First-principles calculation and x-ray diffraction simulation methods have been used to explore crystal structures and reaction mechanisms of the intermediate phases involved in dehydriding of LiBH4. LiBH4 was found to dehydride via two sequential steps: first dehydriding through LiBH, followed by the dehydriding of LiBH through LiB. The first step, which releases 13.1 wt. % hydrogen, was calculated to have an activation barrier of 2.33 eV per formula unit and was endothermic by 1.28 eV per formula unit, while the second step was endothermic by 0.23 eV per formula unit. On the other hand, if LiBH4 and LiBH each donated one electron, possibly to the catalyst doped on their surfaces, it was found that the barrier for the first step was reduced to 1.50 eV. This implies that the development of the catalyst to induce charge migration from the bulk to the surface is essential to make LiBH4 usable as a hydrogen storage material in a moderate temperature range, which is also important to stabilize the low-temperature structure of Pnma (no. 62) LiBH on dehydrogenation. Consequently, the high 13.1 wt. % hydrogen available from the dehydriding of LiBH4 and LiBH and their phase stability on Pnma when specific catalysts were used suggest that LiBH4 has good potential to be developed as the hydrogen storage medium capable of releasing the Department of Energy target of 6.5 wt. % for a hydrogen fuel cell car in a moderate temperature range

Goddard, William A., III

Han, Young Soo

Kang, Jeung Ku

Kim, Se Yun

Muller, Richard P.

