Investigating All-Solid-State Battery Cell Inhomogeneities and Their Coupling to Interfacial Phenomena By Continuum Modelling and Simulation
Authors: Becker-Steinberger Katharina , Neumann Anton , Hein Simon , Urban Karsten , Latz Arnulf 
Published: February 2020

All-solid-state batteries (ASSB) are experiencing a growing scientific interest from both practical and theoretical point of views in recent years as potential next-generation high-voltage batteries with great intrinsic safety. Particularly, the large charge transfer resistances at the various ASSB interfaces are still subject to scientific discussions requiring further investigations. To advance ASSB design, fundamental knowledge of the underlying processes at both the heterogeneous solid electrolyte (SE)-electrode-interfaces and the homogeneous SE-SE-interfaces, i.e., grain boundaries, are essential.
In this contribution, we explore transport and interfacial processes in ASSB cells by a continuum modelling framework and numerical simulations. Special attention is paid to the coupled multi-physics at heterogeneous SE-electrode-interfaces and their feedback to inhomogeneous bulk properties. Local balancing processes lead to charge accumulation at the solid-solid interface and the formation of charged zones, i.e., space charge layers (SCL), in the near interface regions. These local charge distributions cause locally strong electric fields, induce large changes in the mechanical field sizes, and lead to local changes in the energy states. The numerical investigation of these boundary layers on the nanometer scale in a continuum approach requires suitably refined models. Therefore, previous approaches either resolve the SCLs [1] or consider scales beyond the SCL width and use less intense bulk type models to examine the interaction of physical processes and numerically demanding microstructures in composite electrodes [2, 3].
We combine the advantages of both approaches yielding a multi-scale model, which is based upon a free energy model, extending earlier work [1] by including mechanical and configurational contributions of detailed structural properties on the atomic scale. The resulting improved transport model is combined with a new interface model including the electrode SCL. The interfaces are active in the sense that they carry dynamically responding interfacial species. This enables the description of charge transfer and intercalation as a multi-step process and splits the electrical current crossing the interfaces into charge transferred by a defect reaction mechanism and a polarization induced contribution. For completely spatially homogeneous incompressible SEs we obtain an equilibrium ion distribution in the SCLs, which is qualitatively similar to the one postulated by earlier phenomenological models [4]. Due to the coupling of mechanical and Maxwell stress, however any deviation from homogeneity leads to fundamentally different distributions. Our approach thus enables new physical interpretations of an earlier model [4]. On the other hand, our results show the limitations of the previous approaches and underline the importance of mechanical coupling for an adequate description of interface phenomena for real SEs with possible inhomogeneities due to production processes.
In the quasi-static regime, we deduce from this transport model material dependent, non-linear, differential capacities characterizing the SCLs without spatial resolution. These capacities serve as input in 3D microstructure-resolved simulations. This model is used to simulate charge/discharge, cyclic voltammetry and impedance measurements for ASSBs; it is validated with experimental data. Furthermore, we study and identify individual contributions of the different interface processes as well as time scales.
[1] S. Braun, C. Yada, A. Latz, J. Phys. Chem. C., 2015, 19, 22249.
[2] A. Latz, J. Zausch, Beilstein Journal of Nanotechnology, 2015, 6, 987-1007.
[3] M. Finsterbusch, T. Danner, C.-L. Tsai, S. Uhlenbruck, A. Latz and O. Guillion, ACS Applied Materials & Interfaces, 2018, 10, 22329.
[4] A. Kornychev and M. Vorotyntsev, Electrochimica Acta, 1981, 26, 303-323.