nanocomposite single ion conductor based on organic–inorganic hybrid
TRANSCRIPT
www.elsevier.com/locate/ssi
Solid State Ionics 167 (2004) 293–299
Nanocomposite single ion conductor based on organic–inorganic hybrid
Nam-Soon Choi, Yong Min Lee, Baik Hyeon Lee, Je An Lee, Jung-Ki Park*
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology and Center for Advanced Functional Polymers,
373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea
Received 16 June 2002; received in revised form 16 June 2003; accepted 19 June 2003
Abstract
New nanosized silica with the propane lithium sulfonate was systematically synthesized as the lithium ions source of the single ion
conductor. The cross-linked single ion conductor based on the PEGDMA, the modified silica, and the plasticizer (PC/DMSO, 50/50 w/w)
was prepared by UV-curing. Ionic conductivity and interfacial stability toward the lithium electrode of the cross-linked nanocomposite single
ion conductor were investigated by varying the modified silica content. The ionic conductivity of the cross-linked single ion conductor
showed maximum trend with the modified silica content. The ionic conductivity of the cross-linked single ion conductor with 30% modified
silica was 2.2� 10� 4 S/cm at 25 jC. Interfacial stability between the cross-linked nanocomposite single ion conductor and the lithium
electrode was enhanced by introducing the modified silica.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Nanocomposite single ion conductor; Nanosized fumed silica; Sulfonation; Ion conductivity; Interfacial stability
1. Introduction
Solid polymer electrolytes have received special attention
more than two decades due to their potential for application
in a variety of solid state electrochemical devices such as
lithium secondary batteries, sensors, and electrochromic
displays [1–3]. One of the interesting fields in solid
polymer electrolytes is the single-ion conductor in which
only the cation can be mobile under an electric field and
which does not suffer from the concentration polarization
caused by accumulation of anions on the electrode [4,5].
However, conventional single ion conductors showed
low ionic conductivities (f 10� 7 S/cm) due to their large
ion dissociation energy. Recently, it was reported that the
aluminate polymer based single ion conductors showed
relatively high ionic conductivity of 10� 5–10� 6 S/cm
[6,7] and that the ionic conductivity of the single ion
conductor could reach 10� 4 S/cm by introducing the
plasticizer with a high dielectric constant [8].
Besides high ionic conductivities, the interfacial stability
toward the lithium electrode is also an essential factor to
guarantee acceptable performance in the electrochemical
devices. The interfacial properties of conventional single
ion conductor with a plasticizer have rarely been studied.
0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ssi.2003.06.002
* Corresponding author. Tel.: +82-42-869-3925; fax: +82-42-869-3910.
E-mail address: [email protected] (J.-K. Park).
Recently, the effect of the nanosized ceramic additives on
the ion conductivity and the interfacial stability was exten-
sively investigated for solid polymer electrolyte based on
PEO [9–13]. The nanosized ceramic materials such as lithi-
ated aluminate (g-LiAlO2), fumed silica (SiO2), alumina
(Al2O3), and BaTiO3 have been introduced into polymer elec-
trolyte as a minor component to improve the electrochemical
properties of the polymer electrolyte. It will be also interest-
ing to consider the hybrid materials based on inorganic par-
ticles and organic compounds as a new functional lithium salt.
In this work, the nanosized-fumed silica was modified
with the propane lithium sulfonate and the cross-linked
nanocomposite single ion conductor containing the nano-
sized silica end-capped with the lithium sulfonate was
prepared. The effect of the modified silica, which was
introduced as the lithium ion sources, on the ion conduc-
tivity and the interfacial stability of the cross-linked nano-
composite single ion conductor was investigated.
