Effect of Halogen Doping in Sodium Solid Electrolytes Based on the Na–Sn–Si–P–S Quinary System

Finding sodium solid electrolytes (SSEs) with good performance is key to building all-solid-state sodium batteries. In the development of solid electrolytes, halogen doping has been shown to be an effective approach. However, it has rarely been applied to chalcogenide-based SSEs, possibly because of strict structure constraints. In this work, the strategy of halogen doping is successfully demonstrated on Na3.67[Sn0.67Si0.33]0.67P0.33S4, a novel SSE with high structural tolerance. The halogen-doped samples formulated as Na3.57[Sn0.67Si0.33]0.67P0.33S3.9X0.1 (X = Cl, Br, and I) exhibit higher ionic conductivities and lower activation energies than the pristine sample. Na3.57[Sn0.67Si0.33]0.67P0.33S3.9I0.1, the I-doped sample, has the highest ambient ionic conductivity of 1.08 mS cm–1 and the lowest activation energy of 0.24 eV. Structure analysis indicates that the high electronegativity of Cl atoms plays an important role in improving the conduction of Na+ ions in Cl-doped samples. The large atomic radius and high polarizability of I are more important factors to accelerate the transportation of Na+ ions in the I-doped counterpart. Furthermore, the all-solid-state sodium batteries assembled with Na3.57[Sn0.67Si0.33]0.67P0.33S3.9I0.1 as solid electrolytes show decent cyclic stability at 30 °C with a current density of 0.15 A g–1. This work systematically manifests the effect of halogen doping in SSEs, providing a versatile method to develop novel SSEs.