A sodium–sulfur (NaS) battery is a type of molten-salt battery that uses liquid sodium and liquid sulfur electrodes. This type of battery has a similar energy density to lithium-ion batteries, and is fabricated from inexpensive and low-toxicity materials.
In a sodium sulfide battery, molten sulfur is used as the cathode and molten sodium is used as the anode. The electrolyte is a solid ceramic-based electrolyte called sodium alumina. When the battery is discharged each sodium atom gives away one electron forming sodium ions. The electrons take the external circuitry to reach the positive terminal.
Utility-scale sodium–sulfur batteries are manufactured by only one company, NGK Insulators Limited (Nagoya, Japan), which currently has an annual production capacity of 90 MW . The sodium sulfur battery is a high-temperature battery. It operates at 300°C and utilizes a solid electrolyte, making it unique among the common secondary cells.
Lifetime is claimed to be 15 year or 4500 cycles and the efficiency is around 85%. Sodium sulfur batteries have one of the fastest response times, with a startup speed of 1 ms. The sodium sulfur battery has a high energy density and long cycle life. There are programmes underway to develop lower temperature sodium sulfur batteries.
Molten sulfur and molten sodium are used as the electrode materials for the sodium-sulfur batteries. This kind of battery operates at higher temperatures ranging from 300°C to 350°C. An internal machine is employed for heating purposes to provide the required active temperatures in the system. The electrodes are separated by a ceramic layer.
Sodium–metal chloride batteries also use a β″-alumina electrolyte in direct contact with the molten sodium active mass. However, the metal chloride electrode is immersed in a molten salt electrolyte such as sodium chloroaluminate (NaAlCl 4).