Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

Ludueña, Guillermo A.; Wegner, M.; Bjålie, Lars; Sebastiani, Daniel

doi:10.1002/cphc.201000156

Cited by 23 publications

(31 citation statements)

References 39 publications

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“…Our results are therefore in agreement with recent studies by Ludueña et al using first-principles path integral molecular dynamics simulations and solid-state 1 H NMR experiments where they observed significant disorder on the Li sublattice. 48 Experimentally, Li 2 NH was found to be a good ionic conductor, with an activation energy of 0.58 eV. 2 This conductivity has been ascribed to the high mobility of Li ions.…”

Section: A Li-ion Conductionmentioning

confidence: 99%

Mechanisms for the decomposition and dehydrogenation of Li amide/imide

2012

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Reversible reaction involving Li amide (LiNH2) and Li imide (Li2NH) is a potential mechanism for hydrogen storage. Recent synchrotron x-ray diffraction experiments [W. I. David et al., J. Am. Chem. Soc. 129, 1594] suggest that the transformation between LiNH2 and Li2NH is a bulk reaction that occurs through non-stoichiometric processes and involves the migration of Li + and H + ions. In order to understand the atomistic mechanisms behind these processes, we carry out comprehensive first-principles studies of native point defects and defect complexes in the two compounds. We find that both LiNH2 and Li2NH are prone to Frenkel disorder on the Li sublattice. Lithium interstitials and vacancies have low formation energies and are highly mobile, and therefore play an important role in mass transport and ionic conduction. Hydrogen interstitials and vacancies, on the other hand, are responsible for forming and breaking N−H bonds, which is essential in the Li amide/imide reaction. Based on the structure, energetics, and migration of hydrogen-, lithium-, and nitrogen-related defects, we propose that LiNH2 decomposes into Li2NH and NH3 according to two competing mechanisms with different activation energies: one mechanism involves the formation of native defects in the interior of the material, the other at the surface. As a result, the prevailing mechanism and hence the effective activation energy for decomposition depend on the surface-tovolume ratio or the specific surface area, which changes with particle size during ball milling. These mechanisms also provide an explanation for the dehydrogenation of LiNH2+LiH mixtures.

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Section: A Li-ion Conductionmentioning

confidence: 99%

Mechanisms for the decomposition and dehydrogenation of Li amide/imide

2012

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“…However, their equilibrium lattice parameters are inconsistent with the space group symmetry inferred experimentally. On the other hand, the local instability of the ideal antifluorite structure of Li ions at low temperature was confirmed by ab-initio molecular dynamics simulations at 300 K which showed the spontaneous formation of Li Frenkel pairs [16] and strong distorsions of the Li sublattice [17].…”

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confidence: 94%

Static disorder and structural correlations in the low-temperature phase of lithium imide

et al. 2011

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Based on ab-initio molecular dynamics simulations, we investigate the low temperature crystal structure of Li2NH which in spite of its great interest as H-storage material is still matter of debate. The dynamical simulations reveal a precise correlation in the fractional occupation of Li sites which leads average atomic positions in excellent agreement with diffraction data and solves inconsistencies of previous proposals.Lithium amide (LiNH 2 ) and imide (Li 2 NH) have been extensively studied in recent years as promising materials for hydrogen storage [1][2][3][4][5]. Hydrogen release occurs in the mixture LiNH 2 /LiH via a reversible solid state decomposition reaction into lithium imide and molecular hydrogen, LiNH 2 + LiH → Li 2 NH + H 2 . The typical operating temperature for this system is around 280 o C, which is probably too high for on-board applications. Nevertheless, the amide/imide system is under deep scrutiny since it represents a prototypical, relatively simple system, which could shed light on the mechanisms of reversible H-release in the more complex, and technologically more promising, reactive hydrides made of mixtures of amide, borohidrides and/or alanates (e.g. LiNH 2 /LiBH 4 or LiNH 2 /NaAlH 4 ) [4,5]. The search for better performing materials in this class would greatly benefit from a microscopic knowledge of the decomposition mechanism which requires in turn a detailed description of the crystalline phases involved. For several complex hydrides the structure of the phases undergoing the dehydrogenation/rehydrogenation process is still not fully resolved. This is the case for instance of the most studied sodium alanate (NaAlH 4 ) for which Raman spectroscopic data[6] and ab-initio simulations [7] very recently suggested the existence of a new high temperature phase which is expected to mediate the decomposition reaction in place of the low temperature α-phase considered so far. The structure of Li 2 NH as well is still a matter of debate. Structural refinement from neutron and x-rays diffraction data reveals a structure of Li 2 NH with fractional occupation. In spite of a substantial amount of experimental and theoretical investigation, the problem of the actual local structure which yields this long-range disorder is still unsettled. Based on ab-initio simulations, we have identified a model for the local structure of the lowtemperature phase of lithium imide (Li 2 NH) which solves inconsistencies of previous proposals and fully agree with experimental data available.Differential thermal analysis and NMR measurements [8,9] in the late 60's revealed a reversible phase transition at 356 K between an unknown low temperature (LT) structure and a high temperature antifluorite phase of Li 2 NH. Structure refinement from x-ray and neutron diffraction measurements on deuterated imide (Li 2 ND) have been published only very recently [10]. The high temperature phase yields a diffraction pattern consistent with an anti-fluorite structure, in which hydrogen atoms occupy the 192l positions of the F m...