2. Experimental
2.1. Materials
The size and specific area of the nanosized-fumed silica
(CAB-O-SIL, EH 5) obtained from Cabot were 11 nm and
380F 30 m2/g, respectively. The surface of the silica was
N.-S. Choi et al. / Solid State Ionics 167 (2004) 293–299294
very hydrophilic (100% Si–OH). Poly(ethylene glycol)
dimethacrylate (PEGDMA, Mw = 400) was purchased from
Polyscience. Potassium tert-butoxide dissolved in THF, 1,3-
propane sultone, lithium hydroxide monohydrate
(LiOH�H2O) and methacrylic acid (MAA) were purchased
from Aldrich. The mixture of propylene carbonate (PC,
Merck) and dimethyl sulfoxide (DMSO, Aldrich) (50/50, w/
w) was dried with 3 A silica molecular sieves and was
stored in a nitrogen-filled dry box to avoid water contam-
ination before use as a plasticizer. Tetrahydrofuran (THF),
which was used as a solvent for the modification reaction of
the fumed silica, was dried with the sodium–benzophenone
complex.
2.2. Preparation of polymer electrolytes
The UV-curable formulation consists of curable mono-
mer (PEGDMA), plasticizer (300 wt.% based on matrix
polymer), modified silica end-capped by the propane lithi-
um sulfonic acid, and methyl benzoylformate as a photo-
initiator (2 wt.% based on matrix polymer) which is known
to undergo a fast cleavage upon photolysis to generate free
radicals. The above curable solution was cross-linked by
UV irradiation (8 mW/cm2, 365 nm) for 10 min. The
resulting film of the cross-linked nanocomposite single ion
conductor showed a good elastic property.
2.3. Characterization
The thermal analysis of the cross-linked nanocomposite
single ion conductor containing the plasticizer (PC/DMSO
mixture) with change of the modified silica content was
done by using a differential scanning calorimeter (DuPont
TA 2000 DSC). Each sample was scanned at a heating rate
of 10 jC/min within an appropriate temperature range under
nitrogen atmosphere. Solid state 7Li static NMR experi-
ments and 29Si cross-polarization magic angle-spinning
(CP/MAS) NMR experiments were performed on a Bruker
DSX 400 MHz NMR spectrometer. The spin–spin relaxa-
Fig. 1. Schematic illustration for the reaction of the
tion time (T2) was determined using the spin-echo technique
by applying � 90–90j pulse sequences.
2.4. Electrical measurements
The ionic conductivities of the polymer electrolytes were
obtained from bulk resistance measured by AC complex
impedance analysis using a Solartron 1287 frequency re-
sponse analyzer (FRA) over a frequency range of 10 Hz–1
MHz.
The lithium transference number was determined by DC
polarization/AC impedance combination method [14]. A
constant polarization of 10 mV was applied to the cell.
The transference number of the lithium cation was calcu-
lated by the relation, tLi +=[Is(DV� IoRo)/Io(DV� IsRs)],
where Io and Is are the currents at the initial and steady-
state, Ro and Rs are the interfacial resistances at initial and
steady-state, respectively. The characteristics of the interface
between the cross-linked single ion conductor and the
lithium electrode were examined by monitoring the time
dependence of the impedance of the symmetrical Li/single
ion conductor/Li cells (dimension: 2� 2 cm2) over a fre-
quency range of 1 Hz–1 MHz at room temperature.
3. Results and discussion
3.1. Synthesis of nanosized fumed silica end-capping with
propane lithium sulfonic acid
The sulfonation of an organic material, which was
reported by Sepulchre et al. [15] was performed by the
reaction of 1,3-propane sultone with the CH(CH3)2–
(PEG)n–O�K+. Contrary to this, in this work the organ-
ic–inorganic hybrid material was newly designed by the
sulfonation of the nanosized fumed silica. The reaction
scheme of the nanosized fumed silica with the 1,3-propane
sultone is shown in Fig. 1. Potassium tert-butoxide dis-
solved in THF as a catalyst was added to the dispersed
modified silica with the 1,3-propane sultone.
