Impedance Modelling of Parent Metal Anodes in Solid-State Batteries—The Role of Current Constriction at Interface Voids, Heterogeneities and SEI

Establishing and maintaining a conform electric contact between individual materials phases at interfaces presents a severe challenge in solid-state battery (SSB) cells. Various processes, such as charge transfer, morphological and chemical instabilities due to pores or solid electrolyte interphase (SEI) formation, affect the interface properties of such cells. Identifying the dominant interface effect(s) is crucial, as the strategy for improving a SSB device depends on the rate-limiting process, which needs to be overcome. Impedance spectroscopy, as a non-destructive method, is particularly suitable for systematically studying the kinetics of interfaces. However, the correlation between interface transport effects with structural properties of a layered stack is non-trivial.

We developed a 3D impedance network model to systematically investigate the interplay between realistic interface morphologies and microstructures of the solid electrolyte (SE) typical for anodes in SSBs.^1-2 It is shown that the impedance not only depends on the materials properties of the solids involved, but also on the interface structure (e.g., pores) and thus on the preparation method of the system. Moreover, charge-transfer-driven morphological instabilities of interfaces and evolving geometric constriction effects have a major influence on the cycling behavior of SSB cells. Therefore, we performed a thorough theoretical analysis of the dependence of the impedance data on the physical contact area A_WE|SE and on temperature. This lead to universal recipes in the form of hierarchical schemes for analyzing experimental impedance data, e.g., for battery stacks with a porous metal|SE interface and without SEI formation as it is anticipated for Li|Li_6.25Al_0.25La₃Zr₂O₁₂.³ The scheme has been applied to experimental data of Krauskopf et al.⁴ and Ortmann et al.⁵ to demonstrate that the Li|Li_6.25Al_0.25La₃Zr₂O₁₂ and the Na|Na_3.4Zr₂Si_2.4P_0.6O₁₂ interface are dominated by constriction effects, rather than the charge transfer process. This emphasizes the important role of geometric constriction effects at alkali metal|SE interfaces on the way towards the successful implementation of reversible metal anodes in the market.

References
1. Eckhardt, J. K.; Fuchs, T.; Burkhardt, S.; Klar, P. J.; Janek, J.; Heiliger, C., 3D Impedance Modeling of Metal Anodes in Solid-State Batteries–Incompatibility of Pore Formation and Constriction Effect in Physical-Based 1D Circuit Models. ACS Appl. Mater. Interfaces 2022, 14 (37), 42757-42769.
2. Eckhardt, J. K.; Klar, P. J.; Janek, J.; Heiliger, C., Interplay of Dynamic Constriction and Interface Morphology between Reversible Metal Anode and Solid Electrolyte in Solid State Batteries. ACS Appl. Mater. Interfaces 2022, 14 (31), 35545-35554.
3. Eckhardt, J. K.; Fuchs, T.; Burkhardt, S.; Klar, P. J.; Janek, J.; Heiliger, C., Guidelines for Impedance Analysis of Parent Metal Anodes in Solid-State Batteries and the Role of Current Constriction at Interface Voids, Heterogeneities, and SEI. Advanced Materials Interfaces 2023, 10 (8), 2202354-2202373.
4. Krauskopf, T.; Hartmann, H.; Zeier, W. G.; Janek, J., Toward a Fundamental Understanding of the Lithium Metal Anode in Solid-State Batteries—An Electrochemo-Mechanical Study on the Garnet-Type Solid Electrolyte Li_6.25Al_0.25La₃Zr₂O₁₂. ACS Appl. Mater. Interfaces 2019, 11 (15), 14463-14477.
5. Ortmann, T.; Burkhardt, S.; Eckhardt, J. K.; Fuchs, T.; Ding, Z.; Sann, J.; Rohnke, M.; Ma, Q.; Tietz, F.; Fattakhova-Rohlfing, D.; Kübel, C.; Guillon, O.; Heiliger, C.; Janek, J., Kinetics and Pore Formation of the Sodium Metal Anode on NASICON-Type Na_3.4Zr₂Si_2.4P_0.6O₁₂ for Sodium Solid-State Batteries. Advanced Energy Materials 2023, 13 (5).