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. Due to the high operating temperature required (usually between 300 and 350 °C), 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 (USA) and a 50 MW/300 MWh system in Fukuoka, Kyushu, (Japan). Despite their very low capital cost and high energy density (300-400 Wh/L), 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 (although there are reports of 15 year operation with 300 cycles per year). 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 (manufactured after 2011) 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 (up to 500 Ah). A similar type of battery called the ZEBRA battery, which uses a NiCl2/AlCl3 catholyte in place of molten sodium polysulfide, has had greater commercial interest in the past, but As of 2023 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 (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. 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 (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. 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:
2 Na + 4 S → Na2S4 (Ecell ~ 2 V) 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.
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