Indian Journal of Pure & Applied Physics
Vol. 51, May 2013, pp. 372-375
Electrical transport in Li2SO4-Li2O-B2O3 glass-ceramic composites
Munesh Rathore & Anshuman Dalvi*
Physics Department, Birla Institute of Technology and Science, Pilani 333 031, Rajasthan
*E-mail: [email protected]
Received 10 January 2013; revised 25 February 2013; accepted 3 April 2013
Lithium ion conducting glass-ceramic composites have been synthesized in Li2SO4-Li2O-B2O3 system by annealing the
glass above its crystallization temperature. The electrical, structural and thermal characterization of these glass-ceramics
reveals interesting results. The conductivity of the glass-ceramic increases with Li2SO4 content and exhibits a maximum of
~ 10−4 at 200°C interestingly for a composition 1Li2SO4-99(0.67Li2O-0.33B2O3). The glass-ceramic samples are found to be
thermally more stable than those of the glassy samples.
Keywords: Glass-ceramics, Ionic conductivity, Crystallization, Lithium borate glasses
1 Introduction
Fast ion conducting glasses have drawn
considerable attention due to their potential
application as solid electrolyte in solid state ionic
devices1-4. A glassy structure is preferable for the fast
ion transport in solids mainly due to (i) high ionic
conductivity owing to liquid like structure,
(ii) isotropic nature and (iii) highly non-crystalline
structure that enables negligible electronic
conductivity5. As a result, large number of
superionic glasses have been developed and
also used successfully as electrolytes in solid state
batteries6.
The lithium ion conducting glasses have the most
promising candidature as solid electrolytes in all solid
state lithium batteries7-9.To enhance the ionic
conductivity in these glasses, two strategies have been
used. The first one is by using mixed glass formers
where more than one glass former is normally used to
increase the mobility. In the second one, Li+ ion salt
(Li2SO4, LiI) is added to the glass matrix to increase
the number of mobile lithium ions10,11.
Several investigations have been carried out on
lithium silicate, lithium phosphate and lithium borate
glasses to which lithium halides and lithium oxy-salts
have been added10-12. Thus, there are many interesting
studies on salt- Li2O-MxOy type system (salt = LiCl,
Li2SO4 etc). Addition of LiCl is found to affect
electrical and structural properties of Li2O-SiO2-B2O3
glassy sysytem10-12. In a system xLi2SO4- 42.5Li2O(57.5-x)B2O3, notable rise in the conductivity11 is
observed till x = 15 m/o. The incorporation of Li2SO4
in the macromolecular network is responsible for such
a conductivity rise and decrease of glass transition
temperature11-15.
Though glasses exhibit high ionic conductivity,
their stability is limited due to the glass transition and
crystallization. Thus glass-ceramics have recently
drawn considerable attention due to their thermal
stability16,17. These are expected to be thermally more
stable than the glasses and thus are likely to be better
candidates as electrolytes in bulk and thin film solid
state batteries.
In the present study, we focus on the preparation
and characterization of Li+ ion oxide glass-ceramics.
In 67Li2O-33B2O3, the effect of Li2SO4 addition on
fundamental structural, thermal, and electrical
properties is discussed.
2 Experimental Details
The glasses of compositions xLi2SO4-(100-x)
[0.67Li2O-0.33B2O3] with x varying from 0 to 5 m/o
were synthesized by conventional melt-quenching.
Glasses were obtained using high-purity of powders
Li2CO3, H3BO3, and Li2SO4. Appropriate mixture of
these materials was taken in porcelain crucibles and
heated to 450ȠC for 2 h to remove CO2 and H2O from
Li2CO3, H3BO3. The mixture then melted at
900-950ȠC for 30 mins and subsequently pressed
between two copper plates. From these glasses, the
glass-ceramics were obtained by annealing at 600°C
for 6 h followed by slow cooling at room temperature.
