Improved Molten Salt Electrolyte Batteries for use in Space, Military and Commercial Applications

Conventional thermal batteries as used in missile systems operate in excess of 400° C. These batteries typically use the Li (Al)/FeS2 and Li(Si)/FeS2 systems with various molten salts. Typical melting points are between 313° C and 436° C. There is a need for suitable lower melting electrolytes to allow battery operation at a lower temperature (e.g., ~250° C). A number of electrolyte systems have been examined in the past. For example, CsBr systems are usable only above 250° C. Iodide based eutectics are operational as low as 158° C, but are susceptible to oxidation by oxygen. Nitrate based eutectics as low as 90° C 124.5° C, but depend on a passivation layer to prevent reaction of the lithium alloy anodes with nitrate. Additionally, there is a high self discharge and a tendency for exothermic reactions above 250°C with the nitrate salts. Other candidates include ionic liquids that have melting ranges at room temperature and are stable up to 300° C. and imidazolium salts that melt above room temperature. The imidazolium salts have a tendency to react with the lithium based anodes. Work at EEMPI under a U. S. Air Force Phase I SBIR contract, studied the use of a passivating agent lithium bis(oxalato) borate (LiBOB) to stabilize the anode/electrolyte interaction. As part of this effort tetraalkylammonium salts were found to be viable low temperature electrolyte candidates. Additionally, there is a need for molten salt batteries that can operate under high temperature (>400° C) and high pressure (>90 atmospheres) for planetary probes. This paper will present the results of characterization tests involving thermal stabilities and chemical compatibilities of various electrolyte candidates for thermal batteries for missile applications, planetary probes and bore-hole battery applications.