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 Mg1/2BH4, Ca1/2BH4, and Zn1/2BH4 are shown to be I 4 ¯ m 2 , F2dd, and I 4 ¯ m 2 , 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 M1/2BH4 for Mg and Zn substitutions. LiBH4 is destabilized with magnesium borides or LiZn4 formation but its decomposition temperature is higher than that of M1/2BH4. On the other hand, the addition of Ca significantly reduces the H2 releasing temperature due to the formation of highly stable CaB6.