N.-S. Choi et al. / Solid State Ionics 167 (2004) 293–299 295
fumed silica in THF and then 1,3-propane sultone was
introduced. The reaction was allowed to proceed at 80 jCwith stirring for 24 h under nitrogen atmosphere. After the
reaction was completed, the reaction solution was centri-
fuged and the precipitated product was washed three times
with THF to remove the unreacted monomer. The resulting
product was the nanosized fumed silica with the propane
sulfonic acid. The sulfonic acid groups in the nanosized
fumed silica were neutralized with 0.1 N LiOH (aq) solution
as shown in Fig. 1 and the neutralized silica, which can act
as the lithium sources in the plasticized single ion conduc-
tion system, was dried in a vacuum oven at 110 jC.Fig. 2a,b shows the FT-IR spectra of the unmodified
silica (EH 5) and the nanosized fumed silica end-capped by
the propane lithium sulfonic acid. The absorption peaks
appearing in the range of 1000–1300 cm� 1 and around 815
cm� 1 in Fig. 2a correspond to SiUOUSi antisymmetric
stretching and SiUO stretching vibration mode in the
unmodified silica. The peaks centered at 1650 and 1470
cm� 1 in Fig. 2b are assigned to the SMO stretching mode of
the USO3�Li+ in the surface of the modified silica and to the
CH2 scissoring vibration respectively. The absorption peak
in the range 1000–1300 cm� 1 due to the SiUOUC
Fig. 2. FT-IR spectra for the unmodified silica (fumed silica, EH 5) and the
modified silica: (a) full region; (b) SiUOUSi antisymmetric stretching
region.
Fig. 3. Solid state 29Si CP/MAS NMR spectra for (a) the unmodified silica
and the modified silica end-capped by the propane lithium sulfonate; (b)
peak resolutions for the silicones under different environment; (c) silicones
with different environments.
antisymmetric stretching mode was broad and shifted to a
lower wavenumber.
Fig. 3 shows the 29Si CP/MAS NMR spectra of the
unmodified hydrophilic silica and the modified silica end-
capped by the propane lithium sulfonate. For the unmodi-
fied silica (EH 5), the resonance peaks centered at � 92,
� 100, and � 110 ppm in Fig. 3 are assignable to silicon of
C (Q2, geminal silanols, O2Si(OH)2), B (Q3, free silanols,
O3Si(OH)), and A (Q4, siloxane, O4Si) sites in silica,
respectively [16,17]. The difference in the peaks in the
Fig. 5. Ion conductivities of the cross-linked nanocomposite single ion
conductor for (a) Arrhenius plot; (b) room-temperature conductivities as a
function of the modified silica content.
N.-S. Choi et al. / Solid State Ionics 167 (2004) 293–299296
range from � 70 to � 120 ppm between the modified and
unmodified silica is due to the introduction of the USO3�Li+
in the surface of the hydrophilic silica. The relative fraction
of the USO3�Li+ in the modified silica could be calculated
by resolving the resonance peaks in the 29Si CP/MAS NMR
spectra. Three silicon peaks in a different environment in
Fig. 3b,c was represented as D, E, F, and G. The fraction of
peak D (siloxane: Q4 site), peak E (free silanol: Q3 site), and
peak F, G (silicone attached in UO(CH2)3SO3�Li+) was
16.3%, 33.7%, and 50.0%. The fraction of the free silanol
and geminal silanol groups in the surface of the unmodified
silica was about 84%. After the modification of fumed
silica, the conversion of the silanol groups is found to be
about 60%. From the above calculation, the lithium content
of the modified silica was 5.4 mmol/1 g silica.
3.2. Effect of modified silica on ion conductivity
The ion conductivity of the modified silica in PC/DMSO
was 4.3� 10� 4 S/cm at room temperature as shown in
Table 2. To investigate the effect of the modified silica on
the ion conductivity in the polymeric single ion conduction
system, the cross-linked polymer was used as a matrix. The
cross-linked nanocomposite single ion conductor films
containing the modified silica were flexible and semi-
transparent as shown in Fig. 4. The schematic representa-
tions for the microstructure of the cross-linked nanocom-
posite single ion conductor and lithium cation dissociated
from the modified silica surface were also illustrated in Fig.
