In this paper, the in-situ swelling analyzer (SWE) is used to test the thickness of NCM cell(523/graphite cells): theoretical capacity 2400mAh under different constant pressure conditions (50N/500N/1000N) to compare and analyze the swelling behavior of the cells.

During the charging and discharging process of lithium-ion batteries, with the continuous insertion and extraction of lithium-ion, the thickness of the battery cells will expand and contract to a certain extent. Because the structural changes of the positive and negative materials are not completely reversible, with the cycle increasing, the irreversible thickness of the cell is also increasing ^1-3. If a certain pressure is applied to the cell during charging and discharging, the swelling of the cell can be reduced to a certain extent and the cycle rate performance of the cell can be improved.

1.Test Information

1.1 Testing Equipment: In-situ swelling analyzer, model SWE2100 (IEST), can apply pressure range of 50~10000N, adjustable temperature -20℃~80℃.

2.Test Parameters

2.1 Charge and Discharge Process:  25℃ rest 5min; 0.5C CC to 4.35V, CV to 0.025C; rest 5min; 0.5C DC to 2.8V.

2.2 Cell Thickness Swelling Test: Put the cell 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.

3.Result Analysis

3.1 Cell Swelling Curve During Charge and Discharge

The cell is subjected to 0.5C charge and discharge tests under three constant pressure conditions, and the swelling curve is shown in Figure 1. The pressures corresponding to the three pressures are about 0.01MPa, 0.1MPa, and 0.2MPa respectively. Taking into account the slight difference between different batteries, the maximum swelling thickness of the battery under the three pressures is about 1.7%, and the overall trend is that the thickness increases during constant current charging, and the thickness is basically unchanged during constant voltage charging. The thickness is reduced during constant voltage discharging.

Figure 1. The charge-discharge curve and swelling curve of the battery cell under three constant pressures

3.2 Analysis of Swelling Curve and Differential Capacity Curve

Compare the swelling curve and the differential capacity curve of the cell under three constant pressure conditions, as shown in Figure 2. During the charging process, the two inflection points of the swelling curve correspond to the two peaks of the differential capacity curve, indicating that the cell swelling is related to the phase transition of deintercalation of lithium. The swelling curves corresponding to the three pressures have small differences during charging and large differences during discharge. This may be related to the different degrees of irreversible swelling of the cells high voltage range.

Figure 2. Swelling curve and differential capacity curve of battery cell under three kinds of constant pressure conditions

3.3 Analysis of Irreversible Swelling During Charge and Discharge

The swelling and SOC curves of the cell under three kinds of constant pressure conditions are shown in Figure 3. The distance between the swelling curves corresponding to charging and discharging represents irreversible swelling. With the increase of pressure, the irreversible swelling of the cell when fully discharged is 0.22%, 0.07%, and 0.01%, respectively, which shows that increasing the pressure can reduce the irreversible swelling ratio.

Figure 3. Three kinds of expansion curves of different SOC under constant pressure

4.Summary

In this paper, the in-situ swelling analyzer (SWE) is used to analyze the thickness swelling of the NCM cell during charging and discharging under different constant pressure conditions. In the range of 1000N, as the applied pressure increases, the irreversible swelling of the cell gradually decreases. The influence of the applied pressure on the irreversible swelling of the cell can be further explored later.

5.Reference

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.