As an effective electrical energy storage device, lithium-ion batteries have the advantages of high energy density, high specific power, high output voltage, low self-discharge, and long service life. They have been widely used in electric vehicles, electronic products and other fields. However, during the electrochemical cycle, the volume expansion and contraction of the electrode material will be caused by the deintercalation process of lithium ions. Moreover, gas production and heat generation occur inside the battery, which will lead to deformation of the battery as a whole, and this deformation is mainly in the thickness direction. Due to the uneven distribution of lithium concentration inside the battery, the stress when the pouch cells expands are unevenly distributed. At present, for the measurement of lithium-ion battery stress, generally only the overall expansion force of the pouch cells or module can be measured, and the distribution of expansion force at different positions of the pouch cells are rarely measured. In this paper, a thin-film pressure sensor is used to realize in-situ monitoring of the expansion force changes at different positions on the surface of the pouch cells during charging and discharging.

Figure1. Simulation of lithium concentration and stress strain distribution corresponding to lithium-ion pouch cells when they are fully charged.

 1. Test Information

1.1 Test Equipment: In-situ swelling analyzer, model SWE2110 (IEST), which can apply a pressure range of 50~10000N.

Figure 2.Schematic diagram of the in-situ swelling analyzer (SWE series)

2.Test Parameters

2.1 The Pouch Cells Information is Shown in Table 1

Table 1. Cell Information

2.2 Test Process

place the pouch cells in the test chamber of the in-situ swelling analyzer, set the charge and discharge parameters: leave for 60 minutes, charge 0.75C, cut-off current 0.05C, leave for 10 minutes, discharge 0.75C, cut-off voltage 3.0V, Simultaneously turn on the in-situ swelling   analyzer to monitor the change curve of the cell expansion force in real time.

3. Result Analysis

Charge and discharge the cell for three cycles in constant gap mode and collect the maximum expansion force curve of the cell synchronously, as shown in Figure 3. As lithium ions are continuously extracted from the positive electrode and inserted into the negative electrode, the structure of the negative electrode expands, and the pressure continues to rise. The peak pressure of the charging result is 125.3kg. During discharge, lithium ions escape from the negative electrode and return to the positive electrode. The structure of the negative electrode shrinks continuously, and the pressure continues to decrease. At the end of the discharge, the pressure is 56.9kg. As shown in Figure 3 below.

Figure 3. Cell charging and discharging voltage and pressure variation curve

To further analyze the expansion force changes at different positions on the surface of the cell, we divide the cell into small units as shown in Figure 4, Each small unit corresponds to a pressure sensor, and the expansion force change curve of each position during the charging and discharging process of the pouch cells are collected synchronously.

Figure 4. Area division of small cells of the battery cell

The thermal diagram of the force changes of each area unit during the entire charging and discharging process is shown in Figure 5. The darker the color, the greater the expansion force. As the SOC increases, the expansion force in the middle area of the cell can be seen to increase significantly. When discharging, the SOC gradually decreases, and the expansion force corresponding to each area gradually decreases. The overall change trend is consistent with Figure 3. The stress of the cell is strong in the middle and weak at the periphery. This may be since the heat-sealed edge of the aluminum-plastic film itself has a certain limiting effect on the battery. The thickness of the edge of the battery expands relatively little during charging and discharging. In addition, it is also related to the structural design of the wound cell. At the same time, the change of expansion force will also be affected by the glue at the end of the pouch cells and the thickness of the tab.

Figure 5.The thermal diagram of different regions of the cell changing with SOC.

Further detailed analysis of the expansion force distribution corresponding to the zero-charge state and full-charge state of the second cycle charging, as shown in Figure 6, although the initial expansion force at the edge is smaller than the middle area, However, the rate of expansion change after full charge is the largest at the edge, which may be related to the fact that the corners of the wound cells are more prone to stress accumulation or lithium precipitation.

Figure 6.The force distribution (relative value) of small units in each area in the zero-charge state and the full-charge state

Figure 7 shows the expansion force curve of some selected small area units. Judging from the absolute value of the expansion force curves at different positions, the absolute value of the expansion force is the smallest at edge positions 1, 5, and 11,This may not be flat with the initial surface of the cell, so the initial force is less at the edge, Especially at the position 11 close to the tab, there is basically no obvious change in expansion force detected during the charging and discharging process, indicating that this position is basically not in contact with the pressure sensor. The uneven stress distribution in each area of the battery may also be related to the deformation process inside the winding cell. In the constant gap mode, the thickness of the battery expands during charging, and the expansion of the thickness forms a force on the clamp. Maintaining a constant gap between the clamps is equivalent to exerting a certain pressure on the battery. Under the pressure, wrinkles and curls may occur inside the cell.As shown in Figure 8, the stress on each area is not uniform.

Figure 7. Variation curve of expansion force of small elements in some areas

Figure 8. Schematic diagram of the deformation process inside the battery

4. Summary

In this paper, the in-situ swelling analyzer (SWE) is used to characterize the stress distribution characteristics of the NCM system cells during the charging and discharging process, and to further analyze the change trend of each small area unit. It can quantitatively characterize the stress distribution difference on the surface of the cell, and provide a deeper perspective for the stress analysis of the cell. It can help technicians to analyze the internal stress distribution of the cell, explore the cause of the failed cell, and develop safer and more reliable cells.

5. References

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