Network-type ionic conductors based on oligoethyleneoxy-functionalized pentamethylcyclopentasiloxanes

Article abstract of DOI:10.1021/ma050066k

Network-type solid polymer electrolytes (NSPEs) were synthesized containing oligoethylene oxide chains, CH₃(OCH₂CH₂) ₃(CH₂)₃-, within the network structures. Hydrosilylation reactions of precursors 1 and 2 (oligoethyleneoxy partially substituted pentamethylccyclopentasiloxanes (D₅^H)) with an α,ω-diallyloligo(ethylene glycol) were employed for the formation of the cross-linked networks. The conductivities of the network polymer/LiX complexes with variable EO/Li ratios were measured by impedance experiments. NSPE-2, with 36.0% cross-linking density, exhibited higher conductivity than NSPE-1, with 43.8%. The optimum conductivity (σ = 9.24 × 10 ^-5 S/cm at 25°C, 2.11 × 10^-4 S/cm at 37°C) was found for NSPE-2 with lithium bis(oxalato)borate (LiBOB). LiBOB-doped polymers exhibited higher conductivity than those doped with LiTFSI at the same salt concentration.

Full text of DOI:10.1021/ma050066k

5714
Macromolecules 2005, 38, 5714-5720
Network-Type Ionic Conductors Based on
Oligoethyleneoxy-Functionalized Pentamethylcyclopentasiloxanes
Zhengcheng Zhang,† Leslie J. Lyons,‡ Khalil Amine,§ and Robert West*,†
Organosilicon Research Center, Department of Chemistry, University of WisconsinsMadison,
Wisconsin 53706; Department of Chemistry, Grinnell College, Grinnell, Iowa 50112; and Chemical
Technology Division, Argonne National Laboratory, Argonne, Illinois 60439
Received January 12, 2005; Revised Manuscript Received May 2, 2005
ABSTRACT: Network-type solid polymer electrolytes (NSPEs) were synthesized containing oligoethylene
oxide chains, CH3(OCH2CH2)3(CH2)3-, within the network structures. Hydrosilylation reactions of
precursors 1 and 2 (oligoethyleneoxy partially substituted pentamethylccyclopentasiloxanes (D5H)) with
an R,ω-diallyloligo(ethylene glycol) were employed for the formation of the cross-linked networks. The
conductivities of the network polymer/LiX complexes with variable EO/Li ratios were measured by
impedance experiments. NSPE-2, with 36.0% cross-linking density, exhibited higher conductivity than
NSPE-1, with 43.8%. The optimum conductivity (σ ) 9.24 × 10-5 S/cm at 25 °C, 2.11 × 10-4 S/cm at 37
°C) was found for NSPE-2 with lithium bis(oxalato)borate (LiBOB). LiBOB-doped polymers exhibited
higher conductivity than those doped with LiTFSI at the same salt concentration.
Introduction
this article, we describe cross-linked network polymer
electrolytes based on cyclosiloxanes, synthesized by
hydrosilylation of partially functionalized pentameth-
ylcyclopentasiloxanes (D5H) with an R,ω-diallyloligo-
(ethylene glycol) cross-linker. Impedance and differen-
tial scanning calorimetry (DSC) measurements were
carried out to characterize the new materials.
Solid polymer electrolytes (SPEs) have been widely
investigated because of their overall potential as practi-
cal materials for applied electrochemistry,1,2 and a
number of reviews on this field are available.3-5 Used
as both a separator and an electrolyte in a solid-state
battery, SPEs have many merits over their liquid
counterparts such as good mechanical stability, flex-
ibility in shapes and sizes, and increased safety. The
first generation of high molecular weight poly(ethylene
oxide) (PEO) polymers were generally poor conductors
(σ < 10-7 S cm-1) due to the high degree of crystallinity.
