Structure and transport properties in the pseudo-binary phase system Li4SiS4 – Li4SnS4

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 show that o-Li4SiS4 crystallizes in space group Pmn21 (No. 31, a = 7.7694(15) Å, b = 13.731(3) Å, c = 6.1413(12) Å). The crystal structure of o-Li4SiS4 consists of isolated SiS4 tetrahedra arranged in a zig-zag-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 Li4Si1-xSnxS4 (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 tetrahedra 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 Li4Si0.5Sn0.5S4, accompanied by the lowest migration barrier of ~0.37 eV.