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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 8889 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 Investigation of hydrogen absorption in Li 7 VN 4 and Li 7 MnN 4 Guang He, a J. F. Herbst,* b T.N. Ramesh, a F. E. Pinkerton, b M. S. Meyer b and Linda Nazar* a Received 16th December 2010, Accepted 1st February 2011 DOI: 10.1039/c0cp02892d The hydrogen storage properties of Li 7 VN 4 and Li 7 MnN 4 were investigated both by experiment and by density functional theory calculations. Li 7 VN 4 did not sorb hydrogen under our experimental conditions. Li 7 MnN 4 was observed to sorb 7 hydrogen atoms through the formation of LiH, Mn 4 N, and ammonia gas. An applied pressurized mixture of H 2 /Ar and H 2 /N 2 gases was helpful to mitigate the release of NH 3 but could not prevent its formation. The introduction of N 2 also caused weight gain of the sample by re-nitriding the absorbed products LiH and Mn 4 N, which correlated with the presence of Li 2 NH, LiNH 2 , and Mn 2 N detected by X-ray diffraction. While our observed results for Li 7 VN 4 and Li 7 MnN 4 differ in detail, they are in overall qualitative agreement with our theoretical work, which strongly suggests that both compounds are unlikely to form quaternary hydrides. Introduction Onboard hydrogen storage has become a critical technology for the development of fuel cell vehicles (FCVs). 1–4 When various storage systems are compared, solid-state materials are advantageous as they possess a higher volumetric energy density than either compressed gas or liquid hydrogen systems. However, despite decades of studies, no solid state hydrogen storage materials satisfactorily meet the requirements for application to FCVs, such as high storage capacity and fast sorption kinetics in the range of 1–10 bar and 25–120 1C. 5 Conventional transition metal alloys, including the La–Ni (AB 5 ) system, Ti–Cr (AB 2 ) system and Ti–Fe (AB) system, 6 have the desired intermediate thermodynamic affinities for hydrogen. The hydrogen gas molecules split into atoms at the surface of the metal and then enter the metallic lattice in the atomic form, diffuse through the metal, jump between interstitial sites; and finally form a hydride phase with a more or less ordered hydrogen sublattice. The hydrogen s electron can be partly donated to the metal conduction band if the metallic lattice contains d or f electron states at the Fermi level. Hence these bare protons can move relatively freely through the metal lattice due to the electrostatic force screened by electrons at the metal Fermi surface. In the desorption process, the opposite takes place; i.e. two hydrogen atoms recombine to form H 2 again. These systems have better kinetics than complex hydrides, but their low gravimetric capacities and high costs prevent vehicular applications. In recent years alternative hydrogen storage systems have been examined, consisting of light alkali and alkaline earth metal nitrides and/or borides with improved storage capacity. A well-known and highly promising example is lithium nitride, which remarkably stores up to 11.4 wt% hydrogen according to the equation: 7 Li 3 N + 2H 2 2 Li 2 NH + LiH + H 2 2 LiNH 2 + 2LiH. One of the major limitations of this system is that the high storage capacity can only be achieved at a temperature of 400 1C, however the reaction tempera- ture can be deceased by incorporating other light elements such as Mg, B, Al, and K into this system. 8–11 Recently, Langmi et al. 12 have reported a reversible hydrogen storage system based on a Li–Fe–N phase, revealing the possible potential of hydrogen storage by lithium transition metal nitrides. Such materials might be expected to function midway between intermetallic alloy systems and the complex hydrides. Interesting new lithium transition metal nitrides have also been recently discovered that are considered as potential hydrogen storage materials. 13 In this article we report on two lithium metal nitrides—Li 7 VN 4 and Li 7 MnN 4 , as candidate materials for hydrogen storage. The electrochemical behaviour of these compounds as Li-ion battery cathodes have been studied extensively, 14–16 but this is the first report of their hydrogen storage properties, examined both experimentally and computationally. Experimental and computational details Synthesis Li 7 VN 4 and Li 7 MnN 4 were synthesized by a high temperature solid-state reaction of Li 3 N and V/Mn metal. Typically, Li 3 N (0.600 g; 10% molar excess) and V powder (325 mesh, 99.5%) a Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada. E-mail: [email protected] b Chemical Sciences and Materials Systems Laboratory, General Motors Research and Development Center, Mail Code 480-106-224, 30500 Mound Road, Warren, MI 48090-9055, USA PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by University of Waterloo on 07 March 2012 Published on 01 April 2011 on http://pubs.rsc.org | doi:10.1039/C0CP02892D View Online / Journal Homepage / Table of Contents for this issue

