In operando determination of the homogeneous Young’s modulus of a lithium-ion battery pouch cell

Lithium-Ion battery (LIB) pouch cells have the highest gravimetrical energy density of up to 340Wh/kg [1] and are therefore an optimal choice for energy storages where weight plays a prominent role. Over the last years the usage of LIBs in electric vehicles is increasing rapidly and pouch cells considering their high energy density are widely used. Especially in this field the mechanical properties and the structural integrity of LIBs play an important aspect for engineers and designers to be able to engineer a safe and reliable battery pack. In this study the homogeneous Young’s modulus, an important material parameter, necessary to perform simulations and calculations regarding the deformation and therefore estimate possible safety threats, is determined with a novel experimental setup. This setup allows to non-destructively determine the change of the homogeneous Young’s modulus during cycling at different C-rates, which ensures to depict the change of the mechanical properties of a LIB under realistic conditions. Recent publications have shown that the mechanical properties of LIBs including the Young’s modulus, change due to the intercalation and deintercalation of lithium-ions in the electrodes while charging and discharging [2]. Even though the parameters from the materials of which a LIB consists are well-known, determining the Young’s modulus of the whole LIB is not a trivial endeavor, because amongst other things, the Young’s modulus cannot be considered as a constant value, like it is the case for other materials used in a vehicle. Most of the studies that can be found in literature investigate a destructive approach to evaluate the material parameters together with the deformation and the force which is needed to cause a failure of LIBs. This study focuses on a non-destructive approach in which the test-setup depicts a so-called two point bending test, where the LIB represents a cantilever beam. The elastic deflection of the LIB is measured and with the help of classical mechanics the homogeneous Young’s modulus can be calculated. To validate the calculated values, a finite element simulation is carried out which depicts the two-point bending test setup. The obtained values of the homogeneous Young’s modulus are used as defining parameters of the homogeneous battery model and the deflection is simulated for different C-rates during charging and discharging. The simulation results are compared with the measurement and the validity and the necessary assumptions to calculate the homogeneous Young’s modulus with the used bending test-setup are discussed. With this novel experimental setup used in this study the changes of the mechanical properties, especially the homogeneous Young’s modulus can be determined during the cycling of the LIB and therefore valuable information regarding the mechanical behavior which varies depending on the C-rate and the charging/discharging process can be deduced. Acknowledgement: This work received funding from the Austrian Research Promotion Agency (FFG) within the collaborative project MOGLI (Grant Agreement No. 879610). [1] X.-Q. Zhang, T. Li, B.-Q. Li, R. Zhang, P. Shi, C. Yan, J.-Q. Huang, Q. Zhang. A Sustainable Solid Electrolyte Interphase for High-Energy-Density Lithium Metal Batteries Under Practical Conditions. Angewandte Chemie International Edition, 59, 3252, 2020. [2] H.Popp, M. Koller, S. Keller, G. Glanz, R. Klambauer, A. Bergmann. State Estimation Approach of Lithium-Ion Batteries by Simplified Ultrasonic Time-of-Flight Measurement. IEEE Access, 7:170992–171000, 2019