Structure and Transport Properties in the Pseudobinary Phase System Li₄SiS₄–Li₄SnS₄

High Resolution Image Download MS PowerPoint Slide Thio-lithium superionic conductors (thio-LISICONs) are a family of promising solid electrolyte materials for potential applications in solid-state batteries. The orthorhombic polymorph of the thio-LISICON Li 4 SiS 4 ( o -Li 4 SiS 4 ) has been known for decades, but its complete crystal structure has been reported only recently. Here, using single-crystal X-ray diffraction, we reevaluated the crystal structure of o -Li 4 SiS 4 and showed that o -Li 4 SiS 4 crystallizes in space group Pmn 2 1 (no. 31, a = 7.7694(15) Å, b = 13.731(3) Å, and c = 6.1413(12) Å). The crystal structure of o -Li 4 SiS 4 consists of isolated SiS 4 tetrahedra arranged in a zigzag-type manner, whereas Li atoms are coordinated both tetrahedrally and octahedrally by sulfur atoms of the SiS 4 groups. Structures identified by first-principles calculations support the lower symmetry solution presented here, with the Pmn 2 1 polymorph being more stable at room temperature than a higher symmetry phase. By knowing the accurate crystal structure of o -Li 4 SiS 4, we investigated the solid solution behavior with another group IV thio-LISICON, Li 4 SnS 4 . Rietveld refinements of powder X-ray diffraction data revealed the solid solution Li 4 Si 1– x Sn x S 4 (0 ≤ x ≤ 1, Δ x = 0.1), which shows a nearly ideal Vegard-type behavior for all silicon-containing samples. 29 Si and 119 Sn magic-angle-spinning solid-state NMR and Raman spectroscopy showed the presence of SiS 4 and SnS 4 tetrahedral moieties, with the spectra showing expected behavior consistent with the silicon–tin ratio in the materials. Electrochemical impedance spectroscopy revealed the highest ionic conductivity of 8.4 × 10 –6 S cm –1 at 25 °C for Li 4 Si 0.5 Sn 0.5 S 4, accompanied by the lowest migration barrier of ∼0.37 eV.