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Page 1: Citethis:Phys. Chem. Chem. Phys.,2011,13 ,88898893 PAPER · Citethis:Phys. Chem. Chem. Phys ... 2 also caused weight gain of the sample by re-nitriding the ... 8890 Phys. Chem. Chem

This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 8889

Cite this: Phys. Chem. Chem. Phys., 2011, 13, 8889–8893

Investigation of hydrogen absorption in Li7VN4 and Li7MnN4

Guang He,aJ. F. Herbst,*

bT.N. Ramesh,

aF. E. Pinkerton,

bM. S. Meyer

band

Linda Nazar*a

Received 16th December 2010, Accepted 1st February 2011

DOI: 10.1039/c0cp02892d

The hydrogen storage properties of Li7VN4 and Li7MnN4 were investigated both by experiment

and by density functional theory calculations. Li7VN4 did not sorb hydrogen under our

experimental conditions. Li7MnN4 was observed to sorb 7 hydrogen atoms through the formation

of LiH, Mn4N, and ammonia gas. An applied pressurized mixture of H2/Ar and H2/N2 gases was

helpful to mitigate the release of NH3 but could not prevent its formation. The introduction of

N2 also caused weight gain of the sample by re-nitriding the absorbed products LiH and Mn4N,

which correlated with the presence of Li2NH, LiNH2, and Mn2N detected by X-ray diffraction.

While our observed results for Li7VN4 and Li7MnN4 differ in detail, they are in overall

qualitative agreement with our theoretical work, which strongly suggests that both compounds

are unlikely to form quaternary hydrides.

Introduction

Onboard hydrogen storage has become a critical technology

for the development of fuel cell vehicles (FCVs).1–4 When

various storage systems are compared, solid-state materials are

advantageous as they possess a higher volumetric energy density

than either compressed gas or liquid hydrogen systems. However,

despite decades of studies, no solid state hydrogen storage

materials satisfactorily meet the requirements for application to

FCVs, such as high storage capacity and fast sorption kinetics in

the range of 1–10 bar and 25–120 1C.5 Conventional transition

metal alloys, including the La–Ni (AB5) system, Ti–Cr (AB2)

system and Ti–Fe (AB) system,6 have the desired intermediate

thermodynamic affinities for hydrogen. The hydrogen gas

molecules split into atoms at the surface of the metal and then

enter the metallic lattice in the atomic form, diffuse through the

metal, jump between interstitial sites; and finally form a hydride

phase with a more or less ordered hydrogen sublattice. The

hydrogen s electron can be partly donated to the metal conduction

band if the metallic lattice contains d or f electron states at the

Fermi level. Hence these bare protons can move relatively freely

through the metal lattice due to the electrostatic force screened by

electrons at the metal Fermi surface. In the desorption process, the

opposite takes place; i.e. two hydrogen atoms recombine to form

H2 again. These systems have better kinetics than complex

hydrides, but their low gravimetric capacities and high costs

prevent vehicular applications.

In recent years alternative hydrogen storage systems have

been examined, consisting of light alkali and alkaline earth

metal nitrides and/or borides with improved storage capacity.

A well-known and highly promising example is lithium nitride,

which remarkably stores up to 11.4 wt% hydrogen according

to the equation:7 Li3N + 2H2 2 Li2NH + LiH + H2 2

LiNH2 + 2LiH. One of the major limitations of this system is

that the high storage capacity can only be achieved at a

temperature of 400 1C, however the reaction tempera-

ture can be deceased by incorporating other light elements

such as Mg, B, Al, and K into this system.8–11 Recently,

Langmi et al.12 have reported a reversible hydrogen storage

system based on a Li–Fe–N phase, revealing the possible

potential of hydrogen storage by lithium transition metal

nitrides. Such materials might be expected to function midway

between intermetallic alloy systems and the complex hydrides.

