Molten salt batteries are a class of
primary cellA primary cell is any kind of electrochemical cell in which the electrochemical reaction is not reversible. a common example of a primary cell is the disposable battery. Unlike a secondary cell, the reaction cannot be reversed by running a current into the cell; the chemical reactants cannot be...
and secondary cell high temperature electric battery that use
molten saltMolten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure . A salt that is normally a liquid at STP is usually called a room temperature ionic liquid, although technically molten salts are a class of ionic liquids.-Uses:Molten salts have...
s as an
electrolyteIn chemistry, an electrolyte is any substance containing free ions that behaves as an electrically conductive medium, usually when in a solution...
. They offer both a higher energy density through the proper selection of reactant pairs as well as a higher power density by means of a high
conductivityElectrical conductivity or specific conductance is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current...
molten
saltA salt, in chemistry, is an ionic compound, and can result from the neutralization reaction of acids and bases. Salts are ionic compounds composed of cations and anions so that the product is electrically neutral...
electrolyteIn chemistry, an electrolyte is any substance containing free ions that behaves as an electrically conductive medium, usually when in a solution...
. They are used in services where high
energy densityEnergy density is a term used for the amount of energy stored in a given system or region of space per unit volume, or per unit mass, depending on the context. The latter is more formally known as specific energy...
and high power density are required. These features make rechargeable molten salt batteries a promising technology for powering
electric vehicleAn electric vehicle , also referred to as an electric drive vehicle, is a vehicle which uses one or more electric motors for propulsion. Depending on the type of vehicle, motion may be provided by wheels or propellers driven by rotary motors, or in the case of tracked vehicles, by linear motors...
s. Operating temperatures of to , however, bring problems of thermal management and safety, and place more stringent requirements on the rest of the battery components. Some newer designs operate at a lower temperature range of to .
Primary cells
Referred to as
thermal batteries the electrolyte is solid and inactive at normal ambient temperatures. Thermally activated (“thermal”) batteries were conceived by the
GermansThe German people are an ethnic group, in the sense of sharing a common German culture, descent, and speaking the German language as a mother tongue. Within Germany, Germans are defined by citizenship , distinguished from people of German ancestry...
during WW II and were used in the
V-2 rocketAccording to head of Nazi rocket program Walter Dornberger, the V-2 rocket was the world's first ballistic missile and first human artifact to achieve sub-orbital spaceflight. It was the progenitor of all modern rockets...
s. Dr. Georg Otto Erb is credited with developing the molten-salt battery that used the heat of the rocket to keep the salt liquid during its mission. The technology was brought back to the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
in 1946 and was immediately adapted to replace the troublesome liquid-based systems that had previously been used in
artilleryArtillery is a military combat Arm that employs weapons capable of discharging large projectiles in combat. They are generally capable of adding considerable fire power to the military capability of an armed force...
proximity fuzes. These batteries have been used for ordnance applications (e.g., proximity fuzes) since World War II and, subsequent to that, in nuclear weapons. They are the primary power source for many missiles such as the
AIM-9 SidewinderThe AIM-9 Sidewinder is a heat-seeking, short-range, air-to-air missile carried by fighter aircraft and recently, certain gunship helicopters. Variants and upgrades remain in active service with many air forces after five decades...
,
MIM-104 PatriotThe MIM-104 Patriot is a surface-to-air missile system, the primary of its kind used by the United States Army and several allied nations. It is manufactured by the Raytheon Company of the United States. The Patriot System replaced the Nike Hercules system as the U.S. Army's primary High to Medium...
,
BGM-71 TOWThe BGM-71 TOW is an anti-tank guided missile. "TOW" stands for "Tube-launched, Optically-tracked, Wire-guided". The TOW was first produced in 1970 and is the most widely used anti-tank guided missile in the world.-Design and development:...
,
BGM-109 TomahawkThe Tomahawk Land Attack Missile is a long-range, all-weather, subsonic cruise missile. Introduced by General Dynamics in the 1970s, it was designed as a medium- to long-range, low-altitude missile that could be launched from a submerged submarine. It has been improved several times and, by way of...
and others. In these batteries the electrolyte is immobilized when molten by a special grade of
magnesium oxideMagnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of . It is formed by an ionic bond between one magnesium and one oxygen atom. Magnesium oxide is hygroscopic in nature and care must be taken to...
that holds it in place by
capillary actionCapillary action, capillarity, capillary motion, or wicking refers to two phenomena:1 The movement of liquids in thin tubes.
