Elastic-plastic Analysis Of LCO Powders With Different Particle Sizes During Compaction Process

IEST
11/26/2024
Powder Resistivity & Compaction Density Case

1. Background

In this paper, we experimentally and introduce empirical equations to quantitatively measure the elastic-plastic mechanical behavior of LCO powders during compaction.

Compaction of electrode powder is a process in which the powder is pressed down at a constant rate by an upper indenter in a fixture of specified specifications. The filling of the powder material under pressure is a complex process in which the contact and arrangement of the electrode powder particles change and the density and strength of the powder increases during compaction. The deformation of dense materials under pressure follows the principle of constant mass and constant volume. While powder deformation is more complex than dense materials, the deformation of powder body compaction only obeys the mass invariance, and the deformation of powder body includes the deformation of powder particles, but also includes the change of the pore morphology between particles, i.e., the particles are displaced. In the compaction process, there are the following microscopic deformation mechanisms: particle alignment adjustment: electrode particles are rearranged under stress. Initially irregularly arranged particles are gradually adjusted to a tighter arrangement during compaction. The contact points between the particles increase, filling the original pore space. Particle re-stacking: With the increase of external stress, the originally dispersed electrode particles form localized lumps, and particle aggregates fill the powder locally to form a denser structure. This re-stacking process can reduce the gap between the particles and improve the compactness of the electrode. Particle compression deformation: under the action of stress, the electrode particles will undergo compression deformation. The contact area between the particles increases, causing the particles to come into closer contact and squeeze each other, reducing the number and size of pores. Particle compression crushing: In the compaction process, the secondary electrode particles will be crushed and pulverized. The crushing phenomenon changes the overall morphology design of the powder and related physical properties such as electrical conductivity. These micro-mechanisms interact with each other, leading to an increase in the compactness and strength of the electrode. It is important to note that different electrode powder types and particle compositions lead to subtle differences in micro-mechanisms. When the powder body is deformed, the deformation of the particles may not be the same, and the degree of deformation may vary greatly from particle to particle, with localized regions of actual stress much higher than the apparent stress (apparent compression pressure) applied to the powder body, or even localized regions of high stress that may exceed the strength limit of the powder particles.

2. Experimental Process

The compaction density and compression properties of four kinds of LCO powders were tested using the model PRCD3100 powder resistivity & compaction density tester by IEST. The average particle size of the four LCO Powders is LCO-4<LCO-2<LCO-3<LCO-1. The tested samples and equipment are shown in Figure 1. Test parameters: the upper indenter was sequentially applied 10-200 MPa to LCO Powders at 20 MPa intervals with a holding pressure of 10 s. The following figure shows the test samples, apparatus and operation flow.

Table 1. Average particle size of four LCO powders

Figure 1. Schematic diagram of experimental materials and instruments: (a) Four LCO 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.

3. Mechanical Analysis Tool

The porosity-pressure relationship is commonly expressed by the Heckel equation, which is a semi-empirical formula summarizing the relationship between compressive stress and density change, and has the following expression:

In[1/(1-D)]=kp+A

where p is the pressure; D is the relative density of the powder column when the pressure is p; k and A are constants that can be obtained from the slope and intercept of the straight line portion of the relationship between In[1/(1-D)] and p. The physical significance of A can be understood by A=In[1/(1-Dρ)], where D is the relative density and ρ is the maximum density of the particles before the particle deformation after the particles have been rearranged at low pressure. This value may be closely related to the true density, morphology, particle size distribution, etc. of the Li-ion battery electrode powder. k is a parameter that measures the size of the powder plasticity. the larger the value of k, i.e., the larger the change in density caused by the same change in pressure, the larger the plasticity of the powder. The experimental results show that when k is a constant, In[1/(1-D)] and p are in a straight line, indicating that the relative density change of the powder is caused by plastic deformation; if k is a variable, then In[1/(1-D)] and p are in a curvilinear relationship, indicating that the relative density change is caused by the rearranging, crushing and so on.

4. Experimental Analysis

The stress-compaction density relationship curves and Heckel fitted straight lines for the four LCO powders are plotted in Figure 2, in which LCO-1 and LCO-3 specimens have larger k values, indicating that under the average particle size and particle size distribution of these powders, LCO-1 and LCO-3 have poorer filling of pores by displacement and rearrangement of particles under the same pressure, while the particles undergo a large elasto-plastic deformation.LCO -4 has the smallest k, and LCO-4 lithium cobaltate particle size is the smallest, from which it can be assumed that smaller particles are displaced and filled more densely, and the particles have more contact points with each other. As a result, the density change under increasing the same pressure is small, and more elastic strain and less plastic deformation occurs under particle interaction. Figure 3 shows the deformation pressure curves of the four LCO powders under different pressures. The pressure and displacement curves of the powders can be obtained after a simple conversion, and the area enclosed under the curve is the energy required for the occurrence of strain in the material.

Figure 2. Stress-compaction density relationship curve and Heckel fitted straight line for LCO powders

The area enclosed by the grid area in Figure 3 represents the energy required for plastic deformation of the LCO powder during compaction, the area enclosed by the vertical line through the highest point of the deformation curve and the horizontal axis is the total work done by the indenter on the powder, and the difference between the total work and the energy consumed by the plastic deformation is the energy consumed by the elastic deformation. The following table shows the energy consumed by elastic deformation, the energy consumed by plastic deformation, and their respective percentages for the four lithium cobalt oxide powders under this condition. The powder with smaller particles has a more pronounced rebound phenomenon, while the percentage of energy consumed for plastic deformation is smaller.

Figure 3. Deformation curves at different pressures

Table 2. Energy consumption for elastic and plastic deformation of four LCO powders

5. Summary

Under the same working conditions, the LCO powder with smaller average particle size undergoes more obvious rebound phenomenon, while plastic deformation consumes a smaller percentage of energy. Usually, the proportion of plastic deformation in the deformation process of powder compaction is around 90%.

Under the same working condition, the Heckel equation describes that the particles with smaller average particle size have smaller K-finger and less plastic deformation.

6. References

[1] YANG Shaobin,LIANG Zheng. Principles and applications of lithium-ion battery manufacturing process[J]. [2023-07-08].

[2] Kai W, Jw A, Yx A, et al. High voltage lithium cobalt oxide materials for rechargeable Li-ion batteries[J]. Journal of Power Sources, 460.

[3] Park M, Zhang X, Chung M, et al. A review of conduction phenomena in Li-ion batteries[J]. Journal of Power Sources, 2010, 195(24):7904-7929.

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