Theoretical and Experimental Study of LiBH4-LiBr Phase Diagram

TL;DR

This study combines experimental and theoretical methods to map the LiBH4-LiBr phase diagram, revealing the stability and solubility limits of hexagonal solid solutions with implications for high Li-ion conductivity materials.

Contribution

It provides the first comprehensive phase diagram of the LiBH4-LiBr system, integrating X-ray, calorimetry, and ab initio calculations using CALPHAD.

Findings

Hexagonal Li(BH4)1-x(Br)x solid solution exists from x=0.30 to 0.55 at room temperature.

The enthalpy of mixing for h-Li(BH4)0.6(Br)0.4 is 1.0 kJ/mol.

The phase diagram covers all compositions and temperatures for the system.

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 from x=0.30 to x=0.55 at room temperature. 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 has been determined experimentally equal to 1.0 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 range by the CALPHAD method.