Moisture-Stable Argyrodites with High Ionic Conductivity via Crystal Structure Engineering: Li_6+<i>x</i>M_<i>x</i>As_1–<i>x</i>S₅I (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, Li6+xMxAs1–xS5I (M = Ge, Sn), adopting an argyrodite-type structure with high ionic conductivity and moisture stability. Among these, Li6.333Ge0.333As0.667S5I 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. Li6.333Ge0.333As0.667S5I 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/Li6.333Ge0.333As0.667S5I/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.