Reasonable conductivity in PEO-salt polymer electro-
lytes can be achieved at around 100 °C; however, the
mechanical properties of the polymer are quite poor.1
Better results are obtained for polymers with highly
flexible backbones, bearing oligo(ethylene oxide) (EO)
as side chains. Polyphosphazenes6-12 of this type have
been studied extensively in form of comb polymers, block
copolymers, and cross-linked network polymers. In
recent years, oligo(ethylene glycol)-substituted poly-
siloxanes as ionically conductive polymer hosts have
been widely investigated by Smid,13-15 Shriver,16-18
Acosta,19 and West and co-workers.20-23 The remarkable
improvement in the ionic conductivity for these poly-
mers was ascribed to the highly flexible inorganic
backbone which produced a totally amorphous polymer
host with a very low glass transition temperature.
Plasticized and gel polymer electrolytes24 were synthe-
sized aiming to obtain desirable conductivities.
Experimental Section
Materials. Pentamethylcyclopentasiloxane (D5H) was pur-
chased from Gelest and distilled prior to use. Allyl bromide,
Karlstedt’s catalyst (platinum divinyltetramethyldisiloxane)
(3% in xylene solution), tri(ethylene glycol) methyl ether, and
poly(ethylene glycol) with a molecular weight of 600 were
supplied by Aldrich and used without purification. Sodium
hydride (NaH, 60% dispersion in mineral oil) was obtained
from Acros Organics. THF was dried over sodium benzophe-
none ketyl and was distilled in an atmosphere of dry nitrogen
before use. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)
was a gift from 3M Co. and was dried under vacuum at 120
°C for 24 h prior to use. Lithium bis(oxalato)borate (LiBOB)
was supplied by Chemetall and was purified by recrystalliza-
tion using dried CH3CN as solvent. NMR grade CDCl3 was
stored over 4A molecular sieves.
Tri(ethylene glycol) Allyl Methyl Ether (APEO3M). Tri-
(ethylene glycol) allyl methyl ether was synthesized as previ-
ously reported.21 A solution of tri(ethylene glycol) methyl ether
(MPEO3) (98.4 g, 0.6 mol) in 250 mL of THF was added
dropwise to a suspension of NaH (60% dispersion in mineral
oil) (28.8 g, 0.72 mol) in THF (250 mL) chilled to 0 °C. This
solution was stirred for a further 2 h followed by the dropwise
addition of allyl bromide (87.1 g, 0.72 mol). The resulting
mixture was stirred overnight and then filtered to remove the
NaBr product and excess NaH. All volatile materials were
removed by rotary evaporation to yield an orange oil. This oil
was dissolved in water, and unreacted MPEO3 was extracted
using 3 × 50 mL portions of toluene. The desired product was
then extracted into chloroform from the water layer with 3 ×
200 mL portions of CHCl3; this was dried with MgSO4, and
all volatile materials were removed by rotary evaporation. 110
g (90%) of product was collected by Kugelrohr distillation (80
°C/0.5 Torr): 1H NMR (CDCl3), δ (ppm): 5.85 (m, 1H), 5.15
(dd, 2H), 3.95 (d, 2H), 3.45-3.65 (m, 12H), 3.30 (s, 3H). 13C
NMR (CDCl3), δ (ppm): 134.6, 116.8, 72.1, 71.8, 70.3-70.0,
69.2, 58.9.
In our laboratory we have focused on the synthesis
of cross-linked network conductors that could provide
good mechanical strength as well as high ambient
conductivity. In a previous paper, we demonstrated that
the conductivity of cross-linked linear polysiloxanes22,23
with pendant oligo(ethylene glycol) chains showed ex-
cellent conductive properties, which has driven us to
extend our research to network-type cyclic siloxanes. In
† University of WisconsinsMadison.
‡ Grinnell College.
§ Argonne National Laboratory.
10.1021/ma050066k CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/25/2005

Macromolecules, Vol. 38, No. 13, 2005
Network-Type Ionic Conductors 5715
Cross-Linker: R,ω-Diallyloligo(ethylene glycol). The
diallyl-terminated oligo(ethylene glycol) was prepared using
the method described previously.22 The following reagents and
quantities were used: NaH (9.6 g, 0.24 mol of a 60 wt %
solution in mineral oil), THF (100 mL), PEG (Mw 600, 60.0 g,
0.10 mol), THF (150 mL), allyl bromide (31.46 g, 0.26 mol).
