Plasticizing Li single-ion conductors with low-volatility siloxane copolymers and oligomers containing ethylene oxide and cyclic carbonates

Article abstract of DOI:10.1039/c5ta06042g

To prepare a safe electrolyte for lithium ion batteries, two groups of novel low-volatility plasticizers combining pendant cyclic carbonates and short ethylene oxide chains have been successfully synthesized, as confirmed by ¹H, ¹³C and ²⁹Si NMR spectroscopy. The Fox equation describes the composition dependence of the glass transition temperature (T_g) very well for the random polysiloxane-based copolymer plasticizers (11000 < M < 13000) while the smaller oligomer plasticizers have T_g as much as 20 K lower than the Fox equation prediction because of their lower molecular weight (450 < M < 700). The Landau-Lifshitz mixing rule describes the dielectric constant of the random polysiloxane-based copolymer plasticizers at all temperatures above T_g. Mixing with 20 wt% polysiloxane tetraphenyl borate-Li ionomer (14 mol% borate and 86 mol% cyclic carbonate) increases conductivity relative to the neat ionomer by lowering T_g, increasing dielectric constant and providing better solvation of Li⁺. The best oligomeric plasticizer only has T_g 10 K lower than the Fox prediction but has dielectric constant 30% larger than expected by the Landau-Lifshitz mixing rule, owing to a surprisingly low viscosity, resulting in ambient conductivity 2 × 10^-5 S cm^-1. For both groups of plasticizers, the fraction of cyclic carbonates relative to ethylene oxide governs the magnitude and temperature dependence of the ionic conductivity.

Full text of DOI:10.1039/c5ta06042g

Journal of
Materials Chemistry A
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PAPER
Plasticizing Li single-ion conductors with low-
volatility siloxane copolymers and oligomers
containing ethylene oxide and cyclic carbonates†
Cite this: DOI: 10.1039/c5ta06042g
ab
ac
ad
a
a
Siwei Liang, Quan Chen,‡ U Hyeok Choi, Joshua Bartels, Nanqi Bao,
a
a
James Runt and Ralph H. Colby*
To prepare a safe electrolyte for lithium ion batteries, two groups of novel low-volatility plasticizers
combining pendant cyclic carbonates and short ethylene oxide chains have been successfully synthesized,
1
13
29
as confirmed by H, C and Si NMR spectroscopy. The Fox equation describes the composition
dependence of the glass transition temperature (T ) very well for the random polysiloxane-based
copolymer plasticizers (11 000 < M < 13 000) while the smaller oligomer plasticizers have T as much as
0 K lower than the Fox equation prediction because of their lower molecular weight (450 < M < 700).
The Landau–Lifshitz mixing rule describes the dielectric constant of the random polysiloxane-based
copolymer plasticizers at all temperatures above T . Mixing with 20 wt% polysiloxane tetraphenyl borate –
g
g
2
g
Li ionomer (14 mol% borate and 86 mol% cyclic carbonate) increases conductivity relative to the neat
+
ionomer by lowering T
oligomeric plasticizer only has T
larger than expected by the Landau–Lifshitz mixing rule, owing to a surprisingly low viscosity, resulting in
g
, increasing dielectric constant and providing better solvation of Li . The best
Received 3rd August 2015
Accepted 9th September 2015
g
10 K lower than the Fox prediction but has dielectric constant 30%
DOI: 10.1039/c5ta06042g
ꢁ5
ꢁ1
ambient conductivity 2 ꢀ 10 S cm . For both groups of plasticizers, the fraction of cyclic carbonates
www.rsc.org/MaterialsA
relative to ethylene oxide governs the magnitude and temperature dependence of the ionic conductivity.
Ionomers with weak binding borate anions are particularly
Introduction
7
–10
useful as cation conductors.
