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

Abstract

Copyright © 2020 American Chemical Society.

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 < 0.3 (i.e., F- > 0.7) enables melting at temperatures above 750 °C.