Hybrid ionogel electrolytes for high temperature lithium batteries

Article abstract of DOI:10.1039/c4ta06062h

Hybrid ionogels fabricated using 1 M LiTFSI in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) crosslinked with ladder-like structured poly(methacryloxypropyl)silsesquioxane (LPMASQ) were investigated as high temperature ionogel electrolytes for lithium ion batteries. In addition to the exceedingly low crosslinker concentration (～2 wt%) required to completely solidify the ionic liquids, which provided high ionic conductivities comparable to the liquid state ionic liquid, these hybrid ionogels exhibited superior thermal stabilities (>400°C). Rigorous lithium ion battery cells fabricated using these hybrid ionogels revealed excellent cell performance at various C-rates at a variety of temperatures, comparable with those of neat liquid electrolytes. Moreover, these hybrid ionogels exhibited excellent cycling performance during 50 cycles at 90°C, sustaining over 98% coulombic efficiency. Highly advantageous properties of these hybrid ionogels, such as high ionic conductivity in the gel state, thermal stability, excellent C-rate performance, cyclability and non-flammability, offer opportunities for applications as high temperature electrolytes.

Full text of DOI:10.1039/c4ta06062h

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Materials Chemistry A
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Hybrid ionogel electrolytes for high temperature
lithium batteries†
Cite this: DOI: 10.1039/c4ta06062h
Jin Hong Lee,‡ab Albert S. Lee,‡a Jong-Chan Lee,b Soon Man Hong,ac
ac
Seung Sang Hwangac and Chong Min Koo
*
Hybrid ionogels fabricated using 1 M LiTFSI in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)
imide (BMPTFSI) crosslinked with ladder-like structured poly(methacryloxypropyl)silsesquioxane
(LPMASQ) were investigated as high temperature ionogel electrolytes for lithium ion batteries. In addition
to the exceedingly low crosslinker concentration (ꢀ2 wt%) required to completely solidify the ionic
liquids, which provided high ionic conductivities comparable to the liquid state ionic liquid, these hybrid
ionogels exhibited superior thermal stabilities (>400 ꢁC). Rigorous lithium ion battery cells fabricated
using these hybrid ionogels revealed excellent cell performance at various C-rates at a variety of
temperatures, comparable with those of neat liquid electrolytes. Moreover, these hybrid ionogels
exhibited excellent cycling performance during 50 cycles at 90 ꢁC, sustaining over 98% coulombic
efficiency. Highly advantageous properties of these hybrid ionogels, such as high ionic conductivity in
the gel state, thermal stability, excellent C-rate performance, cyclability and non-flammability, offer
opportunities for applications as high temperature electrolytes.
Received 10th November 2014
Accepted 2nd December 2014
DOI: 10.1039/c4ta06062h
www.rsc.org/MaterialsA
polymers or crosslinked through thermally or photo-initiated
processes.8,9 Extensive reports have detailed the utility of
Introduction
organic polymers as a matrix or crosslinkable oligomers/poly-
mers as crosslinking agents.8–10 Furthermore, GPEs can be
classied depending on the type of solidied liquid electrolyte
solution, as ionogels are GPEs composed of solidied ionic
liquids.11 Many ionogels have been fabricated using a variety of
structural support materials, including polymers, colloidal
particles, carbon nanotubes and crosslinkable small organic
molecules.12 While these approaches have been proved to be
extremely useful for the fabrication of gels and lms, consid-
erable amounts of matrix or crosslinking agent compared with
the liquid electrolyte needed for the solidication of ionic
liquids have had major depreciating effects on the electro-
chemical device performances.12 Therefore, minimal use of the
crosslinking agent, while maintaining non-owing, solid-like
behaviour, has remained a challenge for optimal electro-
chemical devices.
