When the battery cell undergoes a phase change of releasing lithium during charging and discharging, it will cause the expansion of thickness and volume.In the previous public number article, we introduced two in-situ testing methods for volume expansion and thickness expansion of ternary/graphite cells.This article uses the same test process to test the expansion of LFP/graphite cells.Since the LFP material has an olivine structure, there is only a very stable platform during the charging and discharging process, and the structure expansion is small, so the expansion of the LFP/graphite cell structure is mainly related to the lithium release platform of the graphite anode.The conversion of different levels of graphite structure is shown in Figure 1 1-2. The thickness and volume expansion corresponding to each level are compared, which can guide R&D personnel to further understand the expansion mechanism.
Figure 1. Schematic diagram of graphite stage structure 1
1. Experimental equipment
1.1 In-situ volume monitor,Model GVM2200 (IEST Yuanneng Technology),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.
1.2 In-situ expansion analyzer, model SWE2110 (IEST Yuanneng Technology), the appearance of the equipment is shown in Figure 3.
2. Test process
2.1The battery cell information is shown in Table 1.
Table 1. Test batteries information
2.2 Charge and discharge process:25℃ Rest 5min; 0.5C CC to 3.65V, CV to 0.025C; rest 5min; 0.5C DC to 2.5V.
2.3 Volume expansion test of battery cell:Initial weighing of the cell m0 .Put the cell to be tested into the corresponding channel of the device, open the MISG software, set the cell number and sampling frequency parameters corresponding to each channel, and the software automatically reads the volume change, test temperature, current, voltage, capacity and other data.
2.4 Thickness expansion test of battery cell:Put the cell 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.
1. Cell expansion thickness and expansion force curve during charge and discharge
Figure 4 (a) and (b) are the charging and discharging curves of the cell, as well as the thickness and volume change curves.When fully charged, the volume and thickness of the cell both increase by about 1%, and a platform with basically the same thickness and volume will appear at the corresponding charge and discharge voltage platform.The thickness and volume of the battery core remain basically unchanged after a round of charging and discharging, indicating that there is no obvious irreversible expansion of the battery core at this time.
Figure 4 (a) Voltage and volume changes during charging and discharging;
(B) Expansion thickness and expansion volume changes during charging and discharging
2. Analysis of cell expansion volume and expansion thickness and differential capacity curve during charging and discharging
Figure 5 shows the expansion volume and expansion thickness of the battery cell versus the differential capacity curve. Each peak of the differential capacity curve corresponds to the phase change of deintercalated lithium.Since the LFP material has only one voltage platform during the entire lithium deintercalation process, the three peaks seen on the differential capacity curve of the cell are all three different-order deintercalation lithium peaks of graphite.It can be seen from the figure that when charging, the slope of the corresponding volume and thickness curve is larger at the first lithium insertion peak LiC24 phase transition and the third LiC6 phase transition of graphite, indicating that the graphite structure expansion caused by the phase transition at this time Larger.
Figure 5 (a) Expansion volume and differential capacity curve; (b) Expansion thickness and differential capacity curve
3. Analysis of irreversible expansion during charging and discharging process
The volume and thickness expansion curves under different SOC conditions are shown in Figure 6 (a) and (b).The distance between the expansion curves corresponding to charging and discharging represents irreversible expansion.The thickness expansion during the discharge process is greater than the thickness expansion during the charging process.The volume expansion curve of the two processes is consistent with the thickness expansion curve in the low SOC interval, but there is no obvious difference in the high SOC interval. This may be due to the Archimedes buoyancy law used when testing volume expansion.If some slight gas production occurs under high SOC conditions, it will also be monitored.Therefore, if the researcher only wants to study the structural expansion of the battery cell, the thickness expansion measurement method may be given priority, and the volume expansion measurement method may be given priority if he wants to study some gas production reactions.
Figure 6. (a) Volume expansion curve under different SOC conditions; (b) Thickness expansion curve under different SOC conditions
In this paper, an in-situ volume monitor (GVM) and an in-situ expansion analyzer (SWE) are used to analyze the volume and thickness expansion during the charging and discharging process of the soft-packed cell, which can characterize the reversible expansion and irreversible expansion in real time, and help R&D personnel from analysis of cell expansion behavior in different dimensions.
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