The thermodynamical stabilities for the series of metal borohydrides $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$ ($M=\mathrm{Li}$, Na, K, Cu, Mg, Zn, Sc, Zr, and Hf; $n=1–4$) have been systematically investigated by first-principles calculations. The results indicated that an ionic bonding between $M^{n+}$ cations and ${\left[\mathrm{B}{\mathrm{H}}_4\right]}^{-}$ anions exists in $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$, and the charge transfer from $M^{n+}$ cations to ${\left[\mathrm{B}{\mathrm{H}}_4\right]}^{-}$ anions is a key feature for the stability of $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$. A good correlation between the heat of formation $\Delta H_{\mathrm{boro}}$ of $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$ and the Pauling electronegativity of the cation ${\chi }_P$ can be found, which is represented by the linear relation, $\Delta H_{\mathrm{boro}}=248.7{\chi }_P-390.8$ in the unit of kJ/mol $\mathrm{B}{\mathrm{H}}_4$. In order to confirm the predicted correlation experimentally, the hydrogen desorption reactions were studied for $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$ ($M=\mathrm{Li}$, Na, K, Mg, Zn, Sc, Zr, and Hf), where the samples of the later five borohydrides were mechanochemically synthesized. The thermal desorption analyses indicate that $\mathrm{Li}\mathrm{B}{\mathrm{H}}_4$, $\mathrm{Na}\mathrm{B}{\mathrm{H}}_4$, and $\mathrm{K}\mathrm{B}{\mathrm{H}}_4$ desorb hydrogen to hydride phases. $\mathrm{Mg}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_2$, $\mathrm{Sc}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$, and $\mathrm{Zr}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_4$ show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, $\mathrm{Zn}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_2$ desorbs hydrogen and borane to elemental Zn due to instabilities of Zn hydride and boride. A correlation between the desorption temperature $T_d$ and the Pauling electronegativity ${\chi }_P$ is observed experimentally and so ${\chi }_P$ is an indicator to approximately estimate the stability of $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$. The enthalpy change for the desorption reaction, $\Delta H_{\mathrm{des}}$, is estimated using the predicted $\Delta H_{\mathrm{boro}}$ and the reported data for decomposed product, $\Delta H_{\mathrm{hyd}∕\mathrm{boride}}$. The estimated $\Delta H_{\mathrm{des}}$ show a good correlation with the observed $T_d$, indicating that the predicted stability of borohydride is experimentally supported. These results are useful for exploring $M{\left(\mathrm{B}{\mathrm{H}}_4\right)}_n$ with appropriate stability as hydrogen storage materials.

The electrical conductivity of lithium borohydride $\left(\mathrm{Li}\mathrm{B}{\mathrm{H}}_4\right)$ measured by the ac complex impedance method jumped by three orders of magnitude due to structural transition from orthorhombic to hexagonal at approximately $390\mathrm{K}$ . The hexagonal phase exhibited a high electrical conductivity of the order of ${10}^{-3}\mathrm{S}{\mathrm{cm}}^{-1}$ . Furthermore, the conductivity calculated from the Nernst-Einstein equation using the correlation time obtained from ${}^7\mathrm{Li}$ nuclear magnetic resonance was in good agreement with the measured electrical conductivity. It was concluded that the electrical conductivity in the hexagonal phase is due to the Li superionic conduction.

The prerequisite for widespread use of hydrogen as an energy carrier is the development of new materials that can safely store it at high gravimetric and volumetric densities. Metal borohydrides M(BH₄)_n (n is the valence of metal M), in particular, have high hydrogen density, and are therefore regarded as one such potential hydrogen storage material. For fuel cell vehicles, the goal for on-board storage systems is to achieve reversible store at high density but moderate temperature and hydrogen pressure. To this end, a large amount of effort has been devoted to improvements in their thermodynamic and kinetic aspects. This review provides an overview of recent research activity on various M(BH₄)_n, with a focus on the fundamental dehydrogenation and rehydrogenation properties and on providing guidance for material design in terms of tailoring thermodynamics and promoting kinetics for hydrogen storage.