In-situ Analysis of The Volume And Thickness Expansion Behavior Of Pouch Cell-LFP/graphite System Cell

In this paper, the LFP/graphite core is tested for expansion. Since the LFP material has an olivine structure, which has only a very stable platform during charging and discharging, and the structural expansion is small, the structural expansion of the LFP/graphite core is mainly related to the de-embedded lithium platform of the graphite anode. The conversion of different orders of graphite structure is shown in Figure1-2 The thickness and volume expansion corresponding to each order are compared, which can guide R&D personnel to further understand the expansion mechanism.

Figure 1. Schematic diagram of graphite stage structure

Figure 1. Schematic diagram of graphite stage structure1

1. Experimental Equipment and Test Methods

1.1 Experimental Equipment

1.1.1  In-situ volume monitor, Model GVM2200(IEST) ,The testable temperature range is 20℃~85℃, and it supports dual-channel (2 batteries) simultaneous testing. The appearance of the device is shown in Figure 2.

In-Situ Battery Gassing Volume Analyzer (GVM2200)

Figure 2. Appearance of GVM2200 Equipment

1.1.2  In-situ swelling analyzer, model SWE2110(IEST), the appearance of the equipment is shown in Figure 3.

IEST In-Situ Cell Swelling Testing System

Figure 3. Appearance of SWE2110 Equipment

2.1 The Battery Cell Information is Shown in Table 1.

Table 1. Test batteries information

Table 1. Test batteries information

2.2 Charging and discharging process: 25℃ Rest 5min; 0.5C CC to 3.65V, CV to 0.025C; rest 5min; 0.5C DC to 2.5V.

2.3 Core Volume Expansion Test: Initial weighing of the core m0, put the core to be tested into the corresponding channel of the equipment, open the MISG software, set the corresponding core number and sampling frequency parameters of each channel, and the software automatically reads the volume change amount, test temperature, current, voltage, capacity and other data.

2.4 Thickness expansion test of electric core: put the electric core to be tested into the corresponding channel of the equipment, open MISS software, set the corresponding electric core number and sampling frequency parameter of each channel, and the software will automatically read the thickness of the electric core, the thickness change amount, the test temperature, current, voltage, capacity and other data.

3. In-situ analysis of the expansion behavior of pouch cell

3.1 Thickness and force curves of core expansion during charging and discharging processes

Figure. 4(a) and (b) show the charging and discharging curves as well as the thickness and volume change curves of the core. When fully charged, both the volume and thickness of the core increase by about 1%, and there is a plateau at the corresponding charge/discharge voltage plateau where the thickness and volume are basically unchanged. After one round of charging and discharging, the thickness and volume of the core remain basically unchanged, indicating that there is no obvious irreversible expansion phenomenon of the core at this time.

Figure 4. (a) Voltage and volume changes during charging and discharging.
Figure 4. (b) Swelling thickness and swelling volume changes during charging and discharging.

Figure 4. (a) Voltage and volume changes during charging and discharging.(b) Swelling thickness and swelling volume changes during charging and discharging.

3.2 Expansion volume and thickness of the core during charging and discharging with differential capacity curve analysis

Figure. 5 shows the expansion volume and expansion thickness of the core versus the differential capacity curve, and each peak of the differential capacity curve corresponds to the phase transition of the de-embedded lithium. Since the LFP material has only one voltage plateau throughout the de-embedded lithium process, the three peaks seen on the differential capacity curve of this cell are all three different orders of de-embedded lithium peaks of graphite. As can be seen from the figure, the slopes of the corresponding volume and thickness curves are larger at the first lithium embedded peak LiC24 phase transition and the third LiC6 of graphite during charging, indicating that the graphite structure expands more due to the phase transition at this point.

Figure 5. (a) Swelling volume and differential capacity curve.
Figure 5. (b) Swelling thickness and differential capacity curve.

Figure 5. (a) Swelling volume and differential capacity curve. (b) Swelling thickness and differential capacity curve.

3.3 Analysis of irreversible swelling during charging and discharging processes

The volume and thickness expansion curves under different SOC conditions are shown in Figure. 6 (a) and (b). The spacing of the expansion curves corresponding to charging and discharging represents the irreversible expansion. The thickness expansion of both the discharge process is larger than that of the charging process, and the volume expansion curves of the two processes are in agreement with the thickness expansion curve pattern in the low SOC interval, while there is no significant difference in the high SOC interval, which may be due to the Archimedes’ law of buoyancy used in the testing of the volume expansion, and if some slight gas production occurs in the high SOC condition, it will also be monitored. Therefore, if researchers only want to study the structural expansion of the cell, they can prioritize the thickness expansion measurement method, and if they want to study some gas production reactions, they can prioritize the volume expansion measurement method.

Figure 6. (a) Volume swelling curve under different SOC conditions.
Figure 6. (b) Thickness swelling curve under different SOC conditions.

Figure 6. (a) Volume swelling curve under different SOC conditions. (b) Thickness swelling curve under different SOC conditions.

4. Summarize

In this paper, an in-situ volume monitor (GVM) and an in-situ swelling analyzer (SWE) are used to analyze the volume and thickness swelling during the charging and discharging process of the pouch cell, which can characterize the reversible swelling and irreversible swelling in real time, and help R&D personnel from analysis of cell swelling behavior in different dimensions.

5. Reference 

[1]Nalamova V,Guerard D,Lelaurain M,et al. X-ray investigation of highly saturated Li-graphite intercalation compound[J]. Carbon ,1995,33(2):177-181.

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