We report that a heat-treated Li2S–P2S5 glass-ceramic conductor has an extremely high ionic conductivity of 1.7 × 10−2 S cm−1 and the lowest conduction activation energy of 17 kJ mol−1 at room temperature among lithium-ion conductors reported to date. The optimum conditions of the heat treatment reduce the grain boundary resistance, and the influence of voids, to increase the Li+ ionic conductivity of the solid electrolyte so that it is greater than the conductivities of liquid electrolytes, when the transport number of lithium ions in the inorganic electrolyte is unity.
