur, H. Ahmadian, J. Sun, J.Y. Ooi, An experimental and numerical study of packing, compression, and caking behaviour of detergent powders. Particuology, 12 (2014) 2-12.