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.

Figure 1

(Color online) Relaxed structures of (a) ${\text{LiBH}}_4$ and (b) ${\text{Li}}_4{\text{BN}}_3{\text{H}}_{10}$. Large (gray) spheres are Li, medium (blue) spheres B, small (green) spheres N, and smaller (orange) spheres H. Inequivalent hydrogen atoms are labeled as H1, H2, H3, and H4.

Figure 2

(Color online) Total and partial densities of states (DOS) for ${\text{LiBH}}_4$. The zero of energy is set to the highest-occupied state.

Figure 3

(Color online) Total and partial densities of states (DOS) for ${\text{Li}}_4{\text{BN}}_3{\text{H}}_{10}$. The zero of energy is set to the highest-occupied state. The inset enlarges the partial DOS curves.

Figure 4

(Color online) Calculated formation energies of hydrogen-related defects ($V_{\text{H}}$ and ${\text{H}}_i$) in ${\text{LiBH}}_4$ as a function of Fermi energy. Only the formation energies of the lowest-energy configurations are shown. $V_{\text{H}}^0$ and $V_{\text{H}}^{+}$ were initially created on the H3 site and $V_{\text{H}}^{-}$ initially on the H1 site [see Fig. 1a for definition of H1 and H3 sites]. ${\text{H}}_i^0$ and ${\text{H}}_i^{+}$ were initially placed near the midpoint of the B-B bond, and ${\text{H}}_i^{-}$ initially near the center of the void formed by Li atoms and ${\text{BH}}_4$ units.

Figure 5

(Color online) Lattice relaxations induced by (a) a positively hydrogen vacancy $V_{\text{H}}^{+}$ and (b) a positively hydrogen interstitial ${\text{H}}_i^{+}$ in ${\text{LiBH}}_4$. Large (gray) spheres are Li, medium (blue) spheres B, and small (orange) spheres H. The removal of one hydrogen atom and one electron from a ${\text{BH}}_4$ unit to form $V_{\text{H}}^{+}$ leads to formation of one ${\text{BH}}_3{\text{-H-BH}}_3$ complex (two ${\text{BH}}_4$ units that share a common H atom; marked by the arrow), whereas the creation of ${\text{H}}_i^{+}$ leaves the system with a ${\text{BH}}_3$ unit and an ${\text{H}}_2$ molecule (marked by the circle).

Figure 6

(Color online) Calculated formation energies of hydrogen-related defects in ${\text{Li}}_4{\text{BN}}_3{\text{H}}_{10}$ as a function of Fermi energy. Only the formation energies of the lowest-energy configurations are shown. $V_{\text{H}}^0$ and $V_{\text{H}}^{+}$ were initially created on the H3 site, and $V_{\text{H}}^{-}$ initially on the H2 site [see Fig. 1b for definition of H2 and H3 sites]. ${\text{H}}_i^0$, ${\text{H}}_i^{+}$, and ${\text{H}}_i^{-}$ were initially placed at various different locations, as described in the text.

Figure 7

(Color online) Calculated formation energies of (a) Ti, (b) Fe, (c) Ni, and (d) Zn nominally on the B site in ${\text{LiBH}}_4$. Other impurities (Zr, Pd, Pt, and Zn) have similar features. The vertical dashed line is $\epsilon \left(+/-\right)$, the transition level between charge states $+1$ and $-1$ of the impurity.

Figure 8

(Color online) Lattice relaxations induced by (a) ${\text{Ti}}_{\text{B}}^{+}$ and (b) ${\text{Ti}}_{\text{B}}^{-}$ in ${\text{LiBH}}_4$. Large (gray) spheres are Li, medium (blue) spheres B, and small (orange) spheres H. The impurity atom (large red sphere) tends to pull hydrogen atoms from the neighboring ${\text{BH}}_4$ units to form a metal-hydrogen complex.

Figure 9

(Color online) Band lineups and position of the $\epsilon \left(+/-\right)$ levels (thick red lines) for different transition-metal impurities nominally on the B site. Band structures of ${\text{LiBH}}_4$ and ${\text{Li}}_4{\text{BN}}_3{\text{H}}_{10}$ are aligned assuming the universal alignment of the transition level between ${\text{H}}_i^{+}$ and ${\text{H}}_i^{-}$ (Ref. 31). Work functions $\left(\Phi \right)$ of the impurities in their standard metallic states are marked by the (black) dash-dotted lines; ${\mu }_{\text{e}}^{\text{int}}$ levels are presented by the (blue) dotted lines.