Tracking the transition behavior and dynamics of ionic transport in crystalline ionic gel electrolytes

Article abstract of DOI:10.1039/b910600f

The transition behavior and dynamics of ionic transport were strongly influenced by changes in the crystal structure and interaction field of the crystalline ionic gel electrolytes with respect to chemical compositions, as proven by impedance, ⁷Li NMR, PCA and 2D IR COS.

Full text of DOI:10.1039/b910600f

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Tracking the transition behavior and dynamics of ionic transport
in crystalline ionic gel electrolytesw
HoSeok Park,ab Se Ra Kwon,b Young Mee Jung,c Hoon Sik Kim,d Hyun Joo Leee and
Won Hi Hong*b
Received (in Cambridge, UK) 29th May 2009, Accepted 8th September 2009
First published as an Advance Article on the web 24th September 2009
DOI: 10.1039/b910600f
The transition behavior and dynamics of ionic transport were
strongly influenced by changes in the crystal structure and
interaction field of the crystalline ionic gel electrolytes with
respect to chemical compositions, as proven by impedance,
7Li NMR, PCA and 2D IR COS.
NMR, and X-ray crystallography.7 However, it is very difficult
to delicately track the transition of ionic transport under
a specific environment, and to simultaneously monitor the
dynamic process due to the overlapped bands and the restricted
static information using one-dimensional (1D) spectroscopy.
From this standpoint, two very popular and powerful techniques
for spectroscopic studies are used herein: principal component
analysis (PCA) and two-dimensional infrared correlation
spectroscopy (2D IR COS). PCA gives a precise mathematical
estimation of the changes along the sample and variable vectors.8
The 2D IR COS, which is generated by a cross-correlation
analysis of dynamic fluctuations of IR signals induced by
an external perturbation such as temperature, time, stress,
concentration and others, is a well-established analytical technique
to monitor the dynamic processes of complex systems.9
Herein, we first report the dynamics and transition behaviors
of ion transport in binary crystalline ionic gel electrolytes
consisting of 1-methylimidazolium-3-propanesulfonate (ImiZIL)
and lithium bis(trifluorosulfonyl) amide (LiTFSI). Importantly,
PCA is used to track the transition behavior of ionic conduction,
while 2D IR COS is used to verify the dynamic process with
respect to chemical compositions.
Intermolecular interactions of ionic materials with chemical
environments are of fundamental importance for electrolytes,
acid–base chemistry, supramolecular assembly, confinement,
enzymatic proton transfer, biological systems, crystal engineering,
etc.1 In particular, stoichiometric ionic mixtures can reveal
appealing phenomena, such as facilitated ionic transport and
thermal transition due to the perturbed interaction fields.2 The
ionic interactions of ionic liquids (ILs) with other ionic com-
pounds are known to strongly influence not only the structures
and physicochemical properties of the IL itself,3 but also the
geometry and force field of local circumstance.4 Despite extensive
research, however, there are very few reports on the dynamics and
transition behavior of ionic transport at certain discrete composi-
tions because of the complexity and sensitivity of these systems.
ILs have become extremely attractive as a new type of
highly efficient electrolyte, as well as environmentally benign
solvents, due to non-volatility, low viscosity, low melting
point, high boiling point, high ionic conductivity, thermal
and electrochemical stabilities, and non-flammability.5
Furthermore, zwitter-type ionic liquids (ZILs), a new type of
IL, have shown unique transport properties, such as an
increased conductivity and transference number of the target
ion.6 This is due to the complex secondary force field related
to the intrinsic molecular structure, which is composed of
covalently tethered cations and anions.6 Therefore, the transition
behavior and dynamics of binary ionic mixtures, including
ZILs and ionic salts, should be investigated.
