All-Solid-State Na/S Batteries with a Na3PS4 Electrolyte Operating at Room Temperature.

Bulk-type all-solid-state Na/S cells, which are expected to have high capacity, be highly safe, and have low material cost, were fabricated using a Na3PS4 glass-ceramic as a solid electrolyte. The sulfur composite electrodes were prepared by mechanical milling of sulfur active material, a conductive additive (acetylene black), and a Na3PS4 glass-ceramic electrolyte. The all-solid-state Na/S cells used the reaction up to the final discharge product of sulfur active material, Na2S, and achieved a high capacity of ~1100 mAh (g of S)-1 at room temperature. The rate of utilization of sulfur active material was ~2 times higher than that of high-temperature-operating NAS batteries (commercially available NAS batteries, Na/sintered β"-alumina/S), where Na2Sx melts with bridging sulfurs contribute to redox in the sulfur electrodes. The open circuit potential curve of the discharge process of the Na/S batteries operating at room temperature was similar to that of the NAS batteries operating at high temperatures; X-ray diffraction and X-ray photoelectron spectroscopy measurement indicated that amorphous Na2Sx with a structure similar to the structure of these melts contributed to sulfur redox reaction in the all-solid-state Na/S cells. A galvanostatic intermittent titration technique and impedance measurement suggested that the overpotential during the discharge process in the all-solid-state Na/S cells was mainly derived from the sodium diffusion resistance in the solid sulfur active material. The finding would be an effective guide for achieving higher performance for all-solid-state Na/S cells. [ABSTRACT FROM AUTHOR]