Effect of Mg, Ca, and Zn on stability of LiBH₄ through computational thermodynamics

The effect of divalent metal-dopants, Mg, Ca, and Zn, on the stability of LiBH₄ is studied by using the first-principles calculations and CALPHAD (CALculation of PHAse Diagram) modeling. The ground states of Mg _1/2BH₄, Ca_1/2BH₄, and Zn _1/2BH₄ are shown to be I4m2, F2dd, and I4m2, respectively, through first-principles calculations. Positive enthalpy of mixing between Li and the alloying element is predicted, indicating unfavorable solubility of alloying elements in LiBH₄ and thus offering possibility to decrease the stability of LiBH₄. The ionic sublattice model of (Li ⁺, M²⁺, Va)₁(BH₄^-) ₁ is adopted for the metal substituted LiBH₄ phase. It is observed that the addition of Mg or Zn has limited effect as the decomposition temperature is between those of LiBH₄ and M_1/2BH ₄ for Mg and Zn substitutions. LiBH₄ is destabilized with magnesium borides or LiZn₄ formation but its decomposition temperature is higher than that of M_1/2BH₄. On the other hand, the addition of Ca significantly reduces the H₂ releasing temperature due to the formation of highly stable CaB₆.