The crude product was purified by passing through a silica
gel column. Final product: 62.5 g, 92% yield. 1H NMR (CDCl3),
δ (ppm): 5.85 (m, 2H), 5.15 (dd, 4H), 3.95 (d, 4H), 3.45-3.65
(m, 40H), 3.30 (s, 6H). 13C NMR (CDCl3), δ (ppm): 134.6, 116.8,
72.1, 71.8, 70.3-70.0, 69.2, 58.9.
Partially Substituted Pentamethylcyclopentasilox-
anes (1). The cross-linkable cyclic precursors were synthesized
H
by platinum-catalyzed hydrosilylation of D5 with APEO3M.
Figure 1. Lithium salts (LiTFSI and LiBOB) structure and
Into a vacuum flame-dried 100 mL round-bottom flask was
charged pentamethylcyclopentasiloxane (5.0 g, 16.6 mmol) and
APEO3M (10.2 g, 50.0 mmol) through a syringe under an argon
atmosphere. Then 40 µL (100 ppm relative to -Si-H bond) of
Karlstedt’s catalyst was syringed into the flask through the
septum with stirring. The flask was heated to 65 °C for 12 h,
after which Kugelrohr distillation was employed to remove the
low boiling point byproducts. The viscous polymer was then
decolorized by refluxing in toluene with activated charcoal for
12 h. After removal of toluene, a colorless liquid polymer was
obtained, 12.9 g (85%).
diagram of conductivity measurement cell.
were loaded in hermetically sealed aluminum pans. Duplicates
of all samples were measured. Glass transition temperatures
are reported as the onset of the inflection in the heating curve
from -150 to 80 °C at a heating rate of 10 °C/min.
FT-IR spectra were recorded on a Nicolet Nexus 670
spectrometer as cross-linked films placed on the Avatar
multibounce HATR accessory.
1H NMR (CDCl3), δ (ppm): 4.62 (Si-H), 3.65-3.45 (CH2-
CH2O), 3.32 (OCH3), 1.55 (CH2CSi), 0.46 (SiCH2), 0-0.2 (Si-
CH3). 13C NMR (CDCl3), δ (ppm): 73.9, 72.0, 70.6-70.7, 70.1,
59.1, 23.0-23.1, 13.0-13.2, 1.16, 1.15, -0.63, -0.96. 29Si NMR
(CDCl3), δ (ppm): -20.2 to -24.1, -35.5 to -38.7.
All NMR chemical shifts are reported in parts per million
(δ ppm); downfield shifts are reported as positive values from
tetramethylsilane (TMS) as the standard at 0.00 ppm. The 1H
and 13C chemical shifts are reported relative to the NMR
solvent as an internal standard, and the 29Si chemical shifts
are reported relative to an external TMS standard. NMR
spectra were recorded using samples dissolved in CDCl3,
unless otherwise stated, on the following instrumentation.
Carbon-13 NMR was recorded as proton-decoupled spectra,
and 29Si was recorded using an inverse gate pulse sequence
with a relaxation delay of 30 s.
Partially Substituted Pentamethylcyclopentasilox-
anes (2). Precursor 2 was synthesized using the same proce-
dure as described for precursor 1. The following reagents and
quantities were used: D5H (6.25 g, 20.8 mmol); APEO3M (15.3
g, 75.0 mmol); Karlstedt’s catalyst (50 µL). After treatment
with charcoal, the final cyclic precursor was a colorless liquid,
17.88 g (83%). 1H NMR (CDCl3), δ (ppm): 4.62 (Si-H), 3.65-
3.45 (CH2CH2O), 3.32 (OCH3), 1.55 (CH2CSi), 0.46 (SiCH2),
0-0.2 (Si-CH3). 13C NMR (CDCl3), δ (ppm): 73.9, 72.0, 70.6-
70.7, 70.1, 59.1, 23.0-23.1, 13.0-13.2, 1.16, 1.15, -0.63, -0.96.
29Si NMR (CDCl3), δ (ppm): -20.2 to -24.1, -35.5 to -38.7.
