Sulfide-based Li2S-P2S5 solid state electrolytes were prepared by a high energy ball milling process. Characterization of the xLi2S-(100-x)P2S5] system for 70<x<80 was carried out for both amorphous glass and crystalline glass-ceramic materials by X-ray diffraction and electrochemical methods. Both glass and glass-ceramic electrolytes exhibited high conductivity over 10-4 S cm-1 at room temperature. Glass-ceramics were proved to have higher stability against lithium metal than respective glass electrolytes. All-solid-state batteries using the glass and glass-ceramic electrolytes were constructed to evaluate the cell performance as a lithium secondary battery. The cell Li/80Li2S-20P2S5 glass/LiCoO2 performed the best but displayed a large initial irreversible capacity.