Interesting new lithium transition metal nitrides have also

been recently discovered that are considered as potential

hydrogen storage materials.13 In this article we report on

two lithiummetal nitrides—Li7VN4 and Li7MnN4, as candidate

materials for hydrogen storage. The electrochemical behaviour

of these compounds as Li-ion battery cathodes have been

studied extensively,14–16 but this is the first report of their

hydrogen storage properties, examined both experimentally

and computationally.

Experimental and computational details

Synthesis

Li7VN4 and Li7MnN4 were synthesized by a high temperature

solid-state reaction of Li3N and V/Mn metal. Typically, Li3N

(0.600 g; 10% molar excess) and V powder (325 mesh, 99.5%)

aDepartment of Chemistry, University of Waterloo,200 University Avenue West, Waterloo, Ontario N2L 3G1,Canada. E-mail: [email protected]

bChemical Sciences and Materials Systems Laboratory, General MotorsResearch and Development Center, Mail Code 480-106-224,30500 Mound Road, Warren, MI 48090-9055, USA

PCCP Dynamic Article Links

www.rsc.org/pccp PAPER

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8890 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 This journal is c the Owner Societies 2011

(0.338 g) were hand milled for 15 min and pressed into a pellet

at 3 MP pressure under an inert Ar atmosphere. This pellet

was heated under high purity N2 at 700 1C for 24 h. A similar

process was applied to synthesize Li7MnN4, replacing V

powder with Mn (325 mesh, 99+%).

Characterization

X-Ray powder diffraction data (XRD) of the as-synthesized

Li7VN4 and Li7MnN4 were collected on a Bruker D8-advance

diffractometer (Cu Ka radiation, l = 1.5418 A). Both air-

sensitive samples were measured in a hermetically sealed

sample holder.

Hydrogenation experiments were conducted in a Cahn 2151

high pressure thermogravimetric analyzer (TGA). Approximately

200 mg of powdered sample was loaded into an open stainless

steel sample bucket in the Ar glove box and protected during

transfer to the TGA by covering the sample with anhydrous

pentane. The TGA was purged with He gas while the pentane

evaporated. After pressurizing to 8.3 MPa of H2 gas, Li7VN4

was heated to 600 1C at 5 1Cmin�1 in flowing H2 gas. Li7MnN4

was heated to 600 1C in 50 1C steps, with a 45 min soak at each

temperature. The TGA exhaust gas was sampled using an

SRS model CIS-100 mass spectrometer operated in residual gas

analysis (RGA) mode to identify the constituents in the evolved

gas. The RGA monitored mass channels at 2 amu (H2), 4 amu

(He), 16 amu (CH4, NH2 crack of ammonia), 17 amu (NH3, OH

crack of water), 18 amu (H2O), 28 amu (N2), 32 amu (O2), and

44 amu (CO2).

Mixed gas experiments were conducted in a Hiden Model

IGA-3 thermogravimetric analyzer (IGA). Approximately 80 mg

of powdered sample was placed into an open stainless steel

bucket, sealed into a transfer container, and transferred into

the IGA instrument without air exposure using a removable

interlock chamber. Sample densities were obtained from

weight changes due to buoyancy during pressurization to 20 bar.

Li7VN4 was heated in 50 1C steps with variable soak times up

to 425 1C in flowing pure H2 gas at 20 bar. Li7MnN4 was

similarly heated to 400 1C in 50 1C steps in a mixture of 50%

H2/50% N2 at 20 bar, mixed 50% H2/50% Ar at 20 bar, or

pure H2 at 10 bar.

Post-heat treatment XRDmeasurements were performedwith a

Siemens D5000 diffractometer (Cu Ka radiation, l = 1.5418 A).

Sample powders were compacted onto XRD slides and then

sealed under a Kapton film to help protect the sample from

atmospheric exposure during the XRD measurement.

Computational procedures

Total electronic energies were calculated with the Vienna

ab initio Simulation Package (VASP), which implements

density functional theory17 with a plane wave basis set.18,19

Potentials obtained via the projector-augmented wave

approach20,21 were employed for the elements in conjunction

with the generalized gradient approximation of Perdew and

Wang22,23 for the exchange–correlation energy functional.

