Thermal analysis on the Li-Mg-B-H systems

Article abstract of DOI:10.1016/j.jallcom.2007.02.097

Thermal analyses of the mixture of MgH₂ and LiBH₄ doped with TiCl₃, and each element MgH₂ or LiBH₄ doped with TiCl₃ were performed under an inert gas flow and 0.5 MPa H₂-gas conditions. It was indicated that the hydrogen desorption reaction of MgH₂ + 2LiBH₄ to MgB₂ + 2LiH + 4H₂ phases proceeded at temperature above 400 °C under 0.5 MPa hydrogen, whereas, under an inert gas atmosphere, the reaction of the same mixture was transformed into Mg + 2B + 2LiH + 4H₂ phases in a temperature range from 350 to 430 °C. Before these reactions, the dehydrogenation of MgH₂ and the melting of LiBH₄ took place under both hydrogen and the inert gas atmospheres with increasing temperature. In addition, the molten LiBH₄ did not decompose into LiH, B and H₂ below 450 °C under 1 MPa H₂, while the LiBH₄ decomposed even below 450 °C under an inert gas atmosphere. From these results, it is deduced that the reaction producing MgB₂ is a solid-liquid reaction between solid Mg and liquid LiBH₄ above 400 °C without decomposition of molten LiBH₄ under a hydrogen atmosphere, while MgH₂ dehydrogenate. A characteristic solid-liquid reaction is realized under a hydrogen atmosphere for proceeding of the MgB₂ producing reaction in this system.

Full text of DOI:10.1016/j.jallcom.2007.02.097

Journal of Alloys and Compounds 446–447 (2007) 306–309
Thermal analysis on the Li–Mg–B–H systems
Tessui Nakagawaa, Takayuki Ichikawab,∗, Nobuko Hanadab,1
,
Yoshitsugu Kojimab, Hironobu Fujiib
a Department of Quantum Matter, ADSM, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan
b Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan
Received 28 October 2006; received in revised form 3 February 2007; accepted 14 February 2007
Available online 23 February 2007
Abstract
Thermal analyses of the mixture of MgH2 and LiBH4 doped with TiCl3, and each element MgH2 or LiBH4 doped with TiCl3 were performed under
an inert gas flow and 0.5 MPa H2-gas conditions. It was indicated that the hydrogen desorption reaction of MgH2 + 2LiBH4 to MgB2 + 2LiH + 4H2
phases proceeded at temperature above 400 ◦C under 0.5 MPa hydrogen, whereas, under an inert gas atmosphere, the reaction of the same mixture
was transformed into Mg + 2B + 2LiH + 4H2 phases in a temperature range from 350 to 430 ◦C. Before these reactions, the dehydrogenation of
MgH2 and the melting of LiBH4 took place under both hydrogen and the inert gas atmospheres with increasing temperature. In addition, the molten
LiBH4 did not decompose into LiH, B and H2 below 450 ◦C under 1 MPa H2, while the LiBH4 decomposed even below 450 ◦C under an inert gas
atmosphere. From these results, it is deduced that the reaction producing MgB2 is a solid–liquid reaction between solid Mg and liquid LiBH4 above
400 ◦C without decomposition of molten LiBH4 under a hydrogen atmosphere, while MgH2 dehydrogenate. A characteristic solid–liquid reaction
is realized under a hydrogen atmosphere for proceeding of the MgB2 producing reaction in this system.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Liquid–solid reactions; X-ray diffraction; Thermal analysis
1. Introduction
However, the dehydrogenating temperature of LiBH4 was
too high for practical use and the reversibility was not realized
because of its quite slow kinetics. At the next step, they reported
that the thermal decomposition of SiO2-doped LiBH4 has three
dehydrogenation steps on LiBH4 with increasing temperature as
follows:
Lithium borohydride (LiBH4) is one of the most attractive
candidates for hydrogen storage because of its high gravimet-
ric and volumetric hydrogen densities; about ∼18 mass% and
121 kg H2/m3, respectively [1]. The synthesis of pure LiBH4
was firstly performed in 1940 by Schlesinger and Brown [2] and
the thermal analysis of LiBH4 was performed in 1964 by Fed-
neva et al. [3]. However, this material had not been paid attention
as a hydrogen storage material until 21st Century. The first inves-
tigation of LiBH4 as a hydrogen storage material was reported
LiBH4 → LiBH4−ε + 1/2(ε)H2
(2)
(3)
(4)
ꢀꢀ
LiBH4−ε → “LiBH2 + 1/2(1 − ε)H2
ꢀꢀ
“LiBH2 → LiH + B + 1/2H2.