Ionic liquids have been widely investigated as possible solu-
tions for the next generation of electrolytes in electrochemical
cells due to their advantageous properties of high ionic
conductivity, high electrochemical stability, non-ammability,
negligible vapour pressure and high thermal stability.1,2 Despite
these advantages over lithium salt solutions,3,4 ionic liquid
electrolytes still face the problems of leakage and low cycling
properties, which has proved to be a serious concern for some
applications, for instance in fully electronic or hybrid vehicles.2
To solve the problem of leakage in liquid electrolytes, research
on gel polymer electrolytes (GPEs) has been pervasive in elec-
trochemical device applications, such as capacitors and lithium
ion batteries.5–7
Gel polymer electrolytes can be dened as a class of elec-
trolytes in which the liquid electrolyte is either swollen in
Inorganic–organic hybrid materials, such as silicon-oxide
based materials, have also garnered great interest as the
support matrix for ionogels for their favourable properties, for
instance, thermal stability, electrochemical stability and facile
processing.13–22 Several studies have detailed the electro-
chemical studies and Li battery applications with chemically
aCentre for Materials Architecturing, Korea Institute of Science and Technology,
Hwarangno 14-gil 5, Seong-Buk Gu, Seoul 136-791, Republic of Korea. E-mail: koo@
kist.re.kr
bDepartment of Chemical and Biological Engineering, and Institute of Chemical
Process, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-744,
Republic of Korea
cNanomaterials Science and Engineering, University of Science and Technology, 217
modied siloxanes, such as commercially available [RSiO1.5 n
]
Gajungro, 176 Gajung-dong, Yuseong-Gu, Daejeon 305-333, Republic of Korea
polysilsesquioxane-based ORMOCERS,23 or [SiO2]n silica-based
materials, as the ionogel support matrix.13–17 These approaches
have either entailed the use of organic crosslinking function-
alities or the in situ sol–gel gelation of ionic liquids using
† Electronic supplementary information (ESI) available: 1H NMR spectrum of
BMPTFSI, interfacial resistance as a function of time, cyclic voltammogram,
chemical structure of ETPTA, and representative discharge proles of reference
cells. See DOI: 10.1039/c4ta06062h
‡ These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2015
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hydrolytic or non-hydrolytic sol–gel processes.13–17 Notable electrolytes. The polymeric nature of these materials allows for
studies by Vioux15–17 and Panzer13 groups have detailed the in the introduction of several hundreds of organic moieties per
situ gelation of ionic liquids to yield ionogels with tunable polymer chain for a fast and easy curing process.37,38 Recently,
exibility. However, the fabrication of these ionogels by in situ our research group developed a facile, one-pot synthesis of
sol–gel reactions may have a number of long-term problems in LPSQs using a base-catalyzed system.37–40 The products of the
the solid state, as uncondensed silanol groups may produce synthesis were fully condensed high molecular weight LPSQs.
water molecules through secondary condensations. Addition- Compared with the above approaches towards hybrid gel poly-
ally, organic acids, such as formic acid, which are used for mer electrolytes, these materials are highly soluble in non-
catalyzing the sol–gel hydrolysis-condensation rearrangements, aqueous electrolytes and ionic liquids, leading to high homo-
may not be completely removed in vacuo, thus remaining as geneity, whereas they need only a small amount of gelator.
impurities. The long times (5 days to 2 weeks) needed to solidify
the ionic liquid in situ is also a process unfavourable for (methacryloxypropyl)silsesquioxane (LPMASQ) following our
industrial applications.14
method and used it as a crosslinker for an ionic liquid elec-
In this study, we synthesized a novel ladder-structured poly-
Another class of hybrid gel polymer electrolytes include trolyte, 1 M LiTFSI in N-butyl-N-methylpyrrolidinium bis(tri-
composites of oxide particles. Functional silane-coated oxide- uoromethylsulfonyl)imide (BMPTFSI). Thermal crosslinking
based nanoparticles,24–27 which have been rigorously investi- of the methacryl moieties of LPMASQ resulted in homogeneous,
gated by the Archer group, and ‘soggy sand’ electrolytes, which non-leaking hybrid ionogels. The thermal, mechanical and
have been investigated by Bhattacharyya28–30 are representative ame-retardant properties of these inorganic–organic hybrid
examples of such composites. Surface modication of oxide- gel polymer electrolytes were examined and Li battery cells were
based nanoparticles coated with functional silanes (e.g., ionic fabricated to test their applications as thermally stable and high
liquid groups, oligo-polyethylene oxide groups, and single ion performance ionogels.
conducting groups) provides excellent electrochemical stability
and mechanical robustness compared with the neat liquid
Experimental
Materials
electrolytes.24 Soggy sand electrolytes, which comprise oxide
particles dispersed in non-aqueous liquid solutions, have also
been widely investigated as structurally supporting agents to
solidify the liquid electrolytes.28–31 However, for applications in
lithium ion batteries, these hybrid gel polymer electrolytes
should provide impeccable dispersion in liquid electrolytes to
achieve well-dened homogeneous composites. The mechan-
ical properties of these materials are still too low for such an
application. These are issues that still need to be addressed and
thoroughly investigated for practical lithium ion battery
applications.
3-Methacryloxypropyltrimethoxysilane (Shin-Etsu, 98%) and
ethyl acetate (J.T. Baker, HPLC grade) were distilled over CaH2
prior to use. Potassium carbonate (Daejung) was dried at 40 ꢁC.
THF (J.T. Baker, HPLC grade) was distilled over sodium. Azo-
bisisobutyronitrile (Daejung) was recrystallised from methanol.
Lithium bis(triuoromethylsulfonyl)imide (LiTFSI) (Aldrich,
99.9%, battery grade), 1-methylpyrrolidine (Aldrich, 98%), 1-
iodobutane (Aldrich, 99%) and all other solvents were used as
received.