The Li, V, Mn, N, and H potentials contained 3, 13, 13, 5,

and 1 valence electron and were constructed with augmenta-

tion wave energy cutoffs of 272, 264, 270, 700, and 700 eV,

respectively. In all calculations a plane wave cutoff energy of

900 eV was imposed, and the k-point spacings of the reciprocal

space meshes were no larger than 0.1 A�1. At least two full-cell

optimizations of the lattice constants and nuclear coordinates

were performed for each material of interest; the total energies

were converged to 10�6 eV/cell and the forces relaxed to

10�5 eV/A. Calculations for the N2 and H2 molecules were

done with the same potentials in boxes large enough to ensure

isolation.

Results and discussion

Theoretical investigations

Enthalpies of formation DH for the Li7VN4 and Li7MnN4

compounds as well as enthalpies of hydride formation DH* for

model hydrides of them were calculated. Using Li7VN4 as a

specific example, we express DH as:

DH(Li7VN4) = E(Li7VN4) � 7E(Li) � E(V) � 2E(N2), (1)

where E denotes the electronic total energy emerging from the

VASP computations for the relevant materials (Li7VN4, Li

and V metals, and the N2 molecule). Defined this way, DH is

the calculated standard enthalpy of formation at zero tempera-

ture in the absence of zero point energy contributions—which

are small on the scale of the electronic terms—making eqn (1) a

good approximation.24

For the low temperature g-phase of Li7VN4 having the

cubic P-43n (space group No. 218) structure25,26 whose synthesis

is described below we find

DH(g-Li7VN4) = �928 kJ mol�1 f.u.

(f.u. � formula unit). Calculations for the tetragonal P42/nmc

(No. 137) a-phase25 yield

DH(a-Li7VN4) = �923 kJ mol�1 f.u.;

the higher value is consistent with the fact that the a-phase is ahigh temperature modification. Both materials are semi-

conductors with calculated bands gaps of 2.8 eV (g-Li7VN4)

and 2.2 eV (a-Li7VN4).

In the case of Li7MnN4, only known to form in the cubic P-43n

structure,27 non-magnetic as well as magnetic calculations

were performed since susceptibility measurements indicate

weak ferromagnetism below about 8 K.28 In the ferromagnetic

state we obtain

DH(Li7MnN4) = �661 kJ mol�1 f.u.,

which is 49 kJ mol�1 f.u. lower than for the non-magnetic

configuration and thus consistent with experiment. The spin-

only calculated moment is 2.0 mB/Mn, identical to the value

inferred from the measured susceptibility.28

These findings make it clear that both compounds are quite

stable thermodynamically and suggest that Li7VN4, with a

significantly more negative formation enthalpy, is less likely to

sorb hydrogen. To assess hydride formation more specifically,

calculations were performed for five hypothetical quaternary

hydrides whose crystal structures were constructed by placing

hydrogen into the largest holes in the parent compound: (i) 8e

and 24i vacancy sites in cubic Li7MnN4 and (ii) 8g and

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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 8891

two distinct 16h sites in tetragonal a-Li7VN4. Each struc-

ture was fully optimized with VASP and the enthalpy DH*

of hydride formation computed, e.g.:

DH*[Li7MnN4H(8e)] � E[Li7MnN4H(8e)]

� E(Li7MnN4). (2)

As Table 1 shows, we obtain large and positive values of DH*

for all five models, strongly suggesting that neither Li7MnN4

nor Li7VN4 is likely to form a quaternary hydride. The entries

for Li7MnN4Hn are undoubtedly also reflective of g-Li7VN4Hn

having the same cubic structure, and the DH* values for

a-Li7VN4 illustrate that not even the lower tetragonal symmetry

offers any prospect for changing the sign of DH*. As described

below, our experimental results confirm the inferences from our

theoretical modeling work.

