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 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 was 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 is found to be a crucial parameter for ion migration.