2 The flow of liquids through porous media, such as the flow of water through soil....
. This powdered mixture is pressed into pellets to form a separator between the
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
and
cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .A widespread misconception is that cathode polarity is always negative...
of each cell in the battery stack. As long as the electrolyte (salt) is solid, the battery is inert and remains inactive. Each cell also contains a
pyrotechnic heat sourceA pyrotechnic heat source, also called heat pellet, is a pyrotechnic device based on a pyrotechnic composition with a suitable igniter. Its role is to produce controlled amount of heat...
which is used to heat the cell to the typical operating temperature of 400 - 550C.
There are two types of design. One uses a fuze strip (containing
barium chromateBarium chromate is a fine light yellow powder with the formula BaCrO4. The chromium occurs in the +6 oxidation state.-Reactions:It is very insoluble in water, but is soluble in acids:-Uses:...
and powdered
zirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. It is a lustrous, gray-white, strong transition metal that resembles titanium. Zirconium is used as an alloying agent due to its high resistance to corrosion. It is never found as a native metal; it is obtained mainly from...
metal in a ceramic paper) along the edge of the heat pellets to initiate burning. The fuze strip is typically fired by an
electrical igniterAn electric match is a device that uses an externally applied electric current to ignite a combustible compound.-Use:Electric matches can be used in any application where source of heat is needed at a precisely controlled point in time, typically to ignite a propellant or explosive...
or
squibSquib may refer to:*Squib , a small explosive*Squib load, a firearms malfunction in which a bullet becomes lodged in the barrel*Squib , a short article that is intended to ignite thinking and discourse...
by application of electric current through it. The second design uses a center hole in the middle of the battery stack into which the high-energy electrical igniter fires a mixture of hot gases and incandescent particles. The center-hole design allows much faster activation times (tens of milliseconds) vs. hundreds of milliseconds for the edge-strip design. Battery activation can also be accomplished by a percussion primer, similar to a
shotgun shellA shotgun shell is a self-contained cartridge loaded with shot or a slug designed to be fired from a shotgun. Most shotgun shells are designed to be fired from a smoothbore barrel, but with the recent gain in popularity of dedicated shotguns with rifled barrels for firing slugs, there are many...
. It is desired that the pyrotechnic source be gasless. The standard heat source typically consist of mixtures of
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
powder and
potassium perchloratePotassium perchlorate is the inorganic salt with the chemical formula KClO4. Like other perchlorates, this salt is a strong oxidizer and reacts with many organic substances. It is usually obtained as a colorless, crystalline powder...
in weight ratios of typically 88/12, 86/14, and 84/16. The higher the potassium perchorate level, the higher the heat output (nominally 200, 259, and 297 calories/
gramThe gram , ; symbol g, is a unit of mass.Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice" , a gram is now defined as one one-thousandth of the SI base unit, the kilogram, or...
, respectively).
This property of unactivated storage has the double benefit of avoiding deterioration of the active materials during storage and at the same time it eliminates the loss of capacity due to
self-dischargeSelf-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes. Self-discharge decreases the shelf-life of batteries and causes them to have less charge than expected when actually put to use.How...
until the battery is called into use. They can thus be stored indefinitely (over 50 years) yet provide full power in an instant when it is required. Once activated, they provide a high burst of power for a short period (a few tens of seconds) to over 60 minutes or more, with power output ranging from a few
wattThe watt is a derived unit of power in the International System of Units . It measures rate of energy conversion. One watt is equivalent to 1 joule of energy per second....
s to several kilowatts. The high power capability is due to the very high
ionic conductivityIonic conduction in solids has been a subject of interest as early as the beginning of the 19th century. It was established by Michael Faraday that the laws of electrolysis are also obeyed in ionic solids like PbF2 and Ag2S. There were, however, several important discoveries...
of the molten salt, which is three orders of magnitude or more greater than that of sulfuric acid in a
lead-acidLead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having the second lowest energy-to-weight ratio and a correspondingly low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain...
car batteryA car battery is a type of rechargeable battery that supplies electric energy to an automobile. Usually this refers to an SLI battery to power the starter motor, the lights, and the ignition system of a vehicle’s engine...
. Older thermal batteries used
calciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
or
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID . Electrons flow in the opposite direction to the electric current...
s, with cathodes of
calcium chromateCalcium chromate is a bright yellow solid. It normally occurs as the dihydrate.-Properties:Calcium chromate loses water at 200 °C. Calcium chromate reacts with organic matter or reducing agents to form chromium. Calcium chromate will react explosively with hydrazine...
or vanadium or
tungsten oxidesTungsten oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce...
