Synthesis of ZrO2 nanoparticles in microwave hydrolysis of Zr (IV) salt solutions-Ionic conductivity of PVdF-co-HFP-based polymer electrolyte by the inclusion of ZrO2 nanoparticles

Article abstract of DOI:10.1016/j.jpcs.2006.11.005

Nanocrystalline ZrO₂ particles have been prepared by microwave hydrolysis of Zr(IV) salt solutions at 400 ^{{ring operator}} C for 6 h. The paper describes the PVdF - co - HFP - ZrO₂-based NCPEMs prepared by a simple solvent casting technique. The incorporation of ZrO₂ nanoparticles in the PVdF-co-HFP matrix, improved the ionic conductivity due to the availability of a large amount of oxygen vacancies on ZrO₂ surface which may act as the active Lewis acidic site that interact with ClO₄^- ions. On the other hand, a high concentration of ZrO₂ [10 wt(%)] leads to depression in ionic conductivity due to the formation of more crystalline phase in the PVdF-co-HFP matrix. DSC, XRD, SEM and DC-polarization studies were carried out. This paper also explores and proposes a structure-conductivity correlation in the PVdF - co - HFP - LiClO₄ - ZrO₂-based NCPEMs system. The proposed correlation is derived from the interpretation of DSC, XRD and AC-impedance measurements. The temperature dependence of the ionic conductivity of NCPEMs follows the Arrhenius behaviour. Finally, the LSV experiment has been carried out to investigate the electrochemical stability in the polymer electrolytes.

Full text of DOI:10.1016/j.jpcs.2006.11.005

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 264–271
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Synthesis of ZrO2 nanoparticles in microwave hydrolysis of Zr (IV) salt
solutions—Ionic conductivity of PVdF-co-HFP-based polymer
electrolyte by the inclusion of ZrO2 nanoparticles
Ã
N.T. Kalyana Sundaram, T. Vasudevan, A. Subramania
Advanced Materials Research Lab, Department of Industrial Chemistry Alagappa University, Karaikudi 630 003, India
Received 6 June 2006; received in revised form 16 October 2006; accepted 10 November 2006
Abstract
Nanocrystalline ZrO2ꢁ particles have been prepared by microwave hydrolysis of Zr(IV) salt solutions at 400 ꢀC for 6 h. The paper
describes the PVdF
ꢁ
co HFP
ꢁ
ZrO2-based NCPEMs prepared by a simple solvent casting technique. The incorporation of ZrO2
nanoparticles in the PVdF-co-HFP matrix, improved the ionic conductivity due to the availability of a large amount of oxygen vacancies
on ZrO2 surface which may act as the active Lewis acidic site that interact with ClOꢁ4 ions. On the other hand, a high concentration of
ZrO2 [10 wt(%)] leads to depression in ionic conductivity due to the formation of more crystalline phase in the PVdF-co-HFP matrix.
DSC, XRD, SEM andꢁ DC-polarization studies were carried out. This paper also explores and proposes a structure–conductivity
correlation in the PVdF co
ꢁ
HFP
ꢁ
LiClO42ZrO2-based NCPEMs system. The proposed correlation is derived from the interpretation of
DSC, XRD and AC-impedance measurements. The temperature dependence of the ionic conductivity of NCPEMs follows the Arrhenius
behaviour. Finally, the LSV experiment has been carried out to investigate the electrochemical stability in the polymer electrolytes.
r 2006 Published by Elsevier Ltd.
Keywords: A. Polymer; A. Thin film; C. Differential scanning calorimetry; D. Transport properties
1. Introduction
extends over a wide temperature range. Among them, the
addition of inert materials into polymer electrolyte has
attracted considerable attention due to its improved
mechanical stability, enhanced ionic conductivity, lithium
transference number, electrolyte–electrode interfacial sta-
bility, etc. [3–7]. Scrosati et al. [8] reported that an
improvement in the cationic transference number and the
interfacial stability between the polymer electrolyte and
lithium metal electrode were achieved by the addition of
ceramic fillers such as SiO2, TiO2, gLiAlO2, etc. Further,
MgO, CaO, CuO, etc., have also been tried which have
positive free energy of lithium and should avoid the
formation of dentrides growth and passivation of metallic
lithium at the electrode–electrolyte interface and also
enhance the conductivity of the polymer electrolyte which
depends upon the particle size and annealing temperature
[9–13].
The development of a new polymeric system to be used
as an electrolyte in advanced batteries and fuel cells is
critical for the commercial success of these types of power
systems [1]. Because these polymer electrolytes often show
insufficient conductivity, poor mechanical stability and
decrease in interfacial stability. Consistent research efforts
have been devoted to increase the ionic conductivity and to
lower the operation temperature of the polymer electrolyte
to near ambient temperature [2]. The addition of liquid
solvent is one of the approaches. However, this promotes
deterioration of mechanical properties of the polymer
electrolyte and increases its reactivity towards the electro-
des. Generally, various methods have been applied to
reduce the crystallinity of the polymer electrolytes while
maintaining their flexibility and mechanical stability which
The micro-size ceramic oxides will increase the large
degree of preferential orientation in polymer electrolyte
film, which leads to non-conducting planes being
Ã
Corresponding author.
E-mail address: drasubramania@yahoo.co.in (A. Subramania).
0022-3697/$ - see front matter r 2006 Published by Elsevier Ltd.
doi:10.1016/j.jpcs.2006.11.005

