Unraveling Polymorphic Crystal Structures of Li₄SiS₄ for All-Solid-State Batteries: Enhanced Ionic Conductivity via Aliovalent Sb Substitution

Safety concerns regarding organic-based liquid electrolytes in Li-ion batteries have led to extensive research on lithium-ion conductors. Despite cost-effectiveness, thio-silicate Li4SiS4 has been overlooked owing to unclear crystallographic information. This study clarifies the crystal structures and electrochemical properties of two Li4SiS4 polymorphs and their aliovalent substitution series, i.e., Li4–xSi1–xSbxS4. Our findings indicate that the polymorphs differ primarily in their SiS4 tetrahedra stacking configurations, with the high-temperature phase being more orderly than the low-temperature phase. However, they exhibit similar ionic-transport properties, indicating that the tetrahedra stacking minimally affects Li-ion mobility. We found that the dense packing of Li in these structures restricts ion movement, necessitating the creation of Li vacancies through the aliovalent substitution of Sb5+ for Si4+ to enhance Li mobility. The substitution series Li4–xSi1–xSbxS4 with x = 0.15 exhibited a 10-fold conductivity increase, signifying the influence of Li vacancies on ionic transport. Cyclic voltammetry confirmed the suitability of Li3.85Si0.85Sb0.15S4 as a solid electrolyte for all-solid-state batteries. This study suggests that the ionic conductivity in Li4SiS4 depends more on Li-ion concentration than on SiS4 tetrahedra stacking, providing strategic insights for developing more efficient solid-state battery materials.