Since the boron is less electro-
Polymer electrolytes as energy materials are of great interest for negative than carbon, B is positive and the negative charge is
+
lithium-ion battery, fuel cell, solar cell and ionic actuator delocalized, weakening the ion pair energy with Li and other
1
1,12
applications, due to the inherent polymeric merits, such as cations.
mechanical robustness and ease of processing to make thin- polysiloxane single-ion conductors with cyclic carbonate side
lm membranes. Among different types of polymer electrolytes, chains were shown to have exceptionally low conducting ion
Recently developed novel tetraphenyl borate – Li
13

ꢁ
1
single-ion conductors with one type of ion covalently bonded to activation energy (7–10 kJ mol ) from the weak binding
the polymer have recently attracted increased attention, owing between delocalized anions and Li and the strong solvation of
to (1) high selectivity of conducting ions (lithium ion trans- Li from the carbonate oxygens. The nonionic polysiloxane
+
+
ference number near unity) and (2) elimination of anion homopolymer with only cyclic carbonate side chains has
polarization at the electrode/electrolyte interface that weakens dielectric constant ꢂ50 near room temperature, further weak-
the electric eld inside the electrolyte and causes safety ening the ionic interactions. However, it seems that the large
1
–6
problems.
tetraphenyl borate anions repel the cyclic carbonate side chains,
effectively aggregating the ions on the nanoscale, which raises
+
T , lowers segmental mobility, and traps the majority of the Li
g
a
Materials Science and Engineering, The Pennsylvania State University, University
Park, PA 16802, USA. E-mail: rhc@plmsc.psu.edu
+
ions in aggregates, with only between 1% and 5% of the Li ever
b
participating in conduction, as seen in the innite temperature
intercept of the Arrhenius simultaneously conducting Li
Joint Center for Articial Photosynthesis, Lawrence Berkeley National Laboratory, One
+
Cyclotron Road, Berkeley, CA 94720, USA
c
13
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of content.
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
Addition of polar solvating plasticizers can improve the
conductivity through three effects: (1) lowering T to improve
d
Functional Composites Department, Korea Institute of Materials Science, Changwon,
g
6
42-831, Korea
ionic mobility, (2) raising dielectric constant to weaken the
†
Electronic supplementary information (ESI) available: Composition of
+
ionic interactions and (3) providing specic solvation of Li ,
copolymer plasticizers and NMR spectra of all plasticizers. See DOI:
with the former one to enhance ionic mobility and the latter two
1
0.1039/c5ta06042g
+
to increase participation of Li in conduction. By simply
‡
Co-rst author.
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crosslinking the polyelectrolytes swollen in polar solvating Dielectric relaxation spectroscopy (DRS) measurements were
14–19
plasticizers, one makes so-called gel electrolytes.
When conducted on samples sandwiched between two polished
mixing with single-ion conductors, plasticizers appear to solvate brass electrodes, with 50 mm silica spacers placed in between
+
and transport Li cations more efficiently. In particular, cyclic the two electrodes. The sandwiched samples were positioned
carbonates such as propylene carbonate (PC) and ethylene in a Novocontrol GmbH Concept 40 broadband dielectric
carbonate (EC) have been widely used in the Li-ion battery spectrometer and the dielectric permittivity and conductivity
industry, due to their high dielectric constant, strong solvation were measured using an AC voltage amplitude of 0.1 V and
+
ꢁ2
7
of Li and ability to form a stable electrode–electrolyte interfa- 10 to 10 Hz frequency range. Prior to the DRS measure-
ꢃ
cial layer. However, safety issues caused by leakage and evapo- ments, the samples were annealed in the Novocontrol at 120 C
ration of these small-molecule carbonates prompt scientists to in a heated stream of dry nitrogen for 1 hour to drive off any
pursue non-volatile alternatives. Poly(ethylene glycol) (PEG) is moisture picked up during loading of these hygroscopic
considered as a good candidate because it can break ion materials. Data were collected in isothermal frequency sweeps
+
ꢃ
aggregates and solvate cations: when Li leaves an ion aggre- from 120 C to near T
g
. Linear viscoelastic measurements were
gate, the ether oxygen of PEG can solvate and stabilize the free conducted with an Advanced Rheometric Expansion System
+
20–28
ꢁ1
Li .