¨
by Zuttel et al. [4,5] in which thermal decomposition properties
Recently, many researchers have focused on this system
[6–8]. Among them, Orimo et al. reported that the rehydrogena-
tion reaction of LiH and B proceeded to form LiBH4 under
35 MPa H2-gas pressures at 600 ◦C [9].
In 2004, two independent groups developed a new hydrogen
storage system composed of LiBH4 and magnesium hydride
(MgH2), which exhibited a much better reversibility than LiBH4
itself [10–12]. The reaction is expressed as follows:
were examined. They reported that LiBH4 desorbed hydrogen
up to ∼13.5 mass% according to the following reaction:
LiBH4 → LiH + B + 1.5H2.
(1)
∗
Corresponding author. Tel.: +81 82 424 5744; fax: +81 82 424 5744.
E-mail address: tichi@hiroshima-u.ac.jp (T. Ichikawa).
Current address: Karlsruhe Research Center, Institute for Nanotechnology,
D-76021 Karlsruhe, Germany.
1
2LiBH4 + MgH2 ↔ 2LiH + MgB2 + 4H2.
(5)
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.02.097

T. Nakagawa et al. / Journal of Alloys and Compounds 446–447 (2007) 306–309
307
It is noteworthy that a finite hydrogen pressure is necessary
for the reaction of Mg with LiBH4 to yield MgB2. In contrast,
the Mg and B phases instead of the MgB2 phase are produced
after dehydrogenation under vacuum. This reaction is expressed
as follows:
The thermal analyses were performed by TG-DTA under a He
gas flow, p-DSC under ∼0.3 MPa Ar gas flow and 0.5 MPa H2-
gas in the closed system. It can be seen that the decomposition
temperature of the TiCl3-doped MgH2 is ∼280 ◦C under the He-
gas flow (Fig. 1(a)), which is almost the same as that of under
the Ar flow condition (Fig. 1(b)), irrespective of sorts of inert
gases. This result indicates that an inert gas gives no influence
on the dehydrogenation properties of MgH2. On the other hand,
in the p-DSC profile under 0.5 MPa H2 gas pressure (Fig. 1(c)),
an endothermic peak corresponding to decomposition of MgH2
is observed at higher temperature (∼370 ◦C) than that under the
inert gas flow, where the pressure rises to ∼0.6 MPa at this tem-
perature. This indicates that the hydrogen dissociation pressure
is about 0.6 MPa at ∼370 ◦C, suppressing the dehydrogenation
of MgH2 by a reverse reaction.
2LiBH4 + MgH2 → 2LiH + Mg + 2B + 4H2.
(6)
At present, it is not yet clear which role the hydrogen atmo-
sphere does play in the reaction (5). Therefore, in this paper, we
report on the results of thermal analysis for the mixture of MgH2
and LiBH4 under different atmospheric conditions. And finally,
we try to clarify the role of hydrogen in this system.