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“…Almost no effect of nuclear quantum delocalisation was found for Li 2 NH whereas a broader distribution of 1 H chemical shifts in LiNH 2 was found when path integration was applied. [22] We are not, however, aware of previous systematic investigations of the effects of nuclear delocalisation on NMR spectra of organic solids. Isotope effects are one manifestation of the quantum nature of nuclei; zero-point fluctuations lead to differences in vibrationally averaged properties of compounds with isotopic substitution.…”

Section: Introductionmentioning

confidence: 99%

Effects of Quantum Nuclear Delocalisation on NMR Parameters from Path Integral Molecular Dynamics

Dračínský

Hodgkinson

2014

Chemistry A European J

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Citation for published item:hr § ¡ %nsk¡ yD wrtin nd rodgkinsonD ul @PHIRA 9i'ets of quntum nuler delolistion on xw prmeters from pth integrl moleulr dynmisF9D ghemistry X iuropen journlFD PH @VAF ppF PPHIEPPHUF Further information on publisher's website:his is the peer reviewed version of the following rtileX hr § ¡ %nsk¡ yD wF nd rodgkinsonD F @PHIRAD i'ets of untum xuler helolistion on xw rmeters from th sntegrl woleulr hynmisF ghemistry E e iuropen tournlD PH @VAX PPHIEPPHUD whih hs een pulished in (nl form t httpXGGdxFdoiForgGIHFIHHPGhemFPHIQHQRWTF his rtile my e used for nonEommeril purposes in ordne with ileyEgr erms nd gonditions for selfErhivingF Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractThe influence of nuclear delocalisation on NMR chemical shifts in molecular organic solids is explored using path integral molecular dynamics (PIMD) and density functional theory calculations of shielding tensors. Nuclear quantum effects are shown to explain previously observed systematic deviations in correlations between calculated and experimental chemical shifts, with particularly large PIMD-induced changes (up to 23 ppm) observed for atoms in methyl groups. The PIMD approach also enables isotope substitution effects on chemical shifts and J couplings to be predicted in excellent agreement with experiment for both isolated molecules and molecular crystals. An approach based on convoluting calculated shielding or coupling surfaces with probability distributions of selected bond distances and valence angles obtained from PIMD simulations is used to calculate isotope effects. IntroductionThe gauge-including projector-augmented wave (GIPAW) procedure has been recently developed for the prediction of the magnetic resonance parameters in solids, [1] and the power of

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Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

Cited by 23 publications

References 39 publications

Mechanisms for the decomposition and dehydrogenation of Li amide/imide

Mechanisms for the decomposition and dehydrogenation of Li amide/imide

Static disorder and structural correlations in the low-temperature phase of lithium imide

Effects of Quantum Nuclear Delocalisation on NMR Parameters from Path Integral Molecular Dynamics

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