Polysilsesquioxanes (PSQs) with chemical formula [RSiO1.5 n
]
are a class of inorganic–organic hybrid materials in which a
silicon oxide network structure is equipped with an organic
Synthesis of LPMASQ
functionality.32 There are three structural classes of PSQs, LPMASQ was synthesized following a known literature proce-
random-branched network, polyhedral cage and ladder-like dure.37–40 In a typical experiment, potassium carbonate, K2CO3
polymers. Although polyhedral cage structures, also known as (0.04 g, 0.29 mmol), was dissolved in deionised H2O (4.8 mL,
polyhedral oligomeric silsesquioxanes (POSS), have been used 0.27 mol) and 8 g of THF was added. To this solution, 3-meth-
as crosslinkers or additives,33–36 their generally low solubility37 acryloxypropyltrimethoxysilane (19.9 mL, 0.08 mol) was added
has limited their applications as composites with ionic liquids. dropwise under nitrogen ow. The solution was magnetically
In addition, as mentioned above, ionogels fabricated from stirred for 10 days when the molecular weight reached its
random-branched structural PSQ-modied silica particles maximum value. Aer partial evaporation of THF, the resinous
suffer from remnant silanol groups that will condense to form material was dissolved in dichloromethane and extracted
water molecules over time, making their applications in Li several times with water. Collection of the organic layers fol-
batteries practically infeasible.
lowed by drying of the organic layer over anhydrous magnesium
Ladder-like structured polysilsesquioxanes (LPSQs) are a sulphate and evaporation of the solvent under reduced pressure
unique class of silsesquioxanes in which the siloxane bond is yielded a transparent liquid with medium viscosity (15.2 g, 95%
double stranded and the organic functional groups are unidi- crude yield). LPMASQ was found to have excellent solubility in
rectionally positioned to the siloxane bonds.37–42 These mate- the majority of organic solvents.
rials exhibit enhanced solubility and superior thermal stability
1H NMR (CDCl3) (d, ppm): (0.48–0.78, SiCH2CH2CH2-
because of the imperceptible amount of silanol groups, which OCOCCH2CH3), (1.5–1.75, SiCH2CH2CH2OCOCCH2CH3), (3.9–
only reside at the polymer end groups.37–42 The acid resistance 4.1, SiCH2CH2CH2OCOCCH2CH3), (5.4, 5.9, SiCH2CH2CH2-
and insensitivity to thermal history characters can also be said OCOCCH2CH3), (1.75–1.85, SiCH2CH2CH2OCOCCH2CH3), 13C
to be the highly attractive merits of these materials as polymer NMR (CDCl3) (d, ppm): (7.6–10.3, SiCH2CH2CH2OCOCCH2CH3),
J. Mater. Chem. A
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(17.5–18.5, SiCH2CH2CH2OCOCCH2CH3), (21.2–22.4, SiCH2CH2-
The ionic conductivity was determined using a complex
CH2OCOCCH2CH3), (65.4–66.8, SiCH2CH2CH2OCOCCH2CH3), impedance analyzer (Bio-Logics, VMP3) over a frequency range
(124.4–126.1, SiCH2CH2CH2OCOCCH2CH3), (135.5–136.9, from 1 Hz to 1 MHz at an AC amplitude of 10 mV. The elec-
SiCH2CH2CH2OCOCCH2CH3), (166.6–168.6, SiCH2CH2CH2- trochemical stability of the gel polymer electrolytes was exam-
OCOCCH2CH3), 29Si NMR (ppm): –65 to ꢂ68 ppm, Mw ¼ 26 000 ined using
a linear sweep voltammetry system. In the
g molꢂ1, Mw/Mn ¼ 2.1.
experiments, a stainless steel working electrode was used with
lithium metal as both the counter and reference electrodes. The
voltage was swept at a scan rate of 1.0 mV sꢂ1. The time
evolution dependence of the interfacial resistance between the
lithium metal and the ionogel was measured through moni-
toring the AC impedance response over the frequency range
from 100 kHz to 10 MHz of the Li/HI-2/Li symmetric cells.
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, and 5 wt% PVDF). All the cells were assembled in an
argon-charged glove box. Aer 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 in the voltage range of 2.5–4.2 V using a battery cycler
(Wonatech, WBCS3000) at room temperature.
