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 expansion.In this paper, an in-situ expansion 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 cell capacity attenuation and thickness expansion.
Figure 1. LFP crystal structure
² Experimental equipment and test methods
1. Experimental equipment
1.1 In-situ expansion analyzer, model SWE2110 (IEST Yuanneng Technology), the appearance of the equipment is shown in Figure 2.
Figure 2. Appearance of SWE2110 equipment
2. Test process
2.1 The battery cell information is shown in Table 1.
Table 1. Test cell information
2.2 Charge and discharge process: 25℃ Rest 5min; 0.5C CC to 3.8V, CV to 0.025C; rest 5min; 0.5C DC to 2.5V, cycle 50 times.
2.3 Cell thickness expansion test: Put the battery 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, and voltage , capacity and other data.
² In-situ analysis of cell expansion behavior of LFP system
1. The voltage and thickness expansion curve during the cycle
Figure 3 shows the charging and discharging curve of the battery cell and the thickness expansion curve.During the charging and discharging process, the thickness of the battery cell first increases and then decreases, which is mainly related to the phase change of the graphite structure caused by the deintercalation of lithium during the charging and discharging process.As the cycle progresses, the corresponding expansion thickness becomes larger and larger when fully charged, but the rate of increase gradually decreases. When the cycle reaches 50 cycles, the corresponding maximum expansion thickness is about 2%, and there is a tendency to gradually stabilize.
Figure 3 Cell charge and discharge curve and thickness expansion curve
2. Expansion curve of charge and discharge capacity and thickness during cycling
Figure 4 shows the charge and discharge capacity and the expansion thickness curve of each circle of the battery cell.Since the cell used in this experiment is a cell with a formed capacity, the Coulomb efficiency of the cell is lower than 99.8% in the first two laps of the cycle, mainly because the repair of the SEI film loses part of the active lithium.In the next few laps of charge and discharge, both the charge and discharge capacity increase. It may be mainly due to the better dynamic performance of the interface during the pressure cycle of the battery, which reduces the polarization of the cell, so the capacity increases slightly. The cell continues to circulate, and the Coulomb efficiency is basically stable at 99.93%.The corresponding expansion thickness after each cycle of the cell is fully charged and the corresponding expansion thickness curve after full discharge are increasing, which shows that the irreversible expansion thickness of the cell is getting larger and larger, and the reversible expansion thickness gradually decreases in the first 20 cycles. , And then stabilized.
Figure 4 (a) Charge and discharge capacity and corresponding thickness expansion curve;
(B) Coulomb efficiency and corresponding reversible thickness expansion curve of the cell
3. Analysis of capacity loss and irreversible expansion 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 expansion loops of the two cycles of charging and discharging, it is also obvious that the expansion thickness 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 expansion thickness curves (reversible expansion thickness) 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) The differential capacity curve of the two circles before and after;(B) The thickness expansion curve of the two cycles before and after charging and discharging
In this paper, the in-situ expansion 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 expansion thickness 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.
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