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS 87, 111904 2005A candidate LiBH4 for hydrogen storage: Crystal structures and reaction
mechanisms of intermediate phases
Jeung Ku Kanga and Se Yun Kim
Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea
Young Soo Han
LG Electronics, Institute of Technology, Devices and Materials Laboratory, Seoul 137-724, Republic of
Korea
Richard P. Muller
Computational Materials and Molecular Biology, Sandia National Laboratories, Albuquerque,
New Mexico 87185-0996
William A. Goddard III
Materials and Process Simulation Center, California Institute of Technology, Pasadena
California 91125-7400
Received 17 December 2004; accepted 4 August 2005; published online 6 September 2005
First-principles calculation and x-ray diffraction simulation methods have been used to explore
crystal structures and reaction mechanisms of the intermediate phases involved in dehydriding of
LiBH4. LiBH4 was found to dehydride via two sequential steps: first dehydriding through LiBH,
followed by the dehydriding of LiBH through LiB. The first step, which releases 13.1 wt. %
hydrogen, was calculated to have an activation barrier of 2.33 eV per formula unit and was
endothermic by 1.28 eV per formula unit, while the second step was endothermic by 0.23 eV per
formula unit. On the other hand, if LiBH4 and LiBH each donated one electron, possibly to the
catalyst doped on their surfaces, it was found that the barrier for the first step was reduced to
1.50 eV. This implies that the development of the catalyst to induce charge migration from the bulk
to the surface is essential to make LiBH4 usable as a hydrogen storage material in a moderate
temperature range, which is also important to stabilize the low-temperature structure of Pnma no.
62 LiBH on dehydrogenation. Consequently, the high 13.1 wt. % hydrogen available from the
dehydriding of LiBH4 and LiBH and their phase stability on Pnma when specific catalysts were
used suggest that LiBH4 has good potential to be developed as the hydrogen storage medium
capable of releasing the Department of Energy target of 6.5 wt. % for a hydrogen fuel cell car in a
moderate temperature range. © 2005 American Institute of Physics. DOI: 10.1063/1.2042632There is great interest in small and lightweight hydrogen
storage materials.1,2 Hydrogen fuel, which can be produced
from renewable energy sources, contains a much larger
chemical energy per mass 142 MJ kg−1 than any hydrocar-
bon fuel, thus making a hydrogen fuel cell an attractive al-
ternative to the internal combustion engine for transporta-
tion. A hydrogen fuel cell car needs to store at least 4 kg
hydrogen to cover the same range as a gasoline-powered
car.
1 On the other hand, to store this hydrogen at room tem-
perature and atmospheric pressure requires such a large vol-
ume that corresponds to a balloon with a 4.5 m diameter,
which is hardly a practical volume for a small vehicle. To
reduce this problem, one could consider using liquid hydro-
gen for hydrogen storage since it has a high mass density1
70.8 kg m−3. However, to liquefy hydrogen requires expen-
sive processes due to its low condensation temperature1
−252 °C at 1 bar. An additional problem is that heat trans-
fer through the available modern containers can result in a
loss of up to 40% of the energy content in hydrogen.3 Cur-
rently, in this respect, there is much interest in storing hydro-
gen on advanced carbons and lightweight metals. Dillon et
al.4 reported that 6–8 wt. % hydrogen was stored in single-
walled nanotubes SWNTs. However, controversial
results5,6 have been reported concerning the true hydrogen
aElectronic mail: jeungku@kaist.ac.kr
0003-6951/2005/8711/111904/3/$22.50 87, 11190
Downloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject tostorage capacity on advanced carbons. Hirscher and his
coworkers5 argued against Dillon’s report by showing that
titanium hydrides in the SWNT stored large amounts of hy-
drogen, but the hydrogen storage capacity on SWNT itself
was less than 1 wt. %. Likewise, Chen et al.6 showed that the
increase in the hydrogen storage capacity by alkaline metal-
doped carbon NTs was attributed to the formation of metal
hydrides. Putting all of these results together, one could say
that the reversible hydrogen content on the pristine SWNT
still does not satisfy the Department of Energy DOE target1
of 6.5 wt. %.
A promising alternative for hydrogen storage is the metal
hydride system. However, there are still considerable chal-
lenges to meet the requirements1,7 for practical applications
that include 1 increasing the maximum weight percent of
reversible hydrogen to 6.5 wt. %, 2 increasing the maxi-
mum hydrogen capacity per volume to 62 kg m−3, and 3
improving the rate for adsorption/desorption to 1–3 g s−1. In
the search for metal hydrides with higher than hitherto
known gravimetric storage capacities, LiBH4, with light ele-
ments Li and B, has been considered for this purpose due to