ImiZILs were synthesized following our previous procedure.10
Fig. 1 presents the ionic conductivities of binary ImiZIL-
LiTFSI gel electrolytes as a function of temperature. The
activation energies for lithium ion conduction of the
ImiZIL-LiTFSI gel electrolyte were obtained from an Arrhenius
plot. The ImiZIL-LiTFSI gel electrolyte showed an activation
energy of 5.0 kJ molÀ1 at a molar ratio of 3 : 1, 3.3 kJ molÀ1 at a
molar ratio of 2 : 1, 12.9 kJ molÀ1 at a molar ratio of 1 : 1,
The interactions and transport mechanisms of IL systems have
been significantly investigated by a wide range of spectroscopic
methods, such as UV-vis, IR, Raman and SFG spectroscopy,
a Department of Biological Engineering, Massachusetts Institute of
Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
b Department of Chem. & Biomolecular Eng. (BK 21), KAIST,
335 Gwahagno, Yuseong-gu, Daejeon 305-701, Republic of Korea.
E-mail: whhong@kaist.ac.kr; Tel: 82 42 350 3919
c Department of Chemistry, Kangwon National University, Chunchon,
200-701, Korea
d Department of Chemistry and Research Institute of Basic Sciences,
Kyung Hee University, 1 Hoegidong, Dongdamoongu, Seoul, Korea
e Division of Environment and Process Technology, KIST 39-1,
Hawolgok-dong, Seongbuk-gu, Seoul 130-701, Korea
w Electronic supplementary information (ESI) available: Experimental
section and additional characterization data. See DOI: 10.1039/b910600f
Fig. 1 Ionic conductivities of ImiZIL-LiTFSI mixtures as a function
of temperature. The blue box indicates region I at the ImiZIL-rich
phase, while the red box indicates region II at the LiTFSI-rich phase.
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11.4 kJ molÀ1 at a molar ratio of 1 : 2, and 12.6 kJ molÀ1 at a
molar ratio of 1 : 3. These values indicate the transport of lithium
ions via a Grotthus-type mechanism. Despite the gel state of the
binary mixture (see the ESIw), a plot of ionic conductivities
matched with a linear Arrhenius behavior of the temperature-
dependent conductivity better than the curved Vogel-Tamman-
Fulcher (VTF) form, indicating ion hopping within the
crystalline structures. When the content of ImiZIL was decreased
below a molar ratio of 1 : 1, the ImiZIL-LiTFSI gel electrolyte
showed 2–4-fold higher activation energies for the transport of
lithium ions. Guided by different activation energies, the ionic
conduction behaviors of binary gel electrolytes were divided into
ImiZIL-rich phase in region I and LiTFSI-rich phase in region II.
The existence of respective crystalline structures of ionic gel
electrolytes at ImiZIL-rich and LiTFSI-rich phases indicates that
the respective conduction behaviors in both regions I and II
were derived from a structural arrangement by two molecular
structures of binary gel electrolytes (see the ESIw).11 Furthermore,
in a similar manner to variations in the ionic conductivities and
crystal structures, the simultaneous changes in the line widths and
chemical shifts of the 7Li NMR spectra indicate the switch of the
lithium ion conduction mechanism from region I to II with
increaseing concentration of LiTFSI (see ESIw).2a,12 In the view
of force fields, ions in two crystal structures are surrounded by
different neighboring hopping sites and interact with them
through sequential conformational changes. Therefore, the trans-
ition of activation energies, crystal structures, chemical shifts, and
7Li line widths of the binary gel electrolyte elucidates that the
transport of lithium ions occurred through respective conduction
pathways due to different crystal structures and interaction fields.
The concentration-dependent IR spectra of the ImiZIL-LiTFSI
mixture were obtained (see the ESIw). The representative bands
regarding the imidazolium ring and sulfonate group of ImiZIL,
and the TFSI anion of LiTFSI were particularly focused in the
range of 1400 cmÀ1–1000 cmÀ1, because they are closely related to
the transport properties. For the IR spectra of ImiZIL, in-plane
nC–H and twisting nC–H of the imidazolium ring, and naSQO of
sulfonate group were assigned around 1170 cmÀ1, 1130 cmÀ1 and
1050 cmÀ1, respectively. For the IR spectra of TFSI, in-phase
nsSQO and naSQO, and out-of-phase nsSQO and nCF3, were
assigned around 1350 cmÀ1, 1330 cmÀ1, 1200 cmÀ1 and
1100 cmÀ1, respectively. Given that the band related to the
sulfonate, imidazolium ring, and TFSI groups in the ImiZIL-
LiTFSI system was shifted and weakened compared to that of the
pristine ImiZIL, it was confirmed that conformational changes
occurred via hydrogen bonding and/or electrostatic interactions.