Recently, PEG with molecular weight of 600 g mol has (ARES-LS, Rheometric Scientic). Parallel plates with diame-
been shown to greatly enhance conductivity of polysiloxane ters of 25 mm were utilized to conduct dynamic frequency
ꢃ
tetraphenyl borate-Li single-ion conductors with cyclic sweeps at room temperature (20 C). Strain amplitudes lower
22
carbonate side chains; although crystallization of the PEG than 0.1 were applied and conrmed to be in the linear
oligomers near room temperature led to an abrupt drop in response region.
conductivity at lower temperatures when more than ꢂ40 wt%
PEG was added.
Cyclic [(allyloxy) methyl] ethylene ester carbonic acid (CECA)
This paper explores novel plasticizers of low-volatility at
working conditions of lithium-ion batteries. They are composed
CECA was prepared according to the method reported in ref. 13.
Potassium carbonate (3 g, 21.7 mmol) was added to a mixture of
of ether-oxygen units (low T
specic solvation of Li ) and cyclic carbonate units (higher T
g
, moderate polarity and strong
3
-(allyloxy)-propane-1,2-diol (92.4 g, 0.188 mol) and diethyl
+
g
,
ꢃ
carbonate (24.8 g, 0.188 mol). Aer stirring at 120 C for 24 h,
the mixture was ltered to isolate the solid. The residue was
+
higher polarity and similar strong solvation of Li ) with
molecular weight 450 < M < 12 000. The composition of the low-
volatility plasticizers was carefully tuned. Blends of these plas-
ticizers with 20 wt% of a tetraphenyl borate – Li polysiloxane
single-ion conductor exhibit improved ionic conductivity up to
puried by K u¨ gelrohr distillation to yield the pure product as a
1
colorless liquid (23.7 g, 80%). H NMR (in CDCl
3
), d(ppm) 5.87
(
m, 1H, C]CH), 5.25 (d, 1H, cis H of CH ]C), 5.14 (d, 1H, trans
2
H of CH ]C), 4.86 (m, 1H, CCH(C)O), 4.38–4.55 (m, 2H, CH C),
2
2
ꢁ5
ꢁ1
ꢃ
2
ꢀ 10 S cm at 25 C. Furthermore, all the plasticizers and
4
.06 (m, 2H, OCH C]C), 3.60–3.74 (m, 2H, OCCH O).
2
2
their mixtures with the ionomer are strictly amorphous, allow-
ing a wider temperature range for applications.
3
Tri(ethylene glycol) allyl methyl ether (vinyl PEO )
To a mixture of NaH (1.44 g, 60% in mineral oil) dispersed in 20
mL dry THF was added a solution of tri(ethylene glycol) methyl
ether (4.7 mL, 0.03 mol) in 150 mL of THF dropwise at ice-bath
temperature. The mixture was stirred for 3 hours before being
transferred into a solution of allyl bromide (3.58 g, 0.03 mol) in
Experimental
Materials
Dichloromethane, diethyl carbonate, chlorodimethylsilane,
3
triethyl amine (NEt ), tetraethylene glycol, potassium carbonate,
20 mL dry THF. The mixture was allowed to react overnight to
2-(2-(vinyloxy)ethoxy)ethanol, diethyl carbonate, toluene and
complete the reaction. The reaction was quenched by ice water
and extracted by ethyl acetate (20 mL ꢀ 3). The organic phases
were combined and condensed by rotavap. The yellowish liquid
anhydrous acetonitrile were purchased from VWR and used
without further purication. Platinum divinyltetramethyldisilox-
ane complex (Pt[dvs]) (3% in xylene) catalyst, diethyldihy-
drosilane, tri(ethylene glycol) divinyl ether, di(ethylene glycol)
was then puried by vacuum distillation to yield vinyl PEO (5.5 g,
3
1
9
0%). H NMR (in d6-acetone), d(ppm) 5.85 (m, 1H, CH]), 5.2,
divinyl ether, RhCl(PPh
3 3
) catalyst and polymethylhydrosiloxane
(
d, cis H of ]CH
2
), 5.1 (d, trans H of ]CH
2
), 3.95 (d, 2H, C]
).
