Correlation analysis of swell performance of single cell and module cell

Correlation analysis of swell performance of single cell and module cell

With the rapid development of the new energy industry, lithium-ion powered vehicles have been widely used, and the safety performance of lithium-ion batteries is becoming more and more important. The single cells are combined in series and parallel to form a module, during the long-term charge-discharge cycle.
 
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The single battery cell will expand to a certain extent due to lithium de-intercalation and gas production, which will affect the structural strength of the module shell. In the battery pack or vehicle system, if the swell force of the single battery is too large, it may burst the shell and cause a safety risk. Therefore, it is necessary to introduce the monitoring of swell performance in the long-term cycle test of the battery.

Since the battery cells are combined in different numbers and in different series and parallel ways when forming battery modules, the pretightening force of modules with different designs will also be different. Therefore, it is necessary to conduct experiments on several factors that affect the swell performance, and initially explore the swell change law of the battery cell-module, combined with simulation, which can help to better design the module.

This article provides basic data for the prediction and simulation of battery module swell force by comparing the swell thickness and swell force correlation of single-cell and multi-cell during charging and discharging.

Schematic diagram of single cell and module

Figure 1. Schematic diagram of single cell and module

Experimental equipment and test methods

1. Experimental equipment: in-situ swell analyzer, model SWE2110 (IEST Yuanneng Technology), the appearance of the equipment is shown in the figure.

Appearance of SWE2110 equipment

Figure 2. Appearance of SWE2110 equipment

2. Test process:

The cell information is shown in Table 1.

Information of cell
Cathode NCM
Anode Graphite
Capacity 2000mAh
Voltage 3.0~4.2V
Model 345877

3. Cell thickness swell test: 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 will automatically read the cell thickness, thickness variation, test temperature, current, voltage, capacity and other data.

Experimental process and data analysis

As shown in Figure 3, there are generally three modes for cell and module swell tests: (a) measurement of free swell without any constraints; (b) Cell swell measurement with constant preload applied; (c) Cell swell measurement with constant gap. A battery or modular unit can be decomposed into two equivalent stiffness components: the equivalent stiffness ka of the inner cell and the equivalent stiffness kc of the case.

The force analysis of the three cases under balanced conditions is shown in Figure 3. In the first case, the outer shell restricts the swell of the inner winding core, the force on the outer shell and the force on the winding core are balanced, and the external force is zero; In the second case, an external preload load ( F 0 ) is applied to the battery, causing an initial displacement of the battery case (s0 and s0,c in Fig. 3b),

The equivalent stiffness ks in the direction perpendicular to the electrode is increased by the binding plates on both sides of the phase. Under equilibrium conditions, the preload F0 (the same as the force Fs of the binding plate on both sides) is equal to the sum of the forces on the winding core and the battery case; In the third case, when the gap is measured constantly, the swell of the winding core and the battery case when the battery expands is also different from that under free conditions because of the fixed gap condition.

In short, since the module is a combination of multiple batteries, the plastic gasket between the battery case and the battery will shrink and expand during the stress process. The thickness and force tested are the combined result of the swell and contraction of lithium intercalation and extraction of electrodes and the swell and contraction of other components. In this paper, the constant pressure and constant gap test modes are used to study the correlation between the monomer and the module.

Three modes of cell and module unit swell test

Figure 3. Three modes of cell and module unit swell test

1. Research on the relationship between the swell thickness of the monomer and the module

As shown in Figure 4, in order to simulate the interlayer between the single cells, a layer of white PET film was pasted on the cells before testing. The cell superposition test method is shown in Figure 5. Turn on the in-situ swell analyzer (SWE2110), set the 200kg constant pressure mode, charge and discharge in parallel, and test the swell thickness changes of individual cells and stacked cells in situ respectively. As shown in Figure 6: the solid line is the actual swell curve of the cell, and the dotted line is the fitting superposition curve (arithmetic sum).

From the results, both the single cell and the stacked cell show the phenomenon of charge swell and discharge contraction, which is mainly due to the structural swell and contraction of graphite and ternary materials caused by the process of lithium deintercalation. With the increase in the number of stacked cells, the overall swell thickness of the module continues to increase, and the superposition swell curve of multi-cells (blue solid line) is basically consistent with the single-cell superposition arithmetic and swell curve (blue dotted line), and there are only some differences at the end of charge and discharge.

This may be related to the poor consistency of each single cell, and with the increase in the number of stacked cells, the greater the difference between the measured curve and the fitted curve, this shows that the more the module with multiple cells, the higher the consistency requirements for the monomer should be.

Schematic diagram of battery sticking PET film and Schematic diagram of battery stacking

Figure 4. Schematic diagram of battery sticking PET film; Figure 5. Schematic diagram of battery stacking

Variation curve of swell thickness of each battery cell and after stacking

Figure 6. Variation curve of swell thickness of each battery cell and after stacking

2. Exploration on the relationship between the swell force of the monomer and the module

Set the constant gap mode, charge and discharge in parallel, and test the swell force changes of the single cell and the stacked cell in the process of charging and discharging in situ, as shown in Figure 7. From the results, as the number of stacked batteries in the module increases, the total swell force of the module continues to increase.

However, the absolute value of the swell force of the module cell has no multiple relationship with the swell force of the single cell, and is usually smaller than the sum of the swell forces of multiple single cells. and the more the number of stacked cells, the greater the difference in absolute value, which may be the boundary condition for controlling the constant gap, It will make the state of the single battery cell in the module different from that of a single battery cell when charging and discharging, which will affect the electrochemical performance. The reason for the difference needs to be further explored.

The capacity of single cells before grouping and the capacity of single cells after grouping can be taken into consideration and compared and analyzed at the same time. After stacking, the pressure does not increase linearly. It may be because the superimposed pressure of the battery cells after stacking reaches a critical value and compresses the even more microscopic space between the pole pieces, which is bound to be reflected in the battery performance!

Variation curve of swell force of each battery cell and superposition

Figure 7. Variation curve of swell force of each battery cell and superposition

From the above results, it can be seen that the module or PACK is fixedly installed in the battery pack casing, and the gaskets between the single cells will have a relatively large impact on the overall force and swell of the module. Excellent battery module design can eliminate the swell of single cells.

Recently, the Qilin battery launched by CATL ranks first in lithium iron phosphate power battery manufacturers in China integrates the needs of use, combining the horizontal and vertical beams, water-cooled plates and heat insulation pads into one, and integrating them into a multi-functional elastic interlayer. A micro-bridge connection device is built in the interlayer, which can flexibly cooperate with the breathing of the battery core to expand and contract freely, and improve the reliability of the battery life cycle.

Multifunctional elastic interlayer of Qilin battery in CATL

Figure 8. Multifunctional elastic interlayer of Qilin battery in CATL

Summary

In this paper, the in-situ swell analyzer (SWE) is used to analyze the swell thickness and swell force of the same system single cell and different numbers of module cells during the charging and discharging process.

It is found that in the constant pressure mode, the variation trend of the swell thickness of the module cell can be fitted by the arithmetic sum of single cells, but in the constant gap mode, the simple arithmetic fitting method is not satisfied.

This is different from the force of the single cell under the measurement mode of the two boundary conditions. The next step can continue to explore the force model under different test modes and analyze the swell process of the electrode in more detail.

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