Effect of preparative parameters on the electrical conductivity of Li2SO4-Al2O3 composites

Article abstract of DOI:10.1016/S0025-5408(01)00698-5

The effect of various preparative parameters, such as the size and form of alumina and also the time of sintering, on the electrical conductivity of the Li₂SO₄-Al₂O₃ composite system has been investigated. The sintering time appears to be an insignificant preparative parameter. The role of different phases of Al₂O₃ on the electrical conductivity of the composite clearly establishes that the maximum enhancement is achieved for γ-Al₂O₃. The 50 m/o Al₂O₃ composition was found to exhibit the highest conductivity, an enhancement of three orders of magnitude at 500°C. The experimental data indicates higher conduction in the space charge layer near the surface to be the possible mechanism of conductivity enhancement.

Full text of DOI:10.1016/S0025-5408(01)00698-5

Materials Research Bulletin 37 (2002) 2043±2053
Effect of preparative parameters on the electrical
conductivity of Li2SO4±Al2O3 composites
P. Gopalana,*, S. Bhandaria, A.R. Kulkarnia, V.R. Palkarb
aDepartment of Metallurgical Engineering and Materials Science, Indian Institute
of Technology, Powai, Mumbai 400076, India
bMaterials Research Group, Tata Institute of Fundamental Research,
Homi Bhabha Road, Colaba, Mumbai 400005, India
(Refereed)
Received 10 July 2000; accepted 16 May 2001
Abstract
The effect of various preparative parameters, such as the size and form of alumina and also
the time of sintering, on the electrical conductivity of the Li2SO4±Al2O3 composite system has
been investigated. The sintering time appears to be an insigni®cant preparative parameter. The
role of different phases of Al2O3 on the electrical conductivity of the composite clearly
establishes that the maximum enhancement is achieved for g-Al2O3. The 50 m/o Al2O3
composition was found to exhibit the highest conductivity, an enhancement of three orders of
magnitude at 5008C. The experimental data indicates higher conduction in the space charge
layer near the surface to be the possible mechanism of conductivity enhancement.
# 2002 Elsevier Science Ltd. All rights reserved.
Keywords: A. Composites; A. Interfaces; B. Sol±gel chemistry; C. X-ray diffraction; C. Impedance
spectroscopy; D. Ionic conductivity
1. Introduction
In recent times, heterogeneous doping has been employed to enhance the con-
ductivity of solid electrolytes. It involves the dispersal of submicron insulating
particles in the host matrix, thereby forming a composite. In general, in most of
these systems, it is also believed that no chemical reaction is found to occur between
* Corresponding author.
E-mail address: pgopalan@met.iitb.ac.in (P. Gopalan).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 6 9 8 - 5