Experimental studies

The X-ray powder diffraction patterns of the prepared Li7VN4

and Li7MnN4 are shown in Fig. 1. Previous studies29,30 have

classified the crystal structures of lithiated transition metal

nitrides of the first row into two groups: the ionic anti-fluorite

structure for elements in the early series where the metal is

present in its group oxidation state25,28,31,32 and a layered

structure based on a-Li3N for elements in the late series that

exist in the univalent state.33–35 In this paper, the studied

elements vanadium and manganese are within the range of

the early series, therefore they both form an anti-fluorite

structure. For the Li3N/ V system, Li7VN4 (P-43n) was obtained

as a major phase with trace quantities of VN and Li7VN4

(Pa3-). In the case of Li3N/Mn a similar XRD pattern is

observed, which can be assigned to Li7MnN4, an isostructure

of Li7VN4. It is notable that as manganese is in the middle

of the first row transition metals, its lithiated nitrides can

adopt anti-fluorite or layered structures. Moreover, a new

composition LixMn2�xN with an anti-rutile structure was also

prepared by Niewa’s group.36 The various possible structures

of the Li–Mn–N system can complicate the preparation of a

single Li7MnN4 phase, however no layered Li2[(Li1�xMx)N]

or anti-rutile LixMn2�xN structures are observed in the XRD

pattern. Therefore, a pure phase of Li7MnN4 was obtained by

a simple high temperature solid-state process.

Attempts were made to induce H2 absorption in both samples.

Fig. 2 shows the thermogravimetric profile of Li7VN4. The

slight increase in weight at the beginning of the temperature

ramp is a convection cell artifact of the high pressure TGA.

The change in weight during depressurization suggests that the

sample became less dense during the temperature profile.

Excluding these weight changes, only 1 wt% loss is observed

after 7 h, even with high temperature and pressure conditions

(600 1C and 83 bar). IGA measurement similarly observed a

weight loss of about 0.35 wt% even after 47 h at 425 1C in

20 bar H2. These results confirm that Li7VN4 is very stable and

hydrogen cannot be easily absorbed. A concurrent mass

spectrographic measurement shows the small weight change

might be due to the production of N2.

The thermogravimetric profile of Li7MnN4 is given in Fig. 3.

Instead of H2 absorption, there is a weight loss of roughly

26 wt% under 83 bar of H2. A slight weight gain of 2.6%

occurred near 350 1C, corresponding to an uptake of approxi-

mately 4 H atoms per formula unit associated with the

formation of LiH. Niewa et al.28 found that the reduction,

or decomposition reaction of Li7MnN4, took place in the

presence of Li above 250 1C, to produce a series of Li–Mn–N

compounds such as Li5[(Li1�xMnx)]3 and Li2[(Li1�xMx)N]. In

our case, H2 gas played a similar role as Li by reducing Mn(V)

in Li7MnN4 to a lower oxidation state. A quantity of LiH was

probably formed during this process, as some Li atoms were

released from Li7MnN4 with the reduction of Mn(V). The large

Table 1 Enthalpies of hydride formation DH* calculated [see eqn (2)]for quaternary hydrides modeled by inserting H atoms into theindicated vacancy sites in cubic Li7MnN4 (8 f.u. per unit cell) andtetragonal a-Li7VN4 (2 f.u. per primitive cell)

Model hydride H site DH* (kJ mol�1 H2)

Li7MnN4H 8e +33Li7MnN4H3 24i +91Li7VN4H4 8g +170Li7VN4H8 16h1 +118Li7VN4H8 16h2 +202

Fig. 1 XRD patterns of as-synthesised Li7VN4 and Li7MnN4 showing

the (almost) pure single phase nature of the products. Fig. 2 The TGA profile of Li7VN4 in 83 bar of hydrogen.

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8892 Phys. Chem. Chem. Phys., 2011, 13, 8889–8893 This journal is c the Owner Societies 2011

weight loss above 350 1C should be attributed to the complete

decomposition of the Li–Mn–N system to form binary MnxN

and LiH. This assumption is confirmed by an XRD pattern of

the final material after attempted hydrogenation (Fig. 4), that

shows the presence of Mn4N, LiH and some Li2O and MnO2

impurities. The overall sample density of the decomposed

sample is 1.43 g cm�3, determined by pressure buoyancy,

which is in excellent agreement with the expected value of

1.43 g cm�3 for mixed LiH and Mn4N. As mass spectrometer

measurements obtained during the observed weight losses

show the evolved gas to be NH3, the overall reaction of the

process should be:

Li7MnN4 + 73/8 H2 - 7 LiH + 1/4 Mn4N + 15/4 NH3

(3)

According to this reaction, the predicted weight loss is 28 wt%,

which is consistent with the measured weight loss (26 wt%).

