In Situ Synchrotron Radiation Powder X-ray Diffraction study of the 2LiNH2+LiH+KBH4 system

Complex metal hydrides are promising candidates for H2 storage, but high stability and poor kinetics are the important challenges which have to be solved for vehicular applications. Among several systems, the LiNH2/KBH4/LiH mixture in the molar ratio 2:1:1 represents a very promising material: it shows a hydrogen gravimetric capacity of 7.48 wt. % and a theoretical ΔU0 equal of 43.6 kJ/mol H2. The van’t Hoff plot based on the calculations presented in literature shows that for the reaction 2LiNH2 + KBH4 + LiH → Li3BN2 + KH + 4H2 the sorption temperatures are between 300 and 420 K in a range of H2 pressures of 1 and 100 bar, appropriate for reversible hydrogen storage applications. However, such encouraging theoretical results still requires an experimental confirmation. In this work we focus on the 2LiNH2 + KBH4 + LiH system: the phase-structural transformations occurring during the desorption process on the powder mixture prepared by ball milling, are provided by in-situ Synchrotron Radiation Powder X-ray Diffraction (SR-PXD). It is observed that LiNH2 transform into Li2NH during the heating, at about 230°C, while the reflections related to KBH4 disappeared at 360°C. At higher temperature, the formation of Li3BN2 is detected, together with an evident increase of the background, ascribable to the presence of a further phase in the molten state. Patterns of the sample , after cooling down to room temperature, confirmed the presence of Li3BN2 and KH as reported in theoretical study. Details of the microstructural parameters, obtained by diffraction patterns refinement according to Rietveld method, will allow to exhaustively explain the desorption mechanism.