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.