Core shell structure for solid gas synthesis of LiBDengineering.snu.ac.kr/pdf/2010(10)/2010_OF_Core shell structure for... · Received 22nd December 2009, Accepted 12th February 2010

Core shell structure for solid gas synthesis of LiBD4

O. Friedrichs,aJ. W. Kim,

bcA. Remhof,

aD. Wallacher,

dA. Hoser,

dY. W. Cho,

c

K. H. Ohband A. Zuttel

a

Received 22nd December 2009, Accepted 12th February 2010

First published as an Advance Article on the web 17th March 2010

DOI: 10.1039/b927068j

The formation of LiBD4 by the reaction of LiD in a diborane/hydrogen atmosphere was

analysed by in situ neutron diffraction and subsequent microstructural and chemical analysis

of the final product. The neutron diffraction shows that nucleation of LiBD4 already starts at

temperatures of 100 1C, i.e. in its low temperature phase (orthorhombic structure). However,

even at higher temperatures the reaction is incomplete. We observe a yield of approximately

50% at a temperature of 185 1C. A core shell structure of the grains, in which LiBD4 forms

a passivation layer on the surface of the LiD grains, was found in the subsequent

microstructural (electron microscopy) and chemical (electron energy loss spectrometry) analysis.

Introduction

In 2003, borohydrides (M[BH4]x) were proposed as new

hydrogen storage materials.1 Among borohydrides LiBH4

has one of the highest gravimetric and volumetric hydrogen

densities, which exceeds even the ones of gasoline. However,

the hydrogen absorption and desorption mechanism is not yet

understood in detail, and high pressures and temperatures are

required for its formation.

Already in 1953, Hermann I. Schlesinger and Nobel laureate

Herbert C. Brown discovered the important role of diborane

in the formation of borohydrides,2 when they published their

study of the synthesis routes for borohydrides. The synthesis

was based on chemical reactions of diborane with metal

hydrides in a solvent (e.g., diethyl ether). The role of the

solvent was to bring the diborane into contact with the hydride

and more importantly, to dissolve the product, borohydride

formed at the surface of the binary hydride. Schlesinger et al.

claimed that the solvent was mandatory for the preparation.

In our recent investigations we showed, in contradiction to

what was claimed by Schlesinger et al., the synthesis of LiBH4

by a solvent free method.3 By heating LiH in a diborane/

hydrogen atmosphere we were able to synthesize LiBH4 at

150 1C. However, the yield was limited to about 50% and similar

experiments to synthesize other borohydrides as Mg(BH4)2 and

Ca(BH4)2 by this method were not successful. Only by milling

the corresponding metal hydrides in a diborane/hydrogen

atmosphere,4 we succeeded to synthesize LiBH4, Ca(BH4)2 and

Mg(BH4)2 in an almost pure and solvent-free method.

In the present work the results of the investigation of the

reaction of LiD with diborane by in-situ neutron diffraction

are presented. The origin of the incomplete reaction was

analysed by microstructural and chemical characterization of

the resulting product.

Experimental

The synthesis of LiBD4 from LiD and B2D6 was carried out in

a custom made, cylindrical Inconel container (id 10 mm,

length 50 mm), developed and constructed by the DEGAS

laboratory of the Helmholtz Centre for Materials and Energy

(HZB) in Berlin, Germany. A schematic sketch is presented in

Fig. 1.

The container consists of two identical compartments, the

lower one filled with LiD (Sigma-Aldrich), the upper one with

a ball milled mixture of Li11BD4 (Katchem) and ZnCl2(Sigma-Aldrich) in a stoichiometric ratio of 5:2 as diborane

source.4 Milling results in the formation of LiZn2(BD4)5,5

which is known to emit diborane and hydrogen when heated

above 85 1C according to the following reaction:

LiZn2(BD4)5 - 2Zn + 2LiD + 5B2D6 + 4D2 (1)

Fig. 1 Schematic sketch of the custom made Inconel sample con-

tainer. The lower compartment is filled with LiD, the upper one with

a milled mixture of Li11BD4 and ZnCl2 forming LiZn2(BD4)5 as borane

source.

