Direct preparation of LiBH4 from pre-treated LiH + B mixture at high pressure

Article abstract of DOI:10.1016/j.jallcom.2010.09.215

LiBH₄ was prepared from LiH + B mixture at 300-500 °C with a hydrogen pressure of 35 MPa. The LiH + B mixture was pretreated by ball milling under 10 MPa hydrogen pressure for 10 h. The results showed that ball-milling treatment is favorable for the formation of LiBH₄, which could reduce the reaction temperature for about 200 °C. A small quantity of LiBH ₄ could be found in the pre-treated sample. The formation of LiBH₄ from LiH + B mixture is a kinetically hindered progress and a diffusion-control reaction. LiBH₄ formed at 400 °C could release 6.59 wt.% H₂ when it was heated from room temperature to 500 °C. The yield of LiBH₄ is about 59.4%.

Full text of DOI:10.1016/j.jallcom.2010.09.215

Journal of Alloys and Compounds 509 (2011) 3481–3485
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Direct preparation of LiBH4 from pre-treated LiH + B mixture at high pressure
Rugan Chen, Xinhua Wang∗, Lou Xu, Hui Li, Changpin Chen, Lixin Chen, Hongge Pan
Department of Materials Science and Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, Zhejiang Province, China
a r t i c l e i n f o
a b s t r a c t
LiBH4 was prepared from LiH + B mixture at 300–500 ◦C with a hydrogen pressure of 35 MPa. The LiH + B
mixture was pretreated by ball milling under 10 MPa hydrogen pressure for 10 h. The results showed
that ball-milling treatment is favorable for the formation of LiBH4, which could reduce the reaction
temperature for about 200 ◦C. A small quantity of LiBH4 could be found in the pre-treated sample. The
formation of LiBH4 from LiH + B mixture is a kinetically hindered progress and a diffusion-control reaction.
LiBH4 formed at 400 ◦C could release 6.59 wt.% H2 when it was heated from room temperature to 500 ◦C.
The yield of LiBH4 is about 59.4%.
Article history:
Received 27 April 2010
Received in revised form 23 August 2010
Accepted 6 September 2010
Available online 10 December 2010
Keywords:
Hydrides
LiBH4
© 2010 Elsevier B.V. All rights reserved.
Hydrogen storage materials
Complex hydrides
1. Introduction
from practical application. Therefore, many efforts have focused
on incorporating additives, such as metals, metal halides, oxides,
Hydrogen is an environmentally friendly synthetic fuel, but the
storage remains a challenge especially for mobile applications. To
find efficient and economic methods for storing hydrogen is critical
to realizing the hydrogen economy. The US Department of Energy
(DOE) is supporting research to demonstrate viable materials for
on-board hydrogen storage. The DOE goals are focused on achiev-
ing a storage system of 6 wt.% H2 and 45 kg H2 m−3 by 2010 and
developing a system reaching 9 wt.% H2 and 81 kg H2 m−3 by 2015
[1,2].
Nanostructured carbon materials [3] and complex hydrides are
a new class of lightweight materials for hydrogen storage. Since
the first publication on the reversible Ti catalyzed NaAlH4 and
Na2LiAlH6 by Bogdanovic´ and Schwickardi [4] in 1997 complex
hydrides gained a lot of attention in the hydrogen community. One
of the primary classes of materials currently under consideration
for solid-state on board hydrogen storage material, are ionic com-
pounds often with group 1 or 2 cations (e.g., Li, Na, and Mg), and an
anionic “complex” of light metals and hydrogen (e.g., [BH4]− and
[AlH4]−). Interest in low-Z complex hydrides, such as metal boro-
hydries [5–7], Mn+(BH4)n, and metal alanates, Mn+(AlH4)n, stems
from their inherent ability to contain large amounts of hydrogen
by weight (up to 18.5 wt.% in LiBH4).
sulfides, hydrides, nanoporous scaffolds to thermodynamically
destabilize LiBH4 toward lowered desorption temperatures. How-
ever, reversibility of LiBH4 gains little attention. In chemical
industry, LiBH4 is usually synthesized by a wet chemical reac-
tion method using diethyl ether or isopropylamine as solvent, by a
metathesis of NaBH4 with Li halides or chlorides [8,9]. As organic
solvent has side-effect on products and is perilous to handle, a
cleaner and more efficient way to regenerate LiBH4 is in great need.