4. The ion conductivities of the cross-linked nanocomposite
single ion conductors with a different modified silica
content were plotted against a reciprocal absolute temper-
ature in Fig. 5a and the ion conductivities as a function of
the modified silica content was presented in Fig. 5b. The
ion conductivities increased up to a maximum value with
Fig. 4. Photographs for the physical state of the single ion conductor film
and schematic representation for the microstructure of the cross-linked
nanocomposite single ion conductor.
the initial increase of the modified silica content and then
slowly decreased with a further increase of the modified
silica content. For the initial increase of the ion conductivity
up to a maximum, ion conductivity is strongly dependent
on the number of free ions in the cross-linked single ion
conductor. Since the increase of the modified silica content
in the cross-linked single ion conductor can produce a
larger number of free ions as shown in Table 1, it can lead
to a higher ion conductivity. However, after the highest
conductivity was reached, the ion conductivity was deter-
mined by the mobility of free ions and the ion conduction
pathway tortuosity rather than number of free ions. Accord-
ing to the previous study of Chen et al. [18], the more ions
that exist in the solution the higher ion conductivity.
However, after the highest conductivity is reached, the
conductivity no longer depends on the number of ionic
carriers in the solution. They reported that at higher salt
concentration, the ion conductivity is determined by the
mobility of ionic carriers. The mobility of the free ions in
the cross-linked nanocomposite single ion conductor could
be obtained from T2 (spin–spin relaxation time) of 7Li
NMR experiment. Table 1 shows that T2 of the lithium ions
decreased with increasing the modified silica content. This
Table 1
Mole concentration of lithium ion, transference number, T2 and n parameter
of the cross-linked nanocomposite single ion conductor with 300 wt.% PC/
DMSO (50/50, w/w) as a function of the modified silica content
Modified silica
content (wt.%
based on
polymer matrix)
Mole concentration
of lithium/1 g polymer
matrix (mmol/1 g)
TLi + T2 (ms) n Parameter
0a 5.20b 1.00 – 0.98
5 0.27 – 28.30 0.98
10 0.54 0.98 27.48 0.97
20 1.08 – 26.06 0.95
30 1.62 – 16.52 0.94
40 2.16 – 11.49 0.92
50 2.70 0.97 1.40 0.89
a Conventional cross-linked single ion conductor (reference system)
consisting of PEGDMA-based cross-linked matrix with LiMA and PC/
DMSO plasticizer.b Lithium content for 1 g cross-linked matrix consisting of PEGDMA
and LiMA.
Fig. 6. (a) DSC thermograms of the cross-linked nanocomposite single ion
coductor; (b) DHm of the cross-linked nanocomposite single ion coductor as
a function of the modified silica content.
N.-S. Choi et al. / Solid State Ionics 167 (2004) 293–299 297
result indicates that the increase of the modified silica
content in the cross-linked single ion conductor causes an
increase in the viscosity of the plasticizer phase (PC/DMSO
mixture) resulting in the retarded migration of the free ions.
The increase in the viscosity of the plasticizer phase is due
to the reduction of free PC/DMSO by ion–dipole interac-
tion between modified silica and plasticizer. The n param-
eter, which reflects the tortuosity of the conduction
pathway, decreased with increase of the modified silica
content as shown in Table 1. Since the modified silica
existing in the plasticizer phase can interrupt the migration
of free ions, as illustrated in Fig. 4, the connectivity of ion
conducting phase becomes more tortuous with increase of
the modified silica content (Table 2).
Fig. 5a shows that low temperature ion conductivities are
enhanced with increasing the modified silica content. This
seems to be due to the suppression of the crystallization of
the plasticizer by the ion–dipole interaction between the
plasticizer and lithium cations attached on the surface of the
modified silica and by the increase of volume fraction of the
Table 2
Mole concentration of lithium ion, transference number, T2, ionic
conductivity, and interfacial resistance of PC/DMSO and cross-linked
single ion conductora containing modified silica
Modified silica
content based
on PC/DMSO
Mole concentration
of lithium/1 g
PC/DMSO
(mmol/1 g)
TLi + Ion
conductivity
(S/cm)
Interfacial
resistance
(V)
10 wt.% in
PC/DMSO
0.54 0.95 4.3� 10� 4 112
10 wt.% in polymer/
PC/DMSOa
0.54 0.97 2.2� 10� 4 150
Conventional single
ion conductorb5.20 1.00 2.9� 10� 4 103
a Nanocomposite single ion conductor consisting of PEGDMA-based
cross-linked matrix and PC/DMSO plasticizer.b Reference system consisting of PEGDMA-based cross-linked matrix
with LiMA and PC/DMSO plasticizer.
modified silica in the single ion conductor. To affirm effect
of the modified silica on the crystallization behavior of the
plasticizer, DSC thermograms of the cross-linked nano-
composite single ion conductor were obtained as shown in
Fig. 6a. Fig. 6b represents a plot of DHm (heat of fusion)
versus the modified silica content. It is found that the
crystallization of the plasticizer in the cross-linked nano-
Fig. 7. Initial impedance spectra of the cross-linked nanocomposite single
ion conductor at room temperature (2� 2 cm2).
