Determination of the role of Li2O on the corrosion of lithium hydride

Article abstract of DOI:10.1016/j.jallcom.2013.02.046

Lithium hydride (LiH) will efficiently react with moisture, forming lithium hydroxide (LiOH) on the surface. Typically, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy is used for studying the surface corrosion reaction. The presence of a surface lithium oxide (Li₂O) layer will enhance hydroxide formation kinetics; however, interference in the DRIFT spectrum prevents the role of Li₂O on the reaction kinetics from being fully understood. In the current study, Raman spectroscopy has been used to follow the reaction of LiH with moisture, with particular focus on the Li₂O vibrational signature. Three vibrations were observed for Li₂O after thermal decomposition of LiOH on the LiH surface in contrast to the single vibration at 515 cm^-1 for pure Li₂O powder. The multiple peaks are indicative on multiple Li₂O chemical domains and are likely the substrate through which unstable LiOH domains are formed during subsequent hydrolysis of the LiH/Li₂O system.

Full text of DOI:10.1016/j.jallcom.2013.02.046

Journal of Alloys and Compounds 580 (2013) S271–S273
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Determination of the role of Li2O on the corrosion of lithium hydride
⇑
Adalis Sifuentes, Ashley C. Stowe , Norm Smyrl
Y-12 National Security Complex, Oak Ridge, TN, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 5 March 2013
Lithium hydride (LiH) will efficiently react with moisture, forming lithium hydroxide (LiOH) on the
surface. Typically, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy is used for studying
the surface corrosion reaction. The presence of a surface lithium oxide (Li2O) layer will enhance hydrox-
ide formation kinetics; however, interference in the DRIFT spectrum prevents the role of Li2O on the reac-
tion kinetics from being fully understood. In the current study, Raman spectroscopy has been used to
follow the reaction of LiH with moisture, with particular focus on the Li2O vibrational signature. Three
vibrations were observed for Li2O after thermal decomposition of LiOH on the LiH surface in contrast
to the single vibration at 515 cmꢀ1 for pure Li2O powder. The multiple peaks are indicative on multiple
Li2O chemical domains and are likely the substrate through which unstable LiOH domains are formed
during subsequent hydrolysis of the LiH/Li2O system.
Keywords:
Raman
Lithium hydride
Hydrolysis
Published by Elsevier B.V.
1. Introduction
Li2O surface of LiH or the hydrogen evolution rate. In these studies,
it was postulated that water diffusion through the Li2O layer medi-
Lithium hydride (LiH) has received a great deal of interest in
recent years due to its potential as a battery, hydrogen storage fuel,
and nuclear energy [1–5]. It is a light hydrogenous material that
will efficiently react with moisture, releasing hydrogen gas and
heat in the formation of lithium hydroxide (LiOH) (Eq. (1)). The
kinetics of this reaction has been previously investigated [6–11];
however, some questions remain regarding the nature of LiOH
formed on the surface of a bulk LiH solid and, in particular, the role
of lithium oxide (Li2O) on the moisture reaction. Li2O forms
through a reaction of LiH and LiOH, releasing more hydrogen gas
(Eq. (2)).
ates the reaction and that the separation of the LiOH layer from LiH
results in an enhanced interfacial layer with a more exothermic
hydrogen release potential. Raman spectroscopy has previously
been shown to be an effective tool to interrogate the LiH chemical
system for measuring all of the chemical constituents of the LiH
and moisture reaction [17,18]. Stowe et al. showed that excitation
with an ultraviolet (UV) laser provides sufficient fluorescence
reduction and Raman scattering intensity to measure small surface
quantities of LiOH and Li2O [17]. Until the present study, no study
reported had investigated the role of the Li2O layer by interrogat-
ing the Li2O chemical environment directly. Raman spectroscopy
is herein used to simultaneously follow Li2O and LiOH under both
moisture exposure and thermal decomposition regimes.
LiH þ H2O ! LiOH þ H2ðgÞ
LiH=LiOH ! Li2O þ H2ðgÞ
LiH=Li2O þ H2O ! LiOH:
:
ð1Þ
ð2Þ
ð3Þ
:
2. Experimental
Moisture exposure studies were conducted on powder samples of LiH and Li2O
with a TA Instruments Dynamic Vapor Sorption attachment for the Q5000-EM TGA
instrument. Both powders were sieved to isolate particles of 125–200 lm. Into a
It has been reported that subsequent exposure of the LiH solid
with an oxide surface coating to moisture results in hydrolysis
again (Eq. (3)); however, the LiOH formed on the Li2O surface has
some unstable domains, which contain more liable hydrogen
[6,10,11]. In studies of the unstable LiOH species, diffuse reflec-
tance infrared Fourier transform (DRIFT) spectroscopy [12–15],
evolved gas analysis [6], or Rutherford backscattering spectroscopy
[16] was used to understand the multiple LiOH domains on the
dried platinum crucible was placed 3–5 mg of each material. A calibrated stream
of moist argon gas was passed over the sample inside the TGA at a flow rate of
100 cc/min at constant moisture levels of 1%, 25%, and 50% relative humidity for
4 h. Thermal decomposition of the LiOH corrosion layer after moisture exposure
was conducted on a TA Instruments Q500 TGA, which offers a higher temperature
furnace. Samples were heated isothermally at 300 °C for 4 h to decompose all LiOH
on the sample surface.
A Renishaw RX-500 UV Raman spectrometer (325 nm) and an NXR FT-Raman
module attached to a Thermo-Nicolet 6700 Fourier transform infrared (FTIR) spec-
trometer were used for ex situ analysis of the reactions after both moisture exposure
and thermal treatment.
⇑
Corresponding author. Tel.: +1 865 241 0675.
E-mail address: stoweac@y12.doe.gov (A.C. Stowe).
0925-8388/$ - see front matter Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.jallcom.2013.02.046

