Abstract
Because substitutions of BH4– anion with Br– can stabilize the hexagonal structure of the LiBH4 at room temperature, leading to a high Li-ion conductivity, its thermodynamic stability has been investigated in this work. The binary LiBH4–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(BH4)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(BH4)0.6(Br)0.4 solid solution, a value of the enthalpy of mixing (ΔHmix) 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 LiBH4 and LiBr have been determined by ab initio calculations using CRYSTAL and VASP codes. Combining results of experiments and theoretical calculations, the LiBH4–LiBr phase diagram has been determined in all composition and temperature ranges by the CALPHAD method.
Highlights
Due to its high gravimetric and volumetric density of hydrogen, LiBH4 has been largely studied as a solid-state hydrogen storage material.[1,2] It shows a polymorphic transition from an orthorhombic structure at room temperature (RT), space group (s.g.) Pnma, to a hexagonal structure, s.g
The h-Li(BH4)1−α(I)α solid solutions have been reported to be stable at RT in the range of 0.18 ≤ α ≤ 0.50.14 Fast Li-ion conductivity at RT is observed in hLi(BH4)1−α(Br)α hexagonal solid solutions (e.g., ∼10−5 S/cm for h-Li(BH4)0.7(Br)0.3),[13] it is reduced as the bromide content increases above x = 0.29.13,15
The formation of a single hexagonal solid solution is confirmed for sample s2 (i.e., Li(BH4)0.5(Br)0.5), whereas a two-phase mixture has been observed for sample s1
Summary
Due to its high gravimetric and volumetric density of hydrogen, LiBH4 has been largely studied as a solid-state hydrogen storage material.[1,2] It shows a polymorphic transition from an orthorhombic structure at room temperature (RT), space group (s.g.) Pnma, to a hexagonal structure, s.g. A small amount of the hexagonal solid solution was already observed (see Figure S14), suggesting that the reaction might be already initiated during the heating up to the LiBH4 phase transition, but the enthalpy contribution cannot be determined due to sensitivity limitation of the DSC analysis. Considering the experimental value of enthalpy of mixing (i.e., −1.0 ± 0.2 kJ/mol) for sample s5 (0.4LiBr−0.6LiBH4 molar fraction), the interaction parameter for the hexagonal solid solution has been fixed on the basis of a regular solution model and turns out to be HEXΩ = −4167 J/mol. Lower experimental enthalpy values are detected during cooling, possibly because of a larger temperature range in which the crystallization takes place, causing an underestimation of the enthalpy related to the transformation, while integrating the DSC peaks
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