(
PMHS, M
n
¼ 3360) were purchased from Aldrich and used as
CCH
2
), 3.8–3.6 (m, 8H, OCH CH O), 3.35 (s, CH
2
2
3
received. Tetrahydrofuran (THF) from EMD Chemicals was
reuxed over sodium metal before use. The polysiloxane-based
ionomer was prepared by the method reported in ref. 13 with
borate content of 14 mol%.
General procedure for synthesis of homopolymer and random
copolymer plasticizers CPP (0, 19 31, 57, 80, 100)
All starting materials were dried using molecular sieves before
use. PMHS was added into a pre-dried ask equipped with a
condenser. The desired amount of CECA and vinyl PEO were
3
Characterization
1
13
29
H, C and Si NMR spectroscopy were recorded on a Bruker charged into the ask followed by 20 mL anhydrous CH CN and
3
ꢃ
AM 300M spectrometer. Glass transition temperatures (Tg) 0.2 mL Pt catalyst. The reaction mixture was stirred at 70 C. The
1
were determined using a TA Q100 differential scanning calo- completion of the reaction was conrmed by H NMR. The
ꢁ1
rimeter (DSC) at 10 K min
heating and cooling rates. reaction time was in the range of 2–7 days. The mixture was
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0
condensed by rotovap and the residue was dissolved in toluene 4,4 -(6,6,20,20-Tetraethyl-2,9,13,17,24-pentaoxa-6,20-
and precipitated in hexane 3 times. Aerwards, the product was disilapentacosane-1,25-diyl)bis(1,3-dioxolan-2-one) (OP67)
ꢃ
dried in a vacuum oven at 80 C for 24 hours.
The same procedure as synthesis of OP73 was used with
di(ethylene glycol) divinyl ether as the linker. H NMR (in d6-
1
0
4
,4 -(6,6,21,21-Tetramethyl-2,7,10,13,16,19,25-heptaoxa-6,21-
acetone), 4.95 (m, 1H, CCH(C)O), 4.4–4.6 (m, 2H, CH
.8 (m, 10H, CCH OCH CH CH , OCH CH CH and OCH
CECA (10.86 g, 0.068 mol), anhydrous CH CN (20 mL) and CH O), 1.64 (m, 4H, OCH CH CH Si), 1 (m, 6H, OCH CH CH
), 0.67 (m, 6H, CH ) (Fig. S7†).
2
C), 3.45 to
disilahexacosane-1,26-diyl)bis(1,3-dioxolan-2-one) (OP62)
3
2
2
2
2
2
2
2
2
-
3
2
2
2
2
2
2
2
chlorodimethylsilane (7.67 g, 0.081 mol) were added into a pre- and SiCH
2
3
dried ask. The mixture was cooled by ice-bath before 0.3 mL Pt
catalyst was charged. The mixture was allowed to react over-
night to complete the reaction. The solvent was evaporated and
Results and discussion
the residue was vacuum distilled to obtain 4-((3-(chloro- Synthesis
dimethylsilyl)propoxy)methyl)-1,3-dioxolan-2-one as a colorless
Two types of plasticizers were synthesized. The rst type is
1
liquid (15.2 g, 89%). H NMR (in d6-acetone), d(ppm) 4.95 (m,
H, CCH(C)O), 4.4–4.6 (m, 2H, CH C), 3.75 (m, 2H, CCH -
referred to as copolymerized plasticizer (CPP), having (EO) side
3
1
2
2
chains and cyclic carbonate side chains, synthesized by hydro-
OCH CH H ), 3.55 (m, 2H, OCH CH CH ), 1.7 (m, 2H, OCH -
2
2
2
2
2
2
2
silylation using a method described in our previous work
2
9
CH CH ), 0.9 (m, 2H, OCH CH CH ), 0.35 (m. CH ); Si NMR
13
2
2
2
2
2
1
3
(
Scheme 1). The CPP samples are coded according to the
3
(
in d6-acetone) 33 (s); (Fig. S2†) C NMR (in d6-acetone),
d(ppm) 160, 75.1, 74.8, 74, 73.2, 23, 15, 0.2 (Fig. S3†).
