Polyethylene glycol-functionalized siloxane hybrid gel polymer electrolytes for lithium ion batteries

Article abstract of DOI:10.1166/jnn.2017.14086

The compatibility of diacrylate terminated polyethylene glycol-block-polydimethylsiloxane-blockpolyethylene glycol was examined with an ionic liquid solution, 1 M LiTFSI in N-butyl-Nmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide, and through mild UV-curing, hybrid gel polymer electrolytes were fabricated for lithium ion battery application. Obtained hybrid gel polymer electrolytes exhibited good ionic conductivity, electrochemical stability, thermal stability, and mechanical pliancy and the polyethylene glycol domains functioned to increase the ionic dissociation to improve ion conduction compared with gel polymer electrolytes fabricated with a conventional organic crosslinker. Lithium ion battery cell tests with these hybrid gel polymer electrolytes revealed that these hybrid gel polymer electrolytes hold promise as next generation electrolytes.

Full text of DOI:10.1166/jnn.2017.14086

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Nanoscience and Nanotechnology
Vol. 17, 3016–3020, 2017
Copyright © 2017 American Scientific Publishers
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Polyethylene Glycol-Functionalized Siloxane Hybrid Gel
Polymer Electrolytes for Lithium Ion Batteries
1
ꢀ†
1ꢀ2ꢀ†
2
1ꢀ3
Albert S. Lee , Jin Hong Lee
, Jong-Chan Lee , Soon Man Hong ,
1ꢀ3
1ꢀ3ꢀ∗
Seung Sang Hwang , and Chong Min Koo
1
Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
2
Department of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea
3
Nanomaterials Science and Engineering, University of Science and Technology, Daejeon 305-333, Republic of Korea
The compatibility of diacrylate terminated polyethylene glycol-block-polydimethylsiloxane-block-
polyethylene glycol was examined with an ionic liquid solution, 1 M LiTFSI in N-butyl-N-
methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, and through mild UV-curing, hybrid gel polymer
electrolytes were fabricated for lithium ion battery application. Obtained hybrid gel polymer elec-
trolytes exhibited good ionic conductivity, electrochemical stability, thermal stability, and mechan-
ical pliancy and the polyethylene glycol domains functioned to increase the ionic dissociation
to improve ion conduction compared with gel polymer electrolytes fabricated with a conven-
tional organic crosslinker. Lithium ion battery cell tests with these hybrid gel polymer elec-
trolytes revealed that these hybrid gel polymer electrolytes hold promise as next generation
IP: 91.243.90.101 On: Mon, 27 Aug 2018 02:22:49
electrolytes.
Copyright: American Scientific Publishers
Keywords: Polysiloxane, Polymer DE l ee lci vt r eo rl ye t de s b, yI o Inn i cg eL ni qt ua ids, Lithium Ion Batteries.
1
. INTRODUCTION
between polysiloxanes and ionic liquids, these methods are
impractical from the point of view of electrochemical cell
fabrication.
Polysiloxanes, such as polydimethylsiloxane (PDMS) and
their derivatives, have been one the most extensively
researched materials in both industry and academia due
to their many favorable properties such as optical trans-
parency, elasticity, thermal stability, and electrochemical
stability,1 making them an attractive material for hybrid
polymer electrolytes. And as such, substantial solid poly-
mer electrolytes fabricated through blending with inor-
ganic salts, followed by various crosslinking methods,
have been reported for a wide range of electrochemical
devices.3
Polyethylene glycol, or PEG, is a representative poly-
mer for polymer electrolyte and gel polymer electrolyte
applications because its ethylene oxide repeating units
assist with ion dissociation, thus isolating them from
ꢀ2
5
the counter anions. However, high molecular weight
PEG is hampered by high crystallinity, leading to low
5
ꢀ6
ionic conductivity. As such, crosslinkable low molecu-
lar weight PEG materials have been extensively utilized as
7
polymer electrolyte materials.
However, the compatibility of polysiloxanes with ionic
liquids has been known to be poor, due to their low
to nonexistent solubility, limiting their use to solid
In this study, we examined the compatibility of a
UV-curable diacrylate terminated polyethylene glycol-
4
block-polydimethylsiloxane-block-polyethylene
glycol
polymer electrolytes which lack the required ionic con-
ductivity for practical battery performance. While some
studies have investigated various synthetic modifications
and in-situ sol–gel methods to improve the compatibility
(DA-PEG-b-PDMS-b-PEG) with the ionic liquid solu-
tion 1 M LiTFSI in N -butyl-N -methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide, and compared the
electrochemical properties of UV-cured gel polymer elec-
trolytes fabricated with a conventional organic-based
crosslinker, Trimethylolpropane ethoxylate triacrylate
(ETPTA).
∗
†
Author to whom correspondence should be addressed.
These two authors contributed equally to this work.
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J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 5
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doi:10.1166/jnn.2017.14086

