In order to improve the bulk energy density of lithium-ion batteries, researchers mainly focus on two aspects: 1. Increase the gram capacity of cathode and anode electrode materials; 2. Increase the compaction density of cathode and anode electrodes. The compaction density of the electrode plate is related to the true density of the active material, the morphology of the material, the particle size distribution of the material and the electrode process.

Material true density refers to the actual mass per unit volume of solid material in the state of absolute density, that is, the density after removing the internal pores or inter particle voids. This parameter is an ideal state of material accumulation. However, in the actual cell, there must be a certain amount of voids in the accumulation between particles. Only by controlling the porosity through a certain pressure can the electrolyte leaching be balanced Wettability and electron and ion transport properties, as shown in Figure 1, is the pressure of cathode and anode particles before and after rolling. Compaction density and true density have similar trends, which are often used to evaluate the physical properties of different cathode and anode materials and to monitor the batch stability of materials.

Figure 1. Schematic diagram of cathode and anode electrode plate rolling¹

 1.How to Calculate the Compaction Density of Materials？

The Calculation Rormula of Material Compaction Density is as Follows:

Where D refers to the density of compaction, S refers to the area of the fixture containing powder, and L_c /L_r refers to the thickness of powder sample under pressure/release.

There are two ways to test the compaction density of powder: pressure method and pressure relief method. The pressurization method is to apply a certain pressure to the powder and hold the pressure for a period of time, and then test the powder thickness L_c for pressurization. The pressure relief method refers to that the powder thickness L_r is tested and released after a certain pressure is applied and held for a period of time, and then reduced to a lower pressure and kept for a period of time. Due to the elastic deformation of the particles and the slip between the particles, the L_r pressure relief is usually greater than the L_c, and the rebound amount of the powder compaction density can be calculated from the two.

Using the pressure relief method, the stress state of the powder particles is closer to that in the real electrode plate, so it has better reference significance. For example, the national standard GB/T24533-2019, graphite anode materials for lithium ion batteries” uses similar pressure relief method to test the powder compaction density².

2.How to Improve the Accuracy of Compaction Density Test？

1.Accurate weighing of powder quality: weigh the powder with a balance of 1 / 10000 or above, that is, the division value (d) is 0.0001g.

2.The accuracy of sample thickness measurement is ± 5um.

3.The powder should not leak during the process of loading fixture and testing to ensure the accuracy of M and L.

4.Before the test, the accumulation state of the powder in the fixture should be as consistent as possible: the powder is pre vibrated.

3.Test of Compaction Density of Different System Materials

1.Test Equipment: PRCD1100 (IEST) was used to test the compaction density of 5 kinds of cathode and anode electrode materials.The appearance of the equipment is shown in Figure 2.

Figure 2. (a)The Appearance of PRCD1100; (b)The Structure of PRCD1100

2.Test Parameters: The applied pressure range is 10-200 MPa, the pressure interval is 10 MPa, the pressure is maintained for 10 s, the pressure is released to 3 MPa, and the pressure is maintained for 10s.

3.Sampling Quality: Lithium cobalt oxide / ternary powder 2.0000 g, LFP / graphite powder 1.0000 g, activated carbon powder 0.5000g.

4.Results

 Figure 3.Test data of compaction density of five different system materials

Comparing the compaction density of the above five materials：LCO>NCM>LFP>Graphite>Activated Carbon,The results showed that there was rebound in the compacted density of powder after pressure relief, and the rebound amount was: Graphite > activated carbon > NCM > LCO > LFP.

The amount of rebound is usually related to the material structure.The graphite material belongs to the multi-layer carbon structure, which is easy to slip between layers, resulting in a large rebound in the thickness after pressure relief; NCM material is to replace a certain proportion of cobalt on the basic structure of the LCO. The higher the Ni content, the greater the difference between the ternary material structure and the LCO structure, which will affect the mechanical properties of the material itself, just as the NCM material used in this paper has greater elastic deformation than the LCO material; LFP material is olivine structure, the octahedral structure composed of Fe and PO₄ is relatively stable, and the elastic deformation of the material is small.

The results show that the compaction density and rebound amount obtained by pressure and pressure relief methods are not only related to the material system, but also to the particle size and particle size distribution. The influence of this aspect can be further explored in the future.

5.Reference

[1].Sang Gun Lee, Dong Hyup Jeon. Effect of electrode compression on the wettability of lithium-ion batteries. Journal of Power Sources 265(2014) 363-369.

[2].《Graphite anode materials for lithium ion batteries》GB/T 24533-2019.