We carried out synchrotron X-ray diffraction and time-of-flight neutron diffraction experiments for a lithium-ion conductor, (Li2S)(5)(GeS2)(P2S5) glass, prepared by mechanical alloying. The three-dimensional structure and the conduction pathways of Li ions in (Li2S)(5)(GeS2)(P2S5) glass were visualized by combining reverse Monte Carlo (RMC) modeling and the bond valence sum (BVS) approach. The electrochemical properties were characterized; the material demonstrated an electrical conductivity, sigma(300) (K), of 1.5 x 10(-5)S/cm at 300 I( and an activation energy, E-a, of 55 kJ/mol. In addition, it was found that the most advanced lithium-ion conductor, Li10GeP2S12 crystal (sigma(300K) = 1.2 x 10(-2) S/cm and E-a = 24 kJ/mot), could be obtained by aging the (Li2S)(5)(GeS2)(P2S5) glass at 803 K. By comparing the conduction pathways of Li ions in (Li2S)(5)(GeS2)(P2S5) glass with those in Li10GeP2S12 crystal, we found that vertical bar Delta V(Li)vertical bar(max), the maximum BV mismatch value for Li ions, for (Li2S)(5)(GeS2)(P2S5) glass (vertical bar Delta V(Li)vertical bar(max) = 0.13) was significantly larger than that for Li10GeP2S12 crystal vertical bar Delta V(Li)vertical bar(max) = 0.03). This indicates that the potential barrier, E-a, for Li ions moving along the conduction pathways lowers on the transition from (Li2S)(5)(GeS2)(P2S5) glass to Li10GeP2S12 crystal. Here, we propose that the lowering of the vertical bar Delta V(Li)vertical bar(max) is the reason for the structural origin of the massive improvement in sigma(300K) and E-a during the glass-to-crystal transition. (C) 2017 Elsevier B.V. All rights reserved.