Lee et al.
Polyethylene Glycol-Functionalized Siloxane Hybrid Gel Polymer Electrolytes for Lithium Ion Batteries
2
2
. EXPERIMENTAL DETAILS
.1. Synthesis of N-Butyl-N-Methylpyrrolidinium
Bis(trifluoromethylsulfonyl)Imide
Then, the solutions were used to impregnate a polyethy-
lene terephthalate nonwoven separator and cured with
2
8
ultraviolet (UV) irradiation energy of 3 J/cm . The hybrid
gel polymer electrolytes were named as HGPE-PEG 15%
for hybrid gel polymer electrolytes containing 15 wt%
of DA-PEG-b-PDMS-b-PEG and ETPTA 15% for gel
polymer electrolytes containing 15 wt% of ETPTA.
Synthesis of N -butyl-N -methylpyrrolidinium bis(trifluoro-
methylsulfonyl)imide was performed following literature
procedure. In a dry 500 mL round-bottom flask, stoichio-
metric amounts of 1-methylpyrrolidine (50 g, 0.59 mol)
and 1-iodobutane (108 g, 0.59 mol) in 250 mL of ethyl
acetate were magnetically stirred at room temperature for
7
2
.3. Characterizations and Measurements
2
4 h. The product was repeatedly washed with ethyl
Fourier transform infrared spectra were measured with
a Perkin-Elmer FT-IR system Spectrum-GX. Thermal
gravimetric analysis was carried out with TA instrument
acetate and filtered until pure white salt of BMPI was
obtained. BMPI was then dissolved in deionized water
and mixed with a stoichiometric amount of LiTFSI dis-
solved in deionized water. The organic phase was extracted
with methylene chloride and subsequently dried at 100 C
for 24 h to remove any residual water. The resulting
ꢀ
−1
(TGA 2950) at heating rate of 10 C min under N
atmo-
2
7
sphere. Solid-state Li NMR were obtained on a Varian
1
7
ꢀ
INOVA ( H: 400 MHz, Li: 155.45 MHz), with spinning
frequency held constant at 10 kHz with a pulse delay
time of 4 s. Samples were packed into 7.5 mm zirconia
BMP-TFSI had H O content of less than 100 ppm as
2
9
rotors and sealed with KeI-F short caps. The electrochem-
measured with the Karl Fischer method. Ionic liquid solu-
tions of 1 M LiTFSI in N -butyl-N -methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide (BMPTFSI) were made
by dissolving LiTFSI at 60 C for 24 hours and drying at
1
ical stability of the gel polymer electrolytes was examined
using a linear sweep voltammetry system. In the experi-
ments, a stainless steel working electrode was used with
lithium metal as both the counter and reference electrodes.
ꢀ
ꢀ
00 C overnight prior to use.
−1
The voltage was swept at a scan rate of 1.0 mV s .
Electrochemical measurements of the gel polymer elec-
trolyte were conducted using 2032 coin cells consisting of
a nonwoven polyethylene terephthalate (PET) separator, a
graphitic anode, and a LiFePO cathode (90 wt% LiFePO ,
2
.2. Preparation of Hybrid Gel Polymer Electrolytes
Trimethylolpropane ethoxylate triacrylate (ETPTA)
Aldrich) was used as received. Diacrylate terminated
polyethylene glycol-block-polydimethylsiloxane-block-
(
4
4
IP: 91.243.90.101 On: Mon, 27 Aug 2018 02:22:49
polyethylene glycols (DA-PEG-b-P CD oM p Sy -r bi g- hP tE: GA )m ewr iac san Scientific Publishers
assembled in argon-charged glove box. The galvanostatic
5
wt% carbon black, 5 wt% PVDF). All the cells were
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obtained from Gelest and used as received. DA-PEG-
charge–discharge experiments were carried out with volt-
age range of 2.5–4.2 V using a battery cycler (Wonatech,
b-PDMS-b-PEG had a total weight averged molecular
weight of 1,800 g/mol with a PEG weight fraction of
3
ꢀ
WBCS3000) at 90 C.
5 wt%. In an inert, argon-charged glove, solutions with
various amounts of either ETPTA or DA-PEG-b-PDMS-
b-PEG in 1 M LiTFSI in N -butyl-N -methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide (BMPTFSI) were pre-
pared in glass vial. An amount of 1 wt% (relative to
DA-PEG-b-PDMS-b-PEG) of Igracure 184 as radical
photoinitiator was added to this solution. After taking the
solutions out of the glove box, samples were sonicated and
shaken for 10 min or until the solution was homogenous.
3. RESULTS AND DISCUSSION
The gel polymer electrolytes were fabricated according
to Figure 1. As shown, the highly compatible siloxane
compound was utilized to crosslink ionic liquid solu-
tions of 1 M LiTFSI in BMPTFSI at a concentration of
15 wt%. Through the UV-irradiation, mechanically com-
pliant, homogeneous hybrid gel polymer electrolytes were
Figure 1. Fabrication of gel polymer electrolytes with photograph, i, showing the flexibility of the HGPE-PEG 15% gel.
J. Nanosci. Nanotechnol. 17, 3016–3020, 2017
3017

