Enhanced Li-Ion Conductivity and Air Stability of Sb-Substituted Li₄GeS₄ toward All-Solid-State Li-Ion Batteries

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, H 2 S 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 Sb 5+ for Ge 4+ in Li 4 GeS 4 to produce a series of materials with a general nominal composition of Li 4– x Ge 1– x Sb x S 4 . 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 Li 4 GeS 4, and the main phase of the material is determined to be Li 3.68 Ge 0.69 Sb 0.31 S 4 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 Li 3 PS 4 retaining only 10% of the initial conductivity. A press cell composed of a TiS 2 composite cathode, an In–Li alloy anode, and a Li 3.68 Ge 0.69 Sb 0.31 S 4 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.