Molten salt battery

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 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 potassium chlorides.

These batteries are used almost exclusively for military applications ie "one-shot" weapons such as guided missiles.

Secondary cells

Since the mid 1960s much development work has been undertaken on rechargeable batteries using 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 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.

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 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.

See also

• Molten-carbonate fuel cell

References

• Beta Research and Development Ltd
• The ENSER Corporation
• Additional information & application
• Thermal Battery Design
• The Rolls-Royce ZEBRA concept
• Thermal Batteries