The practical use of such metal complexes is limited due to both
thermodynamic and kinetic deficiencies, and reversibility of those
metal complexes have not yet been well resolved. For example,
LiBH4 can be formed according to reaction (1) under a pressure of
35 MPa hydrogen and 600 ◦C [10]:
LiH + B + (3/2)H2 → LiBH4
(1)
The enthalpy and entropy for reaction (1) is calculated to be
ꢀrH0 = 66.6 kJ mol−1 H2 and ꢀrS0 = 97.4 J K−1 mol−1 H2, respec-
tively [11]. From a thermodynamic point of view, hydrogen
absorption in LiH with boron forming LiBH4 should be possible
at 1 bar hydrogen pressure and room temperature, and conclu-
sively the reaction is simply kinetically hindered [12]. Thus, more
activated mixture of LiH and B may result in a lower reaction tem-
perature.
In particular, LiBH4 with a theoretical gravimetric hydrogen
density of 18.5 wt.% is a very interesting material, nevertheless
the thermodynamic, kinetic and reversibility deficiencies limit it
In this paper, LiBH4 is prepared via the pre-activated
mixture of LiH and
To achieve such high hydrogen pressure,
B
under 35 MPa H2 and 300–500 ◦C.
a
a double stage
hydrogen compressor is used in our work which can pro-
duce high-pressure ultrapure hydrogen with a pressure over
70 MPa and purity of 99.9999%, the working mechanism of
∗
Corresponding author. Tel.: +86 571 8795 2716; fax: +86 571 8795 2716.
E-mail address: xinhuawang@zju.edu.cn (X. Wang).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.09.215

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R. Chen et al. / Journal of Alloys and Compounds 509 (2011) 3481–3485
Fig. 2. The XRD patterns of pre-treated LiH + B composite maintained at different
temperatures and at a hydrogen pressure of 35 MPa for 10 h.
Fig. 1. The XRD pattern of LiH + B mixture after ball-milling for 10 h at a hydrogen
pressure of 10 MPa, SiO2 was used to calibrate the position of diffraction peaks.
vate the mixtures rather than just remove inert outer layer of the
reactants.
the compressor has been described in our previous work
[13].
Generally speaking, the break of B–B bond and the forma-
tion of B–H bond are the reaction barrier to reaction (1) [12].
Based on this consideration, additives that will react with boron
to form instable intermediates received considerable attraction. It
has been reported that LiBH4 can be formed from AlB2 and LiH at
moderate temperature and hydrogen pressure due to the low sta-
bility of AlB2, but LiBH4 is absent when AlB2 is replaced by boron
under the same condition [14]. The enthalpy of formation of AlB2
is calculated to be 16 kJ mol−1 by density functional calculations
[15]. However, the stability of the LiBH4/Al system is comparable
to that of pure LiBH4. The small destabilization due to the AlB2
can even be compensated at higher temperatures by an entropy
2. Experimental details
The initial materials, LiBH4 (16949-15-8, 95%), LiH (7580-67-8, 98%) and B
(7440-42-8, 95–97%) were purchased from Aldrich Chemical and used as received.
Influence of impurities in the initial materials on the final experimental results was
not taken into consideration. To prevent samples and raw materials from under-
going oxidation and/or hydroxide formation, they were stored and handled in
an argon-filled glovebox (Mbraun MB200B). LiH and B with a molar ratio of 1:1
and a total weight of about 1.5 g were placed into a stainless vessel with steel
balls. The ball-to-powder ratio was 60:1. The vessel was first evacuated and then
filled with 10 MPa hydrogen and ball-milled for 10 h. Finally, the treated sample
was introduced to a high pressure reactor and heated to target temperatures, and
maintained the temperature constantly under a hydrogen pressure of 35 MPa for
10 h.
decrease of ꢀS = − S0(Al) − (1/2)S0(B) + S0(AlB2) = − 14 J mol−1 K−1
.
