Investigating the Grain Boundary Transport and Resulting Effective Bulk Properties for Polycrystalline Solid Electrolytes

Abstract

All-solid-state batteries (ASSB) are candidates for the next-generation of safe and high energy density storage systems. In order to reach the full potential in terms of energy and power density the current limiting processes on the microscopic interface level as well as the macroscopic cell level need to be overcome. In this contribution, we explore the working principle of ASSBs and their interfaces in a multi-scale approach. We combine our 3D microstructure-resolved simulation tool BEST [1,2] with a supplementary thermodynamically consistent modelling framework [3] to capture the electrochemical performance and gain insight on - homogeneous solid-solid interface processes. Through this combined approach we are able to provide additional microscopic interface and bulk properties which are transferred as theory-based input parameters to our 3D microstructure-resolved simulations. Our focus lies on the transport limiting phenomena occurring at the homogeneous solid-solid interface e.g. grain boundary resistance in the polycrystalline electrolyte. On the microscopic level we resolve the formation of space charge layers in the solid electrolyte under the influence of grains and grain boundaries. This local electrostatic perturbation of the system through the additional polarization of the crystal grain boundaries is approximated by a mean-field approach to derive an effective dielectric constant for the polycrystalline bulk solid electrolyte. The new effective material parameter depends on the magnitude of the induced polarization and therefore relies on the material specific parameters e.g. grain size, grain width, grain volume fraction. This approach provides a link between microscopic material parameters and macroscopic electrochemical transport phenomena on cell level. Besides the effective material parameter estimation of the system, the three dimensional resolved transport through the granular solid electrolyte e.g., the grain and grain boundary transport is of special interest. In order to account for the lithium ion hopping probability between two neighbouring particles we introduce a modified interface flux at the grain interfaces. By explicit modelling of the polycrystalline electrolyte, the experimentally observed impedance characteristics are reproduced and identified. In an extended study for oxidic garnet type systems the correlation between grain size and grain impedance as well as the varying transport properties between boundary and core phase are presented. The comparison between simulations and experiments demonstrates the importance of interfacial processes and gives directions for the development of future ASSBs.