Synthesis of N-butyl-N-methylpyrrolidinium
bis(triuoromethylsulfonyl)imide
Synthesis of BMPTFSI was performed following a literature
procedure.43 In a dry 500 mL round-bottom ask, stoichiometric
amounts of 1-methylpyrrolidine (50 g, 0.59 mol) and 1-iodobu-
tane (108 g, 0.59 mol) in 250 mL of ethyl acetate were magnet-
ically stirred at room temperature for 24 h. The product was
repeatedly washed with ethyl acetate and ltered until a 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 a H2O content of less than 100 ppm as measured
using the Karl Fischer method. The 1H NMR characterization of
BMPTFSI is shown in the ESI, Fig. S1.†
Results and discussion
Inorganic–organic hybrid ionogel fabrication
Preparation of hybrid ionogels
The synthesis and characterization of LPMASQ has been
previously reported.37 This facile and mass-producible method
for production of highly condensed ladder-like structured poly-
silsesquioxanes has been proved to be extremely useful for high
yield production of LPSQs. The obtained LPMASQ was then
dissolved in an ionic liquid solution of 1 M LiTFSI in BMPTFSI
with various amounts of AIBN as a thermal initiator (Scheme 1).
In an inert, argon-charged glove box, solutions with various
amounts of LPMASQ in 1 M LiTFSI in BMPTFSI were prepared
in a glass vial. An amount of 1 wt% (relative to LPMASQ) of AIBN
as a thermal initiator was added to this solution. Aer taking
the solutions out of the glove box, samples were sonicated and
shaken for 10 min or until the solution was homogeneous.
Then, the samples were directly taken to an oven preset at 70 ꢁC
for 3 h. The hybrid ionogels were named as HI-2 and HI-5 for
ionogels containing 2 and 5 wt% of LPMASQ, respectively.
ꢁ
Through mild thermal treatment at 70 C, non-leaking, homo-
geneous hybrid ionogels were able to be easily processed as free-
standing gels (Scheme 1, inset). As shown in the right-hand side
model, we expected that the LPMASQ network being formed
aer crosslinking was loose enough to facilitate high lithium
mobility within the LPMASQ network. The obtained hybrid
ionogels were HI-2, HI-5 containing 2 and 5 wt% of LPMASQ,
respectively.
Through extensive pre-trials, we observed that LPMASQ was
capable of completely solidifying BMPTFSI, even at an exceed-
ingly low concentration of 2 wt% (HI-2). As shown in Fig. 1, the
peaks assigned to the C]C bonds at 1635 cmꢂ1 completely
disappeared aer the curing process, demonstrating that the
cross-linking reaction was completed. The inset photograph in
Characterisation and measurements
Fourier transform infrared spectra were recorded with a Perkin-
Elmer FT-IR system Spectrum-GX. Number average molecular
weight (Mn) and molecular weight distributions (Mw/Mn) of
polymers were measured using a JASCO PU-2080 plus SEC
system equipped with a refractive index detector (RI-2031 plus),
UV detector (l ¼ 254 nm, UV-2075 plus) and a Viscotek SLS
ꢁ
apparatus using THF as the mobile phase at 40 C with a ow
rate of 1 mL minꢂ1. The samples were separated through four
columns (Shodex-GPC KF-802, KF-803, KF-804, KF-805). 1H-
NMR and 29Si NMR spectra were recorded in CDCl3 at 25 ꢁC
using a Varian Unity INOVA (1H: 300 MHz, 29Si: 59.6 MHz).
Thermal gravimetric analysis was carried out witꢂh1a TA instru-
ꢁ
ment (TGA 2950) at a heating rate of 10 C min under a 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 a nitrogen atmosphere.
Scheme 1 Fabrication of hybrid ionogels.
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Fig. 1 FTIR spectra of hybrid ionogel HI-5 before and after thermal
curing with an inset photograph.
Fig. 1 shows that the ionic liquid electrolyte was completely
solidied aer thermal curing. Similar to our previous study,37
we demonstrated that LPMASQ was able to produce mechan-
ically pliant and non-owing gel polymer electrolytes using only
a mere 2 wt% of the gelator. This stable formation of homoge-
neous ionogels led us to hypothesize optimal electrochemical
performance with these hybrid ionogels.
Rheological properties of hybrid ionogels
Fig. 2 Rheological properties of hybrid ionogels (a) frequency sweep
and (b) temperature sweep.
Rheological examinations of gel polymer electrolytes and hybrid
ionogels have recently been found to be an efficient method to
elucidate the mechanical properties of such so materials.24–29
Fig. 2a presents the changes of dynamic viscoelastic properties
of neat BMPTFSI and hybrid ionogels as a function of angular
frequency. For the neat BMPTFSI, the loss modulus G00 was
found to be greater than the storage modulus G0. Both dynamic
modulus values exhibited power law dependency over the
experimental range, which is a typical rheological characteristic
of a viscous uid.29,30 These results clearly demonstrate the
dynamic shi from a liquid to solid-like state; predominantly
elastic behaviour with only minimal addition of LPMASQ due to
the well-developed network structure. Meanwhile, with
increasing LPMASQ concentration, the network structure
becomes denser, resulting in an increase in the gel rigidity, as is
evident by the increase in the dynamic modulus order.