-((3-(Chlorodimethylsilyl)propoxy)methyl)-1,3-dioxolan-2-
one (8.7 g, 0.034 mol) was added dropwise into the mixture of
NEt (7.5 g), tetraethylene glycol (6.67 g, 0.034 mol) and 20 mL
fraction f of carbonate-containing units, as determined by the
1
integrated areas of H NMR peaks (see Fig. S1 and Table S1† for
4
details). For example, CPP-19 means the copolymerized plasti-
cizer having molar fraction f ¼ 19% of carbonate containing
units.
To further improve the performance of the low-volatility
plasticizers, the second type of oligomeric plasticizers (OP) of
3
dry THF over 30 minutes. The reaction mixture was allowed to
stir overnight to complete the reaction. The mixture was ltered
to remove the solid. The liquid was condensed by rotavap and
4
50 < M < 700 were synthesized by condensation reactions
further dried by vacuum oven to yield OP62 as a brown liquid
0
(Scheme 2). To better compare with the CPP, an equivalent f is
1
(
(
13 g). H NMR (in d6-acetone), 4.95 (m, 1H, CCH(C)O), 4.4–4.6
calculated based on the carbonate : EO ratio. Since the CPPs
have carbonate side chains (with one cyclic carbonate group
and one EO) and EO side chains (with four EOs), the ratio of
carbonate : EO ¼ f/[f + 4(1 ꢁ f)]. For OP, by dening carbo-
m, 2H, CH
CH O) 3.55 (m, 2H, OCH
CH ), 0.9 (m, 2H, OCH CH
in d6-acetone) 17.57 (s) (Fig. S4†).
2
C), 3.75 (m, 2H, CCH
CH
CH ), 0.35 (m. CH
2
OCH
2
CH
2 2
H ), 3.63 (s, 16H,
OCH
CH
(
2
2
2
2
CH
2
), 1.7 (m, 2H, OCH -
2
2
9
2
2
2
2
2
3
); Si NMR
0
0
0
0
nate : EO ¼ f /[f + 4(1 ꢁ f )] enables an estimation of f anal-
0
ogous to f of CPP. OPs are coded according to f , for example
0
4
,4 -(6,6,22,22-Tetraethyl-2,9,12,15,19,26-hexaoxa-6,22-
0
OP73 means f ¼ 73%.
disilaheptacosane-1,27-diyl)bis(1,3-dioxolan-2-one) (OP73)
OP62 was synthesized by condensation reaction with triethyl
Diethyldihydrosilane (7.2 g, 0.082 mol), CECA (8.4 g, 0.053 mol) amine as an acid scavenger (Scheme 2(a)). The resulting product
and 10 mL benzene were added into a ask followed by 0.1 g with Si–O–C linkages is moisture sensitive. The rst attempt to
RhCl(PPh ) catalyst. The mixture was stirred at room temper- synthesize OP67 and OP73 (Scheme 2(b)) failed because the
3
3
ature overnight to complete the reaction. The solvent was platinum catalyst that works for most hydrosilylation reactions
evaporated and the residue was vacuum distilled to afford 4-((3- gave nearly no desired product. The well-known Wilkinson Rh
(
diethylsilyl)propoxy)methyl)-1,3-dioxolan-2-one as a colorless
1
liquid (20 g, 100%). H NMR (in d6-acetone), 4.95 (m, 1H,
CCH(C)O), 4.4–4.6 (m, 2H, CH
OCH CH CH , OCH CH CH and SiH), 1.64 (m, 4H, OCH
CH2CH Si), 1 (m, 6H, OCH CH CH and SiCH
CH3); Si NMR (in d6-acetone) ꢁ1.38 (s) (Fig. S5†).