a Empa, Materials Science & Technology, Department ofEnvironment, Energy and Mobility, Div. Hydrogen and Energy,CH-8600 Dubendorf, Switzerland.E-mail: [email protected]; Fax: +41-44 823 4153;Tel: +41-44 823 4022

bDepartement of Materials Science and Engineering,Seoul National University, Seoul 151-742, Republic of Korea

cMaterials Science and Technology Research Division,Korea Institute of Science and Technology, Seoul 136-791,Republic of Korea

dHelmholtz Centre Berlin for Materials and Energy GmbH,Glienicker Strasse 100, 14109 Berlin, Germany

4600 | Phys. Chem. Chem. Phys., 2010, 12, 4600–4603 This journal is �c the Owner Societies 2010

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

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The two parts of the container were separated by a sinter filter

(mesh size: 0.5 mm), suppressing intermixing of the powders

but enabling gas exchange. The quantities of LiD and milled

Li11BD4 and ZnCl2 were selected to ensure a large excess of

emitted diborane during the reaction. The sample container

was filled and sealed in a glove box under inert helium atmos-

phere. To control the temperature, the sample container

was placed into a high-temperature furnace (HTF) which

was finally mounted on the sample stage of the focusing

diffractometer E6 of the HZB. The diffractometer is equipped

with a horizontally and vertically bent monochromator con-

sisting of 105 pyrolytic graphite crystals (20 � 20 � 2 mm)

mounted on a 15 � 7 matrix. The wavelength of 2.445 A was

chosen to maximize the neutron flux at the sample position

(5� 106 neutrons cm–2 s–1). During the experiment, the sample

was heated from rt to 185 1C and the diffraction pattern of

the LiD containing part of the container was recorded

consecutively.

The microstructure of the sample was investigated by secondary

electron imaging, induced by ion or electron irradiation using

a double (ion and electron) beam focussed ion beam (FIB,

FEI, Nova 200). For the chemical composition analysis of the

sample, transmission electron microscopy-electron energy loss

spectroscopy (TEM-EELS) is introduced to detect the light

elements (i.e. Li and B). A cross-sectional TEM sample

was obtained from the specific interest region of the final

product by FIB equipped with a manipulating probe

(100.7t, Omniprobe). A TEM sample preparation process

using FIB and air-lock loading chamber without air-exposure

are described in the previous report.7 Using this technique,

TEM sample which has a final thickness of E50 nm and a

large observation area (10 � 5 mm2) was obtained. The pre-

pared TEM sample was loaded into a 200 keV TEM (FEI,

Tecnai F20) using a portable glove-bag under Ar atmosphere

(99.999%).

Results and discussion

In an in-situ neutron powder diffraction experiment LiD is

heated stepwise from room temperature to 185 1C in diborane/

hydrogen atmosphere, while monitoring the diffraction pattern.

Fig. 2 shows the neutron diffraction pattern measured at

different temperatures.

Heating the cell to 85 1C leads to the decomposition of

LiZn2(BD4)5 and to the formation of the diborane/hydrogen

atmosphere. At this stage of the experiment, the diffraction

pattern of the lower part of the sample container remains

unchanged. The intensities of the LiD reflections6,7 as well

as the background stay constant, no new reflections appear.

Then we heated the sample container to 100 1C. Already

at this temperature, the diborane released from the source

reacts with LiD to form LiBD4 according to the following

reaction:

LiD + 1/2B2D6 - LiBD4 (2)

The corresponding Bragg reflections of the low temperature

phase of LiBH48 can be clearly identified in the diffraction

pattern. Heating to higher temperatures leads to a phase

transition, i.e. to the formation of the high temperature phase

of LiBD4.9,10 The reaction proceeds until about 50% of the

LiD has reacted. Then the reaction stops and further heating

for several hours at the final temperatures of 185 1C has no

more influence on the sample. This is in agreement to our

former studies, where we observed that only about 50% of

LiH is converted to LiBH4 while exposing to diborane at a

temperature of 150 1C.3

In order to analyse the origin of the incomplete transfor-

mation a microstructural study of the final product is carried

out using dual beam FIB. Fig. 3 shows secondary electron

images of the final product which are induced by ion beam

irradiation with an acceleration voltage of 30 keV at a working

distance of 19.5 mm. The corresponding cross-sectional view

of Fig. 3a is prepared by FIB milling treatment using Ga+ ion

beam11 and displayed in Fig. 3b.