, but
lithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number three. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
-alloy anodes replaced these in the 1980s, with lithium-
siliconSilicon is the most common metalloid. It is a chemical element, which has the symbol Si and atomic number 14. A tetravalent metalloid, silicon is less reactive than its chemical analog carbon...
alloys being favored over the older lithium-aluminum alloys. The corresponding cathode for use with the lithium-alloy anodes is mainly iron disulfide (pyrite) with cobalt disulfide being used for high-power applications. The electrolyte is normally a eutectic mixture of
lithium chlorideLithium chloride is a chemical compound with the formula LiCl. The salt is a typical ionic compound, although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorides, such as extraordinary solubility in polar solvents and its hygroscopic...
and
potassium chlorideThe chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state it is odorless. It has a white or colorless vitreous crystal, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are face-centered cubic...
. More recently, other lower-melting, eutectic electrolytes based on
lithium bromideLithium bromide, or LiBr, is a chemical compound of lithium and bromine. Its extreme hygroscopic character makes LiBr useful as a desiccant in certain air conditioning systems.-Production and properties:...
,
potassium bromidePotassium bromide is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries. Its action is due to the bromide ion . Potassium bromide is presently used as a veterinary drug, as an antiepileptic medication for dogs and cats. It is a white crystalline...
, and lithium chloride or
lithium fluorideLithium fluoride is a chemical compound of lithium and fluorine. It is a white, inorganic, crystalline, ionic, solid salt under standard conditions. It transmits ultraviolet radiation more efficiently than any other known substance. Uses include specialized UV optics, and as a means to record gamma...
have also been used to provide longer operational lifetimes; they are also better conductors. The so-called "all-lithium" electrolyte based on
lithium chlorideLithium chloride is a chemical compound with the formula LiCl. The salt is a typical ionic compound, although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorides, such as extraordinary solubility in polar solvents and its hygroscopic...
,
lithium bromideLithium bromide, or LiBr, is a chemical compound of lithium and bromine. Its extreme hygroscopic character makes LiBr useful as a desiccant in certain air conditioning systems.-Production and properties:...
, and
lithium fluorideLithium fluoride is a chemical compound of lithium and fluorine. It is a white, inorganic, crystalline, ionic, solid salt under standard conditions. It transmits ultraviolet radiation more efficiently than any other known substance. Uses include specialized UV optics, and as a means to record gamma...
(no potassium salts) is also used for high-power applications, because of its high ionic conductivity.
These batteries are used almost exclusively for military applications ie "one-shot" weapons such as guided missiles. However, the same technology was also studied by Argonne National Laboratories in the 1980s for possible use in
electric vehicleAn electric vehicle , also referred to as an electric drive vehicle, is a vehicle which uses one or more electric motors for propulsion. Depending on the type of vehicle, motion may be provided by wheels or propellers driven by rotary motors, or in the case of tracked vehicles, by linear motors...
s, since the technology is rechargeable.
Secondary cells
Since the mid-1960s much development work has been undertaken on rechargeable batteries using
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
(Na) for the
negative- Science and mathematics :* Negative and non-negative numbers* Negative mass* Negative energy * Electrical polarity* Negative lenses, uses to describe diverging optics...
electrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s. Sodium is attractive because of its high
reduction potentialReduction potential is the tendency of a chemical species to acquire electrons and thereby be reduced...
of -2.71
voltThe volt is the SI derived unit of electromotive force, commonly called "voltage". It is also the unit for the related but slightly different quantity electric potential difference...
s, its low weight, its non-toxic nature, its relative abundance and ready availability and its low cost. In order to construct practical batteries, the sodium must be used in liquid form. Since the
melting pointThe melting point of a solid is the temperature range at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point...
of sodium is this means that sodium based batteries must operate at high temperatures, typically in excess of .
Sodium-sulfur batteryA sodium-sulfur battery is a type of battery constructed from sodium and sulfur . This type of battery has a high energy density, high efficiency of charge/discharge and long cycle life, and is fabricated from inexpensive materials...
and
lithium 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 have been demonstrated on the longest and...
comprise two of the more advanced systems of the molten salt batteries. The NaS battery has reached a more advanced developmental stage than its lithium counterpart; it is more attractive since it employs cheap and abundant electrode materials. Thus the first commercial battery produced was the sodium-sulfur battery which used liquid
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
for the positive electrode and a
ceramicA ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
tube of
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) for the electrolyte. 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. A further problem of dendritic-sodium growth in Na/S batteries led to the development of the ZEBRA battery.
