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, 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-xGe1-xSbxS4. 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, similar to 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.

Automated summary

Sulfide inorganic materials with aliovalent substitutions of Sb5+ for Ge4+ in Li4GeS4 showed enhanced ionic conductivity and air stability, making them promising solid electrolytes for Li-ion all-solid-state batteries. The material with x = 0.4 displayed the highest ionic conductivity, around 10-4 S cm-1 at 303 K, and retained 70% of the initial conductivity after exposure to air with a relative humidity of 15% for 70 min. A press cell composed of the electrolyte, TiS2 composite cathode, and In-Li alloy anode showed excellent cycle performance. These findings provide insights into the synthesis design of air-stable solid electrolytes for all-solid-state batteries.