NaS or Sodium-Sulphur battery is a kind of molten metal battery used in non mobile applications like grid energy storage. Sodium-Sulphur battery is made up of Sodium and Sulphur and has very high energy density and very high charge/ discharge efficiency compared to other batteries. The Sodium-Sulphur battery is composed of a solid electrolyte membrane between its anode and cathode. Due to very high energy efficiency, Sodium-Sulphur battery finds applications in grid energy storage and space explorations.
See more in this link http://www.engineersgarage.com/contribution/sodium-sulfur-nas-battery