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.