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265
perpendicular to the current path and thus reduces the
conductivity [14]. To avoid this problem, in the present
investigation, ZrO2 nanoparticles were synthesized and
used for the preparation of NCPEM using poly(vinylidene
fluoride-co-hexafluoropropylene) PVdF-co-HFP as the
polymer matrix. Finally, the DSC, XRD, SEM, AC-
impedance spectroscopy and DC-polarization studies were
also investigated.
where, DHFm is the crystalline melting heat of pure PVdF
(104.7ꢁJ/g), DHm
is the heat of fusion of
ZrO2-based NCPMs. It can be calculated
from the integral area of the baseline and each melting
curve.
PVdF co
ꢁ
HFP
ꢁ
The X-ray diffraction (XRD) patterns were recorded
with JOEL (JDX-8030) diffractometer using nickel filtered
Cu2Ka radiation at a scan rate of 10 ꢀC= min to examine
the nature of crystallinity with respect to ZrO2 nanopar-
ticles in the PVdF-co-HFP polymer network. The surface
morphology of NCPMs was studied by Scanning electron
microscope (JOEL JSM-35CF).
2. Experimental
The ZrO2 nanoparticles were synthesized by the hydro-
lysis of Zr(IV) salts in aqueous–alcohol solution. An initial
aqueous–alcohol solution was prepared from distilled
water and tert butyl alcohol. This solution was mixed with
aqueous solutions of zirconyl salts ZrOðNO3Þ2 in ratios
such that the zirconium concentration was 0.128 M and the
alcohol-to-water ratio was 5:1. The initial solution in a
glass flask was placed in a micro-wave oven (input power
300 W). The solution was heated to the boiling point
(3–5 min) and the oven was switched off. Then the solution
was neutralized with 20% ammonia solution, which
prevented the precipitate formed in the course of micro-
wave hydrolysis from being dissolved on cooling. This
precipitate was centrifuged (4000 rpm, 10 min), washed
with distilled water until the reaction towards NOꢁ3 was
negative, dried at 60 ꢀC for 3 h and annealed at 400 ꢀC for
6 h. The phase purity and the degree of crystallinity of the
resulting ZrO2 sample were monitored by XRD analysis.
Surface morphology of ZrO2 nano-particle was measured
by scanning electron microscopy on a JOEL JSM-35CF
microscope.
The ionic conductivity measurements were performed by
sandwiching the NCPEM in between two stainless steel
electrodes using HIOKI 3522-50 LCR meter over a
frequency range of 1 mHz–100 kHz at a scan rate of
1 mV/s with various temperatures ranging from 298 to
353 K. DC polarization cell was constructed by sandwich-
ing the polymer electrolyte in between the symmetrical
lithium metal electrodes and the experiment was performed
as described earlier [16] to find out the lithium transference
number of NCPEMs. The electrochemical stability of
polymer electrolyte was studied at room temperature by
running a linear sweep voltammetry (LSV) using a two
electrode cell in the configuration of Li/ polymer electrolyte
/SS at a scan rate of 1 mV/s where lithium acts as a counter
and reference electrodes and SS act as a working electrode,
respectively. The cell was assembled in a dry box under
argon atmosphere.
3. Results and discussion
PVdF-co-HFP with an average molecular weight greater
than 5, 00, 000 (Aldrich, USA) and LiClO4 (E-Mercꢁk3,
Germany) were dried in vacuum oven at 50 ꢀC under 10
torr pressure for 48 h. EC&DEC (Across organic, Belgium)
were purifiedꢁbyꢁdistillꢁation under reduced pressure.
The PVdF co HFP ZrO2-based nano-composite poly-
mer electrolyte membranes (NCPEMs) were made by a
simple solvent casting technique. The solution of PVdF-co-
HFP in AR grade acetone was prepared in which ZrO2
nanoparticles was dispersed and sonicated. After sonica-
tion, a homogeneous colloidal solution was obtained which
was cast and dried into a film form of about 50280 mm
thickness. The amount of inclusion of ZrO2 nanoparticles
used in the polymer matrix is 2–10 wt(%) to prepare nano-
composite polymer membranes (NCPMs). Finally, the
NCPMs were soaked in 1 M LiClO4 containing (1:1 (v/v))
ratio of EC-DEC for 18 h to get NCPEMs.
3.1. XRD and SEM analysis of ZrO2 nano-particles
The XRD pattern for the synthesized ZrO2 nanoparticles
is shown in Fig.1. The diffractogram reveals the formation
of highly crystalline product with high phase purity on
Thermal behaviour of the NCPMs was studied by using
a differential scanning calorimeter (Dupont TA-2000
DSC). Each sample was scanned from 0 to 200 ꢀC by
heating at a scan rate of 10 ꢀC= min under N2 atmosphere.
The crystallinity of the NCPMs were calculated using Eq.
(1) from the DSC curves [15],
20
30
40
50
60
70
80
2θ
Xcð%Þ ¼ ðDHm=DHFmÞ ꢂ 100,
(1)
Fig. 1. XRD pattern of ZrO2 nanoparticles.