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P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
the matrix and the dispersoid. Following the discovery of the phenomenon by Liang in
1973 [1], enhancements in conductivity, between one and three orders of magnitude,
have been effected for a number of composite systems by the dispersal of an
insulating second phase [2±6].
A number of theoretical models on this subject have evinced suf®cient attention
[6±14], and continue to be worked upon. It is widely accepted that unlike homo-
geneous doping, local deviation from electroneutrality plays an important role in
heterogeneous doping [7].
Various mechanisms have been reviewed in literature in recent times to explain the
``composite effect'' [6,7,12]. The interface mechanisms involve one or more of the
following scenarios: space charge layer formation at the matrix±particle interface,
enhanced conduction at the core of the interface, or an interfacial phase formation and
effects of adsorbed surface moisture and impurities. By a similar token, the matrix
mechanisms, on the other hand, involve enhanced charge transportation along grain
boundaries and dislocations, the stabilization of highly conducting metastable phases
due to the addition of the dispersoid, or a homogeneous doping of the matrix.
As for the host system, the focus has changed from past studies involving mainly
halides of alkali metals or silver to recent investigations predominantly on sulfates,
carbonates, nitrates, tungstates, and other salts of alkali metals [15±22]. These
investigations have led to new features in the conductivity±composition and con-
ductivity±temperature behavior. Increasingly, materials like BaTiO3 and other
ferroelectric phases with the perovskite structure [18±22] are being used as dis-
persoids. Much of the literature, however, comprises of work involving alumina
(in the g-Al2O3 form) and silica, though, to a lesser extent. Surprisingly, very
recently, we have reported that even the use of a-Al2O3 as a dispersoid has been
responsible for a signi®cant enhancement in conductivity of Na2SO4±Al2O3
composite system [23].
Of greater interest among the alkali sulfates is the high-temperature form of lithium
sulfate that offers an exceptionally large conductivity. Lithium sulfate exists at room
temperature in a monoclinic form and undergoes a phase transition at 5758C to a fcc
structure (Fm3m) having a high degree of rotational disorder. The ionic conductivity
of the high-temperature phase, a-Li2SO4, is of the order of ꢀ1 OÀ1 cmÀ1 at 6108C
[24]. Forland and Krogh-Moe [25] have investigated the structure of a-phase and
found that a molten sublattice-type mechanism is responsible for the large con-
ductivity in a-Li2SO4.
Thus far, there has been only a single investigation on the Li2SO4±Al2O3
composite system. It has been established clearly by Zhu et al. [26] that the
Li2SO4±Al2O3 composites exhibit much higher conductivity relative to pure Li2SO4.
However, the role of the form of alumina, size, and sintering characteristics remain
undetermined.
In the present work, we have experimented with the role of the preparatory
parameters and the role of the phase of Al2O3. Through a ®tting of various existing
theoretical models to our experimental data, we have also aimed to identify a possible
mechanism of conductivity for this system. This work also unambiguously establishes

P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
2045
the effect of the phase and content of Al2O3 on the conductivity of Li2SO4±Al2O3
composite. While investigating the role of preparative parameters, we have been able
to analyze the effect of factors like the time and temperature of sintering, as well as the
size and form of alumina on the electrical conductivity of the composite.
2. Experimental
Lithium sulfate was procured from Loba Chemie (India) and Aldrich (Milwau-
Ê
kee, WI, USA). By employing the sol±gel process, very ®ne size alumina (ꢀ40 A)
was prepared in the laboratory. Initially, ultra®ne Boehmite, AlO(OH) was synthe-
sized by the action of deionized water on amalgamated aluminum. A stable aqueous
suspension (sol) was formed by adding a few drops of acetic acid and peptizing at
808C for 15 min. The sol was gelled with a dehydrating agent (2-ethyl hexanol) in
the presence of a surfactant (Span 80). The gel was dried and calcined at different
temperatures to obtain alumina. The inevitable particle size distribution in the
product was narrowed down considerably by a size fractionation procedure using a
pipette centrifuge (Ladal Analysette Model 21.002). This instrument, normally used
for the measurement of particle size distribution, consisted of a shallow stainless
steel bowl that holds the sample in the form of a suspension. The bowl was rotated
about its central axis at speed up to 1500 rpm, and size separation occured during
the centrifugal motion. Samples were sucked out at speci®ed time intervals from
points at a ®xed distance from the axis. The samples collected at different times
were dried and analyzed for particle size using TEM and surface area measurement
techniques.
Dispersal Sol-P3 (Al2O3 ꢀ70%, acetic acid ꢀ6±7.5%), a water dispersible alumina
hydrate, obtained from Condea Chemie (Germany), was also processed to obtain
alumina with a particle size of about 1 mm. The alumina sol was prepared by pouring
dispersal Sol-P3 powder into water and subjecting it to intensive stirring. After
stirring for 15 min, the sol was synthesized and was then characterized to determine
its particle size.
Lithium sulfate and alumina were mixed in appropriate mole percentages and
stirred thoroughly in deionized water. The mixtures were then dried to achieve a ®ne
dispersion of second phase particles. The samples were then ground into ®ne powders
and pelletized at pressure of around 32 kN cmÀ2. The resulting pellets were sintered
at different temperatures for 12 h and furnace cooled. They were then gold coated to
provide good contact with the Pt electrodes for the impedance measurements.
The DSC measurements were made using DuPont 9900 calorimeter at a heating
rate of 108C minÀ1. The room temperature XRD patterns were recorded with a
Ê
PW1820 Philips diffractometer using Cu Ka (l ˆ 1:54 A) radiation. The impedance
measurements were carried out as a function of temperature using a computer
controlled Solartron impedance gain/phase analyzer, Model Schlumberger SI
1260. The measurements were made in the frequency range of 1 Hz±32 MHz, with
a signal amplitude of 100 mV.