Polyethylene Glycol-Functionalized Siloxane Hybrid Gel Polymer Electrolytes for Lithium Ion Batteries
Lee et al.
Figure 4. Static solid-state 7Li NMR spectra of HGPE-PEG 15% and
ETPTA 15% at room temperature.
Figure 2. FTIR spectra of the HGPE-PEG 15% before and after
UV-curing.
To confirm the proposed ion transport behavior, static
obtained as highly bendable gels (Fig. 1 photograph shown
as i). FT-IR analysis (Fig. 2) showed that no discernible
peaks assigned to the C C bonds at 1637 cm were
present, indicating that curing process was successfully
conducted.
7
solid-state Li NMR single-pulse experiments were carried
10
7
out. As shown in Figure 4, the Li NMR peaks for HGPE-
PEG 15% were noticeably narrower than of that of the
ETPTA gel, indicating that the solubilization of the lithium
doped ionic liquids into the PEG chains assisted the ion
−1
The temperature dependency of the ionic conductiv-
ity of ionogel electrolyes was investigated and compared
with that of the gel fabricated with a previously stud-
+
−
dissociation of Li cation from its TFSI anion. The high
ionic conductivity, along with the ion dissociation ability
of the HGPE-PEG 15% compared with the ETPTA gel, led
us to expect superior electrochemical performance.
1
0
ied organic crossliker, ETPTA. As shown in Figure 3,
the HGPE-PEG 15% provided higher ionic conductivity
than the ETPTA 15 wt% gel over a wide range of tem-
Electrochemical stability of the ionogel was evaluated
using linear sweep voltammetry. As shown in Figure 5,
no peak or remarkable oxidation currents associated with
IP: 91.243.90.101 On: Mon, 27 Aug 2018 02:22:49
peratures. Given the identical 15 wt% of crosslinker used
Copyright: American Scientific Publishers
for the fabrication for both gel polymer electrolytes, the
conductivity was undoubtedly influenced by both the num-
ber of charge carriers and structural factors. We hypoth-
esize, with respect to the latter, the change in network
structure play a crucial role in the increase in ionic con-
ductivity rather than the number of charge carrier ions
because we fabricated both ionogels with equal concentra-
tion of crosslinker to eliminate mitigating factors for the
crosslinker concentration. In the context of the proposed
hypothesis, the ethylene oxide chains are considered as
ion-conducting facilitator, which in turn exerts a beneficial
effect on the ion dissociation and conduction.
the decomposition of the electrolyte was observed below
Delivered by Ingenta
+
.0 V (V vs. Li/Li ꢁ, which indicates that the hybrid gel
5
polymer HGPE-PEG 15% was electrochemically stable up
to 5.0 V, suggesting that the electrochemical stability of
ionogel is sufficient for application in lithium batteries.
The thermal stability of ionogel was also investigated. As
shown in TGA curve in Figure 6, the ionogel exhibited an
ꢀ
initial degradation temperature at 400 C, which indicates
that hybrid gel polymer electrolyte could be utilized as an
electrolyte material at high temperature.
Next, we proceeded to evaluate the electrochemical
performance of ionogels. For the test, we fabricated
Figure 3. Temperature dependent ionic conductivities for HGPE-PEG
Figure 5. Linear sweep voltammograms of the neat ionic liquid and
15% and ETPTA 15%.
HGPE-PEG 15%.
3018
J. Nanosci. Nanotechnol. 17, 3016–3020, 2017