The experimental results are in accord with our calculations

insofar as Li7MnN4 is much less stable than Li7VN4 and can

be hydrogenated at elevated temperatures.Moreover, conducting

VASP computations for Mn4N and the NH3 molecule, we find

that the left side of reaction (3) is thermodynamically unstable

with respect to the right side (by 104 kJ mol�1 Li7MnN4),

consistent with the inference of reaction (3) from our measure-

ments. The accompanying release of NH3 during the absorp-

tion makes Li7MnN4 unlikely to be used in FCVs for

hydrogen storage. As we have had previous success in

suppressing NH3 formation with a partial pressure of N2

mixed with the H2, this was attempted with the IGA, which

can supply multiple gases simultaneously. Fig. 5 shows the

weight change in 10 bar of pure H2 (red curve), 20 bar 50%

H2/50% N2 (blue curve), and 20 bar 50% H2/50% Ar (green

curve). A second gas mixed with H2, whether N2 or Ar,

does appear to suppress the NH3 production by moving

the reaction to a higher temperature. All samples still lost

weight due to nitrogen release in the form of NH3. However,

when N2 gas is present the sample gains weight due to

re-nitriding of the materials above 350 1C, as shown in the

blue curve of Fig. 5. A similar result can be obtained by

treating a fully desorbed sample (red curve) with a gas mixture

containing N2 (magenta curve), which exhibits similar absorp-

tion temperature and weight gaining. The XRD pattern of the

final product produced by desorption in H2 followed by a

subsequent nitriding (Fig. 6) shows the presence of Mn2N plus

Li2NH and LiNH2, which further corroborates the re-nitriding

assumption.

Conclusions

Lithium transition metal nitrides Li7VN4 and Li7MnN4 were

prepared by a high temperature solid-state synthesis. No

hydrogen absorption is observed for Li7VN4 at high tempera-

ture and pressure conditions, while Li7MnN4 can react with

H2 and produce LiH, Mn4N, and NH3. Although as many as

7 H atoms can be absorbed per unit of Li7MnN4, the release of

NH3 and the relatively high absorption temperature prevent

its application in FCVs. A mixture of H2/N2, or H2/Ar was

found to be helpful for suppressing the release of NH3 by

shifting the reaction to a higher temperature. In addition, a

weight gain is observed above 350 1C in the presence of N2 gas

due to the re-nitriding of the desorbed products. Computational

modeling work indicates that Li7VN4 and Li7MnN4 are

unlikely to form quaternary hydrides, in qualitative agreement

with the experimental results.

Fig. 3 The TGA profile of Li7MnN4 in 83 bar of hydrogen.

Fig. 4 XRD pattern of Li7MnN4 after attempted hydrogenation.

Fig. 5 TGA results of the attempts to suppress NH3 formation using

N2 or Ar gas, and subsequent nitriding above 350 1C when N2 gas is

present.

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Acknowledgements

L.F.N. gratefully acknowledges General Motors Canada for

funding, and NSERC through its Discovery, Canada Research

Chair and Collaborative Research & Development programs.

References

1 L. Schlapbach and A. Zuttel, Nature, 2001, 414, 353.2 U. Eberle, M. Felderhoff and F. Schuth, Angew. Chem., Int. Ed.,2009, 48, 6608.

3 C. L. Aardahl and S. D. Rassat, Int. J. Hydrogen Energy, 2009,34, 6676.

4 S. I. Orimo, Y. Nakamori, J. R. Eliseo, A. Zuttel and C. M. Jensen,Chem. Rev., 2007, 107, 4111.

5 P. Jena, J. Phys. Chem. Lett., 2011, 2, 206; US Department ofEnergy’s Energy Efficiency and Renewable Energy Website:http://www.eere.energy.gov/hydrogenandfuelcells/.

6 G. Sandrock, J. Alloys Compd., 1999, 293–295, 877.7 P. Chen, Z. T. Xiong, J. Z. Luo, J. Y. Lin and K. L. Tan, Nature,2002, 420, 302.

