Unveiling the Crystal Structures of Na₂SiS₃Polymorphs and Na₆Si₃OS₈as Sodium Ionic Conductors

All-solid-state Na-ion batteries are promising energy storage systems due to the abundance and cost-effectiveness of sodium resources. This study unveils three crystal structures of sodium ionic conductors via ab initio structure determination using powder X-ray and neutron diffraction data: two polymorphs of Na₂SiS₃and Na₆Si₃OS₈. The high-temperature (HT) Na₂SiS₃polymorph crystallizes in the tetragonal space group of P4₂/mcm and features isolated Si₂S₆units consisting of two edge-sharing SiS₂S_2/2tetrahedra. The low-temperature (LT) polymorph adopts the orthorhombic Pbca space group and contains infinite chains of corner-sharing SiS₄tetrahedra, where each tetrahedron shares two sulfur atoms with neighboring units. Na₆Si₃OS₈crystallizes in the monoclinic P2₁/c space group and contains isolated Si₃OS₈units comprising one corner-sharing SiS₂S_2/2tetrahedron and two SiS₂S_1/2O_1/2tetrahedra linked via a bridging oxygen atom. HT-Na₂SiS₃exhibits a sodium ionic conductivity of 1.85 × 10^–7S cm^–1at 303 K with an activation energy of 0.36 eV, while Na₆Si₃OS₈shows a lower conductivity of 1.10 × 10^–9S cm^–1and a higher activation energy of 0.43 eV. Bond valence energy landscape calculations revealed three-dimensional sodium-ion diffusion pathways in HT-Na₂SiS₃and Na₆Si₃OS₈, characterized by relatively low energy barriers. In contrast, the LT polymorph features more restricted pathways with higher diffusion barriers. These results provide valuable insights into the relationship between crystal structure and ion mobility, offering guidance for the design of next-generation sodium solid electrolytes. While this work establishes fundamental structure–property relationships, further studies are needed to assess their electrochemical performance in practical battery systems.