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A. Sifuentes et al. / Journal of Alloys and Compounds 580 (2013) S271–S273
Subsequent thermal decomposition of the corroded LiH/LiOH
sample resulted in a slight mass loss due to reaction of LiOH with
LiH, which resulted in the release of hydrogen gas (Eq. (2)). A sur-
face layer of Li2O formed as expected; however, the Raman spec-
trum of baked LiH/LiOH (LiH/Li2O sample) did not show the
single Li2O vibration at 515 cmꢀ1. As Fig. 2 shows, three vibrations
were observed at 515, 540, and 565 cmꢀ1. The two new vibrations
are from different Li2O chemical environments on the LiH surface.
These multiple vibrations for Li2O have not previously been ob-
served. There is a large lattice mismatch between LiH and Li2O,
such that as Li2O grows at the decomposition interface of LiOH
and LiH, multiple domains are required to relieve some of the lat-
tice strain. Some domains will be highly crystalline, whereas others
will have missing lattice crystallinity. Dinh et al. showed that for
thick Li2O layers formed during thermal decomposition on the
LiH surface, blisters and cracks in the Li2O layer appear to release
the stress. Under those conditions, it is expected that the lattice
vibrations would be different from the bulk because of defective
or missing crystalline bonding sites [19]. A similar phenomenon
has been observed for LiOH vibrations in the ternary system where
LiH/Li2O was exposed to moisture (LiH/Li2O/LiOH sample); how-
ever, since Li2O has never been specifically probed with a vibra-
tional spectroscopic tool, the Li2O precursor had not been
observed. This strained Li2O is quite likely the cause of the unstable
LiOH phases.
Fig. 1. Comparison of mass gained during moisture exposure of LiH and Li2O
powder.
A final test scenario was prepared by exposing the decomposed
LiH/Li2O sample to a moist atmosphere. This experiment aimed to
replicate the findings of multiple LiOH vibrational states related to
unstable phases which exhibit greater hydrogen outgassing poten-
tial. Fig. 3 shows the Raman spectrum of the LiH/Li2O/LiOH sample
after moisture exposure. Reference spectra of LiOH and LiOHꢁH2O
standards are also included. It is clear from the Raman spectrum
that a new vibration is observed in the LiH/Li2O/LiOH sample that
is not represented in the LiOH or LiOHꢁH2O standards.
It is interesting to note that moisture exposure to pure Li2O
powder did not result in formation of unstable LiOH domains. Only
when Li2O was grown on a LiH surface during thermal decomposi-
tion of the LiH/LiOH system were the conditions appropriate for
unstable LiOH to be formed during moisture. With this new under-
standing of the LiH/Li2O/LiOH system, it is possible to develop a
conditioning program that would transform LiOH into a Li2O layer,
which minimizes multiple domains and thereby minimizes subse-
quent hydrogen outgassing potential from the hydroxide layer
formed from moisture exposure. Perhaps a high-temperature sin-
tering step could be considered after the initial thermal decompo-
sition of the LiOH corrosion layer.
Fig. 2. The Raman spectrum of Li2O formed during the thermal decomposition of
LiH/LiOH is shown as well as Li2O and LiOH standards.
3. Results and discussion
LiH and Li2O powder reacted favorably with moisture at room
temperature. As Fig. 1 shows, the reaction rate for LiH was mea-
surably faster than Li2O, as expected. At 50% RH, the LiH sample
increased in mass by 40% while the Li2O sample gained only
13%. The reaction of LiH with water (Eq. (1)) results in twice
the mass increase compared to Li2O with water. It should be
noted that this Li2O reaction with water is similar to Eq. (3); how-
ever, in this TGA experiment, pure Li2O powder was reacted
rather than LiH with a small surface Li2O layer. As such, if the
reaction rate was similar, a 20% mass increase is expected for
the Li2O sample with 50% RH under the experimental conditions
employed. The trend is similar at lower moisture contents. A LiOH
layer of a few microns was grown during exposure. Subsequent
Raman spectroscopy indicated
a single LiOH vibration at
3670 cmꢀ1 on both samples similar to pure LiOH powder. The sin-
gle LiOH vibration is expected for the corroded LiH sample; how-
ever, it has been previously reported that multiple vibrational
signatures are present indicating LiOH grown on a Li2O surface
[6,10,11]. In these studies, Li2O was an interfacial layer between
LiH and the reacting water, which limited the reaction rate via
diffusion.
Fig. 3. Raman spectrum of LiOH formed during moisture corrosion of LiH with an
intermediate Li2O layer as well as LiOH and LiOHꢁH2O standards.

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4. Conclusions
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Raman spectroscopy is the ideal tool to investigate the role of
Li2O in the moisture reaction with LiH. For the first time, multiple
domains of Li2O have been observed; they are representing slightly
different chemical sites within the lattice on the LiH surface. These
sites form during thermal decomposition of surface LiOH corrosion
on the hydride surface. This fact is evident in the observation of
three Li2O vibrations in the Raman spectrum. Subsequent exposure
to water vapor results in propagation of the strained Li2O lattice.
The LiOH layer forms interfacial with the Li2O layer rather than
LiH itself. Strained LiOH on the incomplete Li2O lattice is therefore
responsible for the unstable LiOH domains, which are represented
by the new LiOH vibration in the Raman spectrum and result in
more readily released hydrogen than LiOH powder or a LiOH
surface layer of LiH in the absence of a Li2O intermediate surface.
Acknowledgments
Funding was provided by the Plant Directed Research and
Development program at the Y-12 National Security Complex,
managed by B&W Y-12, LLC, for the US Department of Energy
under Contract DE-AC05-00OR22800.
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