Two electrolytes have been tested for use in Li-Si/FeS2 cells: the iodide-containing LiF-LiCl-LiI electrolyte and the classical LiF-LiBr-KBr composition. Their melting points are very close, which allowed us to compare their performances as electrolytes for thermal battery applications. The main interest of this iodide alternative composition is that it contains the single lithium cation which improves the ionic conductivity. In this work, both electrolytes were tested in Li-Si/FeS2 single cells. The temperature range (400-580degreesC) for the experiments was close to that used in thermal batteries under operating conditions. At high temperatures, the LiF-LiCl-LiI alternative composition showed a behavior similar to that of the LiF-LiBr-KBr electrolyte. At lower temperatures in the 400-450degreesC range, the LiF-LiCl-LiI electrolyte exhibited the best performances. The LiF-LiCl-LiI electrolyte can be considered as a promising electrolyte for thermal batteries. (C) 2005 The Electrochemical Society. All rights reserved.