Hence the thermodynamic driving force is similar for both systems
[14,16]. So the enabled hydrogenation using AlB2 instead of B is
probably a kinetic effect, which indicates improving the kinetics
properties is the key task in synthesizing LiBH4. Many previous
studies [17–19] on high-energy ball milling of metallic materials
have shown that considerable enthalpies can be stored in the ball-
milled metals. The stored enthalpy can be as high as 5–10 kJ mol−1
and has been attributed to the presence of high-density structural
defects and a large area of grain boundaries. Atoms at the defect
sites and grain boundaries have higher energies than those located
at the perfect lattice sites, and thus have higher activity. The acti-
vation energy will be decreased if the transformation from LiH–B
to LiBH4 starts from these active sites.
The reaction among LiH, B and hydrogen is a solid–solid–gas
reaction. From a kinetic point view, the reaction suffers both
from inert boron and solid diffusion. In this paper, high energy
ball-milling is adopted as a pre-treatment, which has several
advantages. Ball-milling makesthe particles of thecomplexes much
smaller; and induces mechanical-alloying effect on LiH and B to
reduce solid diffusion pathway; eliminates passivation layer of LiH
and B powder and deoxidizes the surface oxides. These reasons are
responsible for the decrease of reaction temperature and formation
of a small quantity of LiBH4 during ball milling under high pressure
as shown in Fig. 1. Similar results had also been reported by Agresti
and Khandelwal [20] and C¸ akanyıldırım and Gürü [21].
The thermal gas desorption properties of the mixtures were determined by ther-
mogravimetry (TG) analyzer (Netzsch STA449F3) upon heating to 500 ◦C at a heating
rate of 5 ◦C/min. The flow rate of argon (99.99% purity) was maintained at 40 mL/min
during the entire heating process. Structure analysis was carried out using a Rigaku
D/max-3B X-ray diffractometer using Cu K␣ radiation at room temperature. The
XRD measurements were carried out by using a specially designed sample holder to
prevent the contact of the sample with air. The hydriding/dehydriding tests on the
prepared samples (∼0.5 g) were carried out on a Sievert’s type apparatus.
3. Results and discussion
Fig. 1 shows the XRD pattern of LiH + B mixture after ball-milling
for 10 h at a hydrogen pressure of 10 MPa, which reveals that a
small quantity of LiBH4 was formed during ball-milling process.
Fig. 2 shows the XRD patterns of the samples formed at 300 ◦C,
400 ◦C and 500 ◦C, respectively. It can be seen from Fig. 2 that LiH
still abound in large quantity for the sample formed at 300 ◦C. As
reaction temperature increases, the diffraction intensity of LiBH4
increases. The samples formed at 400 ◦C and 500 ◦C contain LiBH4,
minor LiH and B. XRD patterns of both samples are quite the same
to each other, which indicated higher reaction temperatures have
little beneficial effect. So we draw a conclusion that LiBH4 can
be directly formed from LiH and B under 400 ◦C and a hydrogen
pressure of 35 MPa, which is about 200 ◦C lower than the value
reported in literature [10]. As no catalyst was applied, ball milling
treatment under high hydrogen pressure should be the key factor
which brings down the reaction temperature. Moreover, our previ-
ous experiments revealed that pre-treatment under Ar atmosphere
could not achieve the same results (not included here). Therefore,
ball-milling pre-treatment under high hydrogen pressure may acti-
When solid reactants have a particle shape as in the present
study, the kinetics of gas/solid and liquid/solid reactions can nor-
mally be investigated based on a shrinking-core model [22,23].
Possible governing steps that control the reaction kinetics of the

R. Chen et al. / Journal of Alloys and Compounds 509 (2011) 3481–3485
3483
Fig. 3. The transformed fraction of LiBH4 as a function of reaction time. The hydrogen
absorption capacity is deduced from the hydrogen pressure reduction of the reactor
as a function of reaction time.
shrinking-core model include (i) diffusion of a gaseous species
through the product layer, (ii) movement of the phase boundary,
(iii) nucleation and growth of the product, or (iv) desorption or
absorption of the gaseous phase at he surface of the solid particle
[24]. In the case of diffusion-control reactions, the transformed frac-
tion, f, as a function of reaction time, t, in an isothermal experiment
can be expressed as
k11/2
r
(1 − f )1/3 = 1 −
t1/2
(2)
where r is the original average radius of the particles, and k1 is
a rate constant. A plot of (1 − f)1/3 vs t1/2 should be linear if the
hydrogenation is indeed controlled by hydrogen diffusion.
