Effect of Particle Size and Pressure on the Transport Properties of the Fast Ion Conductor t-Li7SiPS8

All-solid-state batteries promise higher energy and power densities as well as increased safety compared to lithium ion batteries, by using non-flammable solid electrolytes and metallic lithium as the anode. As the liquid electrolyte is replaced by a solid electrolyte, ensuring permanent and close contact between the various components as well as between the individual particles is key for the long-term operation of a solid-state cell. Currently, there are few studies on how a solid-state electrolyte behaves when compressed by external pressure. Here we present a study in which the compression mechanics and ionic conductivity evolution of the fast solid-state conductor Li7SiPS8 were investigated under pressure on two samples with different particle sizes. In operando electrochemical impedance spectroscopy under pressure allows the determination of the activation volume of Li7SiPS8. In addition to the experiments under pressure, we show that the determined ionic conductivity additionally depends on the contact pressure. Furthermore, we simulate pelletizing using the discrete element method followed by finite volume analysis, where the effect of the pressure dependent microstructure can be distinguished from the atomistic effect of the activation volume. We conclude not only that the pelletizing pressure is an important parameter for describing the ionic conductivity of a solid, but also the particle size and morphology as well as the contact pressure during the measurement affect the impedance of a solid tablet. Furthermore, the relative density of a tablet is a weaker descriptor for the sample's impedance, compared to the particle size distribution.