A number of iodide-based electrolytes (LiCl–LiI, LiI–KI, LiCl–LiBr–LiI, LiCl–LiI–KI and LiF–LiCl–LiI) were considered to be used in thermal batteries. Ionic conductivity of multi-cation and all-lithium electrolytes were evaluated. They were compared to the classical electrolytes (LiCl–KCl, LiF–LiBr–KBr and LiF–LiCl–LiBr) used in thermal battery applications. Measurements were realised by electrochemical impedance spectroscopy (E.I.S.). It was shown that some all-lithium iodide-based electrolytes exhibit interesting ionic conductivities and are suitable as electrolytes in thermal batteries. It was pointed out that activation energy E a was close to 11 kJ mol−1 for multi-cation electrolytes, whereas it was equal to 7 kJ mol−1 for all-lithium electrolytes.