In-situ Expansion Analysis of Pouch Cell: Constant Pressure Vs Constant Gap

1. Preface

The expansion analysis of lithium ion batteries during charging and discharging has two manifestations: thickness and stress. Accurate measurement of expansion thickness and expansion force can help optimize the design of the battery and improve the safety performance of the battery during usage ^1-3. The traditional test method of the constant gap mode is to use a steel plate fixture to fix the battery cell in the middle of the pressure plate, fix the position of the upper and lower pressure plates with bolts, and install a force sensor on the upper pressure plate to monitor the pressure change, but this method is difficult to guarantee the test time The gap between the upper and lower platens is constant, and sometimes the gap will produce fluctuations of tens or even hundreds of microns. As shown in Figure 1, the red curve indicates that during the charging and discharging process of the cell, the gap change of the traditional fixture produces a fluctuation of about 65um. The traditional test method is to place a constant weight on the surface of the cell, but it is difficult to freely adjust different pressures ^4-7. Based on the above problems, we use an automatic pressure and displacement control system to accurately control the test pressure and gap to achieve true constant pressure and constant gap test modes. The green curve in Figure 1 represents the gap of the SWE test system in the constant gap mode. There is almost no change. The structure diagram of the in-situ swelling test system(SWE) is shown in Figure 2.

Figure 1. Thickness control comparison between traditional test method and SWE test system in constant gap mode

Figure 2. Schematic diagram of in-situ swelling test system SWE structure

2. Experimental Equipment and Test Methods

2.1 Experimental Equipment

In-situ swelling analyzer, model SWE2110 (IEST), the appearance of the equipment is shown in Figure 3.

Figure 3. Appearance of SWE2110 equipment

3. Test Information

3.1 Cell Information is Shown in Table 1.

Table 1. The Information of Battery

3.2 Charging and discharging process

25℃ Rest 5min; 0.5C CC to 4.35V, CV to 0.025C; rest 5min; 0.5C DC to 3.0V.

3.3 Constant Pressure Mode Test

Select “Constant Pressure” mode in SWE2110 software, set the applied pressure as 10kg, start the test, and the software will automatically read out the data of cell thickness, thickness variation, pressure, test temperature, current, voltage and capacity.

3.4 Constant Gap Mode Test

Select “Constant Gap” mode on SWE2110 software, set the pressure corresponding to the initial state as 10kg, start the test, and the software automatically reads the data of thickness of the battery cell, the amount of change in thickness, pressure, test temperature, current, voltage, capacity and so on.

4. In-situ Analysis Of The Expansion Behavior Of Pouch Cells

4.1 Charging and discharging process cell expansion thickness and expansion force curve

From the charging and discharging curves as well as the thickness and expansion force curves in Fig. 4, it can be seen that when charging, the expansion force and thickness of the cell increase, the expansion force of the cell reaches about 160 kg when fully charged and the thickness of the cell expands by about 2%, and when discharging, the expansion force and thickness of the cell decrease. The changes in the thickness and force curves during charging and discharging are not completely symmetrical, indicating the existence of irreversible thickness and stress residuals.

Figure 4. (a) Voltage and current changes during charge and discharge; (b) Expansion thickness and expansion force changes during charge and discharge

4.2 Charging and discharging process of the cell expansion thickness and expansion force and differential capacity curve analysis

Figure 5 shows the expansion thickness and expansion force of the cell versus the differential capacity curve, and each peak of the differential capacity curve corresponds to the phase change of the de-embedded lithium. From the figure, it can be seen that during charging, when the first anode embedded lithium peak occurs, the thickness and the slope of the force curve also correspond to the increase, and each subsequent de-embedded lithium peak corresponds to the change of thickness and the slope of the force curve, which indicates that it is due to the phase change of the anode and negative structure that leads to the change of the expansion thickness and the expansion force of the cell. This indicates that it is the phase change of the positive and negative structure that leads to the change of the expansion thickness and expansion force.

Figure 5. (a) Expansion force and differential capacity curve; (b) Expansion thickness and differential capacity curve

5. Summary

In this paper, an in-situ swelling analyzer (SWE) is used to monitor the changes of swelling thickness and swelling force during the charging and discharging process of pouch cells in real time under the constant pressure and constant gap modes, and it can be found that the swelling thickness and swelling force curves are related to the structural phase change during the charging and discharging process. Li-ion R&D personnel can use this in-situ analysis method to analyze the expansion behavior of batteries with different systems and production processes, so as to design batteries with better performance.

6. Reference

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