8 Z. T. Xiong, G. T. Wu, J. Q. Hu and P. Chen, Adv. Mater., 2004,16, 1522.

9 F. E. Pinkerton, G. P. Meisner, M. S. Meyer, M. P. Balogh andM. D. Kundrat, J. Phys. Chem. B, 2005, 109, 6.

10 Z. T. Xiong, G. T. Wu, J. Q. Hu and P. Chen, J. Power Sources,2006, 159, 167.

11 J. Wang, T. Liu, G. Wu, W. Li, Y. Liu, C. M. Araujo,R. H. Scheicher, A. Blomqvist, R. Ahuja, Z. Xiong, P. Yang,M. Gao, H. Pan and P. Chen, Angew. Chem., Int. Ed., 2009,48, 5828.

12 H. W. Langmi, S. D. Culligan and G. S. Mcgrady, Int. J. HydrogenEnergy, 2009, 34, 8108.

13 A. Yamada, S. Mori, H. Iyama, R. Kanno and M. Shibata,J. Ceram. Soc. Jpn., 2009, 117, 52.

14 M. Nishijima, N. Tadokoro, Y. Takeda, N. Imanishi andO. Yamamoto, J. Electrochem. Soc., 1994, 141, 2966.

15 S. Suzuki and T. Shodai, Solid State Ionics, 1999, 116, 1.16 J. Cabana, C. M. Ionica-Bousquet, C. P. Grey and M. R. Palacın,

Electrochem. Commun., 2010, 12, 315.17 W. Kohn and L. Sham, Phys. Rev., 1965, 140, A1133.18 G. Kresse and J. Hafner, Phys. Rev. B: Condens. Matter, 1994,

49, 14251.19 G. Kresse and J. Furthmuller, Comput. Mater. Sci., 1996,

6, 15.20 P. E. Blochl, Phys. Rev. B: Condens. Matter, 1994, 50, 17953.21 G. Kresse and D. Joubert, Phys. Rev. B: Condens. Matter Mater.

Phys., 1999, 59, 1758.22 J. P. Perdew and Y. Wang, Phys. Rev. B: Condens. Matter, 1992,

45, 13244.23 J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson,

M. R. Pederson, D. J. Singh and C. Fiolhais, Phys. Rev. B:Condens. Matter, 1992, 46, 6671.

24 J. F. Herbst and M. S. Meyer, J. Alloys Compd., 2010, 492, 65.25 R. Niewa, D. Zherebtsov and Z. Hu, Inorg. Chem., 2003, 42, 2538.26 K. Kushida, Y. Ichihashi, Y. Suzuki and K. Kuriyama, Phys. B,

2010, 405, 2305.27 R. Juza, E. Anschutz and H. Puff, Angew. Chem., 1959, 71, 161.28 R. Niewa, F. R. Wagner, W. Schnelle, O. Hochrein and R. Kniep,

Inorg. Chem., 2001, 40, 5215.29 R. Juza, K. Langer and K. V. Benda, Angew. Chem., Int. Ed. Engl.,

1968, 7, 360.30 R. Niewa and F. J. Disalvo, Chem. Mater., 1998, 10, 2733.31 R. Niewa, D. Zherebtsov and S. Leoni, Chem.–Eur. J., 2003,

9, 4255.32 A. Gudat, R. Kniep and A. Abenau, J. Less Common Met., 1990,

161, 31.33 J. L. C. Rowsell, V. Pralong and L. F. Nazar, J. Am. Chem. Soc.,

2001, 123, 8598.34 M. Nishijima, T. Kagohashi, Y. Takeda, M. Imanishi and

O. Yamamoto, J. Power Sources, 1997, 68, 510.35 R. Niewa, Z. L. Huang, W. Schnelle, Z. Hu and R. Kniep,

Z. Anorg. Allg. Chem., 2003, 629, 1778.36 R. Niewa, F. J. Disalvo, D. K. Yang, D. B. Zax, H. Luo and

W. B. Yelon, J. Alloys Compd., 1998, 266, 32.

Fig. 6 XRD pattern of Li7MnN4 produced by decomposition in H2

followed by subsequent nitriding.

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