Using a new simulative technique developed by us, we systematically investigated new ternary or quaternary molten salt systems, which are based on LiF-LiCl, LiF-LiBr, and LiCl-LiBr binary systems, for use as electrolytes in thermal batteries, and evaluated their ionic conductivities and melting points experimentally. It was confirmed experimentally that LiF-LiBr-KF (melting point: 425 degrees C, ionic conductivity at 500 degrees C: 2.52 S cm(-1)), LiCl-LiBr-KF (405 degrees C, 2.56 S cm(-1)), LiCl-LiBr-NaF-KF (425 degrees C, 3.11 S cm(-1)), LiCl-LiBr-NaCl-KCl (420 degrees C, 2.73 S cm(-1)), and LiCl-LiBr-NaBr-KBr (420 degrees C, 2.76S cm(-1)) meet our targets for both melting point (350-430 degrees C) and ionic conductivity (2.0S cm(-1) and higher at 500 degrees C). A single cell using the newly developed LiCl-LiBr-NaCl-KCl molten salt as an electrolyte was prepared, and the DC-IR of the cell decreased by 20% than that of a single cell using the conventional LiCl-KCl molten salt. It was therefore concluded that the use of new quaternary molten salt systems can improve the discharge rate-capability in practical battery applications because of their high ionic conductivities. (C) 2011 Published by Elsevier B.V.