Furthermore, as shown in Fig. 2b, the thermal scanning rheo-
logical observation for hybrid ionogel HI-2 showed that storage
temperatures exceeding 500 ꢁC. The residual mass of the hybrid
ionogels at various concentrations also correlated well with the
LPMASQ concentration as ionogels with higher LPMASQ
ꢁ
concentration gave greater residual mass at 800 C. Given the
exceedingly high initial degradation temperature of these
hybrid ionogels, it can be said that these materials exhibit
superior thermal stability. Moreover, thermal shrinkage tests
with the neat ionic liquid compared with a HI-2 impregnated
ꢁ
polypropylene (PP) separator were conducted at 150 C for 30
minutes (Fig. 3b). As shown, the neat ionic liquid impregnated
PP separator showed signicant shrinkage, while the HI-2
impregnated PP separator showed exceptionally low thermal
shrinkage, indicating that these hybrid ionogels have superior
thermal and thermo-mechanical properties for use at elevated
temperatures.
ꢁ
modulus values were stable at temperatures exceeding 250 C.
Thermal stability
Electrochemical properties of hybrid ionogels
The thermal stability of electrolytes is a critical requirement for
their safety and stability in lithium ion battery applications. The
Ionic conductivity. The temperature dependency of the ionic
TGA thermograms of LPMASQ, BMPTFSI and two kinds of conductivity of gel polymer electrolytes produced with various
prepared hybrid ionogels are presented in Fig. 3. As shown, the amounts of cross-linker was examined via the AC impedance
initial degradation temperatures of LPMASQ and BMPTFSI are spectroscopy technique. The ionic conductivity (s) of an elec-
very similar (ꢀ400 ꢁC), but LPMASQ le residual silica at trolyte can be described using the following equation.36
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Fig. 4 Temperature dependency of ionic conductivities of hybrid
ionogels.
BMPTFSI led to the drastic decrease in ionic conductivities and
subsequent poor cell performance to be discussed later.
Electrochemical stability. The electrochemical stability of
electrolytes over the operating voltage of a lithium-ion battery is
also an important requirement for practical battery operation.
The linear sweep voltammetry (LSV) measurements were per-
formed at potentials between 3.0 and 7.0 V (V vs. Li/Li+) under
various temperature conditions. As shown in Fig. 5, upon the
voltage sweep, the onset of the oxidation current increase,
which is related to the electrochemical oxidative decomposition
of the electrolyte, shied closer to 5.0 V with increasing
temperature. Greater slopes of I–E diagrams were also observed
at higher temperatures, indicating that the oxidation current
depends on the test temperature. However, no signicant
oxidation current was observed below 5.0 V even at elevated
temperatures, demonstrating that the obtained gel polymer
electrolytes were electrochemically stable up to 5.0 V, which
could be applied to high-voltage lithium batteries over a wide
temperature range.
Fig. 3 (a) TGA thermograms of BMPTFSI, LPMASQ, and hybrid ion-
ogels and (b) thermal shrinkage tests with neat ionic liquid and HI-2
impregnated polypropylene separators.
s(T) ¼ Sn ꢃ q ꢃ m
where n is the number of charge carriers, q is the charge on the
charge carrier, and m is the mobility of charge carriers. Hybrid
ionogels fabricated through the gelation of the liquid electrolyte
can provide good mechanical properties, though at the cost of
sacricing ionic conductivity. The ionic mobility (m) correlated
with the diffusion of ions is hindered when compared with 1 M
LiTFSI in the BMPTFSI electrolyte due to the formation of a
network structure. Furthermore, a decrease in the number of
the charge carriers (n) per unit volume of the electrolyte, caused
by the introduction of the crosslinking agent, is also a
contributing factor for the restricted ionic conductivity. A
notable advantage of this work is the gelation of the ionic liquid
with 2 wt% LPMASQ. Due to the minimal amount of gelator
content used to fabricate these free-standing hybrid ionogels,
high ionic conductivity similar to the neat BMPTFSI was
expected. As shown in Fig. 4, a decrease in ionic conductivity
was observed for hybrid ionogels with increasing amounts of
LPMASQ. However, the ionic conductivity of HI-2 was observed
to be 0.79 mS cmꢂ1 at 30 ꢁC, which is in close proximity to the
ionic conductivity of the neat ionic liquid electrolyte (0.96 mS
cmꢂ1). In addition, we plotted the ionic conductivities of a
commercially available organic-based crosslinker (ETPTA)
utilized in several previous studies.44 As shown, the high
amounts of ETPTA required for full solidication of 1 M LiTFSI
Fig. 5 Linear sweep voltammograms of hybrid ionogel HI-2 at various
temperatures.