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calcination at 400 ꢀC for 6 h. The pattern is very similar to
that reported for ZrO2 [17]. The average coherent-
scattering domains (CSD) size was calculated by means
of the well-known Debye–Scherer formula [18] is 60 nm.
Fig. 2 shows the SEM photograph of ZrO2 nanoparticles.
From this figure, the grain size of the particle is found to be
ꢃ62 nm with uniform distribution. The value of 62 nm
observed from SEM result was in good agreement with the
XRD result of 60 nm. It reveals that the microwave
hydrolysis makes it possible to obtain particles of
predominantly spherical shape, with a narrower size
distribution.
3.2. DSC analysis of NCPMs
Fig. (a)–(f) shows the DSC curves of
PVdF co HFP ZrO2-based NCPMs with different wt
3
ꢁ
ꢁ
ꢁ
(%) of ZrO2 nano-particles [0, 2, 4, 6, 8 and10 wt (%)] in
polymer matrix. From these curves it was observed that the
amount of ZrO2 nano-particles slightly influences the
crystalline melting temperature (TmÞ, heat of fusion
(DHmÞ and crystallinity (XcÞ as shown in Figs. 3 and 4.
From these figures, it may be seen that the reorganization
of polymer chain may be hindered by the cross-linking
centres formed by the interaction of the Lewis acid group
ceramics with the polar group polymers [19]. The
incorporation of ZrO2 nano-particles, far more compared
to the segmental chain motion of the polymer, stabilizes the
amorphous structure and thus enhances the ionic con-
ductivity of polymer electrolyte [20]. It can be observed
from Figs. 3 and 4 that the Tm, DHm and Xc decreases with
increase in ZrO2 content upto 8 wt(%) to the PVdF-co-
HFP system and was found to be 144:4 ꢀC, 82.5 J/g and
76.8%, respectively. Beyond this concentration [10 wt(%)
of ZrO2], Tm, DHm and Xc increases. It may be explained
by the assumption that immobilization of PVdF-co-HFP
polymer chain segments due to the steric hindrance caused
by both PVdF-co-HFP and ZrO2 nano-particles will
induce crystalline site in the polymer matrix which leads
to lower segmental mobility. Hence, 8 wt (%) of ZrO2
nanoparticles incorporated in PVdF-co-HFP matrix has
been taken as an optimized composition for lithium-ion
battery applications.
Fig. 2. SEM photograph of ZrO2 nanoparticles.
(e)
(f)
(d)
(c)
(b)
(a)
(a) Owt (%) of ZrO2
(b) 2wt (%) of ZrO2
(c) 4wt (%) of ZrO2
(d) 6wt (%) of ZrO2
(e) 8wt (%) of ZrO2
(f) 10wt (%) of ZrO2
Endo
40
60
80
100
120
140
160
180
200
220
Temperature (°C)
Fig. 3. DSC curve of PVdF
ꢁ
coꢁ
HFPꢁZrO2-based NCPMs containing different wt (%) of ZrO2 nanoparticles.

