Highly ion-conductive Li2S-P2S5 glass-ceramic electrolytes were prepared by controlling the compositions and heat treatment temperatures of the glasses. The 70Li(2)S.30P(2)S(5) (mol%) glass-ceramic heated at 360 degrees C showed the highest conductivity of 3.2 x 10(-3) S cm(-1) at room temperature and the lowest activation energy of 12 kJ mol(-1) for conduction in the binary system Li2S-P2S5. The outstanding property was attributed to both the precipitation of the new crystal as a metastable phase and the increase in crystallinity of the phase. With increasing heat treatment temperatures, the metastable phase changed into thermodynamically stable phases such as the Li4P2S6 crystal by heat treatment up to 550 degrees C, resulting in low conductivities of the glass-ceramics. It was, thus, found that the formation of superionic metastable phases by heating the Li2S-P2S5 glasses is responsible for the marked enhancement on the conducting properties of the glass-ceramics. (c) 2006 Elsevier B.V. All rights reserved.