Difference Between Expansion Stiffness And Compression Stiffness Of Cells

1. Preface

With the increasing demand for lithium-ion battery end-products, lithium-ion battery performance also needs to be constantly improved, such as the structural stability of lithium-ion batteries, safety performance and appearance of the hardness and so on. Improving the cell stiffness not only beautifies the appearance, but also improves the safety performance of the cell, such as hard impact, falling from height, extrusion and other performance, and also plays a vital role in the safety design of the module.

Cell stiffness generally includes expansion stiffness and compression stiffness. Compression stiffness refers to the ability of elastic deformation when the cell is not charged and discharged statically, and expansion stiffness refers to the ability of the cell to resist elastic deformation during the charging and discharging process. At present, the usual test method is to squeeze the cell, and record the cell compression displacement and pressure correspondence, so as to obtain the cell compression stiffness under different extrusion deformation amount, and use the compression stiffness is approximately equal to the expansion stiffness of the cell. This approximation solves for how large the cell expansion stiffness difference is, and this paper uses an in-situ swelling analysis system (SWE2110) to quantify the difference between the compression stiffness and expansion stiffness of the same cell.

2. Test Information

2.1 Experimental equipment

In-situ Swelling Analyzer, Model SWE2110 (IEST), as shown below:

Figure 1. Schematic diagram of the in-situ swelling analysis system(SWE Series)

2.2 Cell Information

Table 1. Cell Information

2.3 Charging and Discharging Process

Table 2. Charging and Discharging Process

3. Experimental Data and Analysis of Results

3.1 Expansion Stiffness

The SWE2110 device selects the constant pressure test mode and sets the test pressure at 10kg (0.02MPa), 30kg (0.06MPa), 50kg (0.10MPa), 100kg (0.21MPa) and 200kg (0.42MPa) respectively, turns on the charging and discharging instrument for charging and discharging the battery cell, and monitors the thickness of the battery cell in real time and in situ under different pressure conditions as shown in Figure 2. Real-time in-situ monitoring of the thickness of the cell under different pressure conditions is shown in Fig. 2: with the charging process, the positive electrode is constantly de-lithiated and the negative electrode is constantly embedded in lithium, the thickness of the cell is constantly increasing, and the thickness of the cell is constantly decreasing with the increase of pressure.

Figure 2. Thickness variation curve of charge/discharge of battery cell under different pressures

The thickness of the battery cell under different SOC/DOD states is selected from Figure 2. The initial pressure of 10kg is used as the benchmark. The expansion stiffness of the battery cell under different SOC/DOD states is calculated according to the stiffness calculation formula K=ΔF/Δδ (where F is stress and δ is the thickness of the battery cell), as shown in Table 2: As the pressure increases, the battery cell shows an increasing trend in each SOC/DOD state, indicating that the expansion stiffness of the battery cell is highly dependent on the applied pressure. The expansion stiffness of the battery cell under different SOC/DOD states also varies, as shown in Figure 3 below: The expansion stiffness of the battery cell is large at the beginning of charging, and then decreases and becomes relatively stable as charging proceeds; the expansion stiffness of the battery cell increases first and then decreases during discharge, and reaches the maximum stiffness at a discharge depth of about 30%~50%.

Table 3. Expansion stiffness of battery cells at different SOCs (charging process on the left, discharging process on the right)

Figure 3. Trend of cell expansion stiffness for different SOC/DOD states

3.2 Compression Stiffness:

Respectively adjust the SOC of the cell to 0%, 30%, 50%, 80%, 100%, and use the SWE2100 to adjust the pressure 10kg (0.02MPa), 30kg (0.06MPa), 50kg (0.10MPa), 100kg (0.21MPa), 200kg (0.42MPa) and monitor the thickness change of the cell and test the cell in each state of compression stiffness as shown in Table 3 (left). The compression stiffness of the tested cells in each state is shown in Table 3 (left), which shows that there is a big difference between the compression stiffness and expansion stiffness of the cells, and the expansion stiffness is obviously smaller than the compression stiffness. Therefore, the compression stiffness can be used directly to invert the expansion stiffness, and there may be a large error.

Table 4. Comparison of cell stiffness (left table compression stiffness, right table expansion stiffness)

4. Summary

In this paper, the constant pressure mode of the in-situ swelling analysis system (SWE2100) is used to characterize the expansion and compression stiffness of the cell, and it is verified that the stiffness of the cell is not only related to the state of the cell, but also correlated with the amount of the applied pressure, and it is found that there is a significant difference between the expansion and compression stiffness of the cell.

5. References

[1] Hoeschele P , Heindl SF , Erker S ,et al. Influence of reversible swelling and preload force on the failure behavior of a lithium-ion pouch cell tested under realistic boundary conditions[J].Journal of Energy Storage, 2023.

[2] ZHU Maoyu,HE Jianchao,YU Ao,et al. Method for testing the expansion stiffness of an cell. Patent No. 202310112653[P][2024-01-15].