Determining the limiting effect of secondary phases on the cell performance of all-solid-state-batteries by continuum modelling and simulation

Kurzfassung

All-solid-state-batteries (ASSB) are regarded as a key technology for future battery applications because they prepare the way towards higher energy densities and an increased safety in operation. A crucial part in the development of ASSB is the optimization of the solid electrolyte which determines battery capacity and cycling stability. Due to a high ionic conductivity and a wide electrochemical stability window, the ceramic LLZO garnet is a promising candidate. While providing high capacity at elevated temperature and low current densities, garnet-based ASSB still show a large polarization at room temperature [1]. Latter is attributed to secondary phases, formed due to high temperature exposure and electrochemical degradation of electrolyte and cathode active material (CAM) in the composite cathode. In our contribution, we investigate the limiting effect of secondary phases in the composite cathode by microstructural resolved modelling and simulation [2]. While it is challenging to experimentally determine properties and composition of existing secondary phases, simulations can elucidate the underlying processes and mechanisms. A model for a resistive film between electrolyte and CAM was implemented in the simulation framework. It takes into account the properties of a blocking layer resulting from degradation processes. Furthermore, we consider grain boundaries in the solid electrolyte and anisotropic properties of the CAM and, thus, are able to depict the complex charge transport in the composite cathode [3]. The underlying 3D-microstructure of the composite cathode is reconstructed from FIB-SEM-measurements in order to include the influence of structural properties like tortuosity and pore distribu- tion. Our results show the limiting effect of secondary phases on the cell performance. The slow Li-diffusion in the resistive layer between solid electrolyte and CAM limits the ion transfer across the LLZO/CAM interface which can lead to reduced CAM utilisation. For small assumed diffusivities of the secondary phases, the simulated capacities are in a similar range as measured in experiments.