2. Experimental
For LiBH4 doped with a small amount of TiCl3, three
endothermic reactions are observed in the DTA profile under
a He gas flow (Fig. 2(a)), which is almost the same as under
an Ar flow condition as shown in Fig. 2(b). Therefore, the
inert gases do not show any differences on the dehydrogena-
tion properties of LiBH4. Here, it should be noted that the
measuring temperature was limited up to 450 ◦C, because the
sample pan of the Al metal was used in this work. For this rea-
son, unfortunately, the third reaction peak could not be clearly
observed in the DTA or DSC profiles under the inert gases, but
the endothermic reactions could be recognized above 400 ◦C
since the DSC signals are lower than the base lines. Comparing
In this work, MgH2 powder (95 mass% purity, Gelest Inc.), LiBH4 pow-
der (95 mass% purity, Sigma–Aldrich) and titanium chloride powder (TiCl3,
99.999 mass% purity, Sigma–Aldrich) were used as starting materials. Thermal
analyses were performed for MgH2, LiBH4 and a mixture of MgH2 and 2LiBH4,
in which 3 mol% TiCl3 was added as a catalyst. Prior to thermal analyses, all the
samples were ball-milled by a planetary ball-mill apparatus (Fritsch P7) for 2 h
at 400 rpm under 1.0 MPa highly pure hydrogen gas (7 N). Mixed powders of
∼300 mg and 20 pieces of steel balls with a diameter of 7 mm were loaded into
the milling container (Cr–steel pot with an internal volume of ∼30 ml). The ther-
mal analyses were examined by a pressurized differential scanning calorimetry
(p-DSC) (TA Instruments, DSC Q10P) and a thermogravimetry with differential
thermal analysis equipment (TG-DTA) (Rigku, TG8120). The measurement of
p-DSC was operated under ∼0.3 MPa Ar gas flow, or under 0.5 MPa H2 gas as
initial pressure (final pressure at 450 ◦C was ∼0.7 MPa) in a closed system, while
the TG-DTA measurement was operated under a He gas flow condition. The
heating rate was fixed at 5 ◦C/min for both measurements. The identification of
the products was carried out by X-ray powder diffraction (XRD) measurements
(Rigaku, RINT-2500, Cu K␣). All the processes from the sample preparation to
the thermal analyses were performed in Ar-filled gloveboxes with a recycling
purification system (MP-P60W, Miwa MFG Co., LTD) to avoid the sample
pollutions by water vapor and oxygen (the dew point of water was lower than
−80 ◦C and the oxygen concentration was below 1 ppm).
´
with the results reported by Fedneva et al. [3] and Soulie et al.
[13], three endothermic reactions in Fig. 2(a) and (b) correspond
to the phase transition (∼105 ◦C, orthorhombic to hexagonal),
the melting phenomenon (∼280 ◦C) and the decomposition into
LiH, B and H2 (above 400 ◦C). On the other hand, under 0.5 MPa
H2 gas pressure in Fig. 2(c), two endothermic peaks are observed
below 300 ◦C, but there is no other reaction above 300 ◦C. These
twopeaksarerecognizedatthesametemperaturesasthoseunder
the inert gas conditions, indicating that the hydrogen pressure
does not affect both the temperatures of phase transition and
melt of LiBH4. However, the third reaction above 300 ◦C disap-
pears in the DSC profile under 0.5 MPa H2 gas pressure, where
3. Result and discussion
The thermal analyses for the TiCl3-doped MgH2 and the
TiCl3-doped LiBH4 are shown in Figs. 1 and 2, respectively.
Fig. 1. TG-DTA and DSC profiles of TiCl3-doped MgH2. (a) TG-DTA profile
under a He flow, (b) DSC profile under an Ar flow and (c) DSC profile under
0.5 MPa H2 (initial pressure).
Fig. 2. TG-DTA and DSC profiles of TiCl3-doped LiBH4. (a) TG-DTA profile
under a He flow, (b) DSC profile under an Ar flow and (c) DSC profile under
0.5 MPa H2 (initial pressure).

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T. Nakagawa et al. / Journal of Alloys and Compounds 446–447 (2007) 306–309
Fig. 3. TG-DTA and DSC profiles of TiCl3-doped mixture of MgH2 and
2LiBH4. (a) TG-DTA profile under a He flow, (b) DSC profile under an Ar
flow and (c) DSC profile under 0.5 MPa H2 (initial pressure).
the pressure rises to ∼0.7 MPa at 450 ◦C. These results indicate
that the hydrogen dissociation pressure of LiBH4 is lower than
0.7 MPa at 450 ◦C, suppressing the dehydrogenation of LiBH4
by the H2 absorption process.