its high available 18.5 wt. % hydrogen. The recent experi-
mental x-ray diffraction XRD study8 and theoretical predic-
tions by Miwa et al.9 and Ge et al.10 indicate that LiBH4 is
the most stable when it has orthorhombic symmetry. On the
other hand, detailed knowledge about the mechanisms of re-
© 2005 American Institute of Physics4-1
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
111904-2 Kang et al. Appl. Phys. Lett. 87, 111904 2005moving hydrogen and adding hydrogen has not yet been re-
ported. However, we consider that this knowledge can be
obtained by using density functional theory DFT11,12 calcu-
lations, as they allow accurate determination of crystal struc-
tures and their energies.
All calculations were performed on the PW91 method13
using Vanderbilt pseudopotentials14 with a cutoff energy of
270 eV. First, we considered the B32 Fd-3m structure for
LiB and perform the total energy calculations using 270 eV
9880 k-points and 300 eV cutoff energy 37 671 k-points.
Their total energies per formula unit were found to differ by
only 2.51 kJ/mol, which indicates that the 270 eV cutoff en-
ergy is somehow optimal in terms of both being able to re-
duce the considerably expensive computational cost by using
much larger cutoff energies and also being reasonably accu-
rate in their total energies. The set of k-points used to expand
the wave functions was based on the Monkhorst-Pack
scheme.15 In addition, we used Gillan’s thermal broadening
scheme16 with broadening energies of 0.2 eV. All electronic
structure calculations for optimizing geometries were carried
out using the CASTEP17 program, and simulated XRD18 pat-
terns were determined through simulations of d-spacing, 2,
and intensities for hkl reflections at the optimized geom-
etries.
First, we determined the lowest-energy structure for LiB
through geometry optimizations of several types: namely, the
Fd-3m, I41/a, Pnma, Cmc21, P63mc, and P21/c space
group types. The Pnma structure is determined as the lowest-
energy structure for LiB, with the P21/c structure 0.39 eV,
the Cmc21 structure 0.83 eV, the Fd-3m structure 1.14 eV,
the I41/a structure 2.82 eV, and the P63mc structure 4.38 eV
per formula unit higher. The optimized lattice parameters for
Pnma LiB are obtained as a=6.41 Å, b=3.01 Å, and c
=5.60 Å with =90.0°, =90.0°, and =90.0°, in which the
B–B distance of 1.52 Å and the B–B–B angle of 165.0° sug-
gest that the B–B bonding has a certain character of sp hy-
bridization.
LiBH4 consists of BH4
− anions spaced by Li+ counter
ions. As an initial guess for LiBH4, we used the Fd-3m
LiBH4 structure, but its simulated XRD was found to dis-
agree with the experimental hkl reflections.8 The hkl reflec-
tions from I41/a LiBH4 were also predicted to disagree with
experimental hkl reflections. On the other hand, the simu-
lated XRD see Fig. 1a peaks at the wavelength 
=0.486 Å from orthorhombic LiBH4 Fig. 2a were found
to agree with the experimental patterns. The strongest XRD
peak was 011 with 2=7.66°, and the ratio of the second
strongest 101 peak with 2=5.63° to the strongest peak
was 92.26%, while the third strongest 200 peak with 2
FIG. 1. Simulated XRDs from Pnma and P63mc LiBH4 at the wavelengths
=0.486 Å and =0.949 Å, respectively: a Pnma and b P63mc. Diffrac-
tion angles are 2 values.=7.80° had a ratio of I200 / I011=83.41%. The Pnma
Downloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject tostructure was the lowest-energy structure for LiBH4 having
predicted lattice parameters of a=7.14 Å, b=4.29 Å, and c
=6.85 Å with =90.0°, =90.0°, and =90.0°, which com-
pared to experimental values:8 a=7.18 Å, b=4.44 Å, and c
=6.80 Å with =90.0°, =90.0°, and =90.0° see Table I.
For Pnma LiBH4, each B had four H tetrahedrally coordi-
nated to form BH4
− with the B–H bond distance of
2.21 to 2.22 Å and the H–B–H bond angle of 109.3° to
110.6°, while the Li–H distance was 2.05 to 2.13 Å. Addi-
tionally, the total energies for Cmc21, P63mc, and P21/c
LiBH4 were determined through full geometry optimizations,
but the Cmc21 structure was 2.13 eV, the P63mc structure
was 0.31 eV, and the P21/c structure was 0.16 eV per for-
mula unit energetically less stable than the Pnma LiBH4.
We also determined energies and structures for the plau-
sible intermediate phases such as Li3BH6, LiBH2, LiBH, and
LiB. Li3BH6 is described in terms of three Li+ ions surround-
ing each BH6−3 molecule see Table II. We found that
Li3BH6 was the most stable in the R-3 phase, but that the
dehydriding of LiBH4 through Li3BH6 was endothermic by
3.57 eV per formula unit, while those through the lowest-
energy structures of Pnm LiBH and Pnma LiBH2 were en-
dothermic by 1.28 and 2.89 eV, respectively, per formula
unit. Thus, these results suggest that the dehydriding of
LiBH4 through LiBH is the most preferable reaction. The
reaction barrier of 2.33 eV per formula unit for the dehydrid-
ing of LiBH4 to LiBH was also obtained from the geom-
