Ionic Liquid Functionalized Gel Polymer Electrolytes for Stable Lithium Metal Batteries

Article abstract of DOI:10.1002/anie.202106237

Metallic lithium (Li) is regarded as the ideal anode material in lithium-ion batteries due to its low electrochemical potential, highest theoretical energy density and low density. There are, however, still significant challenges to be addressed such as Li-dendrite growth and low interfacial stability, which impede the practical application of Li metal anodes. In order to circumvent these shortcomings, herein, we present a gel polymer electrolyte containing imidazolium ionic liquid end groups with a perfluorinated alkyl chain (F-IL) to achieve both high ionic conductivity and Li ion transference number by fundamentally altering the solubility of salt within the gel electrolyte through Lewis-acidic segments in the polymer backbone. Moreover, the presence of F-IL moieties decreased the binding affinity of Li cation towards the glycol chains, enabling a rapid transfer of Li cation within the gel network. These structural features enabled the immobilization of anions on the ionic liquid segments to alleviate the space-charge effect while promoting stronger anion coordination and weaker cation coordination in the Lewis-acidic polymers. Accordingly, we realized a high Li ion conductivity (9.16×10^?3 S cm^?1) and high Li ion transference number of 0.69 simultaneously, along with a good electrochemical stability up to 4.55 V, while effectively suppressing Li dendrite growth. Moreover, the gel polymer electrolyte exhibited stable cycling performance of the Li|Li symmetric cell of 9 mAh cm^?2 for more than 1800 hours and retained 86.7 % of the original capacity after 250 cycles for lithium-sulfur (Li-S) full cell.

Full text of DOI:10.1002/anie.202106237

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Ionic Liquid Functionalized Gel Polymer Electrolytes for Stable
Lithium Metal Batteries
Authors: Tianhong Zhou, Yan Zhao, Jang Wook Choi, and Ali Coskun
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202106237
Link to VoR: https://doi.org/10.1002/anie.202106237