ZEBRA battery
The
ZEBRA battery operates at and utilizes
moltenMölten is a comune in the province of Bolzano-Bozen in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano ....
sodium chloroaluminateSodium chloroaluminate, also known as natrium chloroaluminate, is a chemical compound with the formula NaAlCl4. It has been discovered in 20th century.- External links :*...
(NaAlCl
4), which has a melting point of , as the electrolyte. The negative electrode is molten sodium. The positive electrode is
nickelNickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
in the discharged state and nickel chloride in the charged state. Because nickel and nickel chloride are nearly insoluble in neutral and basic melts, intimate contact is allowed, providing little
resistanceThe electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the...
to charge transfer. Since both NaAlCl
4 and Na are liquid at the operating temperature, a sodium-conducting β-alumina ceramic is used to separate the liquid sodium from the molten NaAlCl
4. This battery was invented in 1985 by the Zeolite Battery Research Africa Project (ZEBRA) group led by Dr. Johan Coetzer at the
Council for Scientific and Industrial Research (CSIR)The Council for Scientific and Industrial Research is South Africa's central and premier scientific research and development organisation. It was established by an act of parliament in 1945 and is situated on its own campus in the city of Pretoria....
in Pretoria, South Africa, hence the name ZEBRA battery. In 2009, the battery had been under development for more than 20 years. The technical name for the battery is Na-NiCl
2 battery.
The ZEBRA battery has an attractive specific energy and power (90 Wh/kg and 150 W/kg). For comparison, LiFePO
4 lithium iron phosphate batteries store 90–110 Wh/kg and the more common LiCoO
2 lithium ion batteries store 150–200 Wh/kg. Nano Lithium-Titanate Batteries store energy and power of (116 Wh & 72 Wh/kg) and (1,250 W & 760 W/kg). The ZEBRA's liquid electrolyte freezes at , and the normal operating temperature range is to . The β-alumina solid electrolyte that has been developed for this system is very stable, both to sodium metal and the
sodium chloroaluminateSodium chloroaluminate, also known as natrium chloroaluminate, is a chemical compound with the formula NaAlCl4. It has been discovered in 20th century.- External links :*...
. The primary elements used in the manufacture of ZEBRA batteries,
NaNa may refer to:* Na, the chemical symbol for Sodium* Na, a tribal people also known as the Nga people, from Arunachal Pradesh, the easternmost state of India* Na, a small ethnic group also known as the Mosuo, from Yunnan and Sichuan Provinces in China...
,
ClCL may refer to:- Places :*Chile, ISO country code*Province of Caltanissetta, a province of Italy, ISO 3166-2:IT code*Călăraşi County, a county in Romania, ISO 3166-2:RO code-Computers:*.cl, internet country code top-level domain for Chile...
and
AlÅl is a municipality in Buskerud county, Norway. It is part of the traditional region of Hallingdal. The administrative centre of the municipality is the village of Ål....
have much higher worldwide reserves and annual production than the
LiLi or li may refer to:* Li , the Confucian concept of ritual*: Li , philosophical concept of principle* Li people, an ethnic group...
used in Li-ion batteries. Lifetimes of over 1500 cycles and five years have been demonstrated with full-sized batteries, and over 3000 cycles and eight years with 10- and 20-cell modules.
Vehicles powered by ZEBRA batteries have covered more than 2 million km.
ModecModec is a Coventry, UK electric vehicle manufacturer, specialising in Commercial vehicles in the N2 category. It unveiled its first model in April 2006 and announced its intention to commence series production in March 2007, with the first production vehicles destined for Tesco.-History:2002 :...
Electric Van uses ZEBRA batteries for the 2007 model. The Th!nk City also uses ZEBRA batteries.
When not in use, ZEBRA batteries are typically left under charge so that they will remain molten and be ready for use when needed. If shut down and allowed to solidify, a reheating process must be initiated that may require up to two days to restore the battery pack to the desired temperature and impart a full charge. This reheating time varies depending on the state-of-charge of the batteries at the time of their shut down, battery-pack temperature, and power available for reheating. After a full shut down of the battery pack, three to four days will usually elapse before a fully-charged battery pack loses enough energy to cool and solidify.