A sodium-sulfur battery is a type of molten-salt battery constructed from liquid sodium (Na) and sulfur (S). This type of battery has a high energy density, high efficiency of charge/discharge  and long cycle life, and is fabricated from inexpensive materials. The operating temperatures of 300 to 350 °C and the highly corrosive nature of the sodium polysulfides, primarily make them suitable for stationary energy storage applications. The cell becomes more economical with increasing size.
The cell is usually made in a cylindrical configuration. The entire cell is enclosed by a steel casing that is protected, usually by chromium and molybdenum, from corrosion on the inside. This outside container serves as the positive electrode, while the liquid sodium serves as the negative electrode. The container is sealed at the top with an airtight alumina lid. An essential part of the cell is the presence of a BASE (beta-alumina solid electrolyte) membrane, which selectively conducts Na+. In commercial applications the cells are arranged in blocks for better heat conservation and are encased in a vacuum-insulated box.
During the discharge phase, molten elemental sodium at the core serves as the anode, meaning that the Na donates electrons to the external circuit. The sodium is separated by a beta-alumina solid electrolyte (BASE) cylinder from the container of molten sulfur, which is fabricated from an inert metal serving as the cathode. The sulfur is absorbed in a carbon sponge.
BASE is a good conductor of sodium ions above 250 °C, but a poor conductor of electrons, and thus avoids self-discharge. Sodium metal does not fully wet the BASE below 400 °C due to a layer of oxide(s) separating them; this temperature can be lowered to 300 °C by coating the BASE with certain metals and/or by adding oxygen getters to the sodium, but even so wetting will fail below 200 °C.
When sodium gives off an electron, the Na+ ion migrates to the sulfur container. The electron drives an electric current through the molten sodium to the contact, through the electrical load and back to the sulfur container. Here, another electron reacts with sulfur to form Sn2-, sodium polysulfide. The discharge process can be represented as follows:
As the cell discharges, the sodium level drops. During the charging phase the reverse process takes place. Once running, the heat produced by charging and discharging cycles is sufficient to maintain operating temperatures and usually no external source is required.
Pure sodium presents a hazard, because it spontaneously burns in contact with air and moisture, thus the system must be protected from water and oxidizing atmospheres.
Early on the morning of September 21, 2011, a 2000 kilowatt NaS battery system manufactured by NGK, owned by Tokyo Electric Power Company used for storing electricity and installed at the Tsukuba, Japan Mitsubishi Materials Corporation plant caught fire. Following the incident, NGK temporarily suspended production of NaS batteries.
In 2014 researchers identified a liquid sodium-cesium alloy that operates at 150 °C and produces 420 milliampere-hours per gram. The material fully coated ("wetted") the electrolyte. After 100 charge/discharge cycles, a test battery maintained about 97% of its initial storage capacity. The lower operating temperature allowed the use of a less-expensive polymer external casing instead of steel, offsetting some of the increased cost associated with using cesium.
The NaS battery was one of four battery types selected as candidates for intensive research by MITI as part of the "Moonlight Project" in 1980. This project sought to develop a durable utility power storage device meeting the criteria shown below in a 10-year project.
A consortium formed by TEPCO (Tokyo Electric Power Co.) and NGK (NGK Insulators Ltd.) declared their interest in researching the NaS battery in 1983, and became the primary drivers behind the development of this type ever since. TEPCO chose the NaS battery because all its component elements (sodium, sulfur and ceramics) are abundant in Japan. The first large-scale field testing took place at TEPCO's Tsunashima substation between 1993 and 1996, using 3 x 2 MW, 6.6 kV battery banks. Based on the findings from this trial, improved battery modules were developed and were made commercially available in 2000. The commercial NaS battery bank offers:
A demonstration project used NaS battery at Japan Wind Development Co.'s Miura Wind Park in Japan.
Japan Wind Development opened a 51 MW wind farm that incorporates a 34 MW sodium sulfur battery system at Futamata in Aomori Prefecture in May 2008.
As of 2007, 165 MW of capacity were installed in Japan. NGK announced in 2008 a plan to expand its NaS factory output from 90 MW a year to 150 MW a year.
In 2010 Xcel Energy announced that it would test a wind farm energy storage battery based on twenty 50 kW sodium-sulfur batteries. The 80 tonne, 2 semi-trailer sized battery is expected to have 7.2 MW·h of capacity at a charge and discharge rate of 1 MW. Since then, NGK announced several large scale deployments including a virtual plant distributed on 10 sites in UAE totaling 108 MW/648 MWh in 2019.
In March 2011, Sumitomo Electric Industries and Kyoto University announced that they had developed a low temperature molten sodium ion battery that can output power at under 100 °C. The batteries have double the energy density of Li-ion and considerably lower cost. Sumitomo Electric Industry CEO Masayoshi Matsumoto indicated that the company planned to begin production in 2015. Initial applications are envisaged to be buildings and buses.[failed verification]
Corrosion of the insulators was found to be a problem in the harsh chemical environment as they gradually became conductive and the self-discharge rate increased. Dendritic-sodium growth can also be a problem.
NaS batteries can be deployed to support the electric grid, or for stand-alone renewable power applications. Under some market conditions, NaS batteries provide value via energy arbitrage (charging battery when electricity is abundant/cheap, and discharging into the grid when electricity is more valuable) and voltage regulation. NaS batteries are a possible energy storage technology to support renewable energy generation, specifically wind farms and solar generation plants. In the case of a wind farm, the battery would store energy during times of high wind but low power demand. This stored energy could then be discharged from the batteries during peak load periods. In addition to this power shifting, sodium sulfur batteries could be used to assist in stabilizing the power output of the wind farm during wind fluctuations. These types of batteries present an option for energy storage in locations where other storage options are not feasible. For example, pumped-storage hydroelectricity facilities require significant space and water resources, while compressed air energy storage (CAES) requires some type of geologic feature such as a salt cave.
In 2016, the Mitsubishi Electric Corporation commissioned the world's largest sodium-sulfur battery in Fukuoka Prefecture, Japan. The facility offers energy storage to help manage energy levels during peak times with renewable energy sources.
Because of its high energy density, the NaS battery has been proposed for space applications. Sodium sulfur cells can be made space-qualified: in fact a test sodium sulfur cell flew on the Space Shuttle. The NaS flight experiment demonstrated a battery with a specific energy of 150 W·h/kg (3 x nickel-hydrogen battery energy density), operating at 350 °C. It was launched on the STS-87 mission in November 1997, and demonstrated 10 days of experimental operation.
The Venus Landsailing Rover mission concept is also considering the use of this type of battery, as the rover and its payload are being designed to function for about 50 days on the hot surface of Venus without a cooling system.
The first large-scale use of sodium-sulfur batteries was in the Ford "Ecostar" demonstration vehicle, an electric vehicle prototype in 1991. The high operating temperature of sodium sulfur batteries presented difficulties for electric vehicle use, however. The Ecostar never went into production.
The facility offers energy-storage capabilities similar to those of pumped hydro facilities while helping to improve the balance of supply and demand