Molten salt battery
Molten salt batteries are a class of primary cell and secondary cell high-temperature electric battery that uses molten salts as an electrolyte. In most cases the salt used is a sodium salt and they are then often referred to as a liquid sodium battery. They offer both a high energy density through the proper selection of reactant pairs as well as a high power density by means of a high-conductivity molten salt electrolyte.
These types of batteries are used where high energy density and high power density are required. These features make rechargeable molten salt batteries a preferred energy storage to balance out environment-dependent power plants (solar, wind, etc.). Historically, thermal batteries have often been used in guided weapon systems such as surface-to-air-missiles. Rechargeable molten-salt batteries are a promising technology for powering electric vehicles. With high operating temperatures of 400 °C (752 °F) to 700 °C (1,292 °F), they have problems of thermal management and safety, and they place stringent requirements on the rest of the battery components. Some newer designs, such as the ZEBRA battery, operate at a lower temperature range of 245 °C (473 °F) to 350 °C (662 °F).
Primary cells 
Referred to as thermal batteries, the electrolyte is solid and inactive at normal ambient temperatures. The origin of the thermal battery dates back to World War II when German scientist Dr. Ing. Georg Otto Erb developed the first practical cells, using a salt mixture as an electrolyte. Erb developed batteries for several military applications, including the V-1 flying bomb and the V-2 rocket, and artillery fuzing systems. However, none of these batteries entered field use before the end of World War II. Following the end of the war, Erb was interrogated by British intelligence and his work was reported in a document titled "The Theory and Practice of Thermal Cells". This information was subsequently passed on to the United States Ordnance Development Division of the National Bureau of Standards.
When the technology reached the United States in 1946 it was immediately applied to replacing the troublesome liquid-based systems that had previously been used to power artillery 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 Sidewinder, MIM-104 Patriot, BGM-71 TOW, BGM-109 Tomahawk and others. In these batteries the electrolyte is immobilized when molten by a special grade of magnesium oxide that holds it in place by capillary action. This powdered mixture is pressed into pellets to form a separator between the anode and cathode 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 source which is used to heat the cell to the typical operating temperature of 400–550°C.
There are two types of design. One uses a fuze strip (containing barium chromate and powdered zirconium metal in a ceramic paper) along the edge of the heat pellets to initiate burning. The fuze strip is typically fired by an electrical igniter or squib 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 shell. It is desired that the pyrotechnic source be gasless. The standard heat source typically consist of mixtures of iron powder and potassium perchlorate in weight ratios of typically 88/12, 86/14, and 84/16. The higher the potassium perchlorate level, the higher the heat output (nominally 200, 259, and 297 calories/gram, 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-discharge 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 watts to several kilowatts. The high power capability is due to the very high ionic conductivity of the molten salt, which is three orders of magnitude or more greater than that of sulfuric acid in a lead-acid car battery. Older thermal batteries used calcium or magnesium anodes, with cathodes of calcium chromate or vanadium or tungsten oxides, but lithium-alloy anodes replaced these in the 1980s, with lithium-silicon alloys being favored over the older lithium-aluminium 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 chloride and potassium chloride. More recently, other lower-melting, eutectic electrolytes based on lithium bromide, potassium bromide, and lithium chloride or lithium fluoride have also been used to provide longer operational lifetimes; they are also better conductors. The so-called "all-lithium" electrolyte based on lithium chloride, lithium bromide, and lithium fluoride (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 i.e. "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 vehicles, since the technology is rechargeable.
Secondary cells 
Since the mid-1960s much development work has been undertaken on rechargeable batteries using sodium (Na) for the negative electrodes. Sodium is attractive because of its high reduction potential of -2.71 volts, 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 point of sodium is 98 °C (208 °F) this means that sodium based batteries must operate at high temperatures, typically in excess of 270 °C (518 °F).
Sodium–sulfur battery 
The Sodium–sulfur battery (or NaS battery), along with the related lithium sulfur battery, comprises one of the more advanced systems of the molten salt batteries. The NaS battery is attractive since it employs cheap and abundant electrode materials. Thus the first alkali metal commercial battery produced was the sodium–sulfur battery which used liquid sulfur for the positive electrode and a ceramic tube of beta-alumina solid electrolyte (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.