-((3-(Diethylsilyl)propoxy)methyl)-1,3-dioxolan-2-one (5 g,
.02 mol) and tri(ethylene glycol) divinyl ether (2 g, 0.01 mol)
were mixed followed by 0.1 g RhCl(PPh catalyst. The mixture
2
C), 3.45 to 3.8 (m, 5H, CCH
2
-
2
2
2
2
2
2
2
-
2
9
2
2
2
2
), 0.67 (m, 6H,
2
4
0
3 3
)
was stirred at room temperature overnight to complete the
reaction. The mixture was washed with cyclohexane to afford
1
the product as a brown liquid (7 g). H NMR (in d6-acetone),
4
.95 (m, 1H, CCH(C)O), 4.4–4.6 (m, 2H, CH
2
C), 3.45 to 3.8 (m,
1
2H, CCH OCH CH CH , OCH CH CH and OCH CH O),
2
2
2
2
2
2
2
2
2
1
.64 (m, 4H, OCH CH CH Si), 1 (m, 6H, OCH CH CH and
2 2 2 2 2 2
Scheme 1 Synthesis of siloxane random copolymer plasticizers CPP-
0, 19, 31, 57, 80, 100), where the number is the mol% of cyclic
carbonate containing units, i.e., 100f.
2
9
2 3
SiCH ), 0.67 (m, 6H, CH ); Si NMR (in d6-acetone) 5.25 (s)
(
(
Fig. S6†).
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Scheme 2 Synthesis of oligomer plasticizers (a) OP62, (b) OP73 (n ¼ 2) and OP67 (n ¼ 3), and (c) OP89.
3 3 g
catalyst [RhCl(PPh ) ] suggested by Hartwig et al. exhibited copolymers from the T s of the two homopolymers when the
29
better catalyzing capability. The hydrosilylation for the second copolymers and homopolymers have (1) similar chain sizes and
SiH groups on the same Si atom didn't start until all the rst SiH thus an effect of chain ends, known to cause extra free volume,
1
groups were consumed, as conrmed by H NMR. The synthesis has equal contribution of T , or (2) sufficiently large chain sizes
g
of OP89 (Scheme 2(c)) is straightforward. The reaction was and thus an effect of chain ends is negligible. Since OPs are
impossible to stop at the monosubstitution state, even with oligomers for which chain ends play a big role in reducing T
excess dihydrosilane. the OPs having T well below the Fox equation of the much
The OP and CPP samples were blended with a tetraphenyl longer homopolymers (CPP0 and CPP100) is reasonable. This
g
,
g
borate – Li polysiloxane single-ion conductor synthesized in our argument also explains the overall trend that T
g
is further below
0
previous work, having 14 mol% tetraphenyl borate-86 mol% the Fox prediction at higher f : considering that each OP
+
13
cyclic carbonate and Li counterions. The weight fraction of molecule contains identically two carbonate end groups, larger
0
the ionomer in all mixtures is w ¼ 20 wt%. To prepare the f simply means shorter chain and accordingly stronger chain
mixtures, the ionomer and plasticizer were dissolved in a
common solvent, acetone, and the solvent was evaporated rst
via rotovap and then the blends were put in a vacuum oven at
ꢃ
ꢃ
8
0 C. Since OP evaporates slowly in vacuum at 80 C, 5–10 wt%
extra OP was added in the common solvent, and the sample was
weighed every 2–3 hours aer introducing into the vacuum oven
to record the weight loss of OP with time. More than 10 hours in
the vacuum oven were required to evaporate the extra OP and
obtain the target content of 20 wt% ionomer.
