Experimental and theoretical studies of the LiBH₄-LiI phase diagram

The hexagonal structure of LiBH₄ at room temperature can be stabilised by substituting the BH₄^- anion with I^-, leading to high Li-ion conductive materials. A thermodynamic description of the pseudo-binary LiBH₄-LiI system is presented. The system has been explored investigating several compositions, synthetized by ball milling and subsequently annealed. X-ray diffraction and Differential Scanning Calorimetry have been exploited to determine structural and thermodynamic features of various samples. The monophasic zone of the hexagonal Li(BH₄)_1-x(I)_x solid solution has been experimentally defined equal to 0.18 ≤ x ≤ 0.60 at 25 °C. In order to establish the formation of the hexagonal solid solution, the enthalpy of mixing was experimentally determined, converging to a value of 1800 ± 410 J mol^-1. Additionally, the enthalpy of melting was acquired for samples that differ in molar fraction. By merging experimental results, literature data and ab initio theoretical calculations, the pseudo-binary LiBH₄-LiI phase diagram has been assessed and evaluated across all compositions and temperature ranges by applying the CALPHAD method.