Lee et al.
Polyethylene Glycol-Functionalized Siloxane Hybrid Gel Polymer Electrolytes for Lithium Ion Batteries
Figure 6. TGA thermograms of the neat ionic liquid and
HGPE-PEG 15%.
Figure 8. Cyclability of LiFePO /HGPE-PEG 15%/lithium cells.
4
highly reversible in the system.12 Therefore, it is reason-
ably concluded that the introduction of ethylene oxide
groups allows the solidified ionogels to provide enhanced
battery cycling performance through improved lithium ion
mobility.
LiFePO /lithium cells assembled with both gel polymer
4
electrolytes, and galvanostatic charge/discharge investiga-
tion was performed in the potential range of between 2.5
+
and 4.2 V (V vs. Li/Li ꢁ. Figure 7 shows the cycling
response of the cells with different C rates, where the
cells were charged at a current densitiy of 0.1 C. As
shown, upon a subsequent increase in current density,
the discharge capacity was gradually decreased due to
the polarization effect of cell. It can be seen that the
cell with HGPE-PEG 15% exhibited better rate capabil-
ity over the ETPTA 15% gel. This could be attributed
to the introduction of long ethylene oxide groups, con-
4
. CONCLUSION
In conclusion, an ionic liquid compatible poly-
dimethylsiloxane derivative, diacrylate terminated
polyethylene
glycol-block-polydimethylsiloxane-block-
polyethylene glycol was utilized for the fabrication of
IP: 91.243.90.101 On: Mon h, y2 b7 r iAd u gg e l2 0p 1o 8l y 0m 2e :r2 2e l: e4 c9 trolyte through mild UV-curing.
tributing the relatively larger ionic conductivity and
Copyright: American Scientific Publishers
Obtained flexible hybrid gel polymer electrolytes exhibited
lithium ion mobility, thereby the mitigating growth of cell
polarization.
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1
0
high thermal stability, mechanical pliancy, high ionic con-
ductivity, and electrochemical stability. The incorporation
of the polyethylene glycol segments to polydimethyl-
siloxane allowed for enhanced ion dissociation, leading
to improved lithium ion battery performance at identical
crosslinker concentration.
The cycling performance of the cell further evaluated
at a constant charge/discharge current rate of 0.2 C/0.5 C.
As shown in Figure 8, the cell with HGPE-PEG 15%
exhibited stable cycling behavior and higher capacity
retention compared with that of the ETPTA 15% gel dur-
ing cycles. The retained discharge capacity after 50 cycles
for the cell with HGPE-PEG 15% was 128.9 mAh g ,
which was 85.8% of its largest discharge capacity. More-
over, it should be noted that the Coulombic efficiency of
the system was stabilized around 99% after initial cycles,
demonstrating that the lithium uptake and removal are
−1
Acknowledgment: This work was financially supported
by a grant from the Fundamental R&D Program for Core
Technology of Materials and the Industrial Strategic Tech-
nology Development Program funded by the Ministry of
Knowledge Economy, Republic of Korea. Partial funding
was given by the Materials Architecturing Research Cen-
ter of Korea Institute of Science and Technology and the
Korea Research Fellowship Program funded by the Min-
istry of Science, ICT, and Future Planning through the
National Research Foundation of Korea.
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Figure 7. Rate capabilities of LiFePO /HGPE-PEG 15%/lithium cells.
4
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Polyethylene Glycol-Functionalized Siloxane Hybrid Gel Polymer Electrolytes for Lithium Ion Batteries
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Received: 19 November 2015. Accepted: 12 September 2016.
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J. Nanosci. Nanotechnol. 17, 3016–3020, 2017