Glass transition temperature
Glass transition temperatures (T ) of CPP and OP are plotted
g
0
against f and f , respectively, in Fig. 1 in lled symbols. T of
g
CPP increases with polar carbonate content f. The overall trend
is well predicted by the Fox equation (thick solid curve in Fig. 1):
1
f
1 ꢁ f
g;CPP0
Fig. 1 Glass transition temperatures T
against f and f , respectively (filled symbols). Tg of resulting blends
g
of CPP and OP plasticizers
¼ T
þ
T
(1)
0
T
g
g;CPP100
(
(
80 wt% plasticizer and 20 wt% ionomer) is added for comparison
open symbols). Thick solid and thin dashed curves show predictions
g g
T s of OPs are lower than T s of CPPs and the Fox equation
prediction. The Fox eqn (1) can predict T
g
s of the random of Fox eqn (1) and (2), respectively.
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end effect. Blended with 20 wt% ionomer with signicantly
higher T (¼301 K), the mixtures display only ꢂ5 K higher T
g
g
than the plasticizers alone. The increase is less than the Fox
prediction of eqn (2) shown as the dashed curve in Fig. 1, sug-
gesting that an ion-solvating effect lowers T
g
, which is more
obvious when EO units are present in the plasticizer.
1
w
f ð1 ꢁ wÞ ð1 ꢁ f Þð1 ꢁ wÞ
g;CPP100
¼ T
þ
þ
(2)
T
g
g;ionomer
T
T
g;CPP0
Dielectric constant
In Fig. 2(a), the static dielectric constant 3
content of polar cyclic carbonate groups. The 3
s
of CPP increases with
values of the
s
four CPP random copolymers can be predicted from those of
the two CPP homopolymers (CPP100 and CPP0) very well with
30
the Landau and Lifshitz mixing rule, at all temperatures in the
liquid state (solid curves in Fig. 2(a) and the inset of Fig. 2(b)).
1
s
/3
1/3
1/3
3
¼ f3s,CPP100 + (1 ꢁ f)3s,CPP0
(3)
While the CPP0 homopolymer perfectly obeys the Onsager
31
equation (dotted line in Fig. 2(a)) the CPP100 homopolymer
exhibits strong curvature for reasons that are not apparent, yet
still the Landau–Lifshitz mixing rule works perfectly for all four
random copolymers.
Fig. 2(b) compares the static dielectric constants 3
s
of the
0
four OPs and six CPPs, which are comparable when f and f are
0
comparable (compare the four OPs (f ¼ 62% to 89%) to CPPs of
f ¼ 57% and 80% in Fig. 2(b)). This feature is more clearly seen
ꢃ
in the inset, where 3
against f and f , respectively.
s
of CPPs and OPs at T ¼ 20 C are plotted
0
s
In the Fig. 2(b) inset, 3 of the OPs scatters closely around the
Landau and Lifshitz curve, meaning the dielectric constant for
both CPPs and OPs is governed by the carbonate : EO ratio. The
considerable scattering should be related to the detailed
molecular structure. Considering 3 here reects a response of
s
the dipole moments to applied electric eld, it should be related
to not only the density of dipoles but also how the dipoles
interact with each other, i.e. dipole–dipole correlations. The
latter effect should rely on the molecular structure, in particular
an energy barrier for dipole rotation. For example, OP62 has 3
s
0
considerably higher than that expected from f . The reason
could be related to the ultra-exible Si–O bond that allows the
polar carbonate groups of OP62 to respond more easily to an
s
Fig. 2 (a) Temperature dependences of static dielectric constant 3 of
CPPs (in symbols) and the predictions by the Landau and Lifshitz
mixing rule (eqn (3), solid curves) and the Onsager equation for CPP0
32–35
(
dotted line). (b) Comparison of dielectric constants of the CPPs (in applied electric eld.
