First-principles investigations of F and Cl impurities in NaAlH4

We study halide interactions with NaAlH4 using first-principles calculations based on density functional theory. First, we determine which products can be formed when TiX3 (X=F, Cl) is ball milled with NaAlH4. Then we investigate impurity incorporation, finding that fluorine may substitute for hydrogen, or form a charged interstitial defect. Fluorine substitution for hydrogen has been proposed as a means of destabilizing hydrides, but we find this to be unlikely for either NaAlH4 or Na3AlH6. Fluorine interstitials are most stable as charged defects; however, their concentration will likely be too low to shift the Fermi level. The migration barrier for interstitial fluorine is low enough to easily allow diffusion during cycling through the hydride phases. Chlorine has a large size mismatch with hydrogen and every chlorine-related defect studied is found to have a high formation energy. Chlorine is therefore unlikely to incorporate in NaAlH4.