Theoretical and Experimental Studies of LiBH₄–LiBr Phase Diagram

Because substitutions of BH$_4$$^–$ anion with Br$^–$ can stabilize the hexagonal structure of the LiBH$_4$ at room temperature, leading to a high Li-ion conductivity, its thermodynamic stability has been investigated in this work. The binary LiBH$_4$–LiBr system has been explored by means of X-ray diffraction and differential scanning calorimetry, combined with an assessment of thermodynamic properties. The monophasic zone of the hexagonal Li(BH$_4$)$_1–x$(Br)$_x$ solid solution has been defined equal to 0.30 ≤ x ≤ 0.55 at 30 °C. Solubility limits have been determined by in situ X-ray diffraction at various temperatures. For the formation of the h-Li(BH$_4$)$_0.6$(Br)$_0.4$ solid solution, a value of the enthalpy of mixing ($ΔH_mix$) has been determined experimentally equal to −1.0 ± 0.2 kJ/mol. In addition, the enthalpy of melting has been measured for various compositions. Lattice stabilities of LiBH$_4$ and LiBr have been determined by ab initio calculations using CRYSTAL and VASP codes. Combining results of experiments and theoretical calculations, the LiBH$_4$–LiBr phase diagram has been determined in all composition and temperature ranges by the CALPHAD method.