In 2020, Central South University and CATL jointly studied the cyclic expansion force changes of the ternary system power battery under different design and assembly process conditions, and further combined with 3D simulation to simulate and analyze the expansion force of the battery pack on the module shell after the cycle failure. To guide battery design.
1.Analysis of the cyclic expansion force of measured single cells and modules
Judging from the capacity retention rate and expansion force curve of the single cell, there is no significant difference between the S40 and S60 cells in the first 1000 cycles, indicating that there is a gap between the electrode sheet and the aluminum shell before that. It is enough to support the expansion of the electrode sheet.When the cycle continues to 1300 cycles, the capacity decay rate of the S60 battery is significantly greater than that of the S40 battery, and the increase rate of the expansion force of the S60 is also greater than that of the S40.This shows that when the gap between the electrode and the aluminum shell in the square aluminum shell cell is constant, the cell with more electrode pieces will increase the expansion force faster at the end of the long-term cycle, which will accelerate the battery capacity decay.
Two modular batteries, the gap between the single cells of S40_1P6S is 2.4mm, and the gap between the single cells of S60_1P4S is 2.0mm,Judging from the capacity retention rate and expansion force curves of the two modular batteries, there is no significant difference between the S40 and S60 cells in the first 400 cycles.Explain that before this, the gap reserved between the battery cell and the module housing is still enough to support the expansion of the battery cell.When the cycle continues to 1000 cycles, the capacity decay rate of the S60_1P4S module is significantly greater than that of the S40_1P6S module, and the increase rate of the expansion force of the S60 is also greater than that of the S40.There are two main reasons:One is that the reserved space inside the S60_1P4S module is smaller than that of the S40_1P6S module, so the housing of the module will expand earlier.The other reason is that the S60 single cell battery is due to the large number of electrode pieces，compared with the S40 single cell, the expansion force will be greater, so the expansion force fed back to the module housing is also greater.Therefore, in the module design, the influence of the reserved gap and the expansion force on the attenuation of the battery capacity should be considered in a balanced manner.
2.Simulation analysis of module design
Based on the previous actual expansion force test of single cell and modular battery, the simulation method is used to analyze the maximum expansion force that the three main structural parts of the module (top cover, side plate, top side plate welding point) can withstand, in advance evaluate the reliability of the module structure.Using ABAQUS simulation software, input some measured cell expansion force and material characteristics and other parameters, and finally output the stress that the structural components of the module bear, as shown in Figure 5 and Table 2.Comparing the simulated stress values and failure thresholds of different components given in Table 2, the expansion force of each component of the S60_1P4S module is greater than that of the S40_1P6S module. If the failure threshold of the component is less than the simulated value, it means that the structural component does not meet the design requirements.
This paper uses experimental testing and 3D simulation to analyze the expansion force of the ternary system single battery and module cycle to EOL, and points out the internal reserved gaps, the number of electrode sheets, and the failure stress threshold of the module structure that need to be paid attention to in the battery design. The influence of other parameters on the battery cycle.
SWE series in-situ expansion analysis system (IEST Yuanneng Technology):
u Using a highly stable and reliable automation platform, equipped with high-precision thickness measurement sensors, to achieve the measurement of the thickness change and rate of change during the entire charging and discharging process of the battery, and can achieve long-term stability and accuracy;
u SWE series equipment can simultaneously measure the expansion thickness and expansion force of the battery during the charging and discharging process, quantify the change of the expansion thickness and expansion force of the battery, and assist in the study of the expansion behavior of the battery.
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.