Metal dodecaborates M_2/_nB_(12)H_(12) (n denotes the valence of
the metal M), containing icosahedral polyatomic anion
[B_(12)H_(12)]^(2−), have been attracting increasing interest as potential
energy materials, especially in the context of hydrogen
storage and superionic conductivity. M_2/_nB_(12)H_(12) are
commonly formed as dehydrogenation intermediates from
metal borohydrides M(BH_4)_n, like LiBH_4 and Mg(BH_4)_2,
which are well-known as potential high-density hydrogen
storage materials. The strong B−B bond in the icosahedral
[B_(12)H_(12)]^(2−), however, is regarded to be the key factor that
prevents the rehydrogenation of dodecaborates. In order to
elucidate the mechanism as well as to provide effective
solutions to this problem, a novel solvent-free synthesis route
of anhydrous M_2/nB_(12)H_(12) (here M means Li, Na, and K) has
been developed. Thermal stability and transformations of the
anhydrous single phase Li_2B_(12)H_(12) suggested the formation of
the high temperature polymorph of Li_2B_(12)H_(12) during the
dehydrogenation of LiBH_4, while concurrently emphasized the
importance of further investigation on the decomposition
mechanism of metal borohydrides and metal dodecaborates.
The high stability of icosahedral [B_(12)H_(12)]^(2−), on the other hand,
favors its potential application as solid electrolyte. Recently,
Na^+ conductivity of Na_2B_(12)H_(12) was reported to be 0.1 S/cm
above its order−disorder phase transition at ∼529 K, which is
comparable to that of a polycrystalline β”-Al_2O_3 (0.24 S/cm at
573 K) solid state Na-electrolyte. Mechanistic understanding
on the diffusion behavior of cation and further improvement of
ionic conductivity at a lower temperature, however, are
important in order to facilitate the practical application of
metal dodecaborates as superionic conductors