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P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
3. Results and discussion
3.1. Effect of preparative parameters
It is observed that the pelletizing pressure does not affect the electrical conductivity
of the composite in a signi®cant manner. To investigate the effect of sintering time, the
Li2SO4±43 m/o Al2O3 composition, that exhibited the maximum conductivity in an
earlier study [26], was treated at 2, 6, 12, and 20 h, respectively. Fig. 1 exhibits the plot
of log s versus 1000/T for the Li2SO4±Al2O3 composite sintered for different
durations at 8008C. Among these, the conductivity of the composite treated at
12 h was found to be marginally higher. However, the sintering time does not appear
to be a signi®cant preparative parameter.
Fig. 2 displays the variation of conductivity for Li2SO4±Al2O3 composites for two
different particle sizes of Al2O3. There does not appear to be much variation in the
conductivity of the two composites. The Al2O3 as processed by the Sol-P3 route has a
Ê
size of ꢀ1 mm, while the other processed by the sol±gel route has a size of 40 A.
Although some of the early studies suggest that the conductivity of the composite
increases as the size of the dispersoid decreases [6], more recent studies have
emphasized the correlation of the increase in the conductivity with the increase in
the effective surface area of the dispersed phase. Our results are in agreement with
such a correlation. Although nothing can be said about the surface area for the
alumina with the two particle sizes, it is clear that despite reducing the Al2O3 size by
almost about two orders of magnitude, the conductivity remains unaffected. One
factor, often overlooked, but can help explain our results is that as-received ®ne size
Fig. 1. Variation of log s vs. 1000/T for Li2SO4±43 m/o Al2O3 treated at 8008C for different sintering
times.

P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
2047
Ê
Fig. 2. Variation of log s vs. 1000/T for Li2SO4±50 m/o Al2O3 for 40 A and 1 mm sized alumina.
particles are invariably highly agglomerated. We have already reported such an
agglomeration in case of the Na2SO4±Al2O3 composite system [23]. This agglomera-
tion reduces the effective interface area for less than theoretically dense composites, as
the matrix material is unlikely to ®ll the deep pores and crevices of the irregular cluster.
Unless the samples are 100% dense, the actual interface area is likely to be considered
less than the measured surface area of the second phase particles. Large particles
having a very high surface area can also enhance the conductivity as effectively as
extremely small, non-porous particles with a similar effective surface area.
3.2. Conductivity versus nature of Al2O3 phases
Al2O3 exhibits a number of phases such as g-, y-, k-, d-, and a-Al2O3. Table 1
shows the various phases of Al2O3 identi®ed from the XRD for the Li2SO4±43 m/o
Al2O3 composition treated at different processing temperatures.
Table 1
Temperature of treatment for the
Li2SO4±43 m/o Al2O3 composition (8C)
Phases observed in XRD
500
700
Li2SO4±g-Al2O3
Li2SO4±d-Al2O3
Li2SO4±d-Al2O3
Li2SO4±k-Al2O3 ‡ y-Al2O3
Li2SO4±y-Al2O3
g-LiAlO2
800
1000
1200
1400

