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., Li_4-xSi_1-xSb_xS₄. 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 Li_4-xSi_1-xSb_xS₄ 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 Li_3.85Si_0.85Sb_0.15S₄ 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.