Correlation analysis of single cell and module cell,INITIAL ENERGY SCIENCE&TECHNOLOGY Co.,Ltd（IEST）

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 cell is formed by series and parallel combination. In the long cycle of charge and discharge cycle, the single cell will expand to a certain extent due to lithium removal and gas production, thus affecting the structural strength of the module shell. In the battery pack or vehicle system, if the single battery expansion force is too large, it may break the shell and cause safety risks. Therefore, the monitoring of the expansion performance needs to be introduced in the long-cycle testing of batteries. Because the battery monomer in battery module in different number, different series and parallel combination, different design module tightening force will be different, so need to several factors affecting the expansion performance, preliminary explore the cell module expansion rule, combined with simulation, can help better design the module. This experiment provides the basic data for comparing the expansion thickness and the expansion force correlation between the single cell and the multiple cell.

Figure 1. Schematic diagram of the single cell and the module¹

1. Experimental equipment and test methods

1.1Experimental equipment：in situ expansion analyzer, model SWE2110 (I EST energy technology), the appearance of the equipment as shown in Figure 2.1

Figure 2. SWE2110 Equipment appearance diagram

1.2 Test process：

1.2.1 The electric cell information is shown in Table 1.

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

1.2.2Charge and discharge process: 25℃ R est 60min; 0.5C CC to 4.35V, CV to 0.05C; rest 30min; 1.0C DC to 3.0V.

1.3 Cell thickness expansion test：put the cell to be tested into the corresponding channel of the equipment, open the MISS software, set the corresponding cell number and sampling frequency parameters of each channel, and the software will automatically read the cell thickness, thickness change, test temperature, current, voltage, capacity and other data.

2、Experimental process and data analysis

As shown in Figure 3, there are generally three modes of cell and module expansion testing: (a) free expansion measurement without any constraint, (b) cell expansion measurement with constant pretension force, and (c) cell expansion measurement with constant gap. The battery or module unit can be decomposed into two equivalent stiffness elements: the equivalent stiffness ka of the internal cell and the equivalent stiffness kc of the shell.

Force analysis under equilibrium conditions of the three conditions is shown in Figure 3, In the first case, The housing limits the expansion of the inner coil core, Balance balance between the shell and the coil core, The external resultant force is zero; In the second case, External pretension load (F 0) is applied to the battery, Cause an initial displacement of the battery housing (s0 and s0 in Figure 3b, c), The ding plates king the phase increase the equivalent stiffness k s in the direction perpendicular to the electrode, Under equilibrium conditions, the pretension force F0 (the same force F s as the bound version on both sides) is equal to the sum of the force of the coil core and the battery shell; In the third case, When constantly measuring the gap, Because of the gap fixation conditions, The expansion of the coil core and the battery shell is also different from the expansion under free conditions.

In short, since the module is combined by multiple battery packs, the plastic gasket between the battery shell and the battery will shrink and expand during the stress process. The thickness and force tested are the common result of the electrode lithium-embedded expansion and contraction and the expansion and contraction of other components.In this paper, the correlation between the monomer and the module and the constant gap test mode are studied respectively.

Figure 3. Three modes of electric cell and module unit expansion test²

2.1 Study on the correlation between monomer and module expansion thickness

As shown in Figure 4, to simulate the compartment between the individual cells, a white P ET film was attached before the cell test.The superposition test method of the cells is shown in Figure 5. Open the in situ expansion analyzer (SWE2110), set the 200kg constant pressure mode, and charge and discharge in parallel to test the expansion thickness change of the charge and discharge of individual cells and superimposed cells in situ respectively. As shown in Figure 6, the solid line is the actual expansion change curve of the cell, and the dotted line is the fitted superposition curve (arithmetic and). From the results, both single cells and superimposed cells show the phenomenon of charging expansion and discharge contraction, which is mainly due to the structural expansion and contraction of graphite and ternary materials caused by the process of removing lithium embedding. With the increase of the number of superposition cells, module overall expansion thickness is increasing, and more cell superposition expansion curve (blue line) and single cell superposition arithmetic and expansion curve is basically the same (blue dotted line), only at the end of charging and discharge there are some differences, this may be related to the consistency of the monomer cell, and with the increase of the number of cell superposition, the measured curve and fitting curve difference, this shows that the more cell module, the consistency of the monomer requirements should be higher.

Fig. 4 Cell paste P ET membrane schematic diagram Figure 5 cell stack schematic diagram

2.2 Exploration of the correlation between monomer and module expansion force

The constant gap mode is set to charge and discharge in parallel, and test the expansion force change of the charging and discharge process of individual cells in situ, respectively, as shown in Figure 7. From the result, as the number of cells in the module increases, the total expansion force of the module increasing, but the absolute value of the module cell expansion force and monomer cell expansion force without multiple relationship, usually smaller than multiple monomer cell expansion force added value, and the more the number of cells, the greater the absolute value difference, this may be the control constant gap boundary conditions, will make the monomer cell in the module and a single cell charge and discharge state, affect the electrochemical performance, need to further explore the cause of the difference.Can be set before monomer cell and group after monomer cell capacity into consideration at the same time comparative analysis, stack after the pressure is not linear growth, probably because after stack stack larger pressure after reaching a critical value of the compression between the pole sheet even more microscopic size space, is bound to reflect in the battery performance!

Fig. 7 Change curve of each cell and the superimposed expansion force

According to the above results, the module or PACK is firmly installed in the battery pack shell, and the gasket between the single cells will have a great impact on the overall force and expansion of the module.Excellent battery module design can eliminate the expansion influence of the single cell. Recently, Ningde era launched the Kirin battery integrated use needs, the horizontal and longitudinal beam, water cooling plate and heat insulation pad combined with three into one, set into a multi-functional elastic interlayer. The micron bridge connection device is built in the sandwich, which is flexibly flexible with the cell respiration, so as to improve the reliability of the whole life cycle of the cell.

Figure 8 Ningde Times Kirin battery multi-functional elastic interlayer

3、Summary

This paper uses the in situ expansion analyzer (S WE) for the charging and discharge process of the expansion thickness and expansion force in the same system. It is found that the constant pressure mode can be calculated and fitted by the single cell, and the simple arithmetic fitting mode is not satisfied in the constant gap mode.

4、reference documentation

1.Yongkun Li, Chuang Wei, Yumao Sheng, Feipeng Jiao, and Kai Wu. Swelling Force in Lithium-Ion Power Batteries，Ind. Eng.Chem. Res，2020, 59, 27, 12313–12318.

2.Oh K Y , Epureanu B I , Siegel J B , et al. Phenomenological force and swelling models for rechargeable lithium-ion battery cells[J]. Journal of Power Sources, 2016, 310(Apr.1):118-129.

3. Martin Wünscha,Kaufmana, Dirk Uwe Sauer. Investigation of the influence of different bracing of automotive pouch cells on cyclic liefetime and impedance spectra. Journal of Energy Storage 21 (2019) 149–155.

4. Qiu itao, Chen Chaohai, Jiang Jibing. Influence of foam performance on battery module expansion force in Guangdong Chemical Industry, 2020,47 (22): 1-3.