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P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
Fig. 3. Variation of log s vs. 1000/T for Li2SO4±43 m/o Al2O3 treated at different sintering
temperatures.
Fig. 3 exhibits the variation of conductivity for the various sintering temperatures
chosen in this study. Among all these phases of Al2O3, the composite having the g-
Al2O3 phase exhibits the largest enhancement in conductivity. This may be attributed
to the higher surface activity of the g-phase [26], relative to that for the other phases of
alumina. The conductivity of the composite containing the g-Al2O3 is almost an order
of magnitude higher than the composites exhibiting other phases of Al2O3. Analyses
of X-ray diffractograms of composites under investigation show that no new phase
like lithium aluminates are formed during the mechanochemical treatment and
sintering thereafter. The results are in agreement with the analysis of Uvarov et al.
[27]. There is, however, an exception only for composites processed at temperatures
in excess of ꢀ13008C, the formation of lithium aluminates is observed. The phase
identi®ed in the composite treated at 14008C is g-LiAlO2 (Table 1).
3.3. Conductivity versus composition
Most of the composites studied show a recognizable, uniform type of conductivity
dependence on composition. Like in most other composites (CuCl±Al2O3, LiI±Al2O3),
the conductivity in Li2SO4±Al2O3 composite also increases with increasing Al2O3
content, passing through a maximum at 50 m/o Al2O3.
Fig. 4 exhibits the conductivity plots for the Li2SO4±g-Al2O3 system. A maximum
in conductivity is observed at around 50 m/o Al2O3. Table 2 displays the conductivity
enhancements obtained for compositions with varying contents of alumina at 300 and
5008C, respectively. The enhancement in conductivity for compositions containing

P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
2049
Fig. 4. Variation of log s vs. 1000/T for Li2SO4±X m/o Al2O3 (X ˆ 0, 20, 30, 40, 43, 48, 50, 60, 70,
80).
between 40 and 50 m/o Al2O3 is about three orders of magnitude for this system. The
conductivity plots in Fig. 4 exhibit an Arrhenius type behavior, and the activation
energy derived from the slope of the log s versus 1000/T plot for the compositions
vary between 0.74 and 0.83 eV. The activation energy for the composite is less than
that of pure Li2SO4. However, it is independent of the alumina content, and is fairly
constant. On the other hand, the preexponential factor, as a function of alumina
content, exhibits a maximum for the 50 m/o Al2O3 composition, the value for the
preexponential factor being 1:713 Â 106. Table 3 indicates the variation of activation
energy and preexponential factor as a function of the composition of Al2O3.
The conductivity isotherms (Fig. 5) show a drop in conductivity beyond 50 m/o
Al2O3. For low Al2O3 containing compositions, the conductivity increases sharply
Table 2
m/o Dispersoid
s300 (OÀ1 cmÀ1
)
s500 (OÀ1 cmÀ1
)
s300/spure
s500/spure
Pure Li2SO4
20% Al2O3
30% Al2O3
40% Al2O3
43% Al2O3
48% Al2O3
50% Al2O3
60% Al2O3
70% Al2O3
80% Al2O3
1.02 Â 10À7
6.31 Â 10À5
8.91 Â 10À5
2.63 Â 10À4
3.16 Â 10À4
4.47 Â 10À4
5.49 Â 10À4
1.51 Â 10À4
1.02 Â 10À4
6.30 Â 10À5
1.05 Â 10À5
1.78 Â 10À3
4.26 Â 10À3
1.26 Â 10À2
1.41 Â 10À2
2.14 Â 10À2
2.34 Â 10À2
4.79 Â 10À3
3.24 Â 10À3
2.57 Â 10À3
1
1
617
871
170
407
2571
3091
4366
5371
1479
100
1202
1349
2042
2239
457
309
245
616

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P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
Table 3
Composition
Pure Li2SO4
s0 (OÀ1 cmÀ1
)
Ea (eV)
Temperature range (8C)
3.3515 Â 107
1.38
220±570
580±650
220±570
220±570
220±570
220±570
220±570
220±570
220±570
220±570
220±570
4.68 Â 102
0.357
0.845
0.773
0.808
0.836
0.76
20% Al2O3
30% Al2O3
40% Al2O3
43% Al2O3
48% Al2O3
50% Al2O3
60% Al2O3
70% Al2O3
80% Al2O3
7.764 Â 105
3.676 Â 105
1.876 Â 106
4.242 Â 106
1.483 Â 106
1.713 Â 106
2.242 Â 105
2.186 Â 105
2.260 Â 105
0.77
0.744
0.753
0.783
with increasing alumina content. However, the rate of increase of conductivity
decreases with a further increase in Al2O3 content. The conductivity peak is a broad
one, and there is no sharp jump in enhancement for any particular threshold Al2O3
composition.
The decrease of conductivity at higher concentrations (>50 m/o) of Al2O3 appears
to be due to the ``blocking effect''. Alumina itself is non-conducting. If a fraction, f, of
the area of plane through the electrolyte consists of alumina, only a fraction (1 À f) is
free to conduct, irrespective of any enhancement near an alumina particle, thereby
accounting for the lower conductivity of the compositions containing greater than
50 m/o Al2O3.
To ascertain the mechanism for conductivity enhancement, different theoretical
models were analyzed [6,28±30]. A higher conduction in the space charge layer near
Fig. 5. Conductivity vs. composition isotherms for the Li2SO4±Al2O3 composite system.

