Iodide substitution in lithium borohydride, LiBH4-LiI

The new concept, anion substitution, is explored for possible improvement of hydrogen storage properties in the system LiBH4-LiI. The structural chemistry and the substitution mechanism are analyzed using Rietveld refinement of in situ synchrotron radiation powder X-ray diffraction (SR-PXD) data, attenuated total reflectance infrared spectroscopy (ATR-IR), differential scanning calorimetry (DSC) and Sieverts measurements. Anion substitution is observed as formation of two solid solutions of Li(BH4)(1-x)I-x, which merge into one upon heating. The solid solutions have hexagonal structures (space group P6(3)mc) similar to the structures of h-LiBH4 and beta-LiI. The solid solutions have iodide contents in the range similar to 0-62 mol% and are stable from below room temperature to the melting point at 330 degrees C. Thus the stability of the solid solutions is higher as compared to that of the orthorhombic and hexagonal polymorphs of LiBH4 and alpha- and beta-LiI. Furthermore, the rehydrogenation properties of the iodide substituted solid solution Li(BH4)(1-x)I-x, measured by the Sieverts method, are improved as compared to those of LiBH4. After four cycles of hydrogen release and uptake the Li(BH4)(1-x)I-x solid solution maintains 68% of the calculated hydrogen storage capacity in contrast to LiBH4, which maintains only 25% of the storage capacity after two cycles under identical conditions. (C) 2011 Elsevier B. V. All rights reserved.