PCA score and loading plots were obtained from the
concentration-dependent FT-IR spectra to elucidate the dynamic
transition of the ionic transport at a specific stoichiometry (Fig. 2).
The score values of PC1 and PC2 clearly separate the FT-IR
spectra into two regions. PC1 describes more than 91.9% of the
total absorbance change of the spectra, whereas PC2 captures
almost 7.4% of the remaining variances not described by PC1. In
an analogous manner to the plots of ionic conductivities, crystal
structures and 7Li line widths, PC1 exhibited turnover from
negative to positive values at the molar ratio of 1 : 1. Considering
that by definition the first PC corresponds to some physical
phenomena related to the greatest amount of variation in the
data,8 it is worth noting that the transition of transport properties
Fig. 2 (a) Scores and (b) loading vectors of PC1 (91.9 %) and PC2
(7.4 %) for the concentration-dependent FT-IR spectra of ImiZIL-
LiTFSI mixtures. 0 indicates a molar ratio of ImiZIL-LiTFSA of 4 : 1,
1 of 3 : 1, 2 of 2 : 1, 3 of 1 : 1, 4 of 1 : 2, 5 of 1 : 3, and 6 of 1 : 4.
in ImiZIL-LiTFSI is strongly related to different dynamic
processes of conformations in the two regions. This point is
7
consistent with the transition of crystal structures and Li line
widths from region I of the ImiZIL-rich phase to region II of the
LiTFSI-rich phase. In particular, the observation of high positive
PC1 loadings at 1323, 1193, 1128 and 1055 cmÀ1 proves that the
conformational changes in the imidazolium rings and sulfonate
groups of ImiZILs and TFSI anions in LiTFSIs are responsible
for the transition process of crystalline ionic gel electrolytes.
On the basis of tracking the transition point by PCA plots,
the respective dynamic behaviors of the two regions were
analyzed by 2D IR COS. Fig. 3 shows synchronous and
asynchronous 2D correlation spectra of regions I and II. In
a synchronous 2D correlation spectrum, the strong auto bands
located at the diagonal positions represent the overall suscept-
ibility of the corresponding spectral region to change in the
spectral intensity as an external perturbation is applied to
the system.9 The positions of the bands in the two sets of the
power spectra are in good agreement with those detected by
PC1. The emergence of an additional band at 1196 cmÀ1 in
region II and the unequal change in the absorption intensity
are related to different dynamic behaviors. In particular, the
synchronous 2D correlation spectrum for region II shows a
four-leaf-clover cluster pattern, comprising two auto peaks at
1196 and 1170 cmÀ1, and two negative cross peaks. The
corresponding asynchronous 2D correlation spectrum reveals
a very characteristic cluster pattern for the band position shift
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(imidazolium ring of ImiZIL) - 1345 (TFSI anion of LiTFSI)
- 1056 (sulfonate group of ImiZIL) in region I and 1051
(sulfonate group of ImiZIL) - 1130 (imidazolium ring of
ImiZIL) -1065 (sulfonate group of ImiZIL) - 1121 (imida-
zolium ring of ImiZIL) - 1325, 1309 (TFSI anion of LiTFSI)
in region II. Region I includes four different kinds of imida-
zolium rings and one kind of sulfonate group and TFSI anion,
while region II has two different kinds of imidazolium rings
and two kinds of sulfonate groups and TFSI anions. There-
fore, significantly different variations of sequential spectra
intensity in regions I and II confirm ion transport in respective
dynamic environments created by two conduction pathways.
In summary, we have demonstrated the dynamics and trans-
ition behavior of lithium ion transport of binary crystalline ionic
gel electrolytes due to the changes in the crystal structures and
interaction fields. The analytical method suggested herein pro-
vides a useful and powerful way to capture the transition of
physical, chemical, and biological properties and to monitor the
dynamics in the complex system from a fundamental perspective.
We gratefully acknowledge Genome-based Integrated Bio-
process Project of the Ministry of Science and Technology of Korea.
Notes and references
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