Hybrid ionogel electrolytes derived from polyhedral oligomeric silsesquioxane for lithium ion batteries

Article abstract of DOI:10.1166/jnn.2017.14087

Inorganic-organic hybrid ionogels fabricated with 1 M LiTFSI in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) crosslinked with a fully methacryl-substituted Polyhedral Oligomeric Silsesquioxane (T8-MMA-POSS) were investigated as gel polymer electrolytes for lithium ion batteries. The effect of T8-MMA-POSS on physical properties of the ionogels was characterized in terms of dimensional stability, ion transport behaviour, and thermal stability. A mere 5 wt% concentration of the cross-linker was able to produce non-flowing hybrid ionogels, leading to high ionic conductivity with good mechanical properties. The lithium battery cell fabricated with ionogels revealed high specific capacity and excellent cycling performance with high Coulombic efficiency at elevated temperature, demonstrating that hybrid ionogels could be a promising candidate electrolyte for use in lithium ion batteries.

Full text of DOI:10.1166/jnn.2017.14087

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Vol. 17, 3101–3104, 2017
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Hybrid Ionogel Electrolytes Derived from Polyhedral
Oligomeric Silsesquioxane for Lithium Ion Batteries
Jin Hong Lee1ꢀ2ꢀ†, Albert S. Lee1ꢀ3ꢀ†, Jong-Chan Lee2, Soon Man Hong1ꢀ3,
Seung Sang Hwang1ꢀ3, and Chong Min Koo1ꢀ3ꢀ∗
1Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
2Department of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea
3Nanomaterials Science and Engineering, University of Science and Technology, Daejeon 305-333, Republic of Korea
Inorganic–organic hybrid ionogels fabricated with 1 M LiTFSI in N-butyl-N-methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide (BMPTFSI) crosslinked with a fully methacryl-substituted Polyhe-
dral Oligomeric Silsesquioxane (T8-MMA-POSS) were investigated as gel polymer electrolytes for
lithium ion batteries. The effect of T8-MMA-POSS on physical properties of the ionogels was char-
acterized in terms of dimensional stability, ion transport behaviour, and thermal stability. A mere
5 wt% concentration of the cross-linker was able to produce non-flowing hybrid ionogels, leading
to high ionic conductivity with good mechanical properties. The lithium battery cell fabricated with
ionogels revealed high specific capacity and excellent cycling performance with high Coulombic effi-
ciency at elevated temperature, demonstrating that hybrid ionogels could be a promising candidate
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electrolyte for use in lithium ion batteries.
IP: 185.13.32.163 On: Thu, 06 Apr 2017 06:19:17
Copyright: American Scientific Publishers
Keywords: Silsesquioxane, Ionic Liquids, Lithium Ion Batteries, Gel Polymer Electrolytes.
1. INTRODUCTION
with diameter ∼3 nm.5 This nano-sized core and organic
functional groups allows for solubility and/or impecca-
ble dispersability in a wide variety of solvents. Moreover,
through the myriad of organic functional groups that can
be introduced, various functional materials can be fabri-
cated through thermal or UV-initiated processes.6ꢀ7
The use of POSS-based materials for fabrication
of hybrid electrolytes has been predominately been
reported for solid polymer electrolytes. Various polymer
chemistries,8 as well as blend composites9 have employed
for the manipulation of hybrid electrolytes for lithium ion
batteries. However, solid polymer electrolytes still suffer
from relatively low ionic conductivity, hampering their use
in real-life settings.
Inorganic–organic hybrid electrolytes, such as sol–gel
based solid polymer electrolytes and solidified ionic liq-
uid ionogels, have been a hot topic of materials scientists
for various electrochemical applications, including elec-
trochromic devices, capacitors, and lithium ion batteries,
because of their good thermal and electrochemical sta-
bilities, and mechanical robustness.1–4 However, the chal-
lenges of utilizing these sol–gel derived hybrid materials
are the electrochemical instability of uncondensed silanol
groups from uncontrolled siloxane structure, thus requiring
substantial thermal treatments at elevated temperature.2
Polyhedral Oligomeric Silsesquioxanes, or more com-
monly known as POSS, are inorganic–organic hybrid
materials with well-defined, inorganic siloxane polyhedral
cores, with covalently bound organic functional groups
stemming radially outwards.5 The inner inorganic core is
known as the smallest silica particle, as the free-volume
of the POSS was reported to be as small as a sphere
In this study, we fabricated a hybrid ionogel with a fully
methacryl-substituted POSS (T8-MMA-POSS). At a low
T8-MMA-POSS concentration of 5 wt%, the ionic liquid is
fully solidified with non-leaking property. Examination of
various properties including compatibility with ionic liq-
uids, thermal, mechanical, and electrochemical traits reveal
that these hybrid ionogels are highly promising materials
for next generation hybrid ionogel electrolytes.
∗Author to whom correspondence should be addressed.
