Sodium-sulfur battery
Encyclopedia
A sodium–sulfur battery or liquid metal battery is a type of molten
metal
battery
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
.
membrane between the anode
and cathode
, compared with liquid metal batteries where the anode, the cathode, and also the membrane are liquids.
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.
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 ion
s, 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.
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.
-sodium growth can also be a problem.
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.
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:
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.
There is currently a demonstration project using NGK Insulators’ NAS battery at Japan Wind Development Co.’s Miura Wind Park in Japan.
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.
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. (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.
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.
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. 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 farm
s 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.
NGK Insulators Ltd.
develops sodium-sulfur batteries as grid storage in Japan, France (Île de la Réunion) and the United States.
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.
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.
Mölten
Mölten is a comune in South Tyrol in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano .-Geography:...
metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
constructed from sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
(Na) and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
(S). This type of battery has a high energy density
Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
, 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
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
nature of the sodium polysulfides, such cells are primarily suitable for large-scale non-mobile applications such as grid energy storage
Grid energy storage
Grid energy storage refers to the methods used to store electricity on a large scale within an electrical power grid. Electrical energy is stored during times when production exceeds consumption and the stores are used at times when consumption exceeds production...
.
Construction
The battery has a solid electrolyteElectrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
membrane between the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
, compared with liquid metal batteries where the anode, the cathode, and also the membrane are liquids.
The cell
Electrochemical cell
An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery"...
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
Beta-alumina solid electrolyte
Beta-alumina solid electrolyte is a fast ion conductor material used as a membrane in several types of molten salt electrochemical cell...
) 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.
Operation
During the discharge phase, moltenMölten
Mölten is a comune in South Tyrol in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano .-Geography:...
elemental
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
sodium at the core serves as the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
, meaning that the Na
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
donates electrons to the external circuit. The sodium is separated by a beta-alumina solid electrolyte
Beta-alumina solid electrolyte
Beta-alumina solid electrolyte is a fast ion conductor material used as a membrane in several types of molten salt electrochemical cell...
(BASE) cylinder from the container of molten sulfur, which is fabricated from an inert
Inert
-Chemistry:In chemistry, the term inert is used to describe a substance that is not chemically reactive.The noble gases were previously known as inert gases because of their perceived lack of participation in any chemical reactions...
metal serving as the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
. The sulfur is absorbed in a carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
sponge. BASE is a good conductor of sodium ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s, but a poor conductor of electrons, and thus avoids self-discharge. When sodium gives off an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
, 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
Polysulfide
Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: anions and organic polysulfides. Anions have the general formula Sn2−. These anions are the conjugate bases of the hydrogen polysulfides H2nSn...
. 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. Once running, the heat produced by charging and discharging cycles is sufficient to maintain operating temperatures and usually no external source is required.
Safety aspects
Pure sodiumSodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
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.
Difficulties
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. DendriticDendrite (metal)
A dendrite in metallurgy is a characteristic tree-like structure of crystals growing as molten metal freezes, the shape produced by faster growth along energetically favourable crystallographic directions. This dendritic growth has large consequences in regards to material properties.Dendrites form...
-sodium growth can also be a problem.
USA
Ford Motor pioneered the battery in the 1960s to power early-model electric cars.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.
NaS battery development in Japan
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.- 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
The other three types of batteries were: improved lead–acid, redox flow (vanadium type)
Vanadium redox battery
The vanadium redox battery is a type of rechargeable flow battery that employs vanadium ions in different oxidation states to store chemical potential energy...
, and zinc-bromide batteries
Zinc-bromine flow battery
The zinc–bromine flow battery is a type of hybrid flow battery. A solution of zinc bromide is stored in two tanks. When the battery is charged or discharged the solutions are pumped through a reactor stack and back into the tanks. One tank is used to store the electrolyte for the positive...
.
TEPCO (Tokyo Electric Power Co.)/NGK (NGK Insulators
NGK Insulators
is a Japanese ceramics company. It primarily produces insulators but also produces other products, especially ceramic products. It is listed on the Nikkei 225, which is an index of the Tokyo Stock Exchange. It is also listed in the Osaka Securities Exchange, the Nagoya Stock Exchange, and the...
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:
- Capacity : 25–250 kW per bank
- Efficiency of 87%
- Lifetime of 2,500 cycles (at 100% DOD - depth of discharge), or 4,500 cycles (at 80% DOD)
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.
