Sodium–sulfur battery
A sodium–sulfur 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. Due to the high operating temperature required, as well as the highly reactive nature of sodium and sodium polysulfides, these batteries are primarily suited for stationary energy storage applications, rather than for use in vehicles. Molten Na-S batteries are scalable in size: there is a 1 MW microgrid support system on Catalina Island CA and a 50 MW/300 MWh system in Fukuoka, Kyushu,.
Despite their very low capital cost and high energy density, molten sodium–sulfur batteries have not achieved a wide-scale deployment yet compared to lithium-ion batteries: there have been ca. 200 installations, with a combined energy of 5 GWh and power of 0.72 GW, worldwide. vs. 948 GWh for lithium-ion batteries. Poor market adoption of molten sodium-sulfur batteries has possibly been due to perceived safety and durability issues, such as a short cycle life of fewer than 1000 cycles on average. In contrast to these concerns, a recent technical data sheet indicates a cycle life of 20 years or 7300 cycles with less than 1% energy degradation per year. Also TÜV Rheinland assessed commercial NaS batteries and their safety features coming to the conclusion that "under practical conditions it is not possible to ignite an intact NGK Insulators NaS battery module or to trigger other dangerous scenarios from the outside or from within."
Like many high-temperature batteries, sodium–sulfur cells become more economical with increasing size. This is because of the square–cube law: large cells have less relative heat loss, so maintaining their high operating temperatures is easier. Commercially available cells are typically large with high capacities.
A similar type of battery called the ZEBRA battery, which uses a / catholyte in place of molten sodium polysulfide, has had greater commercial interest in the past, but there are no commercial manufacturers of ZEBRA. Room-temperature sodium–sulfur batteries are also known. They use neither liquid sodium nor liquid sulfur nor sodium beta-alumina solid electrolyte, but rather operate on entirely different principles and face different challenges than the high-temperature molten NaS batteries discussed here.
Construction
Typical batteries have a solid electrolyte membrane between the anode and cathode, compared with liquid-metal batteries where the anode, the cathode and the membrane are liquids.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 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.
For operation, the entire battery must be heated to, or above, the melting point of sulfur at 119 °C. Sodium has a lower melting point, around 98 °C, so a battery that holds molten sulfur holds molten sodium by default. This presents a serious safety concern; sodium can spontaneously ignite in air, and sulfur is highly flammable. Several examples of the Ford Ecostar, equipped with such a battery, burst into flame during recharging, leading Ford to abandon the attempted development of molten NaS batteries for cars. Stationary NaS batteries by NGK Insulators use hermetically sealed cells and multiple safety features on module level, such as sand for fire suppression. According to the manufacturer, these are sufficient to avoid that a fire can spread from one to neighboring cells.
Operation
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 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 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. Before the cell can begin operation, it must be heated, which creates extra costs. To tackle this challenge, case studies to couple sodium–sulfur batteries to thermal solar energy systems. The heat energy collected from the sun would be used to pre-heat the cells and maintain the high temperatures for short periods between use. Once running, the heat produced by charging and discharging cycles is sufficient to maintain operating temperatures and usually no external source is required.
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.
Safety
Pure sodium presents a hazard, because it spontaneously burns in contact with air and moisture, thus safety features are required to avoid direct contact with water and oxidizing atmospheres.2011 Tsukuba Plant fire incident
Early on the morning of September 21, 2011, a 2000 kilowatt NaS battery system manufactured by NGK Insulators, 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. According to a report by TÜV Rheinland additional safety measures were adopted afterwards: "NGK implemented additional safety measures on module and battery level, additional automated quality controls were introduced during cell production, the number of cells per module was reduced and additional fuses installed. The interconnection/wiring of the cells was changed so that in case of an internal short-circuit subsequent propagation with serious consequences can be reasonably ruled out. The additional safety measures implemented mean that the occurrence of incidents with consequences similar to those which occurred in 2011 and earlier can reasonably be excluded."Development
United States
pioneered the battery in the 1960s to power early-model electric cars. In 1989 Ford resumed its work on a Na-S battery powered electric car, which was named Ford Ecostar. The car had a 100-mile driving range, which was twice as much as any other fully electric car demonstrated earlier. 68 of such vehicles were leased to United Parcel Service, Detroit Edison Company, US Post Office, Southern California Edison, Electric Power Research Institute, and California Air Resources Board. Despite the low materials cost, these batteries were expensive to produce, as the economy of scale was not achieved during that time. Also, the battery life was estimated to be only 2 years. However, the program was terminated in 1995, after two of the leased car batteries caught fire., a lower temperature, solid electrode version was under development in Utah by Ceramatec. They use a NASICON membrane to allow operation at 90 °C with all components remaining solid.
In 2014, researchers identified a liquid sodium–caesium alloy that operates at 150 °C and produces 420 milliampere-hours per gram. The material fully coated 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 caesium.
Japan
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.- 1,000 kW class
- 8 hour charge/8 hour discharge at rated load
- Efficiency of 70% or better
- Lifetime of 1,500 cycles or better
A consortium formed by TEPCO and 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 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:
- Capacity: 25–250 kWh per bank
- Efficiency of 87%
- Lifetime of 2,500 cycles at 100% depth of discharge, or 4,500 cycles at 80% DOD
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.
In 2024, only one company produced molten NaS batteries on a commercial scale. BASF Stationary Energy Storage GmbH, a wholly owned subsidiary of BASF SE, acts as a distributor and development partner for the NaS batteries produced by NGK Insulators. In 2025, NGK discontiued production.