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Interfacial stability towards the Li metal electrode. To eval-
uate the compatibility of Li metal electrodes with hybrid ion-
ogels, the interfacial resistance of the Li/HI-2/Li test cell was
monitored during a period of 25 days. Fig. 6 shows the time
evolution of the AC impedance spectra under open-circuit
potential conditions. The intercept with the Zreal axis at high
frequency presents the bulk resistance (Rs), which remained
stable and constant with the storage time, conrming the
stability of ionogels against the lithium metal. In comparison,
the mid-frequency semicircle associated with the interfacial
resistance (Rint) gradually increased with time until a steady
state was reached (ESI, Fig. S2†). Such a response could be
explained by the formation of a passivation layer on the lithium
metal electrode surface that suppresses continuous electro-
chemical reactions between the lithium metal electrode and
electrolyte, resulting in the stabilization aer a few days of
storage.45,46 The electrochemical behaviour of the hybrid ionogel
toward lithium metal was further investigated by conducting
cyclic voltammetry (CV) of a symmetrical Li/HI-2/Li cell. As ESI
Fig. S3† reveals, the cell underwent a reversible redox process
with high coulombic efficiency, and the peak current of the CV
prole tended to decrease with increase in the cycle number.
These results also conrm the stabilization at the interface,
suggesting that the compatibility of the ionogel towards the
lithium metal electrode is sufficient for application in lithium
batteries.
Cell performance. To characterize the electrochemical
performance of hybrid ionogels, we fabricated coin cells using
LiFePO4 as the cathode and metallic lithium counter electrodes
assembled with HI-2 (98 wt% of ionic liquid containing 2 wt%
LPMASQ). Fig. 7a presents the discharge capacity of the cells at
various temperatures. During the examination, the cells were
charged at a constant current density of 0.1 C (1 C rate corre-
sponded to a current density of 155.1 mA gꢂ1) and discharged at
various current densities in a voltage range of 2.5–4.2 V. As
shown, the cells delivered stable discharge capacity upon
repeated cycling at various current rates. However, the discharge
Fig. 7 (a) Discharge capacity at various temperatures and C-rates for
LiFePO4/HI-2/Li cells, (b) representative discharge profile at different
temperatures for LiFePO4/HI-2/Li cells cycled under 0.1 C charge–0.1
C discharge conditions.
capacity and voltage of cells gradually decreased when the dis-
charging current density rate increased. This lower discharge
capacity is attributed to the increase in cell polarization caused by
the poor kinetics at the electrode–electrolyte interface.47 With
increasing temperature, as expected, the discharge capacity of the
cell increased. For instance, in the initial cycle at 50 ꢁC, the
discharge capacity with a current density of 0.5 C was 88.3 mA h
gꢂ1, whereas cells cycled at 90 C delivered a greater discharge
ꢁ
capacity of 156.2 mA h gꢂ1, which is close to the theoretical
discharge capacity of the cathode active material in this potential
region (170 mA h gꢂ1 for LiFePO4).
Typical discharge proles of the cell fabricated using HI-2 at
various temperatures with 0.1 C charge–0.1 C discharge are
displayed in Fig. 7b. In general, a at voltage plateau at
approximately 3.4 V (vs. Li/Li+) corresponds to the conversion
between LiFePO4 and FePO4.47,48 A well-dened voltage plateau
was observed above 50 ꢁC, whereas the truncated at voltage
region and the voltage drops were observed at 30 ꢁC. These
results clearly show that the limited conversion process is
mainly due to the low diffusion of lithium-ions in both elec-
trolytes and the cathode material, indicating that a sufficient
mobility of lithium ions is required to maintain the electro-
chemical performance of batteries.
Fig. 6 Nyquist plot of AC impedance for a Li/HI-2/Li cell.
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electrolyte in Li batteries. Using an extremely simple mix and
cure system, these hybrid ionogels exhibited thermal stability
ꢁ
beyond 400 C, stable mechanical properties at high temper-
atures and ionic conductivities on a par with the neat ionic
liquid. Li battery cell tests revealed that these hybrid ionogels
could deliver high specic capacity and good cycling perfor-
mance at elevated temperature, demonstrating the possible
application of hybrid ionogels in Li batteries.
Acknowledgements
This work was nancially supported by a grant from the
Fundamental R&D Program for Core Technology of Materials
and the Industrial Strategic Technology Development Program
funded by the Ministry of Knowledge Economy, Republic of
Korea, and partially by a grant from Centre for Materials
Architecting of the Korea Institute of Science and Technology
(KIST).
Fig. 8 Cyclability of HI-2 compared with a conventional organic
ionogel fabricated using ETPTA, with the inset figure showing the
representative discharge profiles for HI-2.