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267
100
95
90
85
80
75
75
80
85
90
95
Heat of fusion
Crystallinity
-2
0
2
4
6
8
10
12
ZrO2 content in wt (%)
Fig. 4. Crystallinity and heat of fusion of PVdF
ꢁ
coꢁHFP
ꢁ
ZrO2-based NCPMs containing different wt (%) of ZrO2 nanoparticles.
(e)
(f)
(d)
(c)
(b)
Fig. 6. SEM photograph of NCPM containing 8 wt (%) of ZrO2 content.
10 wt(%)] are shown in Fig. 5 (a)–(f). From the XRD
patterns, the following distinct features were observed:
(a)
15
20
25
30
35
40
45
1. The decrease in relative intensity of the apparent peaks
were observed with increase in ZrO2 content upto 8 wt%
[Fig. 5 (a)–(e)]. This is because decrease in crystallinity of
PVdF-co-HFP matrix created the space charge layer at
the filler–polymer interface to assist the transport of ions
and ZrO2 may influence the recrystallization kinetics of
PVdF-co-HFP chain to promote localized amorphous
region which is in good agreement with DSC results.
2. Further increase in concentration of ZrO2 [10 wt(%)]
increases the relative intensity of apparent peaks. It may
be attributed to the phase separation of zirconia from
the PVdF-co-HFP matrix because the aggregated
2θ
Fig. 5. XRD patterns of PVdF-co-HFP-based NCPMs containing
different wt(%) of ZrO2 nanoparticles. (a) 0 wt (%) of ZrO2, (b) 2 wt
(%) of ZrO2 (c) 4 wt (%) of ZrO2 (d) 6 wt (%) of ZrO2 (e) 8 wt (%) of
ZrO2 (f) 10 wt (%) of ZrO2.
3.3. XRD analysis of NCPMs
X-ray diffraction patterns of PVdF co HFP ZrO2-based
NCPMs obtained from different wt(%) of ZrO2 nanopar-
ticles [0 wt(%), 2 wt(%), 4 wt(%), 6 wt(%), 8 wt (%) and
ꢁ
ꢁ
ꢁ

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268
nanoparticles are not consistent with the polymer matrix
[Fig. 5(f)].
matrix and some tiny pores existed between the filler–po-
lymer interfaces. It may be suggested that the liquid
electrolyte is present both in pores and the PVdF-co-HFP
polymer matrix which may provide the fast ion transport.
3.4. SEM analysis of NCPMs
Fig. 6 shows the SEM photograph of nano-composite
polymer membrane (NCPM) of high ionic conductivity
system. It can be seen from the figure that the ZrO2
nanoparticles are distributed uniformly in the polymer
3.5. AC-impedance analysis of NCPEMs
The particle size and the mass of ceramic filler in
the polymer matrix plays an important role in ionic
152
150
148
146
144
142
140
138
136
-2.3
-2.6
-2.9
-3.2
-3.5
-3.8
-4.1
8
6
10
12
0
4
-2
2
ZrO2 content in wt (%)
Fig. 7. The relationship between Tm and ionic conductivity of NCPEM with respect to ZrO2 content.
-1
-1.5
-2
-2.5
-3
0wt(%) of ZrO2
-3.5
2wt(%) of ZrO2
4wt(%) of ZrO2
6wt(%) of ZrO2
-4
8wt(%) of ZrO2
10wt(%) of ZrO2
-4.5
2.8
2.9
3
3.1
3.2
3.3
3.4
1000/T (K-1)
Fig. 8. Temperature depends on ionic conductivity of NCPEM containing different wt (%) of ZrO2 nanoparticles.