†These two authors contributed equally to this work.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 5
1533-4880/2017/17/3101/004
doi:10.1166/jnn.2017.14087
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Hybrid Ionogel Electrolytes Derived from Polyhedral Oligomeric Silsesquioxane for Lithium Ion Batteries
Lee et al.
2. EXPERIMENTAL DETAILS
2.1. Synthesis of N-butyl-N-methylpyrrolidinium
(TGA 2950) at heating rate of 10 ꢀC min−1 under N2
atmosphere. Rheological properties were examined using
a rheometer (Advanced Rheometric Expansion System,
ARES) instrument with cone-plate geometry (25 mm
diameter). All rheological measurements were performed
in the linear viscoelastic region under N2 atmosphere.
The ionic conductivity was determined using a complex
impedance analyzer (Bio-Logics, VMP3) over frequency
range from 1 Hz to 1 MHz at AC amplitude of 10 mV. The
electrochemical stability of the gel polymer electrolytes
was examined using a linear sweep voltammetry system.
In the experiments, a stainless steel working electrode was
used with lithium metal as both the counter and refer-
ence electrodes. The voltage was swept at a scan rate
of 1.0 mV s−1. Electrochemical measurements of the gel
polymer electrolyte were conducted using 2032 coin cells
consisting of a separator, Li metal and a LiFePO4 cathode
(90 wt% LiFePO4, 5 wt% carbon black, 5 wt% PVDF). All
the cells were assembled in argon-charged glove box. After
fabrication, the cells with pre-gel solution were subjected
to thermal cross-linking for 3 h at 70 ꢀC. The galvanostatic
charge–discharge experiments were carried out with volt-
age range of 2.5–4.2 V using a battery cycler (Wonatech,
bis(trifluoromethylsulfonyl)imide
Synthesis of N -butyl-N -methylpyrrolidinium bis(trifluoro-
methylsulfonyl)imide was performed following literature
procedure.11ꢀ12 In a dry 500 mL round-bottom flask,
stoichiometric 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 temper-
ature for 24 h. The product was repeatedly washed with
ethyl 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 dissolved
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 BMP-
TFSI had H2O content of less than 100 ppm as mea-
sured with the Karl Fischer method. Ionic liquid solu-
tions of 1 M LiTFSI in N -butyl-N -methylpyrrolidinium
bis(trifluoromethylsulfonyl)iꢀmide (BMPTFSI) were made
by dꢀissolving LiTFSI at 60 C for 24 hours and drying at
100 C overnight prior to use.
ꢀ
WBCS3000) at 50 C.
2.2. Preparation of Hybrid Ionogels
T8-MMA-POSS (Fig. 1) was purchased from Hybrid Plas-
tics and used as received. In an inert, argon-charged
3. RESULTS AND DISCUSSION
The inorganic–organic hybrid ionogels were fabricated by
thermal initiation of the unsaturated vinyl group of T8-
MMA POSS (Fig. 1). After curing process, a mechanically
stable and opaque ionogel was obtained. The FT-IR analy-
sis was carried out to characterize the chemical structures
of the cured ionogels. As presented in Figure 2, the char-
acteristic peak centered at 1640 cm−1 derived from the
glove, solutions with various amounts of T8-MMA-POSS
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in BMPTFSI were prepared in gIlPas:s18vi5a.l1. 3A.3n2a.1m6o3unOtno:fThu, 06 Apr 2017 06:19:17
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1 wt% (relative to T8-MMA-POSS) of AIBN as thermal
initiator was added to this solution. After taking the solu-
tions out of the glove box, samples were sonicated and
shaken for 10 min or until the solution was homogenous.
Then, ꢀthe samples were directly taken to an oven preset
at 70 C for 3 h.12 The hybrid ionogel electrolytes were
named as HIE-5 and HIE-10 for ionogels containing 5 and
10 wt% of T8-MMA-POSS, respectively.
C
C stretching mode of the vinyl group disappeared after
thermal treatment, indicating the crosslinking reaction was
complete.
Gel formation was further confirmed by observing rhe-
ological response before and after thermal curing process.
2.3. Characterizations and Measurements
Fourier transform infrared spectra were measured with
a Perkin-Elmer FT-IR system Spectrum-GX. Thermal
gravimetric analysis was carried out with TA instrument
Figure 2. FTIR of HIE-5 before and after thermal treatment with inset
photographs of the pre-gel hybrid ionogel electrolyte solution and hybrid
ionogel electrolyte.
Figure 1. Chemical structure of T8-MMA-POSS.
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Lee et al.
Hybrid Ionogel Electrolytes Derived from Polyhedral Oligomeric Silsesquioxane for Lithium Ion Batteries
Figure 3. Rheological properties of the neat ionic liquid and hybrid
ionogel electrolytes.
Figure 5. Linear sweep voltammograms for the ionic liquid and HIE-5.
network structure that restricts the mobility of the ions.
