A sodium–sulfur battery or liquid-metal battery[1] is a type of molten-metal battery[2] constructed from sodium (Na) and sulfur (S). This type of battery has a high energy density, high efficiency of charge/discharge (89–92%) and long cycle life, and is fabricated from inexpensive materials. However, because of the operating temperatures of 300 to 350 °C and the highly corrosive nature of the sodium polysulfides, such cells are primarily suitable for large-scale non-mobile applications such as grid energy storage.
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The battery has a solid electrolyte membrane between the anode and cathode, compared with liquid-metal batteries where the anode, the cathode, and also the membrane are liquids.[2]
The cell is usually made in a tall 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+. The cell becomes more economical with increasing size. In commercial applications the cells are arranged in blocks for better conservation of heat 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, but a poor conductor of electrons, and thus avoids self-discharge. 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.[3]
Pure sodium presents a hazard because it spontaneously burns/explodes in contact with water, thus the system must be protected from moisture. In modern NaS cells, sealing techniques make fires unlikely.
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.
Ford Motor pioneered the battery in the 1960s to power early-model electric cars.[4]
A lower temperature version is being developed (2009) in Utah by Ceramatec. They use a new NaSICON membrane to allow operation at 90°C with all components remaining solid.[5] [6]
The NaS battery was one of the 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.
The other three types of batteries were: improved lead–acid, redox flow (vanadium type), and zinc-bromide batteries.
TEPCO (Tokyo Electric Power Co.)/NGK (NGK Insulators Ltd.) consortium declared their interest in researching the NaS battery in 1983, and have become the primary drivers behind the development of this type ever since. TEPCO chose the NAS battery because all its component elements (Sodium, Sulphur, Ceramics) can be abundantly found in Japan. First large-scale prototype 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 performance of the commercial NAS battery bank is as follows:[7]
As of 2008, sodium–sulfur batteries are only manufactured by one group, the NGK/TEPCO consortium, which is producing 90 MW of storage capacity each year.[8]
There is currently a demonstration project using NGK Insulators’ NAS battery at Japan Wind Development Co.’s Miura Wind Park in Japan.[9]
Japan Wind Development has opened a 51 MW wind farm that incorporates a 34 MW sodium sulfur battery system at Futamata in Aomori Prefecture in May 2008.[8][10]
There are already 165 MW of installed capacity base in Japan alone as of 2007, and NGK has just announced plan to expand its NAS factory output from 90 MW a year to 150 MW a year.[11] (Source in Japanese, but with some pictures)
Xcel Energy has announced that it will be testing a wind farm energy storage battery based on 20–50 kW sodium–sulfur batteries from NGK Insulators Ltd of Japan. 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.[12]
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 temperatures under 100 °C. The batteries would have double the energy density of Li-ion and considerably lower cost. Sumitomo Electric Industry CEO Masayoshi Matsumoto indicated that the company aimed to begin production in 2015. Initial applications would be buildings and buses.
As noted above, NaS batteries can be deployed to support the electric grid. In 2010, Presidio, Texas built the world's largest sodium–sulfur battery to provide power when the city's lone line to the United States power grid goes down.[13] 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.[14] 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, it is likely that sodium sulfur batteries could be used throughout the day 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 due to location or terrain constraints. For example, pumped-storage hydroelectricity facilities require a lot of space and a significant water resource, and compressed air energy storage (CAES) requires some type of geologic feature for storage.[15]
NGK Insulators Ltd. develops sodium–sulfur batteries as grid storage in Japan, France (Île de la Réunion) and the United States.
Because of its high energy density, the NaS battery has been proposed for space applications.[16][17] Sodium sulfur cells can be made space-qualified; in fact a test sodium sulfur cell was flown on the Space Shuttle to demonstrate operation in space. The sodium sulfur 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 experiment operation in orbit.[18]
The first large-scale use of sodium–sulfur batteries was in the Ford "Ecostar" demonstration vehicle,[19] an electric vehicle prototype that was demonstrated in 1991. The high temperature of sodium sulfur batteries presented some difficulties for electric vehicle use, however, and with the development of other battery types better suited to automotive use, the Ecostar never went into production.