Unveiling the Crystal Structures of Na₂SiS₃ Polymorphs and Na₆Si₃OS₈ as 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 Na2SiS3 and Na6Si3OS8. The high-temperature (HT) Na2SiS3 polymorph crystallizes in the tetragonal space group of P42/mcm and features isolated Si2S6 units consisting of two edge-sharing SiS2S2/2 tetrahedra. The low-temperature (LT) polymorph adopts the orthorhombic Pbca space group and contains infinite chains of corner-sharing SiS4 tetrahedra, where each tetrahedron shares two sulfur atoms with neighboring units. Na6Si3OS8 crystallizes in the monoclinic P21/c space group and contains isolated Si3OS8 units comprising one corner-sharing SiS2S2/2 tetrahedron and two SiS2S1/2O1/2 tetrahedra linked via a bridging oxygen atom. HT-Na2SiS3 exhibits a sodium ionic conductivity of 1.85 × 10–7 S cm–1 at 303 K with an activation energy of 0.36 eV, while Na6Si3OS8 shows a lower conductivity of 1.10 × 10–9 S cm–1 and a higher activation energy of 0.43 eV. Bond valence energy landscape calculations revealed three-dimensional sodium-ion diffusion pathways in HT-Na2SiS3 and 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.