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In-situ swelling analysis of NCM batteries-different charging rates

Author:INITIAL ENERGY SCIENCE&TECHNOLOGY Co.,Ltd(IEST) Click: Time:2020-12-14 14:11:21

During the charging and discharging process of lithium ion batteries, with the continuous insertion and extraction of lithium ions, the thickness of the battery cells will expand and contract to a certain extent. Because the lithium extraction process of cathode and anode materials is not completely reversible, with the cycle increases, the irreversible thickness of the cell will continue to increase 1-3. The greater the charge rate, the greater the current density and the greater the concentration of lithium ions that will react. If the rate is too large, a large amount of lithium ions will accumulate on the surface of the anode electrode, which will easily cause lithium ions to deposite on the surface of the anode electrode and increase the thickness of the battery. Figure 1 is a schematic diagram of analyzing the lithium deposition process from different dimensions4. In this paper, an in-situ swelling analyzer (SWE) is used to test the thickness of NCM523/graphite cell (3446106, theoretical capacity 2400mAh) under different charging rate conditions (0.04C/0.2C/0.5C/1.0C/1.5C) and compare the swelling behavior of the battery cell.

 

Figure 1. Schematic diagram of lithium analysis4

 

² Test information

1. Test equipment: in-situ swelling analyzer, model SWE2110 (IEST), can apply pressure range 50~10000N, the appearance of the equipment is shown in Figure 2.

 

Figure 2. Appearance of SWE2110 equipment

2. Parameters

2.1 Charge and discharge test: 25℃, voltage range 2.8~4.35V, theoretical capacity 2400mAh, charging rate is 0.04C/0.2C/0.5C/1.0C/1.5C, and discharge rate is 0.5C.

2.2 Cell thickness swelling test: Put the cell to be tested into the corresponding channel of the device, open the MISS software, set the cell number, sampling frequency, test pressure and other parameters corresponding to each channel, and the software automatically reads the cell thickness, thickness change, test data such as temperature, current, voltage, and capacity.

² Results

1. Charge and discharge curve analysis

The charge-discharge curve and differential capacity curve of the cell are shown in Figure 3(a) and 3(b). Using different charge rates to charge the battery, it can be seen that as the charge rate increases, the peak position of the differential capacity curve shifts to the right. This is mainly because the large rate increases the battery polarization and increases the electrochemical reaction potential. When the rate is increased to 1.5C, the differential capacity curve has a split peak, which may be due to the large rate causing the battery to analyze lithium, and then as the voltage increases, the precipitated lithium will be further embedded in the graphite. When using 0.5C rate discharge, the peak position and peak intensity of the differential capacity curve are almost the same, indicating that after charging at different rates, the small rate discharge can be restored to the original capacity without the occurrence of dead lithium. 

Figure 3. Charge-discharge curve (a) and differential capacity curve (b) of the battery cell at five rates


2. Analysis of swelling curve and differential capacity curve

The correspondence between the thickness change curve of the cell and the voltage and differential capacity curve is shown in Figure 4(a) and 4(b). The battery is charged with different charging rates. It can be seen from Figure 4(a) that with the increase of the number of cycles, the initial thickness of each cycle increases. This shows that after different rates of charge and discharge, there is irreversible structure phase change causes the thickness swelling. When the magnification is less than 0.5C, the thickness of the battery increases during constant voltage charging, but when the rate increases to 1.0C and 1.5C, the thickness decreases during the constant voltage charging stage. This may be due to constant voltage charging. As the current decreases, the internal polarization of the battery gradually decreases, and the lithium concentration distribution in the anode electrode structure gradually becomes uniform, showing that the thickness of the battery decreases. From the correspondence between the differential capacity curve in Figure 4(b) and the battery thickness change curve, it can be seen that the change in the slope of the thickness curve corresponds to the peak of the differential capacity curve one-to-one, and as the charge rate increases, the thickness growth rate also increases accordingly.

 

Figure 4. Cell thickness & voltage curve (a) and differential capacity & thickness curve (b) at five rates


² Summary

In this paper, an in-situ swelling analyzer (SWE) is used to analyze the thickness swelling of NCM523 cells during charging and discharging under different charging rates. As the charge rate increases, the thickness change of the battery cell increases, and the slope of the thickness change curve also increases. The relationship between the charge rate, lithium evolution and battery thickness swelling can be further explored later.

² References


1. Yongkun Li, Chuang Wei, Yumao Sheng, Feipeng Jiao, and Kai Wu. Swelling Force in Lithium-Ion Power Batteries. Ind. Eng. Chem. Res, 2020, 59, 27, 12313–12318.

2. Ximing Cheng and Michael Pecht. In Situ Stress Measurement Techniques on Li-ion Battery Electrodes: A Review. Energies, 2017, 10, 591.

3. Amartya Mukhopadhyaya, Anton Tokranova, Xingcheng Xiaoc, Brian W. Sheldona. Stress development due to surface processes in graphite electrodes for Li-ion batteries: A first report. Electrochimica Acta, 2012,66, 28–37.

4. Thomas Waldmann, Björn-Ingo Hogg, Margret Wohlfahrt-Mehrens. Li plating as unwanted side reaction in commercial Li-ion cells – A review. J. Power. Source. 2018, 384:107–124.


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@copyrigth 2020 INITAL ENERGY SCIENCE&TECHNOLOGYCo.,Ltd(IEST)  technical support:zacnet

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