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High Temperature Batteries
Since the mid 1960s much development work has been undertaken on batteries using Sodium 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. All these factors offer the prospect of batteries with very high power and energy densities.
Unfortunately in order to construct practical batteries using sodium electrodes, 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. This in turn brings problems of thermal management and safety and places more stringent requirements on the rest of the battery components.
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 for the electrolyte.
Corrosion of the insulators can be a problem in the harsh chemical environment since they gradually become conductive and the battery self-discharge rate increases.
Round trip efficiency is around 87% and cycle life is around 2,500 cycles at 100% depth of discharge (DOD), or 4,500 cycles at 80% DOD
Later developments included the Zebra Battery which used a solid metal positive electrode of Nickel Chloride. This was intrinsically safer and allowed larger cells to be made.
The special safety precautions and complex thermal management required for these batteries conspire to prevent the achievement of the very high energy densities theoretically possible. With the advent of the newer Lithium technologies enthusiasm for these high temperature batteries is beginning to wane.
High temperature technology is also used in Thermal Batteries . These batteries us an electrolyte which is solid and inactive at normal ambient temperatures. The batteries only become active at high temperatures by the application of heat from an external source to liquefy the electrolyte. These batteries are use almost exclusively for military applications