Fig. 3 shows a TG-DTA profile under a He gas flow and p-
DSC profiles under ∼0.3 MPa Ar gas flow and 0.5 MPa H2 gas
pressure for the TiCl3-doped mixture of MgH2 and 2LiBH4.
Under an inert gas conditions in Fig. 3(a) and (b), there are
three endothermic peaks at ∼105, ∼280 and ∼410 ◦C, respec-
tively. The two peaks below 350 ◦C are identified with similarity
to above results as the phase transition of LiBH4 (∼105 ◦C)
and superposition of the melt of LiBH4 and dehydrogenation of
MgH2 (∼280 ◦C), whereas, the endothermic peak at ∼410 ◦C
is due to the dehydrogenation of LiBH4. Actually, LiH and Mg
phases were confirmed in the X-ray profile (Fig. 4(b)) of the
product after the DSC measurement at 450 ◦C under the Ar gas
flow condition. Under 0.5 MPa H2 gas pressure in Fig. 3(c),
four endothermic reactions were observed around ∼105, ∼280,
∼370 and above 400 ◦C. The first and the second reactions (the
peaks at ∼105 and ∼280 ◦C) are due to the phase transition
and melt of LiBH4 because these peak temperatures are almost
the same as those in Fig. 2(c). The third peak temperature at
∼370 ◦C is similar to that of the peak in Fig. 1(c), so that the
third reaction is due to the decomposition of MgH2. Actually, the
Mg phase is observed in the XRD profile as shown in Fig. 4(d)
later, which was examined after heating the product up to 380 ◦C
under 0.5 MPa H2 gas pressure. Here, it should be noted that
the most important point is appearance of the fourth endother-
mic peak above 400 ◦C. This peak was neither observed in the
case of DSC measurement of MgH2 nor LiBH4 under 0.5 MPa
H2 pressure. This indicates that the dehydrogenation reaction
depends on the difference whether hydrogen pressure is applied
or not on the dehydrogenation process.
Fig. 4. XRD patterns of TiCl3-doped mixture of MgH2 and 2LiBH4. (a) After
milling, (b) after heat treatment under an inert gas flow at 450 ◦C, (c) after
heat treatment under 0.5 MPa H2 (initial pressure) at 380 ◦C and (d) after heat
treatment under 0.5 MPa H2 (initial pressure) at 450 ◦C.
we could not find any peaks originated from B related phases,
because the B peaks corresponding to single phase are too weak
to be observed in our experiment. Therefore, it was clarified that
the MgB2 was generated in the forth reaction (above 400 ◦C).
From the above results, we conclude that the reaction (5)
proceeds in hydrogen pressure without the decomposition of
LiBH4, though LiBH4 is under the molten phase at the tem-
perature above 400 ◦C with Mg. This result indicates that the
suppression of the LiBH4 decomposition by hydrogen pressure
plays the important role in the proceeding of the reaction (5).
In addition, the reaction (5) proceeds by a solid–liquid reaction
between the solid Mg phase and molten LiBH4 phase, where the
liquidphaseofLiBH4 leadstoamuchbettercontactwithMgand
the Li+ and (BH4)− ions than a solid–solid reaction. This reac-
tion is different from other typical hydrogen storage systems, so
that this system may exhibit a characteristic reaction. In addi-
tion, hydrogen gas atmosphere could bring another contribution
for proceeding of a direct reaction between condensed materials
just like Mg and LiBH4. As suggested by Hayashi et al., hydro-
gen in materials introduces some defects and reduces activation
barriers for the mobility of atoms [14]. Similarly, some defects
in the composite mixture of Mg and LiBH4 could be introduced
by exchanging hydrogen between the gas phase and condensed
phase, and the activation barriers for the mobility of atoms in the
composite mixture of Mg and LiBH4 might be reduced, possibly
yielding MgB2 and LiH in this system.