Network Polymer Electrolyte Films Formation. A
representative procedure for NSPE-1 was as follows: The
stoichiometric amounts of precursor 1 (0.5 g), R,ω-diallyloligo-
(ethylene glycol) cross-linker (0.334 g, 0.49 mmol), LiN(CF3-
SO2)2 (4.38 mL, 0.1547 M THF solution), and Karstedt’s
catalyst (3 µL, 3% xylene solution) were injected into a flame-
dried Schlenk flask. The homogeneous solution was then
evacuated for 12 h on a standard Schlenk line and then further
evacuated on a high-vacuum line (∼10-5 Torr) to make the
mixture fully dry. The flask was transferred into a glovebox,
where the dry liquid electrolyte was loaded into the O-ring of
a conductivity measurement cell. The liquid mixture was cured
into a solid by heating at 75 °C for 48 h to ensure that the
cross-linking reaction was as complete as possible. Light
yellowish self-standing films with thickness of 2.0 mm were
obtained, which were subjected to impedance experiments.
Measurements. The ac impedance measurements were
performed under computer control using a Princeton Applied
Research model 273A potentiostat/galvanostat, a Princeton
Applied Research model 1025 frequency response analyzer for
frequency control (75 Hz-100 kHz), and Princeton Applied
Research PowerSine impedance software for data acquisition.
Subsequently, the data obtained were analyzed on a PC with
Microsoft Excel. Room temperature conductivity measure-
ments were at 23 ( 1 °C while variable-temperature measure-
ments (25-70 °C) were made by placing the electrochemical
cell (Figure 1) in a jacketed holder and circulating ethylene
glycol/water from a Lauda RMT6 circulating bath. Actual
temperatures were determined via an Omega thermocouple
attached directly to the cell.
Results and Discussion
Synthesis. APEO3M and R,ω-diallyloligo(ethylene
glycol) were prepared in a one-pot synthesis from tri-
(ethylene glycol) monomethyl ether and oligo(ethylene
glycol) by reacting alkoxides of the reagents with allyl
bromide. Precursors with variable amount of -Si-H
functionalities were accomplished by altering the rela-
H
tive feeding ratios of D5 and APEO3M, as depicted in
Scheme 1. The formation of platinum colloids25,26 was
indicated by a yellow color of the solution during the
hydrosilylation which changes to dark brown27,28at the
end of the reaction. The extent of hydrosilylation reac-
tion (disappearance of CH2dCH-CH2-OR proton sig-
nals) and the purification (disappearance of cis- and
1
trans-CH3CHdCH2-OR) were monitored by H NMR.
The cyclic oligomer solutions were decolorized with
activated charcoal, passed through a silica gel column,
and evaporated. The structure and composition (Si-R
and Si-H ratios) of the purified precursors were ana-
lyzed and summarized in Table 1. Representative
1
spectra of H and 29Si NMR are shown in Figure 2.
Scheme 2 outlines the strategy used for the produc-
tion of NSPEs by hydrosilylation cross-linking reactions
in the presence of lithium salt. The precursor, cross-
linker, and LiTFSI were dissolved in THF. The complete
removal of THF was then achieved by evacuating to
pressure <5 × 10-5 Torr, after which the viscous
mixture was loaded into an electrochemical cell with
stainless steel ion blocking electrodes and sealed with
O-rings. The dry liquid was cross-linked into a solid
membrane upon heating. Interestingly, our gelation test
demonstrated that precursors with Si-H molar content
lower than 30% (precursors 3 and 4) could not be made
Thermal measurements were recorded on a Perkin-Elmer
Pyris Diamond DSC operated under computer control. Low
temperatures were achieved by using the TA Instruments
liquid nitrogen cooling accessory. Samples of preformed gels

5716 Zhang et al.
Macromolecules, Vol. 38, No. 13, 2005
Figure 2. Representative NMR spectra of precursor 2: 1H NMR (top); 29Si NMR (bottom).
Scheme 1. Synthesis of Monomer, Cross-Linker, and Cyclic Siloxane Precursors 0-4
absorption at 2160 cm-1 in FT-IR spectra (Figure 3),
indicating the completion of network formation.