If the formation of LiBH4 is controlled by the movement of the
LiH–B/LiBH4 phase boundary, the transformed fraction as a func-
tion of reaction time should be [22]
k2
(1 − f )1/3 = 1 −
t
(3)
r
where k2 is a rate constant.
For the nucleation/growth-controlled reaction, the rate equa-
tion would be [25]
f = 1 − exp(−k3tm
)
(4)
Eq. (4) is known as a Johnson–Mehl–Avrami (JMA) equation, where
m is a numerical exponent whose value can vary from 1 to 4 depend-
ing on the detail of nucleation and growth of LiBH4. k3, on the other
hand, depends on the nucleation and growth rates and is therefore
very sensitive to temperature. Eq. (4) could be rewritten as
ꢀ
ꢁ
ꢂꢃ
f
ln ln
= ln k3 + m ln t
(5)
1 − f
Fig. 3 shows the experimental results of transformed fraction
of LiBH4 as a function of reaction time at 400 ◦C. The pre-treated
composite absorbs about 8.26 wt.% of hydrogen at 400 ◦C, which
means the yield of LiBH4 is about 59.4%. And Fig. 4 shows the curves
of (1 − f)1/3 vs t1/2, (1 − f)1/3 vs t and ln(ln(f/(1 − f))) vs ln t in which
the linear equation obtained via curve fitting through the least-
squares method and the R-squared value is included. From Fig. 4,
we can obtain that R2 = 0.979, so Eq. (2) can be rewritten as
Fig. 4. The transformed fraction of LiBH4, f, as function of the reaction time, t, shown
in Fig. 3 is re-plotted in a chart with the axes of (a) (1 − f)1/3 vs t1/2, (b) (1 − f)1/3 vs t
and (c) ln(ln(f/(1 − f))) vs ln t. The straight lines and their linear equation obtained via
curve fitting of the experimental data along with the R-squared value are included
for comparison.
(1 − f )1/3 = 1.08725 − 0.00227t1/2
,
with R2 = 0.979
(6)
For the case of rate-controlling step is the movement of the
LiH–B/LiBH4 phase boundary,
The same approach can be used to examine other possible
rate-controlling mechanisms. The obtained equations and their R-
squared values are listed below.
(1 − f )1/3 = 0.98911 − 1.14995 × 10−5t, with R2 = 0.931
(7)

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R. Chen et al. / Journal of Alloys and Compounds 509 (2011) 3481–3485
tion of LiBH4 at 500 ◦C. On the other hand, the TG curve reveals that
the decomposition of LiBH4 is a multi-step process. The reaction
equation could be the one proposed by Orimo et al. [26] as follows:
LiBH4 + (1/2)Li2B12H12 + (5/6)LiH + (13/12)H2
→ LiH + B + (3/2)H2
(9)
for the D2/D1 (D2 and D1 stand for the hydrogen desorption
capacities of the second and the first stage, respectively) data of
experiment (which is 1.59) is close to that of theoretical calcula-
tion (which is 1.39) according to Eq. (9). The endothermic peak at
about115 ◦CofDSC resultcan beassignedtothe polymorphic trans-
formation of LiBH4. The second endothermic peak at about 262 ◦C
corresponds to the melting of LiBH4. Large exothermic peak ranges
from 270 ◦C to 450 ◦C is assigned to the decomposition of LiBH4
or/and other borides. The decomposition of LiBH4 is supposed to be
an endothermic process by Orimo et al. [10]. However, as the hydro-
genation is not fully achieved, some by-products (borides) may be
formed synchronously which are not detected yet. In fact, thermal
properties of some metal complexes are variable. For example, an
exothermic peak around 290 ◦C in succession to an endothermic
one was found in the DSC curve of MgH2 + 2LiBH4 system by Zhang
et al. [27] while no exothermic peaks were observed by Bösenberg
et al. [28].
