Because of its high safety and stability, LFP cell are often used as power trams or energy storage devices that require high power or high safety. As the capacity of a single cell increases, its swelling during charging and discharging will also increase. Usually, the swelling behavior of the cell is analyzed, and the swelling thickness or swelling force parameters of the cell are used. But the two test modes are different. We have also analyzed the differences between these two aspects of lithium cobalt oxide battery cells. You can refer to the article “In-situ swelling analysis of soft-pack batteries-constant pressure vs constant gap”. This article compares the swelling thickness and swelling force of the large-capacity LFP cell, and explores its swelling behavior during the charging and discharging process.

Figure 1. In-situ XRD characterizes the lattice changes of different materials

1. Experimental Equipment and Test Methods

1.Experimental Equipment: In-situ swelling analyzer, model SWE2110, the appearance of the equipment is shown in Figure 2.

Figure 1. Appearance of SWE2110 Equipment

2. Test Process

2.1 Cell Information is Shown in Table 1.

Table 1. LFP Cell Information

2.2 Charging and Discharging Process: 25℃ Rest 5min; 1C CC to 3.65V, CV to 0.025C; rest 5min; 1C DC to 2.5V.

2.3 Thickness Swelling Test of Cell: Put the power to be tested into the corresponding channel of the device, open the MISS software, set the cell number and sampling frequency parameters corresponding to each channel, and the software automatically reads the cell thickness, thickness change, test temperature, current, voltage, capacity and other data.

3.In-situ Analysis of Cell Swelling Behavior of LFP Cell

3.1 Equipment Stability Verification

This article uses two different test modes of the in-situ swelling device, testing the swelling thickness change of the cell under the constant pressure condition, and testing the swelling force change of the cell under the constant gap condition. The verification of equipment control stability is shown in Figure 2. When the constant pressure is set to 35kg and 400kg, the pressure fluctuation during the whole test is ±1kg. When the constant gap is set, the gap changes during the whole test. The amount is within ±1µm, which shows that the control accuracy and stability of the equipment itself is very good.

Figure 2. Stability curve of control force and gap under constant pressure and constant gap

3.2 Swelling Thickness Curve Under Two Different Pressure Conditions

Figure 3 shows the charge and discharge curve, differential capacity curve and thickness swelling curve of the battery cell. For LFP/graphite battery cells, during the charging process, the thickness curve will also show a slight decrease, and the thickness decrease appears at the second lithium insertion potential of graphite. This is likely to be related to when lithium-ion are extracted from the olivine-structured LFP material, so the thickness of the positive electrode side is reduced, which offsets the increase in the thickness of the negative electrode. Therefore, the entire battery cell shows a decrease in thickness, which can be further analyzed with the help of in-situ XRD. When different constant pressures are applied to the battery core, the maximum swelling thickness of the battery core increases with the increase of the pressure.

Figure 3. The charge and discharge curve, differential capacity curve and thickness swelling curve of the battery cell

3.3 Comparison of Swelling Force and Swelling Thickness Curve During Charging and Discharging

Figure 4 shows the comparison of the swelling force and the swelling thickness curve of the cell under two different test mode conditions. During the charging and discharging process, the swelling trend of force and thickness is the same, and decreases when charging occurs in the range of 25% to 70% SOC. The phenomenon of increase during electricity, this interval is the main deintercalation lithium phase transition interval, and also the interval where the internal resistance of the cell is relatively small. How to further analyze the swelling and contraction mechanism of the positive and negative electrodes requires joint research with other characterization methods.

Figure 4. Comparison of expansion force and expansion thickness curves under two different test modes

4. Summary

In this paper, an in-situ swelling analyzer (SWE) is used to compare the swelling thickness and swelling force of large-capacity LFP cells.It is found that during the charging and discharging process, the swelling trend of force and thickness is the same, and it decreases when charging occurs in the range of 25% to 70% SOC. The phenomenon of increase during discharge, how to further analyze the swelling and contraction mechanism of the positive and negative electrodes, also needs to be combined with other characterization methods to jointly study.

5. Reference Materials

1.Xiujuan Wei, Xuanpeng Wang, Qinyou An, Chunhua Han, and Liqiang Mai. Operando X-ray Diffraction Characterization for Understanding the Intrinsic Electrochemical Mechanism in Rechargeable Battery Materials  Small Methods 2017, 1700083.