Li3PS4 (LPS) glasses in a Li2S–P2S5 binary system demonstrate promise as solid electrolytes for the development of all-solid-state lithium batteries due to their high electrochemical stability and ionic conductivity. However, sulfide electrolytes are hygroscopic; hence, these electrolytes are treated in a dry inert gas atmosphere, thereby increasing the process costs. To improve the chemical stability of LPS in air, in this study, composite electrolytes comprising LPS and ZnO, which is a H2S absorbent, were synthesized. In addition, the effects of the addition of ZnO on the thermal behavior and electrical properties of LPS glasses were investigated. Differential scanning calorimetry (DSC) measurements and ex situ TEM images revealed that the increase in the ZnO content significantly affects the exothermal behavior of LPS glass. In particular, LPS glasses comprising greater than 50 mol% of ZnO exhibit multiple exothermic reactions at temperatures of greater than ca. 250 °C. This result revealed that in LPS–ZnO electrolytes, 10 or 20 mol% of ZnO is an optimum additive to ensure a good balance among the thermal stability and chemical stability in air and conductivity.