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A new method for the mechanical characterization of automotive lithium-ion pouch cells

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Summary:

Large-format pouch cells are widely used in the automotive industry and stacked in modules to increase energy density on battery level. Since pouch cells are packaged in a flexible foil (pouch bag) a mechanically stable module design is crucial for battery safety and optimum lifetime [1]. Design of such requires information about the mechanical properties of the cell stack itself since the change of stress inside the cell stack depends on change of cell thickness but also mechanical stiffness. Both are known to change during cell operation and with aging of cells [2]. Both also determine final energy density of the cell stack, since more layers can be fit into the casing at given maximum stress.
However, exact and reproducible determination of compressive mechanical cell properties is difficult due to the mechanically complex composite structure of lithium-ion cell components consisting of porous, electrolyte-soaked electrode and separator networks. A novel method is proposed in this contribution that not only allows for determination of mechanical properties of lithium-ion pouch cells but also measures their evolution in electrical operation. The proposed method allows particularly for measuring viscoelastic properties in a way so cell properties are not changed during measurement. This also enables distinction of different mechanical phenomena that may occur simultaneously like plastic deformation and/ or poroelastic phenomena [2][3]. Measurement provides that a sinusoidal stress is applied onto the cell stack in thickness direction while measuring cell thickness response at the same time. Through phase-shift and amplitude amplification a complex mechanical modulus is measured – the mechanical cell impedance – similar to mechanical modulus measured in polymer science [4] but at constant temperature.
Measurement results can be plotted in a nyquist plot that shows storage modulus (elastic properties) on the x-axis and loss modulus (dissipative properties) on the y-axis. Results are gained by stress oscillation in a range of 200 kPa ± 85 kPa exhibiting a mechanical modulus of approximately 120 MPa for the investigated cell. After charging an increased storage modulus is measured suggesting more dense rather then viscous mechanical properties. This is likely due to lithiated graphite particles in the anode that lead to a densification through increased particle volume.
The approach contributes not only to more accurate measurement of mechanical changes within the cell but also enables new options for the mechanical modeling of electrode stacks. Similar to electrical equivalent circuit modeling mechanical rheological models can be parameterized using its transfer function. Two modeling approaches for viscoelastic materials are proposed: Maxwell chains and Kelvin-Voigt-chains. Both are valid for the modeling of viscoelastic problems but the Maxwell chain seems to show better description of the compressive cell behavior.

References
[1] J. Cannarella, C.B. Arnold, JPS 2014, 245, 745–751. DOI: 10.1016/j.jpowsour.2013.06.165.
[2] T. Deich, L. S. Hahn, S. Both, K. P. Birke, A. Bund, JES 2020, 28, 101192. DOI: 10.1016/j.est.2020.101192
[3] G. Y. Gor, J. Cannarella, J. H. Prévost, C. B. Arnold, JECS 2014, 161, F3065-F3071. DOI: 10.1149/2.0111411jes
[4] K. P. Menard, CRC Press LLC 1999, Dynamic mechanical analysis: A Practical Introduction, ISBN: 0849386888

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