There is currently a demonstration project using NGK Insulators’ NAS battery at Japan Wind Development Co.’s Miura Wind Park in Japan.
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.
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. (Source in Japanese, but with some pictures)
Xcel Energy
Xcel Energy
Xcel Energy, Inc. is a public utility company based in Minneapolis, Minnesota, serving customers in Colorado, Michigan, Minnesota, New Mexico, North Dakota, South Dakota, Texas, and Wisconsin. Primary services are electricity and natural gas...
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.
In March 2011, Sumitomo Electric Industries
Sumitomo Electric Industries
is a major manufacturer of electric wire and optical fiber cables. It holds the lead in market share for electric cables within Japan. Its headquarters are in Chūō-ku, Osaka, Japan. The company's shares are listed in the first section of the Tokyo, Osaka, and Nagoya Stock Exchanges, and the Fukuoka...
and Kyoto University
Kyoto University
, or is a national university located in Kyoto, Japan. It is the second oldest Japanese university, and formerly one of Japan's Imperial Universities.- History :...
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.
Electricity storage for grid support
As noted above, NaS batteries can be deployed to support the electric grid. In 2010, Presidio, TexasPresidio, Texas
Presidio is a city in Presidio County, Texas, United States. It stands on the Rio Grande , on the opposite side of the U.S.-Mexico border from Ojinaga, Chihuahua. The population was 4,167 at the 2000 census....
built the world's largest sodium–sulfur battery to provide power when the city's lone line to the United States power grid
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
goes down. Under some market conditions, NaS batteries provide value via energy arbitrage
Arbitrage
In economics and finance, arbitrage is the practice of taking advantage of a price difference between two or more markets: striking a combination of matching deals that capitalize upon the imbalance, the profit being the difference between the market prices...
(charging battery when electricity is abundant/cheap, and discharging into the grid when electricity is more valuable) and voltage regulation
Voltage regulation
In electrical engineering, particularly power engineering, voltage regulation is the ability of a system to provide near constant voltage over a wide range of load conditions.-Electrical power systems:...
. NaS batteries are a possible energy storage technology to support renewable energy generation, specifically wind farm
Wind farm
A wind farm is a group of wind turbines in the same location used to produce electric power. A large wind farm may consist of several hundred individual wind turbines, and cover an extended area of hundreds of square miles, but the land between the turbines may be used for agricultural or other...
s 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
Pumped-storage hydroelectricity
Pumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps...
facilities require a lot of space and a significant water resource, and compressed air energy storage (CAES
Compressed air energy storage
Compressed Air Energy Storage is a way to store energy generated at one time for use at another time. At utility scale, energy generated during periods of low energy demand can be released to meet higher demand periods....
) requires some type of geologic feature for storage.
NGK Insulators Ltd.
NGK Insulators
is a Japanese ceramics company. It primarily produces insulators but also produces other products, especially ceramic products. It is listed on the Nikkei 225, which is an index of the Tokyo Stock Exchange. It is also listed in the Osaka Securities Exchange, the Nagoya Stock Exchange, and the...
develops sodium-sulfur batteries as grid storage in Japan, France (Île de la Réunion) and the United States.
Space applications
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 was flown on the Space ShuttleSpace Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
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
STS-87
STS-87 was a Space Shuttle mission launched from KSC pad 39-B on 19 November 1997. It was the 88th flight of the Space Shuttle, and the 24th flight of Columbia...
mission in November 1997, and demonstrated 10 days of experiment operation in orbit.
Transport and heavy machinery
The first large-scale use of sodium–sulfur batteries was in the Ford "Ecostar" demonstration vehicle, an electric vehicleElectric vehicle
An electric vehicle , also referred to as an electric drive vehicle, uses one or more electric motors or traction motors for propulsion...
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.
See also
- Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryThe Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus...
- Lithium sulfur batteryLithium sulfur batteryThe lithium–sulfur battery is a rechargeable galvanic cell with a very high energy density. By virtue of the low atomic weight of lithium and moderate weight of sulfur, Li–S batteries are relatively light; about the density of water. They were demonstrated on the longest and highest-altitude...
- Molten salt batteryMolten salt batteryMolten salt batteries or liquid sodium battery are a class of primary cell and secondary cell high-temperature electric battery that use molten salts as an electrolyte. They offer both a higher energy density through the proper selection of reactant pairs as well as a higher power density by means...