High Hydrogen Mobility in an Amide–Borohydride Compound Studied by Quasielastic Neutron Scattering

Abstract

The hydrogen storage performance of reactive hydride composite Mg(NH2)2+2LiH can be significantly improved by the addition of LiBH4 and the subsequent formation of an amide–borohydride compound Li4(BH4)(NH2)3 during hydrogen release. Herein, an investigation into the structure and anion motions of Li4(BH4)(NH2)3 using synchrotron radiation powder X-ray diffraction (SR-PXD; 295–573 K) and quasielastic neutron scattering (QENS; 297–514 K) is described. The highest temperature studied with QENS (514 K) is above the melting point of Li4(BH4)(NH2)3. The neutron measurements confirm a long-range diffusive motion of hydrogen-containing species with the diffusion coefficient 𝐷≈10−6 cm2 s−1. Interestingly, this value is comparable to that of Li+ diffusion inferred from conductivity measurements. SR-PXD confirms the recrystallization of Li4(BH4)(NH2)3 from the melt into the α-phase upon cooling. At temperatures below 514 K, localized rotational motions are observed that are attributed to (BH4)− tetrahedra units mainly undergoing rotations around the 𝐶3 axes. The activation energy for this thermally activated process is found to be 𝐸a=15.5±0.9 and 17.4±0.9 kJ mol−1 respectively for the two instrumental resolutions utilized in the QENS measurements, corresponding to observation times of 55 and 14 ps.