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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ New ternary or quaternary molten salt electrolyte systems have been developed. ▶ These electrolyte systems have no instable anions nor expensive cations. ▶ Some of electrolyte systems have no environmentally unfavorable F- anion. ▶ LiCl-LiBr-NaCl-KCl system improves the high rate discharge capability more than 20%.