Halogen-Dependent Ionic Conductivity in Na11Sn2PS12: Reduction by Cl and Enhancement by Br Substitution

Sodium-ion conductors with high ionic conductivity are essential materials for the realization of all-solid-state sodium batteries, a promising next-generation secondary battery technology. This study reports on halogen-substituted derivatives Na11–zSn2PS12–zXz (X = Cl, Br) of the sulfide-based sodium-ion conductor Na11Sn2PS12, which is structurally analogous to the lithium-ion conductor Li10GeP2S12. Among the synthesized compositions, Na10.95Sn2PS11.95Br0.05 achieved a bulk conductivity of 2.1 × 10–3 S cm–1 at 298 K. Structural analysis revealed that the crystal structure of Na10.95Sn2PS11.95Br0.05 adopts the same tetragonal symmetry as that of Na11Sn2PS12. Furthermore, the probability density distribution of sodium ions, calculated from both crystallographic data and molecular dynamics simulations, confirmed that Na10.95Sn2PS11.95Br0.05 possesses a three-dimensional diffusion pathway consisting of chain-like channels along the c-axis interconnected by pathways within the ab-plane. These results suggest that halogen substitution with bromine enhances ionic conductivity through optimization of the sodium concentration, without altering either the local coordination environment or the overall crystal structure of the sodium sublattice.