Sulfide inorganic materials have the potential to be used as solid electrolytes (SEs) in Li-ion all-solid-state batteries (ASSBs) owing to their high ionic conductivity and mechanical softness. However, H2S gas release in ambient air is a critical issue for realizing scalable production of these materials. In the present study, we designed aliovalent substitutions of Sb5+ for Ge4+ in Li4GeS4 to produce a series of materials with a general nominal composition of Li4–x Ge1–x Sb x S4. With increasing Sb substitution up to the solubility limit (x = 0.4), the unit cell expands, the ionic conductivity increases, and the activation energy decreases. Among the series, the material with x = 0.4 displays the highest ionic conductivity, ∼10–4 S cm–1 at 303 K, 2 orders of magnitude higher than that of the unsubstituted Li4GeS4, and the main phase of the material is determined to be Li3.68Ge0.69Sb0.31S4 by the X-ray Rietveld refinement. It also shows high air stability: 70% of the initial ionic conductivity is retained without any structural degradation after exposure to air with a relative humidity of 15% for 70 min at 303 K, in contrast to a control sample of Li3PS4 retaining only 10% of the initial conductivity. A press cell composed of a TiS2 composite cathode, an In–Li alloy anode, and a Li3.68Ge0.69Sb0.31S4 electrolyte showed excellent cycle performance, demonstrating the electrolyte as a dry-air-stable SE candidate for ASSBs. These results provide insights into the synthesis design of air-stable SEs with appropriate compositions and improved performance.

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