Thermal batteries, lithium alloys

Lithium alloys have been used for a number of years in the high-temperature "thermal batteries" that are produced commercially for military purposes. These devices are designed to be stored for long periods at ambient temperatures before use, where their self-discharge kinetic be-... 

Other common anode materials for thermal batteries are lithium alloys, such as Li/Al and Li/B, lithium metal in a porous nickel or iron matrix, magnesium and calcium. Alternative cathode constituents include CaCr04 and the oxides of copper, iron or vanadium. Other electrolytes used are binary KBr-LiBr mixtures, ternary LiF-LiCl-LiBr mixtures and, more generally, all lithium halide systems, which are used particularly to prevent electrolyte composition changes and freezing out at high rates when lithium-based anodes are employed. 

Within the past five years, thermal batteries that can operate at high spin rates (300 rps) have been developed and successfully demonstrated. These batteries have been based on the now standard lithium (alloy)/iron disulfide eouple employed in most thermal batteries (see Chap. 21 for a detailed discussion of these chemistries). 

Introduced in the mid-1970s, lithium has become the most widely used anode material in thermal batteries. There are two major configurations of lithium anodes lithium alloy and lithium metal. The most commonly used alloys are lithium aluminum, with about 20 weight percent lithium and lithium (silicon), with about 44 weight percent lithium. Lithium-boron alloy has also been evaluated, but has not been used widely because of its higher cost. 

Elements from column III which have been studied for Li-ion anodes are essentially aluminum (Al) and gallium (Ga). Although the boron-lithium phase diagram shows several compounds [122], Li poorly reacts with B at room temperature [123]. Actually, B or rather Li-B alloys have been studied for application in the so-called thermal batteries, which are primary devices working at high temperature (350-450 °C) with molten salt electrolytes [124, 125]. 

Estimated data on a number of ternary lithium systems theoretically investigated as extensions of the Li-Si binary system are included in Table 14.2. Also included are comparable data for the binary Li-Si alloy that are currently being used in commercial thermal batteries. 

The self-discharge reaction of calcium with calcium chromate is highly exothermic, forming complex chromium(m) oxides. Above about 600°C the selfdischarge reaction accelerates, probably due to the markedly increasing solubility of the chromate in the chloride electrolyte. This acceleration increases the rate of fonnation of calcium-lithium alloy. The resulting thermal runaway is characterized by short battery lives, overheating and cell step-outs, shorts, and noise characteristics of excess alloy. 

To a large extent calcium anode thermal batteries are being displaced by ones using lithium rich alloys as discussed below. Lithium types avoid unwanted side reactions which characterize the calcium types which have electrochemical elEciencies down to 20% of theoretical values. 

Since lithium metal is molten at thermal battery discharge temperatures, it is retained on high surface area metals by immersion of the metal matrix in molten lithium to form anodes. Often this structure is contained within a metal cup to prevent leakage during cell operation. Another method is the fabrication of lithium alloy anodes, such as lithium—boron, lithium—aluminium and lithium—silicon, which are solid at battery discharge temperatures and thus offer the possibility of simpler construction. However, the lithium alloys are more difficult to fabricate than the metal matrix anodes and do not achieve this same peak current density. Most of the lithium anode batteries currently use the lithium chloride—potassium chloride electrolyte and an iron disulphide (FeS2) cathode. 

Table 27.4 presents data on two types of lithium-iron disulphide thermal battery and illustrates the advantage of the lithium anode systems. While the liquid lithium anode battery shows better performance than the lithium alloy battery, particularly in its rate... 

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