Fig. 8. Interfacial stability of the cross-linked single ion conductor
containing a different modified silica content with storage time.
N.-S. Choi et al. / Solid State Ionics 167 (2004) 293–299298
composite single ion conductor is hindered with increasing
the modified silica content.
3.3. Effect of modified silica on interfacial stability
To investigate the effect of the modified silica on the
interfacial resistance and interfacial stability with storage,
the cross-linked single ion conductor without containing the
modified silica was prepared by the UV-curing of poly(eth-
ylene glycol) dimethacrylate (PEGDMA), lithium methac-
rylate (LiMA), and 300 wt.% plasticizer (PC/DMSO) of the
matrix polymer. LiMA was prepared by following the
procedures described elsewhere [19]. The mole ratio of
PEGDMA to LiMA was 20–80 and the ion conductivity
of this system was 2.9� 10� 4 S/cm at room temperature.
This value was similar ion conduction level with the nano-
composite single ion conduction system containing 30 wt.%
modified silica based on the matrix polymer.
Fig. 7 shows the impedance diagram obtained for the Li/
the cross-linked single ion conductor/Li symmetrical cell
with a different content of the modified silica content. The
cross-linked single ion conductor without containing the
modified silica showed the lowest initial interfacial resis-
tance of 103 V at initial state among the tested ones. This
seems to be due to easier charge transfer reaction owing to
better physical adhesion of the single ion conductor without
containing the modified silica to the lithium electrode.
When the modified silica is added, the interfacial resistance
of the cross-linked nanocomposite single ion conductor
decreased with increasing the modified silica content. Since
a larger number of free ions can lead to easier charge
transfer reaction at the Li/single ion conductor interface,
the cross-linked nanocomposite single ion conductor with a
higher modified silica content showed a lower interfacial
resistance than that with a lower amount of modified silica.
The interfacial stability between the cross-linked nano-
composite single ion conductor and the lithium electrode
was investigated by monitoring the impedance response of a
Li/the single ion conductor/Li symmetrical cell for a period
of 24 days.
Fig. 8 represents the interfacial resistance of the cross-
linked nanocomposite single ion conductor with a different
content of the modified silica. The interfacial resistance of
the cross-linked single ion conductor without containing the
modified silica sharply increased with a storage time.
However, the interfacial resistance of the cross-linked single
ion conductor with the modified silica showed a relatively
stable value with storage. This result indicates that the
modified silica not only plays as a lithium ion source but
also inhibits the interfacial reaction of the plasticizer such as
PC and DMSO with the lithium electrode. It is well known
that the nanosized ceramic materials will tend to minimize
the area of the lithium electrode exposing to the plasticizer
and thus can reduce passivation on the lithium electrode
surface. The surface of the modified silica could have been
well wetted by the ion–dipole interactions between the ion
groups of the modified silica and the plasticizer, PC/DMSO.
This could minimize the growth of passivation layer by the
decomposition reaction of PC and DMSO at the Li/single
ion conductor interface resulting in the enhancement of
interfacial stability.
4. Conclusions
The nanosized-fumed silica was modified by the reaction
of 1,3-propane sultone with the surface hydroxyl group and
the modified silica was characterized by FT-IR and solid
state 29Si CP/MAS NMR spectra. The introduction of the
modified silica as an organic–inorganic hybrid lithium salt
led to an increase in the ion conductivity, but at higher
content of the modified silica the pathway for the conduct-
ing phase of the free lithium ions became more tortuous.
The interfacial stability of the cross-linked single ion con-
ductor with the modified silica was also improved through
prevention of the growth of the passivation layer at the Li/
single ion conductor interface.
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