This argument is in accordance with
black symbols) and OPs (in blue symbols). Inset: static dielectric the observation that OP62 and OP89 having Si–O bonds exhibit
ꢃ
constant 3
s
at T ¼ 20 C increases with the fraction of carbonate units f
3
s
higher than those of CPPs, while OP67 and OP73 having no
Si–O bond exhibit 3 lower than those of the CPPs. The dielectric
constants of the mixtures with 20 wt% ionomer are shown in
for the six CPPs (open symbols). The solid curve is the Landau and
Lifshitz mixing rule, eqn (3). 3
comparison (filled symbols). The significantly larger 3 of OP62 seems
s
0
s
s
vs. f of the four OPs are added for
to play a role in its superior conductivity. (c) Dielectric constant of Fig. 2(c), with blend dielectric constant increasing with cyclic
mixtures of 20 wt% of 14 mol% lithium tetraphenyl borate/86 mol% carbonate content.
13
cyclic carbonate random copolymer ionomer with 80 wt% of the
various plasticizers.
Fig. 3(a) shows the temperature dependence of ionic
conductivity sDC of the ionomer (red symbols) and its 20 wt%
mixtures with CPP (black symbols) and OP (blue symbols). For
CPP mixtures, the conductivity shows an interesting crossover,
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Fig. 3 (a) Temperature dependence of ionic conductivity for the ionomer (red ꢀ symbols) and its blends with CPP (black symbols) and OP (blue
ꢃ
symbols) at 20 wt% ionomer. Inset: sDC versus 1/h
conductivity divided by static dielectric constant.
0
at T ¼ 20 C. (b) T
g g
/T dependence of the ionic conductivity. (c) T /T dependence of ionic
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ꢃ
g s 0
Table 1 Molecular weight M, glass transition temperature T , and 20 C values of static dielectric constant 3 and zero-shear viscosity h of
plasticizers
CPP0
CPP19
CPP31
CPP57
CPP80
CPP100
OP62
OP67
OP73
OP89
a
ꢁ1
M (g mol
)
12 700
187
5.9
12 400
194
11
12 200
202
15
11 800
218
27
11 400
229
38
11 100
248
54
626
213
37
695
214
24
651
212
29
450
220
48
T
g
(K)
3
s
h
0
(Pa s)
0.70
2.83
2.39
2.21
14.8
68.6
0.18
0.87
0.51
0.30
a
Number-average molecular weight of CPP, calculated from f, assuming number-average DP ¼ 52.
increasing with f at sufficiently high T while decreasing with f at plasticizer OP62 has the lowest viscosity, the most exible
sufficiently low T. This result reects a tradeoff between structure and nearly the highest dielectric constant. Polymer
reducing Tg via increasing the EO content (Fig. 1) and single-ion conductor blends containing those plasticizers and a
ꢁ
5
ꢁ1
increasing 3 via increasing the cyclic carbonate content (Fig. 2). borate ionomer have conductivity up to 2 ꢀ 10 S cm at
s
ꢃ
The reduction of T
g
(faster segmental motion) boosts ionic 25 C. By mixing with low-volatility polar plasticizers, these
mobility and dominates at low T close to T
soens ionic interactions and facilitates ion dissociation; this electrolytes for lithium-ion rechargeable battery separators.
effect dominates at high T, sufficiently far above T . The effect of
increasing with cyclic carbonate content is more clearly seen
g
. The increase of 3
s
ionomers are potential candidates to make safer gel polymer
g
3
s
Acknowledgements
in Fig. 3(b) where temperature has been normalized by T . At
g
constant T /T, s increases with f for the CPP mixtures. This The authors gratefully acknowledge the nancial support of the
g
DC
trend is also seen in the OP mixtures: the conductivity decreases Department of Energy under Grant BES-DE-FG02-07ER46409.
in the order OP89 ꢂ OP62 > OP73 > OP67, in accordance with Janna Maranas, Karl Mueller (both at Penn State) and Karen
the order of their 3
s
.
Winey from the University of Pennsylvania are thanked for
In Fig. 3(a), sDC is larger for the OP mixtures than the CPP helpful discussions.
0
samples even if f and f are comparable, reecting that the
smaller OP molecules exhibit lower viscosity than the larger CPP
molecules (Table 1). The lower viscosity is due partly to the
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