Enhanced Li-Ion Conductivity and Air Stability of Sb-Substituted Li₄GeS₄toward 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, 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 Li₄GeS₄to produce a series of materials with a general nominal composition of Li_4-xGe_1-xSb_xS₄. 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^-4S cm^-1at 303 K, 2 orders of magnitude higher than that of the unsubstituted Li₄GeS₄, and the main phase of the material is determined to be Li_3.68Ge_0.69Sb_0.31S₄by 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 Li₃PS₄retaining only 10% of the initial conductivity. A press cell composed of a TiS₂composite cathode, an In-Li alloy anode, and a Li_3.68Ge_0.69Sb_0.31S₄electrolyte 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.