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269
conductivity. Fig. 7 shows the ionic conductivity of
different wt (%) of inclusion of ZrO2 nanoparticles in the
PVdF-co-HFP matrix at room temperature. It can be
observed from Fig. 7 that the ionic conductivity increases
with increase in ZrO2 content up to 8 wt(%) of the PVdF-
co-HFP matrix which bring localized amorphous regions
and ceramic–salt interactions. Hence, ionic conductivity of
NCPEM increases [20]. Beyond this concentration, ionic
conductivity decreases. It may be probably because
increasing the crystallinity and viscosity of PVdF-co-HFP
matrix may trap the ion mobility.
composite. Such interaction prevents PVdF-co-HFP mole-
cules from crystallinity and also releases more free Liþ
ions. Besides, the ClOꢁ4 anions are also bound to the Lewis
acid sites (O-vacancies) through coordination, which
separates the LiþClO4ꢁ pairs. These two modes of
coordination produce more dissociated Liþ ions as charge
carrier and more amorphous region for the carrier to
transfer. Hence, ionic conductivity of the NCPEMs was
increased.
Fig. ꢁ8
shows
the
ionic
conductivity
of
ZrO2 matrix as a function of different wt
PVdF
ꢁ
co HFP
ꢁ
The relationship between Tm and ionic conductivity
with respect to ZrO2 content is shown in Fig. 7. The rise
value of Tm with inclusion of ZrO2 over 8 wt(%) may
increase the crystallization site or cause a molecular
interaction between the polymer chains which leads to
lower mobility. On the basis of the characterization results,
it is feasible to postulate a modelꢁtoꢁdescrꢁibe the Lewis
acid–base interaction in PVdF co HFP ZrO2ꢁLiClO4
(%) of ZrO2 content at various temperatures in the range
of 298 to 353 K. The 8 wt(%) ZrO2 incorporation of PVdF-
co-HFP matrix shows relatively high ionic conductivity
over a wide temperature range even at a lower temperature.
It may be due to increasing the dissociation of lithium salt
resulting in the enhancement of conductivity. But at
higher concentrations of ZrO2 [10 wt(%)] will build
up a continuous non-conductive phase and hence this
27.5
a
26.5
25.5
24.5
23.5
0
4000
8000
12000
16000
Time (Sec)
b 27.5
26.5
25.5
24.5
23.5
0
4000
8000
12000
16000
Time (Sec)
Fig. 9. DC-polarization curve of PVdF-co-HFP-based (a) GPE and (b) NCPEM of high ionic conductivity system.

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2.5
a
2.0
1.5
1.0
0.5
0.0
2
2.5
3
3.5
4
4.5
5
5.5
Voltage (V) vs Li/Li+
0.40
0.30
0.20
0.10
0.00
b
2
2.5
3
3.5
4
4.5
5
5.5
6
Voltage (V) vs Li/Li+
Fig. 10. LSV curve of PVdF-co-HFP-based (a) GPE and (b) NCPEM of high ionic conductivity system.
electrically inert filler would block up lithium-ion transport
resulting in an increase of total resistance of the NCPEMs.
its impedance measurements were obtained before and
after DC-polarization measurements with an applied
potential difference of 10 mV/s. It can be seen from the
Fig. 9(b) that the initial current value (IoÞ is 26:95 mA and
steady-state value (IsÞ is 23:94 mA within about 3.21 h and
its corresponding AC-impedance values such as initial
resistance of interface (R ꢀi Þ and steady-state resistance of
interface (Rsi ) are 218.34 and 300:21 O, respectively, which
3.6. Transference number
The transference number of PVdF-co-HFP-based gel
polymer electrolyte (GPE) and NCPEM of high ionic
conductivity system was found out by means of chron-
oamperometric technique as shown in Fig. 9(a) and (b) and
gives the transference number (i ) ca. 0.729. Fig. 9(a)
þ

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271
shows that the initial current value (IoÞ is 26:89 mA and
steady-state value (Is) is 23:07 mA within about 4.20 h and
its corresponding AC-impedance values such as initial
resistance of interface (R ꢀi Þ and steady-state resistance of
interface (Rsi ) are 264.56 and 460:27 O, respectively, which
ductivity system was found to be 0.729. The LSV study
revealed that the NCPEM is electrochemically stable up to
5.0 V vs Li/Liþ. From the results, it is proposed that, the
PVdF
ꢁ
co
ꢁ
HFP
candidate for high voltage lithium-ion batteries.
ꢁZrO2-based NCPEMs is a new promising
gives the transference number (i Þ ca. 0.574 and also this
þ
relaxation time of nano-composite polymer electrolyte
membrane (NCPEM) is much faster than the GPE.
Theoretically, this phenomenon implies that the ionic
mobility of NCPEM is more facile than the conventional
gel polymer electrolyte. Hence, the inclusion of ZrO2 onto
the PVdF-co-HFP polymer matrix that provides more
liquid pathway is probably a positive factor leading to
superior ionic transport as well.
Acknowledgements
The authors gratefully acknowledge the DST, New Delhi
for the financial support.
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PVdF
ꢁco
ꢁ
HFP ZrO2-based NCPEM of high ionic con-