Table 1. Cross-Linking Test Results
precursor
x
y
Si-H content (%)
film consistency
Ionic Conductivity. The impedance spectra were
determined in an equivalent circuit of a parallel capaci-
tance and resistance. The magnitude of the resistance
was calculated at the points where the phase angle in
the impedance data was closest to zero using the
equation R ) Z cos(θ), where R is the resistance, Z the
impedance, and θ the phase angle. The conductivity, σ,
was calculated from the relationship σ ) 1/R × l/A,
0
1
2
3
4
2.32 2.68
2.81 2.19
3.20 1.80
3.49 1.51
4.15 0.85
53.6
43.8
36.0
30.2
17.0
rigid film
rigid film
soft film
very viscous liquid
liquid
into solid films, and the precursor with 20% Si-H only
resulted in viscous liquid (Table 1), presumably due to
the deficient cross-linking sites. Two NSPEs were
produced from precursors 1 (43.8% Si-H) and 2 (36.0%
Si-H); both films did not dissolve but were swollen by
the deuterated solvents. No Si-H resonance at 4.67 ppm
or CH2dCH- resonances at 5.1 and 5.9 ppm were
observed in 1H NMR spectra, nor Si-H stretching
where l/A values were typically 0.2 cm-1
.
The room temperature conductivities were slightly
lower than those of cross-linked linear polysiloxane
electrolytes as reported by us previously.22,23 Conductiv-
ity data of NSPE-1 and NSPE-2 are summarized in
Table 2. These NSPEs show typical behavior as ionic

Macromolecules, Vol. 38, No. 13, 2005
Network-Type Ionic Conductors 5717
Scheme 2. Formation and Network Structure of Cross-Linked Polymer Electrolytes
conductors: the conductivities increase to an optimum
value as the salt content increases, with the conductivity
dropping off at higher salt levels, as illustrated in Figure
4. This phenomenon is most likely due to the smaller
number of free ions present at lower salt concentration
and formation of anchored bulky ion clusters at higher
salt level, which hindered the conductivity.
6. NSPE-1 and NSPE-2 showed typical increased con-
ductivity with increasing temperature, as observed for
many other polymer systems.7-14,16,19,21-23 The type
of ion transport suggested by all the plots can be
described by the WLF equation or the Vogel-Tamman-
Fulcher3,29-31 equation, σ ) AT-1/2 exp(-B/(T - T0)),
producing parameters for the activation energy (Ea) and
empirical glass transition temperature (T0), as listed in
Table 2.
The variable-temperature ionic conductivities for the
solid polymer electrolytes are shown in Figures 5 and
Figure 3. Representative FT-IR spectra of network polymer
electrolytes: NSPE-1-2, EO/Li ) 20 (top); NSPE-2-2, EO/Li
) 20 (bottom).
Figure 5. Temperature-dependent ionic conductivities of
cross-linked networks: (A) NSPE-1 and NSPE-2; (B) compari-
son of maximum conductivity for NSPE-1 (2) and NSPE-2 (b).
Figure 4. Plots of ambient conductivities of NSPE-1 and 2
at various EO/LiTFSI.

5718 Zhang et al.
Macromolecules, Vol. 38, No. 13, 2005
shown in Table 2 and Figure 5B. Within the tempera-
ture measurement range, the optimum conductivities
for NSPE-2 were 35% higher than those for NSPE-1,
as expected because this system has lower cross-linking
density which allows more flexible chain motion within
the network structure.
The type of anion in the metal salt also influenced
the conductivity. Larger ions induce better ion-pair
separation, in addition to generating a larger free
volume.32 Thus, lithium ions become more mobile
resulting in increased conductivity. This effect was
detected for NSPE-2 by doping with lithium bis(oxala-
to)borate) (LiBOB)33 and LiN(CF3SO2)2 (LiTFSI), which
are widely investigated electrolyte salts. Data in Table
2 reveal that with same doping level of 20/1 the polymer/
LiBOB complex displayed higher conductivity than the
LiTFSI-doped material both at room temperature and
at elevated temperatures (Figure 6). The superior
conductivity associated with LiBOB is presumably due
to the electron-withdrawing boron providing a very
weakly basic bulky anion,34,35 leading to the easy
dissociation into free ions.
