New molten salt systems for high temperature molten salt batteries: Ternary and quaternary molten salt systems based on LiF-LiCl, LiF-LiBr, and LiCl-LiBr

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 °C, ionic conductivity at 500 °C: 2.52 S cm⁻¹), LiCl-LiBr-KF (405 °C, 2.56 S cm⁻¹), LiCl-LiBr-NaF-KF (425 °C, 3.11 S cm⁻¹), LiCl-LiBr-NaCl-KCl (420 °C, 2.73 S cm⁻¹), and LiCl-LiBr-NaBr-KBr (420 °C, 2.76 S cm⁻¹) meet our targets for both melting point (350-430 °C) and ionic conductivity (2.0 S cm⁻¹ and higher at 500 °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.