Molten salt battery
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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°C however bring problems of thermal management and safety and places more stringent requirements on the rest of the battery components.
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[edit] 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 Germans during WW II and were used in the V2 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 Sidewinder, Patriot, Tow, Tomahawk, Cruise, etc. 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 (match) by application of a voltage across 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 shot-gun 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 tungsten 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 and potassium chlorides. More recently, other lower-melting, eutectic electrolytes based on lithium bromide, potassium bromide, and lithium chloride or 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 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 vehicles, since the technology is rechargeable.
[edit] 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 this means that sodium based batteries must operate at high temperatures, typically in excess of 270°C.
Sodium/sulfur and lithium/sulfur batteries 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 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.
[edit] Zebra battery
The zebra battery, which operates at 250°C, utilizes molten chloroaluminate (NaAlCl4), which has a melting point of approximately 160 °C, 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 a group led by Dr. Johan Coetzer at the CSIR in Pretoria, South Africa, hence the name zebra battery (for the Zeolite Battery Research Africa Project), and has been under development for almost 20 years. The technical name for the battery is Na-NiCl2 battery.
The ZEBRA battery has an attractive specific energy and power (90 Wh/kg and 150 W/kg). The liquid electrolyte freezes at 157 °C, and the normal operating temperature range is 270–350 °C. The β-alumina solid electrolyte that has been developed for this system is very stable, both to sodium metal and the sodium chloroaluminate. 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. Modec Electric Van uses ZEBRA batteries for the 2007 model.
When not in use, zebra batteries typically require being left under charge, in order to be ready for use when needed. If shut down, a reheating process must be initiated that may require up to two days to restore the battery pack to the desired temperature, and full charge. This reheating time will however vary 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 usually elapse before a fully-charged battery pack loses all of its significant heat.
