  We report the results of the first-principles calculation on the intermediate
compounds of LiBH$_4$. The stability of LiB$_3$H$_8$ and Li$_2$B$_n$H$_n
(n=5-12)$ has been examined with the ultrasoft pseudopotential method based on
the density functional theory. Theoretical prediction has suggested that
monoclinic Li$_2$B$_{12}$H$_{12}$ is the most stable among the candidate
materials. We propose the following hydriding/dehydriding process of LiBH$_4$
via this intermediate compound : LiBH$_4 \leftrightarrow
{1/12}$Li$_{2}$B$_{12}$H$_{12} + {5/6}$ LiH $+ {13/12}$H$_2 \leftrightarrow
$LiH $+$ B $+ {3/2} $H$_2$. The hydrogen content and enthalpy of the first
reaction are estimated to be 10 mass% and 56 kJ/mol H$_2$, respectively, and
those of the second reaction are 4 mass% and 125 kJ/mol H$_2$. They are in good
agreement with experimental results of the thermal desorption spectra of
LiBH$_4$. Our calculation has predicted that the bending modes for the
$\Gamma$-phonon frequencies of monoclinic Li$_2$B$_{12}$H$_{12}$ are lower than
that of LiBH$_4$, while stretching modes are higher. These results are very
useful for the experimental search and identification of possible intermediate
compounds.