The cycling stability of the cells at 90 ꢁC was evaluated
further through measurement of the discharge capacity. The
cells were charged at a current density of 0.2 C and discharged
at 0.5 C. Fig. 8 shows the cycling performance of the cell
prepared using our hybrid ionogels and the discharge proles
at various cycles (Fig. 8, inset), in comparison with the cells
prepared using a well-known organic crosslinker, ETPTA-based
ionogel.44 The structure of ETPTA is shown in the ESI, Fig. S4.†
The cells containing HI-2 ionogels exhibited relatively stable
cycling performance. In addition, the discharge capacities for
HI-2 were comparable to cells containing the liquid 1 M LiTFSI
BMPTFSI (ESI, Fig. S5†). The discharge capacity was main-
tained at a value of 147.1 mA h gꢂ1 aer 50 cycles, which was
94.9% of its largest discharge capacity and the columbic effi-
ciency of the system was found to be above 98% with the
exception of cell activation. Moreover, no obvious change in the
charge–discharge proles or signicant capacity decay were
observed, conrming the thermal stability and electrochemical
stability of the ionogels. In comparison, the cells containing an
organic crosslinker ETPTA-based ionogel exhibited a rapid
capacity loss over a repeatable cycling process and the
discharge capacity decreased to 118.7 mA gꢂ1, which is a
capacity retention of 82.7%, the greatest discharge capacity
aer being cycled merely 50 times. It should be noted that HI-2
was able to fully solidify the ionic liquid at a mere 2 wt%,
whereas the organic ionogel fabricated using ETPTA required a
high concentration of the cross-linker (15 wt%), which caused
the signicantly lower ionic conductivity compared with HI-2
(Fig. 4). Therefore, the improved electrochemical performances
of the cell with ionogels could be ascribed to the less dense
network-structure formed by LPMASQ, which could help to
minimize the depreciation of the lithium-ion diffusion of the
gel polymer electrolyte, resulting in enhanced electrochemical
performances.
References
1 M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and
B. Scrosati, Nat. Mater., 2009, 8, 621–629.
2 Y.-M. Chiang, Science, 2010, 330, 1485–1486.
3 K. Xu, Chem. Rev., 2004, 104, 4303–4417.
4 Z. Chen, Y. Ren, A. N. Jansen, C. K. Lin, W. Wang and
K. Amine, Nat. Commun., 2013, 4, 1513.
5 J. M. Tarascon and M. Armand, Nature, 2001, 414, 359–367.
6 N. S. Choi, Z. Chen, S. A. Frruenberger, X. Ji, Y. K. Sun,
K. Amine, G. Yushin, L. F. Nazar, J. Cho and P. G. Bruce,
Angew. Chem., Int. Ed., 2012, 51, 9994–10024.
7 A. Yoshino, Angew. Chem., Int. Ed., 2012, 51, 5798–5800.
8 J. Y. Song, Y. Y. Wang and C. C. Wan, J. Power Sources, 1999,
77, 183–197.
9 A. M. Stephan, Eur. Polym. J., 2006, 42, 21–42.
10 A. M. Stephan and K. S. Nahm, Polymer, 2006, 47, 5952–5964.
11 M. P. Singh, R. K. Singh and S. Chandra, Prog. Mater. Sci.,
2014, 64, 73–120.
12 D. M. Tigelaar, M. A. B. Meador and W. R. Bennet,
Macromolecules, 2007, 40, 4159–4164.
13 A. I. Horowitz and M. J. Panzer, J. Mater. Chem., 2012, 22,
16534–16539.
14 A. K. Gupta, M. P. Singh, R. K. Singh and S. Chandra, Dalton
Trans., 2012, 41, 6362–6371.
15 L. Viau, M. A. Neouze, C. Biolley, S. Wolland, D. Brevet,
P. Gaveau, P. Diudonne, A. Galarneau and A. Vioux, Chem.
Mater., 2012, 24, 3128–3134.
16 F. Gayet, L. Viau, F. Leroux, S. Monge, J. J. Robin and
A. Vioux, J. Mater. Chem., 2010, 20, 9456–9462.
17 M. A. Neouze, J. L. Bideau, P. Gaveau, S. Bellayer and
A. Vioux, Chem. Mater., 2006, 18, 3931–3936.
18 T. Echelmeyer, H. W. Meyer and L. Wullen, Chem. Mater.,
2009, 21, 2280–2285.
Conclusions
Inorganic–organic hybrid ionogels were fabricated using 19 K. Matsumoto and T. Endo, Macromolecules, 2009, 42, 4580–
thermally curable LPMASQ as a high temperature gel polymer
4584.
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A

View Article Online
Journal of Materials Chemistry A
Paper
20 R. K. Donato, L. Matejka, H. S. Schrekker, J. Plestil, 35 J. Shim, D. G. Kim, J. H. Lee, J. H. Baik and J. C. Lee, Polym.
