Solid electrolyte Na₃AsS₄ with high conductivity and moisture tolerance synthesized by mechanochemical process

Abstract

The sodium-ion conducting sulfide electrolyte Na₃MS₄ (M is a group 15 central element) has been developed and is promising for solid-state battery applications. Na₃PS₄ is a standard electrolyte with moderate conductivity greater than 10⁻⁴ S cm⁻¹ and good ductility, which allows close solid-solid contact with active materials. The conductivity of Na₃SbS₄ is ten times higher than that of Na₃PS₄, and partial replacement of Sb with W significantly increases conductivity to a magnitude of 10⁻² S cm⁻¹. This electrolyte has a high tolerance to moisture, and the structure is resistant to hydrolysis without generating harmful H₂S gas. The group 15 element As, which is located between P and Sb in the periodic table, has not been studied in detail as an endmember of Na₃MS₄ electrolytes. Thus, we synthesized and characterized Na₃AsS₄. The cubic Na₃AsS₄ phase was directly obtained by a mechanochemical process and exhibited a high Na⁺ ion conductivity of 10⁻³ S cm⁻¹ at room temperature, higher than that of the tetragonal Na₃PS₄ phase. Humidity stability tests showed that the crystal structure did not change when exposed to moisture. Cubic Na₃AsS₄ is an important sulfide endmember of group 15 central elements, and further increases in conductivity are expected by optimizing its composition through elemental substitution.