Moisture-Stable Argyrodites with High Ionic Conductivity via Crystal Structure Engineering: Li_6+xM_xAs_1−xS₅I (M = Ge, Sn)

Sulfide solid electrolytes (SSEs) are promising alternatives to liquid electrolytes in lithium-ion batteries due to their high ionic conductivity and reduced flammability. However, their chemical instability under humid conditions poses significant challenges. This study introduces a substitution series, Li_6+xM_xAs_1−xS₅I (M = Ge, Sn), adopting an argyrodite-type structure with high ionic conductivity and moisture stability. Among these, Li_6.333Ge_0.333As_0.667S₅I achieves ∼3 mS cm⁻¹ at 303 K, an improvement of 3 orders of magnitude over pristine Li₆AsS₅I. Powder X-ray and neutron diffraction patterns reveal additional lithium-ion sites enhancing 3D diffusion pathways, significantly lowering the activation energy. Li_6.333Ge_0.333As_0.667S₅I also demonstrates superior moisture stability, releasing minimal toxic H₂S gas (70 ppm) after exposure to 27% relative humidity at 303 K for 1 h, outperforming Li₆PS₅Cl (160 ppm). Additionally, it retains ∼70% of its initial discharge capacity over 40 cycles of galvanostatic testing (In/InLi/ Li_6.333Ge_0.333As_0.667S₅I/TiS₂). However, cycling beyond the electrochemical stability window leads to capacity fading. These findings provide insights into the interplay between crystal structure, ionic conductivity, and moisture stability, offering a pathway to high-performance solid electrolytes for next-generation all-solid-state batteries.