Microstructure-resolved modelling of solid-state batteries: The importance of anisotropy and secondary phases in the composite cathode

Abstract

All-solid-state batteries are being considered to play an important role in future battery systems due to an increased safety in operation and the possible deployment of Li-metal at the anode of the cell. A promising candidate for the solid electrolyte is the garnet-type oxide LLZO, which shows a high stability towards Li-metal while providing a high ionic conductivity. However, thermal and electrochemical induced degradation of the electrolyte and active material in the composite cathode can lead to the formation of secondary phases, which hinder the lithiation of the active material and limit the cell performance. The optimization of the composite cathode is further complicated by the existence of grain boundaries in the electrolyte and anisotropic properties of the often- deployed layered oxides, which may show a preferential crystallographic orientation in the microstructure. In this contribution we investigate the influence of secondary phase formation and anisotropy of the cathode active material on the battery performance by continuum-modelling and simulation. By including an interface model in our simulation environment, we determine the limiting effect of existing secondary phases on the cell performance. Furthermore, we include anisotropic transport parameters for the active material to account for the hindered Li-transport in the direction of the crystallographic c-axis. Our results show a strong reduction of battery capacity due to secondary phases forming in the composite cathode. The slow Li-diffusion at the interface between active material and electrolyte results in an increasing blocking of the active material which could explain the small measured capacities at room temperature. While the anisotropic properties of the active material play a minor role for homogeneous composite cathodes with moderate active material fractions, there is a stronger effect when considering inhomogeneous microstructures. Longer diffusion paths resulting from an unfavourable particle orientation hinder the Lithiation of the active material and cause smaller capacities.