A. Jigounov, J. Brus and M. Slouf, J. Mater. Chem., 2011, 21,
13801–13810.
21 F. Gayet, L. Viau, F. Leroux, F. Mabille, S. Monge, J. J. Robin
and A. Vioux, Chem. Mater., 2009, 21, 5575–5577.
22 A. Vioux, L. Viau, S. Volland and J. L. Bideau, C. R. Chim.,
2010, 13, 242–255.
Chem., 2014, 5, 3432–3442.
36 D.-G. Kim, J. Shim, J. H. Lee, S.-J. Kwon, J.-H. Baik and
J.-C. Lee, Polymer, 2013, 21, 2812–5820.
37 A. S. Lee, J. H. Lee, J. C. Lee, S. M. Hong, S. S. Hwang and
C. M. Koo, J. Mater. Chem. A, 2014, 2, 1277–1283.
38 A. S. Lee, S. S. Choi, H. S. Lee, H. Y. Jeon, K. Y. Baek and
S. S. Hwang, J. Polym. Sci., Part A: Polym. Chem., 2012, 50,
4563–4570.
23 M. Popall, M. Andrei, J. Kappel, J. Kron, K. Olma and
B. Olsowski, Electrochim. Acta, 1998, 43, 1155–1161.
24 Y. Lu, K. Korf, Y. Kambe, Z. Tu and L. A. Archer, Angew. 39 S. S. Choi, A. S. Lee, H. S. Lee, H. Y. Jeon, K. Y. Baek and
Chem., Int. Ed., 2014, 53, 488–492.
25 J. L. Schaefer, D. A. Yanga and L. A. Archer, Chem. Mater.,
2013, 25, 834–839.
26 Y. Lu, S. S. Moganty, J. L. Schaefer and L. A. Archer, J. Mater.
Chem., 2012, 22, 4066–4072.
27 S. S. Moganty, S. Srivastava, Y. Lu, J. L. Schaefer, S. A. Rizvi
and L. A. Archer, Chem. Mater., 2012, 24, 1386–1392.
28 S. K. Das, S. S. Mandal and A. J. Bhattacharyya, Energy
Environ. Sci., 2011, 4, 139101399.
29 M. Patel, M. Gnanavel and A. J. Bhattacharyya, J. Mater.
Chem., 2011, 21, 17419–17424.
30 S. K. Das and A. J. Bhattacharyya, J. Phys. Chem. C, 2009, 16,
6699–6705.
31 C. Pfaffenhuber, M. Gobel, J. Popovic and J. Maier, Phys.
Chem. Chem. Phys., 2013, 15, 18318–18335.
32 R. H. Baney, M. Itoh, A. Sakakibara and T. Suzuki, Chem.
Rev., 1995, 95, 1409–1430.
33 M. Li, W. Ren, Y. Zhang and Y. Zhang, J. Appl. Polym. Sci.,
2012, 126, 273–279.
S. S. Hwang, J. Polym. Sci., Part A: Polym. Chem., 2011, 49,
5012–5018.
40 S. S. Choi, H. S. Lee, S. S. Hwang, D. H. Choi and K. Y. Baek, J.
Mater. Chem., 2010, 20, 9852–9854.
41 Z. X. Zhang, J. Hao, P. Xie, X. Zhang, C. C. Han and R. Zhang,
Chem. Mater., 2008, 20, 1322–1330.
42 S. Chang, T. Matsumoto, H. Matsumoto and M. Unno, Appl.
Organomet. Chem., 2010, 24, 241–246.
43 Z. B. Zhou, H. Matsumoto and K. Tatsumi, Chem.–Eur. J.,
2006, 12, 2196–2212.
44 S.-H. Kim, K.-H. Choi, S.-J. Cho, E.-H. Kil and S.-Y. Lee, J.
Mater. Chem. A, 2013, 1, 4949–4955.
45 M. A. Navarra, J. Manzi, L. Lombardo, S. Panero and
B. Scrosati, ChemSusChem, 2011, 4, 125–130.
46 V. Gentili, S. Panero, P. Reale and B. Scrosati, J. Power
Sources, 2007, 170, 185–190.
47 M. Wetjen, M. A. Navarra, S. Panero, S. Passerini, B. Scrosati
and J. Hassoun, ChemSusChem, 2013, 6, 1037–1043.
48 L.-X. Yuan, Z.-H. Wang, W.-X. Zhang, X.-L. Hu, J.-T. Chen,
Y.-H. Huang and J.-B. Goodenough, Energy Environ. Sci.,
2011, 4, 269–284.
34 S.-J. Kwon, D.-G. Kim, J. Shim, J. H. Hong, J.-H. Baik and
J.-C. Lee, Polymer, 2014, 12, 2799–2808.
J. Mater. Chem. A
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