The present invention is an improvement in the iron-lithium anode disclosed in U.S. Pat. No. 4,221,849. Thermal batteries typically comprise a plurality of thermal cells each of which includes an anode, cathode, electrolyte and an integral ignitable heat source. The electrolyte is usually an eutectic mixture of LiCl and KCl and the cathode (depolarizer) normally comprises materials which are reduced in the electrochemical cell such as phosphates, metal oxides, borates and chromates. Examples of typical thermal cells are disclosed and taught in U.S. Pat. Nos. 3,677,822; 3,425,872; 3,527,615; 3,367,800; 3,891,460; 3,930,888 and 4,119,769. The latter patent, U.S. Pat. No. 4,119,769 teaches the use of a mixture of iron pyrite and binder as a depolarizer which has significant advantages over the conventional depolarizer materials of calcium chromate or vanadium pentoxide. U.S. Pat. No. 4,044,192 discloses a thermal battery having a central opening filled with a solid fuse material.
U.S. Pat. No. 4,221,849, discloses a further improvement in thermal batteries in which the anode comprises a pyrometallurically combined iron-lithium having a ratio of lithium to iron of about 15% to 35%. In those preferred ratios, the iron particles are held together by the surface tension of the lithium rather than being alloyed thereto. Typically, only about 20% by weight of the anode is lithium; however, the material retains the essential characteristics of lithium and is easily rolled and shaped. The present invention utilizes the improvement in the anode material of U.S. Pat. No. 4,221,849.
Thermal batteries are constructed by placing the iron-lithium anode in a metal cup such that the anode contacts the base thereof. The cup-anode assembly also includes an insulator, normally made by Fiberfrax.RTM., which is positioned in the cup and against the anode to act as a separator between the two-layered depolarizer (preferably comprising an anolyte layer and a catholyte layer) and the anode. Adjacent to the depolarizer is positioned an ignitable heat source. Each anode, depolarizer, ignitable heat source comprise a unit, a large number of which are positioned in a stack array to produce a thermal battery.
It has been found that upon activation of these batteries that noise of a few seconds duration, with peak to peak values of greater than 1/2 volt between 3 and 15 KHz are exhibited. This noise was greatly exaggerated on cold conditioned batteries relative to hot conditioned batteries. In many cases, the low level, high frequency noise is construed as a failure mode of the battery during sampling resulting in the rejection of otherwise good batteries. Furhter, such noise has been found to seriously degrade the final activation rate of the thermal battery. Accordingly, it is an object of the present invention to provide a noise-free activation of thermal batteries over a fairly large temperature rage as well as to enhance the iron-lithium anode electrical contact. In addition, it is an object of the present invention to afford greater utilization of the anode as well as the cathode material and to provide lower resistance battery operation.