Hydrogen-related defects and the role of metal additives in the kinetics of complex hydrides: A first-principles study

We report first-principles studies of hydrogen-related point defects and impurities in ${\text{LiBH}}_{4}$ and ${\text{Li}}_{4}{\text{BN}}_{3}{\text{H}}_{10}$, two promising materials for hydrogen storage. In both systems, hydrogen vacancies and interstitials are found to be positively or negatively charged, and hence their formation energies are Fermi-level dependent. One can therefore tailor the formation energies of these point defects (hence the kinetics of the systems) by shifting the Fermi level. This can be accomplished by adding appropriate impurities that are electrically active into the systems. We have identified a number of transition-metal impurities that are effective in shifting the Fermi level of ${\text{LiBH}}_{4}$ and ${\text{Li}}_{4}{\text{BN}}_{3}{\text{H}}_{10}$. A comparison of our calculations with experimental results for the effects of addition of impurities on the kinetics of ${\text{LiBH}}_{4}$ and ${\text{Li}}_{4}{\text{BN}}_{3}{\text{H}}_{10}$ shows qualitative agreement, providing validation for our interpretation of the results and for our proposed model for enhancement of kinetics.