In-situ Analysis of the Thickness Swelling of the LFP System Cell During Cycling

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 swelling and cell capacity attenuation.

Due to its safety and stability, LFP batteries are becoming more and more popular in the new energy storage and electric vehicle industries. Currently commonly used LFP system batteries, the positive electrode is LFP material with olivine structure, and the negative electrode is graphite material. In the long-term cycling process, due to the increase of the internal resistance of the battery polarization, the film-forming repair of the negative electrode SEI or the Fe ion dissolution of the positive electrode LFP material, the capacity of the battery will decrease, and the thickness of the battery will increase with the swelling.

Figure 1. LFP crystal structure

Figure 1. LFP crystal structure

1.Experimental Instrument and Test Methods

1.1 Experimental instrument

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

IEST In-Situ Cell Swelling Testing System

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

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 swelling 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 Swelling Curve During the Cycle

Figure 3 shows the charging and discharging curves as well as the thickness swelling 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 swelling at full charge is getting larger and larger, but the rate of increase is gradually decreasing, and the corresponding maximum thickness swelling is about 2% when the cycle reaches 50 turns, and there is a tendency of gradual stabilization.

Figure 3. Cell charge and discharge curve and thickness swelling curve

Figure 3. Cell charge and discharge curve and thickness swelling curve

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

Figure. 4 shows the charging and discharging capacity versus thickness swelling 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 swelling corresponding to each full charge and full discharge of the cell is increasing, which indicates that the irreversible thickness swelling of the cell is getting larger and larger, while the reversible thickness swelling gradually decreases in the first 20 cycles, and then tends to be stabilized.

(a) Charge and discharge capacity and corresponding thickness swelling curve
(b) Coulomb efficiency and corresponding reversible swelling thickness curve of the cell

Figure 4. (a) Charge and discharge capacity and corresponding thickness swelling curve;(b) Coulomb efficiency and corresponding reversible thickness swelling curve of the cell

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 the thickness swelling loops of the two cycles of charging and discharging, it is also obvious that the thickness swelling 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 swelling curves (reversible thickness swelling) 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.

In-situ Analysis of the Thickness Swelling of the LFP System Cell During Cycling
(b) The thickness swelling curve of the two cycles before and after charging and discharging.

Figure 5. (a) The differential capacity curve of the two circles before and after. (b) The thickness swelling curve of the two cycles before and after charging and discharging.

3. Summary

In this paper, the in-situ swelling analyzer (SWE) is used to analyze the capacity and thickness swelling during the cycle of the LFP system cell. It is found that as the cycle progresses, the corresponding thickness swelling becomes larger and larger when fully charged, but the rate of increase is gradually decreasing, further analysis of the thickness swelling 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. Reference

[1]LiFePO4 – The Unexpected Battery Success Story

[2]M Lewerenz,A Marongiu, A Warnecke, DU Sauer. Differential voltage analysis as a tool for analyzing inhomogeneous aging: A case study for LiFePO4|Graphite cylindrical cells. Journal of Power Sources 368 (2017) 57~67.

[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.

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