In situ X-ray powder diffraction studies of hydrogen storage and release in the Li–N–H system

Abstract

We report the experimental investigation of hydrogen storage and release in the lithium amide-lithium hydride composite (Li-N-H) system. Investigation of hydrogenation and dehydrogenation reactions of the system through in situ synchrotron X-ray powder diffraction experiments allowed for the observation of the formation and evolution of non-stoichiometric intermediate species of the form Li1+xNH2-x. This result is consistent with the proposed Frenkel-defect mechanism for these reactions. We observed capacity loss with decreasing temperature through decreased levels of lithium-rich (0.7 ≤ x ≤ 1.0) non-stoichiometric phases in the dehydrogenated material, but only minor changes due to multiple cycles at the same temperature. Annealing of dehydrogenated samples reveals the reduced stability of intermediate stoichiometry values (0.4 ≤ x ≤ 0.7) compared with the end member species: lithium amide (LiNH2) and lithium imide (Li2NH). Our results highlight the central role of ionic mobility in understanding temperature limitations, capacity loss and facile reversibility of the Li-N-H system.

The Inside‐Outs of Metal Hydride Dehydrogenation: Imaging the Phase Evolution of the Li‐N‐H Hydrogen Storage System

“…Key to the widespread deployment of hydrogen, particularly for mobile applications, is high‐density reversible storage. Complex metal hydrides possess both large gravimetric and volumetric hydrogen densities, up to 15 wt% and over 100 kg m −3 1a,2. The two major challenges preventing the widespread use of complex metal hydrides are the sluggish kinetics at low temperatures and the formation of stable intermediate phases that reduce the capacity over time and shift thermodynamic phase equilibria unfavorably.…”

“…In addition, knowledge of phase conversions is an essential aspect of a comprehensive understanding of hydrogenation and dehydrogenation because it provides evidence that can help determine which process limits the rate of the phase transformation. The formation of Li 2 NH from LiNH 2 involves the diffusion or migration of highly mobile Li + from LiH to LiNH 2 to counter the transport of hydrogen from one NH 2 − to another and eventually to the hydride in LiH 2,8b,10. The phases have been proposed to form in a roughly core–shell arrangement, with the imide forming on the exterior of a LiNH 2 ‐rich core which shrinks during dehydrogenation as the imide propagates inward.…”

Hydrogen storage properties of LiNH 2 –LiH system with MgH 2 , CaH 2 and TiH 2 added

“…The PCI displays a slope down toward the complete dehydrogenation. This behavior in the PCI curve of the LiNH 2 eLiH system can be interpreted on the basis of recent investigations [30,31]. These studies have demonstrated that the LiNH 2 and Li 2 NH are the dominant phases in the hydrogenated and dehydrogenated states respectively, and different phases with intermediate stoichiometry exist between both compounds.…”

“…In this context, David et al [30] proposed a Frenkel defect model based on the migration of Li þ and H þ ions for dehydrogenation (and hydrogenation). The recent work of Makepeace et al [31] demonstrates the formation and evolution of non-stoichiometric intermediate species of the form Li 1þx NH 2Àx . Then, the sloping behavior of the PCI curve can be related with the conversion of LiNH 2 into Li 2 NH, involving the formation of non-stoichiometric species.…”

Recent Progress and New Perspectives on Metal Amide and Imide Systems for Solid-State Hydrogen Storage

“…David and coauthors, noted that the non-stoichiometric intermediates reported as Li 1+x NH 2−x species are formed during desorption and absorption of the Li-N-H system, in accordance with the Frenkel defect model [24].…”