Figure 6. Variable temperature conductivities for NSPE-2
with LiTFSI and LiBOB at salt concentration (EO/Li ) 20).
The conductivity behavior was influenced by the
cross-linking density. For NSPE-1 (x ) 2.81, y ) 2.19)
and NSPE-2 (x ) 3.20, y ) 1.80), the maximum
conductivities and their temperature dependence are
Figure 7. DSC thermograms of NSPE-1 and NSPE-2 with various lithium salt concentrations.

Macromolecules, Vol. 38, No. 13, 2005
Network-Type Ionic Conductors 5719
Table 2. Conductivity, Ea, and T0 Values and Thermal Analysis of NSPE-1,2
conductivity
conductivity
polymer
EO/Li
25 °C (S cm-1
)
37 °C (S cm-1
)
Eaa (kJ/mol)
T0a (K)
Tgb (K)
NSPE-1-1
NSPE-1-2
NSPE-1-3
NSPE-1-4
NSPE-1-5
NSPE-2-1
NSPE-2-2
NSPE-2-3
NSPE-2-4
NSPE-2-5c
NSPE-2-6c
15:1
20:1
24:1
28:1
32:1
15:1
20:1
24:1
32:1
20:1
24:1
3.48 × 10-5
4.90 × 10-5
5.12 × 10-5
5.48 × 10-5
3.97 × 10-5
4.55 × 10-5
6.12 × 10-5
7.38 × 10-5
3.04 × 10-5
9.24 × 10-5
6.33 × 10-5
8.53 × 10-5
9.30 × 10-5
9.68 × 10-5
1.14 × 10-4
7.46 × 10-5
8.86 × 10-5
9.38 × 10-5
1.92 × 10-4
5.75 × 10-5
2.11 × 10-4
9.25 × 10-5
7.33
5.13
5.48
6.12
5.02
5.15
6.04
3.45
6.70
4.24
6.56
192.97
204.81
156.23
150.80
196.86
146.12
153.25
136.78
160.71
149.64
152.44
220.88
217.77
213.45
211.69
210.20
215.62
212.05
210.59
207.62
215.43
212.14
a Two parameters fit of VTF equation. b Measured by DSC with a scanning rate of 10 °C/min. c Doped with LiBOB.
The glass transition temperatures, Tg, for cross-linked
polymer electrolytes NSPE-1 and 2 were studied by DSC
analysis. The DSC curves of the salt-free as well as the
salt-complexed polymers confirmed that these materials
are completely amorphous over the temperature range
of -150 to 50 °C. As seen in Figure 7, glass transitions
ranging from -65.38 to -52.12 °C were observed for
each polymer. Tg’s of all the polymers are reported in
Table 1. These values are well below the room temper-
ature, and the low values reflect the good flexibility of
the siloxane segments. Both polymers showed increases
in Tg values with increases in LiTFSI salt concentration,
presumably due to transient coordinative cross-linkings
between the oligoether chains. The Tg values of NSPE-
1/LiTFSI complexes were higher than those of NSPE-2
complexes, indicating that NSPE-2 has higher segmen-
tal flexibility, thus facilitating the ion transfer. The
result is consistent with the ionic conductivity results
discussed earlier.
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A new type of solid polymer electrolyte based on cross-
linked pentamethylcyclopentasiloxane (D5H) precursors
was synthesized and studied. These SPE’s, with a
unique cross-linked network structure, were synthesized
by hydrosilylation of partially oligoethyleneoxy-substi-
tuted pentamethylcyclopentasiloxane (D5H) precursors
with an R,ω-diallyloligoethyleneoxy cross-linking re-
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Acknowledgment. This work was supported by the
U.S. Department of Commerce, National Institute of
Standards and Technology, Advanced Technology Pro-
gram. We thank 3M Co. for the gift of LiN(CF3SO2)2.
Instrumentation at Grinnell was funded by the NSF-
MRI program (Grant 0116159). The NMR instruments
in this research were funded by NIH (Grants NIH 1 S10
RRO 8389-01, 1 S10 RRO8389-01, 1 S10 RRO4981-01,
and CHE-9629688).
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