For the structural characterization, the X-ray
diffraction
(RIGAKU
MiniFlex
II,
X-Ray
diffractometer) and scanning electron microscopy
(Ziess ultra-60 Field emission type) were used. Glass
RATHORE & DELVI: ELECTRICAL TRANSPORT IN GLASS-CERAMIC
transition and crystallization temperatures were
obtained by Differential scanning calorimetry
(DSC-60, Shimadzu). The electrical conductivity was
measured using computer controlled HIOKI LCR
meter model 3532-50 and Libratherm programmable
PID temperature controller.
3 Results and Discussion
The XRD patterns for glasses as well as glassceramics were obtained for various compositions.
Figure 1 shows the XRD patterns for sample with
x = 1 and 5 m/o Li2SO4 content. Fig. 1 (a and c)
shows the XRD results for as prepared melt quench
glassy samples. The absence of any prominent Bragg
peak in Fig.1 (a) confirms the amorphous/glassy
nature of the sample, whereas for the sample with 5
m/o there exist prominent peaks correspond to Li2SO4
that may have remained undissolved in glass matrix.
Further, Fig. 1(b and d) shows the XRD patterns of
glass-ceramic samples (annealed at 600°C).
Appearance of significant peaks confirms the
existence of crystallites viz., Li4B2O5 and Li2SO4 in
the glass matrix. Using debye-scherrer relation, size
of crystallites is calculated to be ~ 22-30 nm.
To understand the microstructural changes during
crystallization, scanning electron microscopy (SEM)
images of the glasses and glass-ceramics were
obtained for various compositions. Figure 2 shows
SEM images of 67Li2O-33B2O3 composition for (a) as
prepared glassy film and (b) glass-ceramic sample at a
resolution of 200 nm. For the glassy film, a good
homogeneity is seen on the surface. However, for the
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glass-ceramic, apparent growth of rod shaped textured
crystallites surrounded by connective tissues of the
glass matrix (Fig. 2b) is noticed.
In order to understand the thermal stability of the
glasses and glass-ceramics, differential scanning
calorimetry (DSC) scans have been performed at a
typical heating rate of 10°C/min and shown in
Figs 3 and 4, respectively.
As apparent in Fig. 3(a), the glassy sample (with no
Li2SO4 content) exhibits a glass transition at ~ 350°C.
Fig. 2 — SEM images of 67Li2O-33B2O3 composition (a) glass,
and (b) glass-ceramic composite
Fig. 1 — X-ray diffraction patterns for samples: (a) as prepared
glass of composition 1Li2SO4-99(0.67Li2O-0.33B2O3), (b) glassceramic composite of composition 1Li2SO4-99(0.67Li2O0.33B2O3), (c) as prepared 5Li2SO4-95(0.67Li2O-0.33B2O3) and
(d) glass-ceramic composite of composition 5Li2SO495(0.67Li2O-0.33B2O3)
Fig 3ʊ DSC scans at a heating rate of 10°C/min for composition
67Li2O-33B2O3: (a) glass; (b) glass-ceramic composite
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INDIAN J PURE & APPL PHYS, VOL 51, MAY 2013
Interestingly, the sample exhibits multiple (at least
two) crystallization at 396° and 442°C. Thus, the
presence of Tg and Tc in DSC scans suggests that
sample is purely glassy in nature and may have
different surroundings (at least two) about Li+ ions.
Fig. 3(b) shows DSC pattern for the glass-ceramic
sample of the same composition. Apparently, subtle
peaks appear at Tp1~ 385°C and Tp2 ~ 425°C.
Fig. 4 shows the similar DSC patterns for glass and
glass-ceramic samples with composition 1Li2SO499(0.67Li2O-0.33B2O3). In Fig. 4(a), the Tg and Tc
followed by two merged-up crystallization peaks are
seen at 287° and 330°C, respectively, which confirms
the glassy nature. In case of the glass-ceramic sample
of the same composition, no significant thermal event
is observed [Fig. 4(b)] till 500°C.
From Figs. 3 and 4, it may be suggested that
(i) addition of very little amount of Li2SO4 reduces the
Tg and Tc of the glass significantly (ii) thermal events
are not prominently seen in glass-ceramics and thus
they appear to be thermally more stable and (iii) XRD
and DSC results are found to be in good agreement
with each other.