Fig. 5. TG/DSC curves of LiBH4 formed at 400 ◦C. The heating rate is 5 ◦C/min.
For the case of rate-controlling step is nucleation and subse-
quent growth of LiBH4,
ꢀ
ꢁ
ꢂꢃ
1
1 − f
ln ln
= −6.99628 + 0.67418 ln t, with R2 = 0.965
(8)
4. Conclusions
LiBH4 is formed from a pretreated LiH + B mixture at 300–500 ◦C
with a hydrogen pressure of 35 MPa. The results show that
high-energy ball milling treatment under high hydrogen pres-
sure could bring down the synthesizing temperature of LiBH4 as
ball milling treatment introduces high-density structural defects
and a large area of grain boundaries which are favorable to
improve the reaction kinetics, on the other hand, the LiH/B
mixture may be well activated upon pre-treated under high hydro-
gen pressure. The formation of LiBH4 through LiH and B is a
diffusion-controlled hydrogenation process and the transformed
fraction as a function of reaction duration can be expressed as
(1 − f)1/3 = 1.08725 − 0.00227t1/2. The yield of LiBH4 at 400 ◦C is
around 59.4% and the synthesized LiBH4 can release hydrogen of
6.59 wt.% in two steps.
Comparisons among these relationship and their R-squared val-
ues indicate that the diffusion-controlled mechanism provides the
best explanation for the observed reaction kinetics. First, the R-
squared value of Eq. (6) is close to 1, which indicates the presence
of a good linear relationship between (1 − f)1/3 and t1/2. Second,
the first term on the right-hand side of Eq. (6) is nearly equal to
1, which is one of the key requirements matching the theoretical
equation derived from the underlying physical process. In contrast,
analyses of fitting line equations of phase-boundary-controlled
equation and nucleation/growth-controlled equation do not lead
to satisfactory results. For the phase-boundary-controlled hydro-
genation Eq. (7), the R-squared value deviates greatly from 1. For the
nucleation/growth-controlled hydrogenation Eq. (8), its m value
equals 0.67418, which cannot be explained by the physical process
of the JMA equation. Although the JMA equation allows m values of
0.5 or 1 for a constant number of nuclei and a one-direction growth,
and we consider 0.67418 as the approximation of 1 or 0.5, then Eq.
(8) represents a hydrogenation process with LiBH4 nucleated at
the beginning of the reaction, and the subsequent transformation
is controlled by the one-direction growth of LiH/B composite into
LiBH4 core, therefore the hydrogenation kinetics will be slow and
the product may have dendrite structure. However, a large amount
of LiBH4 is formed within 90 min as shown in Fig. 3 and the as-
prepared LiBH4 is observed to have fine morphology (which is not
shown in this paper).
In conclusion, it should be pointed out that the present anal-
ysis only reveals that diffusion is the rate-limiting step for solid
state hydrogenation of the LiH + B mixture. Whether hydrogenation
proceeds with one or multiple elementary steps cannot be derived
from this kinetics analysis. In fact, the mixture contains two solid
reactants and hydrogen diffusion in either reactant or the necessary
diffusion between LiH and B may be the rate-limiting factor.
Fig. 5 shows TG/DSC curves of LiBH4 formed at 400 ◦C. It is
clear that the LiBH4 formed at 400 ◦C decomposed by two steps.
About 2.54 wt.% weight loss occurred from 270 ◦C to 370 ◦C, another
4.05 wt.% weight loss began at about 390 ◦C. A total weight loss of
about 6.59 wt.% is achieved when heated to 500 ◦C at a heating rate
of 5 ◦C/min. The difference between maximum absorption capacity
and desorption capacity attributes to the incomplete decomposi-
Acknowledgments
This work was supported by National Natural Science Foun-
dation of China (Nos. 50671094 and 50631020) and National
Basic Research Program of China (Nos. 2007CB209706and
2010CB631304).
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