Analysis of Swelling Thickness and Swelling Force in LiFePO4 Cells

1. Introduction

LiFePO4 cells are often used as power trams or energy storage devices with high power or high security requirements because of their high security and stability. As the capacity of a single cell rises, its expansion during charging and discharging increases. Usually, to analyze the expansion behavior of a cell, the cell expansion thickness or expansion force parameter is used, but the test modes of these two are different, and we have previously analyzed the differences between these two aspects of lithium cobalt oxide system cells¹. For more information, please refer to the article, “In-situ Expansion Analysis of Pouch Cell: constant Pressure Vs Constant Gap”.

Figure 1. In-situ XRD characterization of lattice changes in different materials ¹

2. Experimental Equipment and Test Methods

2.1 Equipment

The In-situ Swelling Analyzer, model SWE2110 (IEST), was used for this investigation. The equipment is shown in Figure 2. The system supports two measurement modes:

Figure 2. Appearance of SWE2110 Equipment

2.2 Testing Procedure

• Cell Information: The test cell specifications are summarized in Table 1.

• Charge-Discharge Protocol: The test was conducted at 25°C using the following sequence: Rest 5 min; 1C constant current (CC) charge to 3.65 V, followed by constant voltage (CV) charge until the current dropped to 0.025C; Rest 5 min; 1C constant current (DC) discharge to 2.5 V.

• Swelling Measurement: The cell was placed in the corresponding channel of the SWE2110. The MISS software was used to set the cell ID and sampling frequency, automatically recording data for cell thickness, thickness variation, temperature, current, voltage, and capacity.

3. In-situ Analysis of Swelling Behavior of LiFePO4 Cell

3.1 Equipment Stability Verification

Two different test modes of the In-Situ Swelling Analyzer (SWE2110) were used to test the variation of expansion thickness under constant-pressure conditions and the variation of expansion force under constant-gap conditions. Control stability verification is shown in Figure 3:

This ±1 kg / ±1 μm control precision is what makes the subtler swelling curve features discussed below — including the slight thickness decrease during charging and the 25%–70% SOC phase-change behavior in Section 4.3 — reliably distinguishable from equipment noise, rather than obscured by measurement uncertainty.

Figure 3. (a) Control stability curves for constant force and constant gap modes

Figure 3. (b) Stability curve of gap under constant pressure and constant gap

3.2 Swelling Thickness Curve Under Two Different Pressure Conditions

Figure 4 shows the cell’s charge-discharge curves, differential capacity (dQ/dV) curve, and thickness expansion curve. A notable observation for this LFP/graphite system is a slight decrease in thickness during the charging process. This thickness decrease coincides with the second lithium intercalation stage of graphite. A plausible explanation is that lithium extraction from the olivine-structured LiFePO₄ cathode causes contraction on the cathode side, which partially offsets the expansion from lithium intercalation into the graphite anode — resulting in a net decrease in overall cell thickness. This phenomenon could be further investigated using techniques like in-situ XRD.

Furthermore, the maximum swelling thickness observed during the cycle increased when a higher constant pressure was applied to the cell — this low-swelling-thickness-under-lower-pressure behavior is an important consideration when interpreting thickness data across different mechanical boundary conditions.

Figure 4. Charge-discharge curves, differential capacity curve, and thickness swelling curves

3.3 Comparative Analysis of Swelling Force vs. Swelling Thickness

Figure 5 presents a direct comparison of the swelling force and swelling thickness curves obtained from the two different testing modes:

This complex behavior — involving opposing expansion and contraction mechanisms from the anode and cathode — necessitates further investigation using complementary characterization techniques to fully deconvolute the individual electrode contributions.

Figure 5. Comparison of swelling force and swelling thickness curves under two different test modes

5. Summary

In this paper, an in-situ battery swelling analyzer (SWE2110) is used to perform a comparative analysis of swelling thickness and swelling force on a high-capacity LiFePO4 pouch cell. The results demonstrate that the general swelling trends for force and thickness are consistent during cycling. A distinct thickness decrease during charging and increase during discharging was observed within the 25%–70% SOC range. Further analysis, combining this method with other characterization techniques, is required to fully elucidate the intricate expansion/contraction mechanisms of the positive and negative electrodes in this system.

6. References

[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.

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