Scaling Relations for Ionic and Thermal Transport in the Na+ Ionic Conductor Na3PS4

Solid-state Na+ and Li+ batteries are promising energy storage technologies, suggesting increased operational safety due to the replacement of the flammable organic electrolyte with a nonflammable solid-electrolyte, leading to a significant advantage over conventional battery systems. While the thermal properties of conventional battery systems are studied extensively, the thermal properties of solid-state batteries and their components often remain elusive, and, consequently, not much is known about thermal runaway in solid systems. Moreover, these battery systems are often composed of complex multiphase components, e.g., the cathode composite consisting of solid electrolyte, active material, coatings and additives, which are of significant importance for their performance. Consequently, modeling of the thermal and ionic transport properties of such multiphase components is of tremendous interest. An often-neglected fact is that porosity has an additional influence on their transport. In order to shed light onto some of these issues, both the thermal conductivity and the ionic conductivity of the model ionic conductor Na3PS4 are characterized as a function of porosity and evaluated using the Bruggemann model of the (differential) effective medium theory. It is found that the Bruggemann power-law (tortuosity factor) describing the density dependence differs significantly between thermal and ionic conductivity. This is unexpected within the current paradigm of the effective medium theory and motivates further study. Moreover, it is confirmed that Na3PS4, despite its relatively simple crystal structure, has an astonishingly low thermal conductivity, comparable to common thermoelectric materials and thermal barrier coatings, which can be explained by the diffusive nature of thermal transport by so-called "diffusons" rather than the usually known phonon transport.