Ionic conductivity and thermal stability of lithium salt / potassium bifluoride electrolytes for thermal batteries

The incorporation of potassium bifluoride (KF–HF) as an additive to the lithium-halide electrolyte for thermal batteries was investigated. Different mixtures of lithium fluoride, lithium chloride and lithium sulfide with KF-HF were studied using electrochemical impedance spectroscopy and differential scanning calorimetry. The results from these tests were combined to obtain a better understanding of the relationship between ionic conductivity, melting point, and electrolyte composition. The results indicated that it is feasible to maintain a relatively high ionic conductivity at temperatures lower (250–300 °C) than current thermal battery electrolytes (400–550 °C). Mixtures of lithium fluoride and potassium bifluoride provided the best ionic conductivity at 260 °C. The effect of mixing these lithium halides on the thermal decomposition of the KF-HF was also investigated. It was observed that it can increase decomposition temperature by up to 15 °C while at the same time reduce the melting point of the mixture and increase the ionic conductivity.