The LiBD4 particle shows a glazed, smooth morphology

on the outside with a size range of 30 to 100 mm. In the cross-

sectional view, a core shell structure with a shell thickness of

about 3 mm is clearly observed.

In order to analyse the chemical compositions of the final

product using TEM-EELS, a uniformly thin TEM sample is

prepared by an FIB milling process. Fig. 4a and Fig. 4b

show an intermediate stage of the sample preparation, and a

Fig. 2 Selected diffraction pattern from the reaction of LiD and B2D6

observed by in situ neutron diffraction. The sample is heated from rt to

185 1C and the assigned phases correspond to LiBD4 high temperature

phase9,10 (�), LiBD4 low temperature phase8 (|), LiD6,7 (J) and Al

(+) from the sample holder.

Fig. 3 Secondary electron images (induced by 30 keV ion beam) of

LiD after reaction with diborane showing a grain (a) with its corres-

ponding cross sectional view (b) of a core shell structure.

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prepared final TEM sample, respectively. Both secondary

electron images induced by electron beam are obtained with

an acceleration voltage of 5 keV.

In Fig. 4b, two different contrast regions are clearly observed,

which implies that the two regions have different chemical

compositions apart from the platinum (Pt) protective layer on top.

The sample is transferred from FIB to TEM and the chemical

compositions are analysed by TEM-EELS with 200 keV

operating voltage. The spectra from Li (K edge at 51 eV)

and B (K edge at 188 eV) absorption are displayed in Fig. 4c

and Fig. 4d, respectively. The outer region of the particle

showing the darker contrast shows signals originating from Li

and B, while the inner region contains Li but no B. This is

explained by the formation of LiBD4 on the surface of LiD

forming a core shell structure observed by the ion beam

analysis displayed in Fig. 3.

Fig. 5 shows a schematic illustration of the core shell

formation during the synthesis of LiBD4.

LiD reacts on the surface with diborane and forms LiBD4.

A surface layer of LiBD4 is formed and grows to a certain

thickness. Then the reaction stops and leaves a core shell

structure with LiD in the interior and LiBD4 on the outside.

For the formation of LiBD4 either boron in the form of a B–H

species has to diffuse to the interior passing the already formed

layer of LiBD4 or Li has to diffuse to the exterior. Thereby the

overall charge neutrality of the reaction has to be conserved.

We explain the reason for the incomplete formation by a

limited diffusion of either species with the increasing thickness

of the LiBD4 layer. Fig. 5 illustrates one possible reaction

mechanism in which BH4� ions are diffusing into the interior

while D� ions are diffusing to the exterior. In this mechanism

the reaction is limited by the diffusion of BH4� and D� species

through the LiBH4 layer. A mechanism based on Li diffusion is

favored by the high mobility of Li even at low temperatures.12

With less mobile elements such as Mg and Ca no formation of

the corresponding borohydride could be observed under similar

experimental conditions.

Conclusions

LiBD4 forms at the surface of LiD in a diborane/hydrogen

atmosphere. The reaction already starts in the temperature

range of the orthorhombic phase of LiBD4 and stops after

about 50% of LiD is consumed for the formation of LiBD4. A

core-shell structure of lithium hydride surrounded by lithium

borohydride is observed. The reaction stops most probably

due to diffusion problems of either B–H species into the grain

or Li towards the exterior. The results are in agreement with

the passivation layer proposed by Schlesinger et al., who

synthesized different borohydrides in solvents in order to

prevent the formation of the passivation layer. It also explains

the new method to synthesize borohydrides by milling metal

hydrides in diborane atmosphere.4 By the milling procedure

the passivation layer is broken and further reaction is enabled

as we presented on the solvent free synthesis of LiBH4,

Ca(BH4)2 and Mg(BH4)2.

Acknowledgements

Financial support from the 6th Framework Program of the

European Commission (NESSHY Contract No.: 518271), the

Swiss National Science Foundation (SNF-Project 200021-

119972/1), the Swiss Federal Office of Energy and the integrated

project of ICC-IMR is gratefully acknowledged. We would also

like to thank for financial support from the European Commis-

sion through the Key Action: Strengthening the European

Research Area, Research Infrastructure (contract number

RII3-CT-2003-505925).

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4602 | Phys. Chem. Chem. Phys., 2010, 12, 4600–4603 This journal is �c the Owner Societies 2010

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