However, we were able to solidify ionic liquid solution
using only a small amount of gelator (∼5 wt%), leading
to mitigate the decrease in ionic mobility. The ionic con-
ductivity of HIE-5 was observed to be 0.51 mS cm−1 at
Figure 3 shows the dynamic viscoelastic properties of neat
ionic liquid and hybrid ionogels as a function of the angu-
lar frequency. For neat ionic liquid, the loss modulus Gꢁꢁ
was found to be greater than storage modulus Gꢁ, and
both dynamic moduli values exhibited frequency depen-
dence, Gꢁ ∼ ꢁ2 and Gꢁꢁ ∼ ꢁ, confirmed a viscous fluid.
In contrast, hybrid ionogels fabricated with HIE-5 exhib-
ited a pronounced plateau region of Gꢁ was observed and
Gꢁ > Gꢁꢁ, demonstrating predominantly solid-like behav-
ior due to the formation of crosslinked network structure.
ꢀ
30 C, which was close to that of the neat ionic liquid
solution.11
Before evaluating the electrochemical performances, we
investigated the electrochemical stability of ionogels by
using linear sweep voltammetry (LSV) measurements.
As shown in Figure 5, in the anodic region, LSV pro-
Meanwhile, with further increase in POSS crosslinker con-
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files of both the neat ionic liquid and hybrid iono-
tent, the network structure was found to be increasingly
IP: 185.13.32.163 On: Thu, 06 Apr 2017 06:19:17
gels were found to be almost identical and stable up to
5.0 V (V vs. Li/Li+ꢂ, indicating that the incorporation of
crosslinkable groups into the ionogels does not give rise to
any adverse effect on the electrochemical stability of the
system. Furthermore, as Figure 6 reveals, the initial weight
Copyright: American Scientific Publishers
dense, which was supported by the increase in mechanical
properties.
The ionic conductivity of the hybrid ionogels also
depended on their network structures. Figure 4 presents
the measured ionic conductivity as a function of temper-
ature. All samples exhibited qualitatively similar Arrhe-
nius increases in ionic conductivity with increasing tem-
perature, suggesting the ion conduction was mainly gov-
erned by the mobility of free ions. Simultaneously, it is
apparent that the gelation of ionic liquid resulted in the
decrease in ionic conductivity because the formation of a
ꢀ
loss occurred at temperature above 350 C, indicating the
excellent thermal stability of the hybrid ionogels.
To characterize the electrochemical performance of the
ionogels, LiFePO4/HIE-5/lithium cells were fabricated,
and the charge–discharge capacities at various C-rates
were investigated at 50 ꢀC. Figure 7 presents the rate
capabilities of the cell with hybrid ionogel electrolyte.
Figure 4. Temperature dependent ionic conductivities of the neat ionic
liquid and hybrid ionogels electrolytes.
Figure 6. TGA thermograms for the ionic liquid and HIE-5.
J. Nanosci. Nanotechnol. 17, 3101–3104, 2017
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Hybrid Ionogel Electrolytes Derived from Polyhedral Oligomeric Silsesquioxane for Lithium Ion Batteries
Lee et al.
to be above 96% during cycles, demonstrating that these
hybrid ionogel electrolytes could be promising candidate
electrolyte for use in lithium batteries.
4. CONCLUSION
Fully solution-processable organic–inorganic hybrid
ionogel electrolytes were fabricated with a crosslink-
able methacrylate-substituted polyhedral oligomeric
silsesquioxane with nano-sized inorganic core. Through
use of mere 5 wt% of the polyhedral oligomeric
silsesquioxane crosslinker, hybrid ionogel electrolytes with
high ionic conductivity, high electrochemical stability,
thermal stability, and mechanical robustness were pre-
pared. Through evaluation of the lithium ion battery tests,
fabricated hybrid ionogel electrolytes exhibited promising
performance for next generation electrochemical devices.
Figure 7. Rate capabilities of LiFePO4/HIE-5/lithium cells with inset
showing the various charge–discharge profile at corresponding C-rates.
For the test, the cell was charged at a current density
of 0.1 C and discharged at different current densities. As
shown, we were able to achieve stable discharge capacity
upon repeated cycling, and the initial discharge capacity
of the cell was ∼154 mAh g−1, which is in close prox-
imity to the theoretical discharge capacity of the LiFePO4
cathode material.13 Moreover, the charge and discharge
profile exhibited a well-defined potential plateaus, indicat-
ing a reversible cycling process. In the following cycles,
the discharge capacity and plateau continuously declined
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
with increasing the current densitiDesedlivueerteodthbeyiInncgreeansteaitno: York University Libraries
National Research Foundation of Korea.
IP: 185.13.32.163 On: Thu, 06 Apr 2017 06:19:17
cell polarization. The cell delivered a moderate discharge
Copyright: American Scientific Publishers
capacity of 88 mAh g−1 at 0.5 C, which was 57% of the
discharge capacity of 0.1 C.
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Figure 8. Cyclability of LiFePO4/HIE-5/lithium cells.
Received: 14 November 2015. Accepted: 26 August 2016.
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