Force Development Prediction Model for Constrained Lithium-Ion Pouch Cells
Authors: Hahn Severin Lukas , Theil Sebastian , Grimsmann Florian , Birke Kai Peter 
Published: February 2020

Mechanical stability of lithium-ion battery modules is crucial for the safety of electric vehicles. This is challenging to ensure as cells show irreversible thickness growth over lifetime. Therefore, mechanical bracing of cells and the consequent force on the module housing increases over lifetime. Furthermore, both too low and too high mechanical pressure on cells have been shown to have negative impact on their lifetime due to layer delamination and lithium plating, respectively [1]. Consequently, an integrated design should realize medium cell pressures over the entire lifetime. Thus, in order to properly design a module for a specific cell in a given installation space a force prediction model is necessary which may consider the potential addition of pressure pads. Unfortunately, both a comprehensive modeling technique as well as automotive module force development data over lifetime are unavailable in literature.
Cell thickness growth is mainly attributed to solid electrolyte interface (SEI) growth on anode particles with consequent cycling [2]. Slightly grown particles cause the macroscopic electrode to expand upon mechanical reorganization of the microstructure caused by the cyclic swelling. Thus, cell thickness growth correlates directly with loss of active lithium to the SEI and – in the limited case – to the capacity loss of the cell [2]. A novel one-dimensional mechano-electrochemical model is presented which considers the cell stack in force equilibrium with its bracing module. For this, a fixed bracing stiffness and an aging-dependent cell stack stiffness is considered where the cell stack may consist of cells with or without pressure pads. Care is taken to implement the aging anode lithiation degrees in the graphite anode affected by lithium loss influencing the reversibility of SOC dependent swelling.

The experimental data required for the model are the module dimensions, stiffness data of components and the cell thickness growth of cells over aging. The latter is measured in a constant-force cell aging experiment where the cell thickness evolution is measured. Based on this, the model may predict force development of the module over lifetime and can thus a priori validate designs or considered pressure pads. For model validation, automotive modules with and without pressure pads are aged in representative as well as accelerated aging conditions. The force development is determined at several points during aging by correlating 3D-deformations of module capping plates to a reference experiment. Such measurements of force development over lifetime in automotive modules have not been reported yet.
Both accelerated and representative aging conditions show similar force evolution as correlated to capacity loss but not to energy throughput. Crucially, the predictions of the model are validated by the force evolution in the modules in both cases. As designed modules with pressure pads show significantly attenuated force development which is also successfully predicted by the model. Remaining errors exist at higher pressures which have to be investigated in detail in the future. The presented model is a powerful tool to quantitatively validate module and pressure pad designs as well as predict force evolution in given bracing conditions.

References

[1] J. Cannarella, C.B. Arnold, Journal of Power Sources
2014, 245, 745–751.
[2] A.J. Louli, L.D. Ellis, J.R. Dahn, Joule
2019, in press.