Fig. 4 shows XRD patterns of TiCl3-doped mixture of MgH2
and 2LiBH4. When a hydrogen pressure of 0.5 MPa was applied
in a dehydrogenation process, the two phases of MgB2 and LiH
were produced (Fig. 4(c)). On the other hand, when no hydrogen
pressure, suchasaninertgasatmosphere, isapplied, threephases
of Mg, B and LiH were produced (Fig. 4(b)). Unfortunately,

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309
4. Summary
References
[1] L. Schlapbach, A. Zu¨ttel, Nature 414 (2001) 353.
[2] H.J. Schlesinger, H.C. Brown, J. Am. Chem. Soc. 62 (1940)
3429.
[3] E.M. Fedneva, V.L. Alpatova, V.I. Mikheeva, Russ. J. Inorg. Chem. 9 (6)
(1964) 826.
[4] A. Zu¨ttel, P. Wenger, S. Rentsch, P. Sudan, Ph. Mauron, Ch. Emmenegger,
J. Power Sources 118 (2003) 1.
[5] A. Zu¨ttel, S. Rentsch, P. Fischer, P. Wenger, P. Sudan, Ph. Mauron, Ch.
Emmenegger, J. Alloys Comp. 356 (2003) 515.
[6] K. Miwa, N. Ohba, S. Towata, Y. Nakamori, S. Orimo, Phys. Rev. B 69
(2004) 245120.
[7] Y. Nakamori, S. Orimo, J. Alloys. Comp. 370 (2004) 271.
[8] J.K. Kang, S.Y. Kim, Y.S. Han, R.P. Muller, W.A. Goddard III, Appl. Phys.
Lett. 87 (2005) 111904.
[9] S. Orimo, Y. Nakamori, G. Kitahara, K. Miwa, N. Ohba, S. Towata, A.
Zu¨ttel, J. Alloys. Comp. 404–406 (2005) 427.
In this work, we performed the thermal analyses of the
TiCl3-doped mixture of MgH2 and LiBH4, and each element
MgH2 or LiBH4 doped with TiCl3 under an inert gas flow
and 0.5 MPa H2-gas conditions. The results indicated that the
reaction [2LiBH4 + MgH2 → 2LiH + MgB2 + 4H2] proceeded
at 450 ◦C under hydrogen atmosphere of 0.5 MPa, whereas,
the reaction [2LiBH4 + MgH2 → Mg + 2B + 2LiH + 4H2] was
induced under the inert gas atmosphere. This indicated that
MgB2 was produced by the solid–liquid reaction between solid
Mg and liquid LiBH4 at temperature above 400 ◦C, where the
molten LiBH4 phase remained without the dehydrogenation into
B + LiH + 1.5H2 under hydrogen atmosphere, while MgH2 was
dehydrogenated. Probably, a characteristic reaction between Mg
metal and (BH4)− ion was realized in this solid–liquid reaction
system.
[10] J.J. Vajo, S.L. Skeith, F. Mertens, J. Phys. Chem. B 109 (2005)
3719.
[11] J.J. Vajo, F.O. Mertens, S.L. Skeith, M.P. Balogh, International Patent
pending, Publication number: WO2005/097671 A2.
[12] G. Barkhordarian, T. Klassen, R. Bormann, International Patent pending,
Publication number: WO 2006/063627 A1, 2007.
Acknowledgement
This work was supported by the project “Development for
Safe Utilization and Infrastructure of Hydrogen Industrial Tech-
nology” of NEDO, Japan.
ˇ
[13] J-Ph. Soulie´, G. Renaudin, R. C erny`, K. Yvon, J. Alloys. Comp. 346 (2002)
205.
[14] E. Hayashi, Y. Kurokawa, Y. Fukai, Phys. Rev. Lett. 80 (1998) 5589.