Wednesday, April 29, 2009

Molten salt battery

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Molten salt secondary (rechargeable) battery, Museum Autovision, Altlu?heim
Molten salt batteries 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 of a high conductivity molten salt electrolyte. They are used in services where high energy density and high power density are required. These features make rechargeable molten salt batteries a promising technology for powering electric vehicles. Operating temperatures of 400 to 700, 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 270350 .
1 Primary cells
2 Secondary cells
2.1 ZEBRA battery
3 References
Primary cells
Referred to as thermal batteries the electrolyte is solid and inactive at normal ambient temperatures. Thermally activated (hermal) batteries were conceived by the Germans during WW II and were used in the V-2 rockets. 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 States in 1946 and was immediately adapted to replace the troublesome liquid-based systems that had previously been used in 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 - 550C.
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 perchorate 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 tungste oxides, but lithium-alloy anodes replaced these in the 1980s, with lithium-silicon 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 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...(and so on)

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