Further, to investigate the thermal events,
conductivity-temperature cycles are obtained at
controlled heating rate of 1 ȠC/min and scrutinized in
the thermally unstable region, i.e, T > Tg and T > Tc.
Fig. 5 shows the electrical conductivity as a function
of temperature for glasses as well as for glass-ceramic
samples of composition xLi2SO4-(100-x)(0.67Li2O0.33B2O3) with x = 0 and 1.
For clarity, the whole ı-T cycles are divided in
three regions, viz. I, II and III. Figure 5 (a and b)
shows ı-T plot for glassy samples. The conductivity
exhibits an Arrhenius behaviour for both the glassy
compositions as shown (region I) reversible up to
~ 300°C. Interestingly, in region II, an anomalous rise
Fig. 4 — DSC scans at a heating rate of 10°C/min for the
composition 1Li2SO4-99(0.67Li2O-0.33B2O3): (a) glass, and
(b) glass-ceramic composite
in conductivity is observed at temperature 318° and
304°C, respectively, for both samples. On further
increase in temperature, the conductivity falls
drastically due to massive crystallization in multiple
stages. In region III, the crystallization completes and
the conductivity again increases linearly as a function
of temperature which once again confirms the ionic
nature of the glass-ceramic samples formed during
crystallization.
The conductivity temperature behaviour for glassceramic samples of the two compositions is shown in
Fig. 5 (c and d). For both the samples, conductivity
exhibits an Arrhenius behaviour as can be seen in
region I. In addition, both the samples exhibit a small
but notable fall in the conductivity at 370°C possibly
due to crystallization of the left over glassy state in
glass-ceramic samples. However, the fall in conductivity
at crystallization is drastically suppressed.
Fig. 5 — Temperature dependence of the conductivity samples:
(a), (b) glasses; (c), (d) glass-ceramic composite
Fig. 6 — Electrical conductivity (200°C) and activation energy for
glass and glass-ceramic as function of Li2SO4 content
RATHORE & DELVI: ELECTRICAL TRANSPORT IN GLASS-CERAMIC
For both, glasses as well as glass-ceramics, the
highest conductivity and minimum activation energy
is obtained for the composition 1Li2SO4-99(0.67Li2O0.33B2O3) as shown in Fig. 6. For x > 1 m/o of Li2O4
the samples are partially crystalline (as suggested by
XRD) and hence, conductivity decreases.Thus, it may
be suggested that low Li2SO4 content samples do
receive Li+ ions form the added salt. Further added
Li2SO4 does not dissolve in glass matrix, as a result
conductivity decreases. Thus, it is also expected that
the overall disorder in glass-ceramics also decreases
for x > 1 m/o of Li2SO4 and thus similar trend is seen
in the composition dependence of conductivity of
glass-ceramics.
4 Conclusions
The glass-ceramic samples exhibit pure ionic
transport and good thermal stability. The addition of
very small amount of Li2SO4 in 67Li2O-33B2O3
system enhances the ionic conductivity. The highest
conductivity for the glassy sample as well as glassceramic sample is found to be 10−4 and 10−6 Ω−1cm−1,
respectively, at 200°C for the composition 1Li2SO499(0.67Li2O-0.33B2O3). The XRD results do infer
that, annealing the glassy samples results into
precipitation of tiny crystallites in glassy matrix. SEM
results confirm the composite nature of the glassceramic samples. Li2SO4 addition also found to be
suppressing the crystallization. These glass-ceramics
may be promising candidate for high temperature
electrolytic applications with engineering of the
battery components. Efforts are on to further enhance
the ionic conductivity of Li+ ion conducting glassceramics in our laboratory.
375
Acknowledgement
This work is supported by DST FIST and UGC
Special assistance programme. We sincerely thank
Dr Dinesh Deva and Barkha Awasthi of Nanoscience
laboratory (IIT-Kanpur, India) for scanning electron
microscopy measurements.
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