Temperature is an important factor affecting the rate charging performance of Li on lithium battery cells. Charging the cells in a low temperature environment may cause lithium precipitation in the cells, resulting in failure or even loss of control of the cells. The effect of the lithium window is significant for professional battery manufacturer.
(1) Test equipment
In-situ expansion analyzer, model SWE2100, can apply pressure in the range of 50~10000N, and can adjust the temperature from -20°C to 80°C.
(2) Test parameters
① The cell information is shown in the table:
② Test process
Set the charging current of the lithium battery cell to 0.5C and the discharge current to 1C, put it in the test chamber of the in-situ expansion analyzer in the high and low temperature box, and charge it at the temperature of 25℃, 15℃, 10℃ and 0℃ respectively. Discharge, synchronously use an in-situ expansion analyzer to monitor the cell expansion thickness change curve in real time (5kg constant pressure mode).
The soft-packed cells are fully charged at the same rate at four different temperatures. As shown in the figure, the maximum thickness expansion at 25°C, 15°C, 10°C, and 0°C is 70.6μm and 80.4μm respectively. , 80.1 μm, 92.4 μm, and the expansion ratios were 2.08%, 2.35%, 2.35%, and 2.71%, respectively. It can be seen from the figure that the 0°C curve is significantly different from the 10°C, 15°C, and 25°C curves, especially the slope of the expansion thickness under the high SOC state. It is initially suspected that lithium precipitation occurs in the 0°C cell.
The charging curve and thickness change curve of the battery cell at different temperatures
Further analyze the differential capacity curves at different temperatures, as shown in the figure. It can be seen from the figure that the peak positions of the differential capacity curves corresponding to the four temperatures are synchronized with the sudden change of the thickness expansion rate, indicating that the thickness change during the charging process of the cell is caused by the phase change of the positive and negative electrode materials. And as the temperature decreases, the phase transition peak keeps moving to the right, indicating that the polarization is increasing. Comparing the thickness expansion curves at different temperatures, in the thickness curve corresponding to 0°C, the thickness expansion corresponding to the position where the slope has a sudden change is significantly larger than that of the other three temperatures, and the thickness expansion curve is differentiated, as shown in the figure. , the expansion thickness of the unit capacity of the cell at 0°C is slightly larger than that of the other three groups when the charging capacity is greater than 30% SOC and the voltage reaches 4.27V. With the increase of ZnO, lithium is deposited on the surface of the negative electrode, resulting in a larger slope of thickness expansion. In order to verify whether there is lithium precipitation in the battery cells at 0°C, the color of the negative electrode surface was observed after disassembling the fully charged cells. As shown in the figure below, it was found that there was no obvious lithium precipitation phenomenon on the negative electrode surface corresponding to the fully charged cells at 25°C and 10°C. , and a large amount of gray lithium is obviously deposited on the negative electrode surface of a fully charged cell at 0°C, indicating that the cell has undergone significant lithium precipitation under this condition.
Cell charging differential capacity and thickness change curve
Cell expansion curve at constant current stage
In order to further verify the location where the lithium precipitation begins, a series of charging experiments at different temperatures were carried out: after the cells were charged at 0.5C with constant current to 40% SOC at 0°C, they were transferred to a 25°C environment to be fully charged with constant current and constant voltage, and then disassembled. After the cell, it was found that there was indeed a slight lithium precipitation on the surface of the negative electrode, which indicated that the lithium precipitation began to occur before the slope of the thickness expansion curve increased significantly.
Cell ladder temperature charging curve and disassembly diagram
In this paper, an in-situ expansion analyzer (SWE) was used to analyze the expansion behavior of the NCM system cells under the same rate and different ambient temperature conditions. Under the environment, use 0.5C rate constant current to charge to 30% SOC, and lithium precipitation will occur when the voltage reaches 4.27V. Accurately quantifying the lithium evolution voltage and SOC window of cells at different temperatures can effectively guide developers to formulate appropriate fast charging strategies and help improve the charging efficiency of lithium battery cells at different ambient temperatures.