P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
2051
the surface appears as a possible mechanism for the ionic conductivity enhance-
ment. This mechanism has been proposed in almost all the known models [6±14].
This conduction mechanism is quite possible, because at thermodynamic equili-
brium most internal interfaces and free surfaces of an ionic crystalline material
have some net charge. This charged interface is compensated with a diffuse layer
containing an excess of the oppositely charged ionic defects. An enhanced
conductivity is attributed to the excess concentration of point defects in the diffuse
layer. Therefore, dispersed ionic conductors can be regarded as three component
systems, because of their distinct interfacial properties. The resistor model [28]
consists of non-conducting, normal conducting, and highly conducting bonds. The
bonds are distributed in space in a manner that corresponds to a random distribution
of insulating material in a normally conducting matrix, with a highly conducting
interface separating them. The resistor model was mapped in the conventional
manner on a random walk, which in turn, was solved by means of Monte Carlo
simulations. According to this model, the normalized conductivity is given by the
expression
Smax
ˆ K expꢁb DE†
(1)
S0
with the value of DE ꢂ 0:66 eV.
The calculated conductivity values at various temperatures for the Li2SO4±50 m/o
Al2O3 composite using the resistor model are shown in Table 4.
The theoretical data as per the resistor model also appears to be in fair agreement
with the experimental data con®rming the existence of a highly conducting interface
between the insulating alumina particles and the normally conducting lithium sulfate
matrix.
Table 4
Temperature (8C)
s or Smax
log s (theoretical)
log s (experimental)
(theoretical) (OÀ1 cmÀ1
)
214
256
282
305
325
354
375
401
424
457
475
502
524
552
5.99 Â 10À5
2.62 Â 10À4
4.91 Â 10À4
8.16 Â 10À4
1.23 Â 10À3
2.12 Â 10À3
3.042 Â 10À3
4.612 Â 10À3
6.487 Â 10À3
1.02 Â 10À2
1.278 Â 10À2
1.761 Â 10À2
2.25 Â 10À2
0.03
À4.22
À4.44
À3.582
À3.309
À3.088
À2.91
À3.841
À3.516
À3.249
À3.035
À2.749
À2.558
À2.338
À2.157
À1.917
À1.795
1.623
À2.674
À2.517
À2.336
À2.188
À1.993
À1.893
À1.754
À1.648
À1.521
À1.491
À1.334

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P. Gopalan et al. / Materials Research Bulletin 37 (2002) 2043±2053
4. Conclusion
Composite solid electrolytes in the Li2SO4±Al2O3 composite system, obtained by
mechanochemical treatment of Li2SO4 and ®ne sized Al2O3, have been investigated,
and the highest conductivity is observed for the composite containing the g-Al2O3
phase. This has been attributed to the high surface activity of g-Al2O3. Conductivity
measurements show that the sintering time does not have a signi®cant effect on the
conductivity of the composite.
The composite containing 50 m/o Al2O3 has the maximum conductivity, about
three orders of magnitude higher than that of pure Li2SO4. The values of the
conductivity for this composition are 1:76 Â 10À2 and 6 Â 10À5 OÀ1 cmÀ1 at 500
and 2008C, respectively, with activation energy being 0:78 Æ 0:05 eV. A higher
conduction in the space charge layer near the surface appears to be the possible
mechanism for the enhancement in ionic conductivity.
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