Superionic Germanium Substituted Lithium Thioarsante Li ₆₊ <i> _x </i> Ge <i> _x </i> As _1‐ <i> _x </i> S ₅ Br Argyrodites With High Air Stability

ABSTRACT Lithium argyrodite solid electrolytes are promising for all‐solid‐state batteries due to their high ionic conductivity and low elastic modulus. However, poor chemical stability in humid conditions remains a major hurdle towards large‐scale practical implementation. This motivates substituting phosphorus with softer acids to stabilize sulfur, where the challenge is to maintain high Li‐ion mobility. In this work, we explore the effects of aliovalent (Ge) substitutions on the structure, ionic transport, and air stability of Li 6 AsS 5 Br. The induced structural modification releases the rate‐limiting step in long‐range Li‐ion transport, as demonstrated by Molecular Dynamics and percolation simulations. As a result, the ionic conductivity reaches 13 mS/cm for Li 6.5 As 0.5 Ge 0.5 S 5 Br, a 6‐fold improvement over pristine Li 6 AsS 5 Br. The key enabler for combining high Li‐ion mobility and improved stability towards moisture is the increased polarity of the Ge─S bonds compared to As─S or P─S bonds. This reduces Li‐ion trapping effectively flattening the energy landscape for diffusion and thermodynamically suppresses H 2 S formation upon exposure to moisture, as experimentally demonstrated. Finally, stable cycling performances in combination with high nickel ternary cathodes are demonstrated. Hereby, this research provides a deeper understanding of the role of composition on the ionic conductivity and moisture stability of lithium argyrodites.