Fast Sodium Ion Conductivity in Pristine Na₈SnP₄: Synthesis, Structure and Properties of the Two Polymorphs LT-Na₈SnP₄ and HT-Na₈SnP₄

Achieving high ionic conductivities in solid state electrolytes is crucial for the development of efficient all-solid-state-batteries. Considering future availability and sustainability, sodium materials hold promises for an alternative for lithium materials in all-solid-state batteries, due to the higher abundance. Here, we report on a sodium phosphide ion conductor Na₈SnP₄ with a conductivity of 0.53 mS cm⁻¹ at room temperature as a pristine material. Due to the simple tetrahedral SnP₄ structure units, Na₈SnP₄ has potential for optimization through aliovalent substitution as successfully applied in sulfide-based materials. Na₈SnP₄ is easily accessible from exclusively abundant elements and forms a high- and low-temperature polymorph, which further allows for a fundamental understanding of the structure-property relationship. Both polymorphs are structurally characterized by synchrotron X-ray powder diffraction and MAS–NMR spectroscopy. Ion conductivity and activation energy for ion mobility is determined by temperature dependent impedance spectroscopy and static ²³Na-NMR measurements. Both MEM analysis of scattering densities as well as structure determination by Rietveld methods hint for ionic motion between special Na positions in the structure and that ion migration proceeds along pathways passing triangular faces of neighboring tetrahedral and octahedral voids. The specific voids filling in the disordered HT-phase are found to be a crucial parameter for ion migration.