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 Na 3 PS 4 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 Na 3 PS 4 , 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.

Aliovalent substitution is a common strategy to improve the ionic conductivity of solid electrolytes for solid-state batteries. The substitution of SbS4 3– by WS4 2– in Na2.9Sb0.9W0.1S4 leads to a very high ionic conductivity of 41 mS cm–1 at room temperature. While pristine Na3SbS4 crystallizes in a tetragonal structure, the substituted Na2.9Sb0.9W0.1S4 crystallizes in a cubic phase at room temperature based on its X-ray diffractogram. Here, we show by performing pair distribution function analyses and static single-pulse 121Sb NMR experiments that the short-range order of Na2.9Sb0.9W0.1S4 remains tetragonal despite the change in the Bragg diffraction pattern. Temperature-dependent Raman spectroscopy revealed that changed lattice dynamics due to the increased disorder in the Na+ substructure leads to dynamic sampling causing the discrepancy in local and average structure. While showing no differences in the local structure, compared to pristine Na3SbS4, quasi-elastic neutron scattering and solid-state 23Na nuclear magnetic resonance measurements revealed drastically improved Na+ diffusivity and decreased activation energies for Na2.9Sb0.9W0.1S4. The obtained diffusion coefficients are in very good agreement with theoretical values and long-range transport measured by impedance spectroscopy. This work demonstrates the importance of studying the local structure of ionic conductors to fully understand their transport mechanisms, a prerequisite for the development of faster ionic conductors.