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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.

A diode requires the combination of p‐ and n‐type semiconductors or at least the defined formation of such areas within a given compound. This is a prerequisite for any IT application, energy conversion technology, and electronic semiconductor devices. Since the discovery of the pnp‐switchable compound Ag10Te4Br3 in 2009, it is in principle possible to fabricate a diode from a single material without adjusting the semiconduction type by a defined doping level. Often a structural phase transition accompanied by a dynamic change of charge carriers or a charge density wave within certain substructures are responsible for this effect. Unfortunately, the high pnp‐switching temperature between 364 and 580 K hinders the application of this phenomenon in convenient devices. This effect is far removed from a suitable operation temperature at ambient conditions. Ag18Cu3Te11Cl3 is a room temperature pnp‐switching material and the first single‐material position‐independent diode. It shows the highest ever reported Seebeck coefficient drop that takes place within a few Kelvin. Combined with its low thermal conductivity, it offers great application potential within an accessible and applicable temperature window. Ag18Cu3Te11Cl3 and pnp‐switching materials have the potential for applications and processes where diodes, transistors, or any defined charge separation with junction formation are utilized.

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