Native Defect Concentrations in NaAlH₄ and Na₃AlH₆

Abstract

Titanium-doped sodium alanate has been shown to have potentially useful properties for storing hydrogen in fuel cell vehicles. To quantitatively explain the kinetic rates and activation energies for hydrogen release and absorption, it is necessary to calculate the rate of metal diffusion that is required for forming the dehydrogenation products, Na₃AlH₆ and Al. The bulk defects existing in large concentrations will likely play a dominant role in the transport of metal species. In the first of a series of papers, we use first-principles density functional theory (DFT) calculations to determine the formation free energies and concentrations of native defects in NaAlH₄ and Na₃AlH₆ as functions of temperature, including vibrational and H₂ gas-phase entropy contributions. We find that at low temperatures the largest concentrations of native defects in NaAlH₄ are positively charged AlH₄ vacancies and negatively charged Na vacancies, which can be thought of as the primary defect types for NaAlH₄. At high temperatures (near 100 °C), neutral AlH₃ vacancies, positively charged AlH₄ vacancies, and negatively charged interstitial hydrogen ions and hydrogen vacancies have the largest concentrations at an interface between NaAlH₄ and Al. At all temperatures, an interface between NaAlH₄ and Na₃AlH₆ remains predominantly populated by positively charged AlH₄ vacancies and negatively charged Na vacancies. In Na₃AlH₆, the highest defect concentrations belong to negatively charged Na vacancies and positively charged H vacancies throughout the entire temperature range considered. Inclusion of the gas-phase free energy of hydrogen is shown to be crucial for obtaining quantitative estimates of defect free energies and concentrations.