All-solid-state Na/S cells with high safety, capacity, and low material costs are desirable for smart grid systems. We report sulfur composite electrodes prepared by the mechanical milling of sulfur, Ketjen black, and P2S5 or Na3PS4 for high-capacity cells. A cell using P2S5, which is not ion conductive, in the sulfur electrode exhibits a high reversible capacity of 340 mAh (g-sulfur electrode)(-1) at 0.04 C rate at 25 degrees C, which is much larger than that (37.3 mAh (g-sulfur electrode)(-1)) obtained in a conventional cell using a high ion-conductive Na3PS4 electrolyte. To investigate the reaction mechanism of the sulfur composite electrode containing P2S5, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements and scanning and transmission electron microscopy (SEM/TEM) observations of the electrodes were conducted. The results indicate that a crystalline Na3PS4 component is automatically produced by the electrochemical reaction with Na in the amorphous S-P2S5 electrode and it mixing with the sulfur redox parts at the nanoscale. The mixing degree is higher than that at microscale in the conventional S-Na3PS4 electrode, which results in the high capacity of the cells containing the S-P2S5 electrode. Partial substitution of P2S5 for SiS2 in the sulfur electrode suppresses the nanocrystallization and further increases the reversible capacity up to 390 mAh (g-sulfur electrode)(-1) under 0.04 C rate, which is the highest in reported all-solid-state Na batteries to date.