etries, as seen in Fig. 2, where the H–H bond distance for a
desorbing H2 is 1.463 Å. However, it should be noted that a
high temperature 400 K would be needed to obtain a
practical amount of released hydrogen from LiBH4 due to its
relatively high barrier of 2.33 eV. On the other hand, the
low-temperature structure of Pnma LiBH might be unstable
at this high temperature. Consequently, dehydrogenation of
LiBH4 and hydrogenation of LiBH should be “reversible”
only if the dehydrogenation temperature could be reduced to
the extent that Pnma LiBH is stabilized. The recent experi-
mental study by Vajo et al.19 showed that LiBH4 could be
developed as reversible hydrogen storage when specific cata-
lysts were used. We also found that the barrier was reduced
to 1.50 eV when LiBH4 donated one electron, possibly to the
catalyst doped on the hydrogen storage material surface,
FIG. 2. Color online Intermediate structures involved in dehydriding of
LiBH4. a LiBH4, b the transition state connecting LiBH4 to LiBH, and
c LiBH. Li and B atoms are shown in dark gray and green, respectively,
while H atoms except desorbing H atoms in blue are shown in white. Ener-
gies eV per formula unit are not to scale, where the barrier in parenthesis
is determined from their ionized structures.where the H–H bond distance of a desorbing H2 is 1.159 Å. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
111904-3 Kang et al. Appl. Phys. Lett. 87, 111904 2005The 1.50 eV barrier was compared to the 1.62 eV obtained
experimentally from SiO2-catalyzed LiBH4 by Zuttel and his
co-workers.20 In this respect, we can conclude that the devel-
opment of the catalyst to induce charge migration from the
bulk to the surface is essential to lower the activation barrier
and the temperature for dehydrogenation of LiBH4, thus also
to stabilize Pnma LiBH on dehydrogenation. Moreover, an-
other good finding was that the 13.1 wt. % hydrogen was
available from dehydriding of LiBH4 through LiBH.
In summary, metal hydride systems such as LiAlH4 and
NaAlH4
21,22
with high hydrogen storage capacities are suf-
fering from formation of intermediate structures having dif-
ferent phases on dehydrogenation, which results in slow ki-
netics on dehydrogenation. In this respect, the phase stability
of LiBH4 and LiBH on the Pnma structure when specific
catalysts were used and the 13.1 wt. % hydrogen available
from dehydriding of LiBH4 to LiBH are very important as-
pects. Consequently, we conclude that the catalyst-doped
LiBH4 could be a very attractive hydrogen storage material
capable of releasing the DOE target1 of 6.5 wt. % for a hy-
TABLE I. Atomistic details and energies per formula unit for the lowest-en
Phase
Atomic Coordinates
x /a ,y /b ,z /c
LiBH4a Li: 0.16 0.25 010,
B: 0.30 0.25 0.42
H: 0.91 0.25 0.94, 0.39 0.25 0.27, 0.20 0.02
Li3BH6c Li: 0.95 0.22 0.31,
B: 0.00 0.00 0.00, 0.00 0.00 0.50
H: 0.85 0.83 0.10, 0.65 0.48 0.43
LiBH2a Li: 0.24 0.25 0.14,
B: 0.45 0.25 0.42
H: 0.07 0.25 0.94, 0.51 0.25 0.23
LiBHa Li: 0.24 0.25 0.21
B: 0.45 0.25 0.51
H: 0.75 0.25 0.96
LiBa Li: 0.26 0.25 0.24
B: 0.50 0.25 0.52
aOrthorhombic phase Pnma space group of no. 62. Total energies
P21/c−333.656 eV, P63mc −333.514 eV, I41/a −333.154 eV, Fd-3m
bReaction energy calculated without zero-point and thermal corrections.
cHexagonal phase R-3 space group of no. 148.
TABLE II. Summary of reaction energies and barriers per formula unit from
this work.
Reaction Energy eV Barrier eV
Thermal decomposition of LiBH4
LiBH4a→1/3Li3BH6b+2/3Bc+H2 3.57d
LiBH4a→LiBHa+3/2H2 1.28d 2.33 1.50e
Thermal decomposition of LiBH
LiBHa→LiBa+1/2H2 0.23d
aOrthohombic phase Pnma space group of no. 62.
bHexagonal phase R-3 space group of no. 148.
cFace-centered cubic lattice.
dReaction energy calculated without zero-point and thermal corrections.
eReaction barrier calculated without zero-point and thermal corrections. The
value in parenthesis is the barrier determined from the ionized unit cells.
Downloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject todrogen fuel cell car in a moderate temperature range.
This research was supported by General Motors Global
Alternative Proposal Center.
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structures of LiBH4, Li3BH6, LiBH2, LiBH, and LiB from this work.
Lattices
a ,b ,cÅ , ,°
Energy
eV
a=7.1 −333.816b
b=4.3
c=6.9
===90
a=b=7.4 −744.316
c=8.8
==90, =120
a=8.1, −299.208
b=3.0
c=5.9
===90
a=6.2, −284.979
b=3.0,
c=6.3
===90
a=6.4 −268.901
b=3.0
c=5.6
===90
formula unit of other space group types for LiBH4 are also given:
.577 eV, and Cmc21 −331.687 eV.ergy
0.42
per
−332121, 10623 2004.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


A candidate LiBH4 for hydrogen storage: Crystal structures and reaction mechanisms of intermediate phases

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A candidate LiBH4 for hydrogen storage: Crystal structures and reaction mechanisms of intermediate phases

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