Physicochemical Characterisation of a Na-H-F Thermal Battery Material

Fluorine-substituted sodium hydride is investigated for application as a thermal energy storage material inside thermal batteries. A range of compositions of NaHxF1–x (x = 0, 0.5, 0.7, 0.85, 0.95, 1) have been studied using synchrotron radiation powder X-ray diffraction (SR-XRD), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and nuclear magnetic resonance spectroscopy (NMR), with the thermal conductivity and melting points also being determined. SR-XRD and NMR spectroscopy studies identified that the solid solutions formed during synthesis contain multiple phases rather than a single stoichiometric compound, despite the materials exhibiting a single melting point. As the fluorine content of the materials increases, the Na–H(F) bond length decreases, increasing the stability of the compound. This trend is also observed during melting point analysis where increasing the fluorine content increases the melting point of the material; that is, x 0.7) enables melting at tempe...