Abstract

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 Li₄SiS₄ (o-Li₄SiS₄) 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-Li₄SiS₄ and showed that o-Li₄SiS₄ crystallizes in space group Pmn2₁ (no. 31, a = 7.7694(15) Å, b = 13.731(3) Å, and c = 6.1413(12) Å). The crystal structure of o-Li₄SiS₄ consists of isolated SiS₄ tetrahedra arranged in a zigzag-type manner, whereas Li atoms are coordinated both tetrahedrally and octahedrally by sulfur atoms of the SiS₄ groups. Structures identified by first-principles calculations support the lower symmetry solution presented here, with the Pmn2₁ polymorph being more stable at room temperature than a higher symmetry phase. By knowing the accurate crystal structure of o-Li₄SiS₄, we investigated the solid solution behavior with another group IV thio-LISICON, Li₄SnS₄. Rietveld refinements of powder X-ray diffraction data revealed the solid solution Li₄Si_1-xSn_xS₄ (0 ≤ x ≤ 1, Δx = 0.1), which shows a nearly ideal Vegard-type behavior for all silicon-containing samples. ²⁹Si and ¹¹⁹Sn magic-angle-spinning solid-state NMR and Raman spectroscopy showed the presence of SiS₄ and SnS₄ 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 ℃ for Li₄Si_0.5Sn_0.5S₄, accompanied by the lowest migration barrier of ~0.37 eV.