Due to its safety and stability, LFP batteries are increasingly popular in the new energy storage and electric vehicle industries. Currently commonly used LFP system batteries, the cathode material is olivine structure of LFP material, the anode material is graphite material, in the long-term cycling process, due to the increase in internal resistance of the battery polarization, the anode SEI film repair or cathode LFP material Fe ions dissolved, etc.^1~3, will cause the capacity of the battery to decline, accompanied by an increase in battery thickness expansion.

In this paper, an in-situ swelling monitor is used to test the capacity and thickness changes of LFP/graphite cells during normal temperature cycling, so as to analyze the correlation between thickness expansion and cell capacity attenuation.

Figure 1. LFP crystal structure¹

1. Experimental Instrument and Test Methods

1.1 Experimental instrument: In-situ swelling analyzer, model SWE2110(IEST), the appearance of the instrument is shown in Figure 2.

Figure 2. Appearance of SWE2110 Equipment

1.2 Test Process

1.2.1 The Battery Cell Information is Shown in Table 1.

Table 1. Test cell information

2.2 Charging and discharging process: 25℃ Rest 5min; 0.5C CC to 3.8V, CV to0.025C; rest 5min; 0.5C DC to 2.5V, cycle 50 turns.

2.3 Core thickness expansion test: put the electricity to be tested into the corresponding channel of the equipment, open the MISS software, set the corresponding core number and sampling frequency parameters of each channel, and the software automatically reads the core thickness, thickness change amount, test temperature, current, voltage, capacity and other data.

2. In-situ Analysis of Cell Swelling Behavior of LFP System

2.1 The Voltage and Thickness Expansion Curve During the Cycle

Figure 3 shows the charging and discharging curves as well as the thickness expansion curves of the battery cell. During the charging and discharging process, the thickness of the core increases and then decreases, which is mainly related to the phase change of the graphite structure caused by the de-embedded lithium during the charging and discharging process. As the cycle proceeds, the corresponding thickness expansion at full charge is getting larger and larger, but the rate of increase is gradually decreasing, and the corresponding maximum thickness expansion is about 2% when the cycle reaches 50 turns, and there is a tendency of gradual stabilization.

Figure 3. Charge and discharge curve and thickness expansion curve of the cell

2.2  Swelling Curve of Charge and Discharge Capacity and Thickness During Cycling

Figure. 4 shows the charging and discharging capacity versus thickness expansion curves for each turn of the cell. Since the battery cell used in this experiment is the one after the chemical capacity, therefore, in the first two laps of the cycle, the Coulomb efficiency of the cell is lower than 99.8% mainly due to the repair of the SEI film depletes a part of the active lithium, and in the next few laps of charging and discharging, the charging and discharging capacity increase, which may be mainly related to the battery in the application of pressure to the cycle, the dynamics of the interface is better to make the polarization of the battery cell reduced, so the The capacity increases slightly and the cell continues to cycle with the Coulombic efficiency basically stabilized at 99.93%. The thickness expansion corresponding to each full charge and full discharge of the cell is increasing, which indicates that the irreversible thickness expansion of the cell is getting larger and larger, while the reversible thickness expansion gradually decreases in the first 20 cycles, and then tends to be stabilized.

Figure 4. (a) Charge and discharge capacity and corresponding thickness expansion curve

Figure 4. (b) Coulombic efficiency of the battery cell and the corresponding reversible thickness expansion curve

2.3 Analysis of Capacity Loss and Irreversible Swelling During Cycling

Comparing the differential capacity curves of the second circle and the fiftieth circle of the battery cell, the three peaks during charging and discharging correspond to the three phase transitions of LiC24, LiC12, and LiC6 in the process of lithium extraction from graphite. The three peak positions of the fiftieth circle all shifted to the right when charging, and shifted to the left when discharging, indicating that the polarization of the cell increased after the fifty-circle cycle. According to Figure 5(b), comparing thet hickness expansion loops of the two cycles of charging and discharging, it is also obvious that the thickness expansion of the fiftieth cycle of charging and discharging is greater than that of the second cycle. In addition, the distance between the charge and discharge thickness expansion curves (reversible thickness expansion) is also significantly reduced, which may be because the continuous thickening of SEI increases the macroscopic thickness of the cell, increases the internal resistance, and decreases the capacity.

Figure 5. (a) Differential capacity curves of the two cycles before and after

Figure 5. (b) Thickness expansion curves during the first and second cycles of charge and discharge

3. Summary

In this paper, the in-situ swelling analyzer (SWE) is used to analyze the capacity and thickness expansion during the cycle of the LFP system cell. It is found that as the cycle progresses, the corresponding thickness expansion becomes larger and larger when fully charged, but the rate of increase is gradually decreasing, further analysis of the thickness expansion corresponding to the full charge and full discharge of each circle, it is speculated that the continuous thickening of SEI makes the macroscopic thickness of the cell increase, the internal resistance increases, and the capacity decreases.

4. References

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[3] D. Anse_an, M. Dubarry, A. Devie, B.Y. Liaw, V.M. García, J.C. Viera, M. Gonz_alez. Operando lithium plating quantification and early detection of a commercial LiFePO4 cell cycled under dynamic driving schedule   Journal of Power Sources 356 (2017) 36~46.