Altering the chemical state of boron towards the facile synthesis of LiBH₄ via hydrogenating lithium compound-metal boride mixture

Boron sources in forms of SiB₄/FeB/TiB₂ were used to react with LiF/LiH under hydrogen atmosphere to investigate their effectiveness for synthesizing LiBH₄, a promising hydrogen storage material. Fourier transform infrared (FTIR) study revealed the formation of B–H bond vibrations in these hydrogenated systems, and it demonstrated the generation of LiBH₄. When using FeB and TiB₂, few amounts of B–H bonds were formed in the hydrogenated samples either reacting with LiH or LiF. When utilizing SiB₄, the formation of B–H bonds was promoted for both systems mixing with LiH and LiF. The results imply that a stepwise process of LiBH_4-x→LiBH₄ possibly took place during the hydrogenation process. Importantly, SiB₄–LiH system exhibited the best hydrogenation performance. At moderate conditions of 250 °C and 10 MPa H₂, LiBH₄ was successfully synthesized from this system. A facile synthesis pathway, SiB₄(s) + 4LiH(s) + 6H₂(g) → 4LiBH₄(s) + Si(s), having a Δ_rH_m of −65 kJ/mol H₂, was proposed. This study supports that the chemical state of boron in the reactant is an important factor affecting the generation of LiBH₄. A hydrogenation reaction between SiB₄ and CaH₂ or MgH₂ may be also applicable for synthesizing Ca(BH₄)₂ or Mg(BH₄)₂, which are regarded as potential hydrogen storage materials.