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.

Primary cells

Referred to as thermal batteries the electrolyte is solid and inactive at normal ambient temperatures. In these batteries the electrolyte is usually stored separately from the electrodes which also remain in a dry inactive state. The battery is only activated when it is actually needed by introducing the electrolyte into the active cell area and elevated to high temperatures by the application of heat from an external source, generally a pyrotechnic charge. This is achieved by burning electrically fired pellets of gas-less thermite. Activation takes between 0.2 second and a few seconds, depending on the size of the stack, and is initiated by a percussive primer. Other methods use an electric heater, or a pyroelectric material, like iron powder potassium perchlorate/zirconium barium chromate placed between the cells in the battery to obtain the required temperature.

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 yet provide full power in an instant when it is required. Activated they provide a high burst of power for a short period (A few tens of seconds to 20 minutes or more.) with power output ranges from a few watts to several kilowatts. Older batteries used calcium or magnesium anodes, but lithium anodes are now common. Typical chemistry is lithium iron disulphide. The electrolyte is normally a eutectic mixture of lithium and potassium chlorides.

These batteries are used almost exclusively for military applications.

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/Sulphur battery which used liquid sulphur 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.

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 beta-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), 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.

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.

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