Because of their high specific power, NaS batteries have been proposed for space applications. A test of a NaS battery for space use was successfully demonstrated on the space shuttle mission STS-87 in 1997, but the batteries have not been used operationally in space. NaS batteries have also been proposed for use in the high temperature environment of Venus.
The NaS battery has reached a more advanced developmental stage than its lithium counterpart. The possibility of construction of Potassium-ion battery by molten electrolyte has been recently patented.
ZEBRA Na-NiCl2 battery, Museum Autovision, Altlußheim, Germany
|Specific energy||90 Wh/kg|
|Energy density||160 Wh/l|
|Specific power||155 W/kg, peak power 335 C |
|Time durability||>8 years|
|Cycle durability||~3000 cycles|
|Nominal cell voltage||2.58 V|
The ZEBRA battery operates at 245 °C (473 °F) and utilizes molten sodium aluminumchloride (NaAlCl4), which has a melting point of 157 °C (315 °F), as the electrolyte. The negative electrode is molten sodium. The positive electrode is nickel 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 resistance to charge transfer. Since both NaAlCl4 and Na are liquid at the operating temperature, a sodium-conducting β-alumina ceramic is used to separate the liquid sodium from the molten NaAlCl4. 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) in Pretoria, South Africa. In 2009, the battery had been under development for more than 20 years. The technical name for the battery is Na-NiCl2 battery.
The ZEBRA battery has a specific energy of 90 Wh/kg and a specific power of 150 W/kg. For comparison, LiFePO4 lithium iron phosphate batteries store 90–110 Wh/kg and the more common LiCoO2 lithium ion batteries store 150–200 Wh/kg. Nano Lithium-Titanate Batteries store 72 Wh/kg energy and can provide a power of 760 W/kg . The ZEBRA's liquid electrolyte freezes at 157 °C (315 °F), and the normal operating temperature range is 270 °C (518 °F) to 350 °C (662 °F).
The β-alumina solid electrolyte that has been developed for this system is very stable, both to sodium metal and the sodium aluminumchloride. The primary elements used in the manufacture of ZEBRA batteries, Na, Cl and Al have much higher worldwide reserves and annual production than the Li used in Li-ion batteries. Lifetimes of over 1,500 cycles and five years have been demonstrated with full-sized batteries, and over 3,000 cycles and eight years with 10- and 20-cell modules.
In 2010 General Electric announced a Na-NiCl2 battery that it called a sodium-metal halide battery, with a 20-year lifetime. The cathode structure of a GE cell consists of a conductive Ni network, molten salt electrolyte, metal current collector, carbon felt electrolyte reservoir, and the active sodium-metal halide salts.
Low-temperature molten salts 
Sumitomo developed a battery using a salt that is molten at 57 °C (135 °F) far lower than sodium based batteries. The battery offers energy densities as high as 290 Wh/L. The battery employs only nonflammable materials and will not ignite on contact with air, nor is there thermal runaway. This eliminates waste-heat storage or fire- and explosion-proof equipment, and allows closer packing of cells. The company expects that the battery requires half the volume of lithium-ion batteries and one quarter that of sodium–sulfur batteries.
Modec Electric Van uses ZEBRA batteries for the 2007 model and the IVECO daily 3.5 ton delivery vehicle was announced in mid 2010. Th!nk City offers a ZEBRA battery option. In 2011, the US Postal Service began testing five delivery vans that had been converted to all-electric power, one of which uses a ZEBRA battery.
When not in use, ZEBRA batteries are typically continuously charged so that they remain molten and ready for use. If shut down and allowed to solidify, reheating 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 shutdown, battery-pack temperature, and power available for reheating. After shutdown a fully charged battery pack loses enough energy to cool and solidify in 3–4 days.
Ongoing research 
Magnesium–antimony cells 
In 2009, Donald Sadoway and his team proposed a very low cost molten salt battery originally based on magnesium and antimony separated by a salt that could be potentially used in Grid energy storage systems. Research on this concept is being funded by ARPA-E, Bill Gates, Khosla Ventures and Total S.A. Experimental data showed 69% storage efficiency, it had good storage capacity (over 1000mAh/cm2) and relatively low leakage (< 1 mA/cm2) and high maximum discharge capacity (over 200mA/cm2). They planned to have a commercial prototype built by 2014.
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