10.1002/anie.202106237
Angewandte Chemie International Edition
RESEARCH ARTICLE
Ionic Liquid Functionalized Gel Polymer Electrolytes for Stable
Lithium Metal Batteries
Tianhong Zhou,[a]† Yan Zhao,[a]† Jang Wook Choi*[b] and Ali Coskun*[a]
[a]
T. Zhou, Y. Zhao, Prof. A. Coskun
Department of Chemistry, University of Fribourg, Chemin de Musee 9, Fribourg 1700, Switzerland.
E-mail: ali.coskun@unifr.ch
[b]
Prof. J. W. Choi
School of Chemical and Biological Engineering, Department of Materials Science and Engineering, and Institute of Chemical Processes, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea.
E-mail: jangwookchoi@snu.ac.kr
[†]
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of the document.
Abstract: Metallic lithium (Li) is regarded as the ideal anode material
in lithium-ion batteries due to its low electrochemical potential, highest
theoretical energy density and low density. There are, however, still
significant challenges to be addressed such as Li-dendrite growth and
low interfacial stability, which impede the practical application of Li
metal anodes. In order to circumvent these shortcomings, herein, we
present a gel polymer electrolyte containing imidazolium ionic liquid
end groups with a perfluorinated alkyl chain (F-IL) to achieve both high
ionic conductivity and Li ion transference number by fundamentally
altering the solubility of salt within the gel electrolyte through Lewis-
acidic segments in the polymer backbone. Moreover, the presence of
F-IL moieties decreased the binding affinity of Li cation towards the
glycol chains, enabling a rapid transfer of Li cation within the gel
network. These structural features enabled the immobilization of
anions on the ionic liquid segments to alleviate the space-charge
effect while promoting stronger anion coordination and weaker cation
coordination in the Lewis-acidic polymers. Accordingly, we realized a
high Li ion conductivity (9.16 × 10−3 S cm−1) and high Li ion
transference number of 0.69 simultaneously, along with a good
electrochemical stability up to 4.55 V, while effectively suppressing Li
dendrite growth. Moreover, the gel polymer electrolyte exhibited
stable cycling performance of the Li|Li symmetric cell of 9 mAh cm−2
for more than 1800 hours and retained 86.7% of the original capacity
after 250 cycles for lithium-sulfur (Li-S) full cell.
technological barriers to overcome, such as Li dendrite growth
and resulting uncontrolled solid electrolyte interphase (SEI)
formation. This interfacial instability is also associated with the
high reactivity of metallic Li during cycling and overheating, which
can potentially lead to a poor cycling performance and safety
hazards.[2]
In an attempt to address these shortcomings, a variety of
approaches have been employed; optimizing electrolyte
additives,[3] introducing artificial SEI layers,[4] designing three-
dimensional Li hosts[5] and implementing solid-state[6] and cross-
linked gel polymer electrolytes (CGPEs),[7] etc.. While the majority
of these approaches have had a marked impact, the development
of CGPEs incorporating glycol chains is remarkable owing to their
potential to facilitate an effective, relatively homogeneous Li+ flux
while aiding to mitigate the Li dendrite growth and enabling control
over the surface microstructure of Li metal.[8] Conventional
CGPEs, which are dual ion conductors, generally show Li ion
transference numbers (t+Li) below 0.5, an indication that the ionic
conductivity is mainly governed by the free anion motion, leading
to a concentration gradient of anions and further causing an
exacerbated dendrite propagation when applied to a Li metal
anode. With the same logic involved, in order to increase t+Li , the
free movement of anions needs to be limited. Whereas single-ion
conducting CGPEs incorporating anionic sites/traps exhibited tL+i
close to unity, they generally exhibited relatively low ionic
conductivities. Therefore, the simultaneous realization of high
ionic conductivity and t+Li is the major bottleneck for the further
evolution of CGPEs. The ionic conductivity of CGPEs is fairly
critical for the operation of a battery cell at high rates as well as
uniform Li (de)plating (from) onto the Li metal anodes for LMBs.[9]
In this direction, several promising approaches have been
proposed - namely, adding plasticizers,[10] designing single-ion
conducting CGPEs[11] and introducing self-healing ability.[12]
Ionic liquids (ILs) are salts that exist as liquids at low
temperatures. They are also synthetically highly versatile and
have also been studied to form a smooth Li plating morphology
on the Li metal anode.[13] Aside from being the most
electronegative element,[14] fluorine atoms could facilitate the
transport of cationic species while favorably interacting with the
fluorinated anions and reducing their mobility once incorporated
onto the polymer networks.[15] We reasoned that the integration of
ILs bearing fluorinated alkyl side chains onto the CGPEs could
Introduction
The consumer demand for portable electronic devices, drones
and all-electric vehicles has increased massively in recent years,
and Li-ion batteries (LIBs) have played a pivotal role in this
direction. Conventional LIBs are close to their theoretical limits in
terms of attainable energy density, thus requiring new chemistries
that allow to store Li ions far beyond today`s LIBs that operate
based on the intercalation mechanism. Metallic Li has been
extensively studied as an anode material since the 1970s due to
its high theoretical capacity (3860 mAh g−1) and low electrode
potential (−3.040 V versus standard hydrogen electrode).[1]
Therefore, Li metal batteries (LMBs) that adopt Li metal anodes
can be a game changer to overcome the energy density
limitations of LIBs. There are, however, several inherent
1
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10.1002/anie.202106237
Angewandte Chemie International Edition
RESEARCH ARTICLE
increase the Li ion conductivity by decreasing binding affinity of
oxygen atoms on the glycol chains towards Li+, while
simultaneously improving tL+i and electrochemical stability by
immobilizing Li salt anion.[15] Accordingly, herein, we introduce a
new type of gel polymer electrolyte (F-IL-GEL) composed of IL
bearing a fluorinated alkyl side chain (F-IL), dipentaerythritol
penta-/hexa-acrylate (DPEPA) and poly(ethylene glycol)
methacrylate (PEGMA, average Mn = 500). From the design
perspective, whereas PEGMA is used to achieve high ionic
conductivity through the glycol chains, DPEPA enables the
formation of a highly cross-linked gel network by means of
abundant acrylate functional groups and pendant F-ILs were
introduced to provide multiple features - namely, to effectively
immobilize Li salt anion through noncovalent interactions with the
Lewis acidic sites, to decrease the affinity of Li+ towards oxygen
atoms of glycol chains,[15] and to increase the electrochemical
stability of the CGPE. In this direction, the integration of F-IL and
the different crosslinking density of CGPEs were systematically
investigated to demonstrate the unique role of F-IL to realize high
t +Li , high Li ionic conductivity and enhanced electrochemical
stability window, thus providing valuable insights for the further
development of CGPEs.
(DOL) and 1,2-dimethoxyethane (DME) (1:1 by volume) with 2
wt% LiNO3 additive. The as-prepared transparent solution was
thermally initiated by using azobisisobutyronitrile (AIBN) at 70 °C
through in-situ radical polymerization of C=C bonds on the F-IL,
DPEPA and PEGMA monomers to form F-IL-GEL.[18] As a
comparison, we prepared two control samples - namely, IL-GEL,
the fluorine-free counterpart incorporating 3-butyl-1-vinyl-1H-
imidazol-3-ium, and GEL prepared by the polymerization of only
DPEPA and PEGMA in the LE. Accordingly, we systematically
investigated the effect of the IL end groups and the fluorinated
alkyl chains in CGPEs in the context of promoting stable and
uniform Li (de)plating.
b
a
C=C
=C-H
C=C
F-IL-GEL
DPEPA
PEGMA
F-IL
4000 3200 2400 1600 800
Wavenumber (cm-1)
c -1.8
-2.0
-2.2
-2.4
-2.6
-2.8
-3.0
d
Steady
Initial
70
300
200
100
0
60
50
40
30
20
Rb
Rb+Ri
a
F-ionic liquid
DPEPA
PEGMA
LE
F-IL-GEL-3%
F-IL-GEL-5%
IL-GEL-5%
GEL-5%
0
100
200
300
ISS
Z' (ohm)
IO
t Li+ =0.69
0
1000
2000
3000
3.0 3.2 3.4 3.6 3.8 4.0
1000/T (1/K)
t (s)
Figure 2. a) FT-IR spectra of F-IL, DPEPA and PEGMA monomers and the F-
IL-GEL. b) SEM image of the F-IL-GEL-3%. (Inset) The optical images of LE, F-
IL-GEL-1%, F-IL-GEL-3% and F-IL-GEL-5%. c) Temperature dependent ionic
conductivity plots of LE, F-IL-GEL-3%, F-IL-GEL-5%, IL-GEL-5% and GEL-5%.
d) The chronoamperometry profile of Li|F-IL-GEL-3%|Li cell under a polarization
voltage of 10 mV. (Inset) The corresponding electrochemical impedance
spectra (EIS) before and after polarization.
b
Fourier-transform infrared spectra (FT-IR) measurements
were conducted to verify the polymerization of F-IL, DPEPA and
PEGMA monomers in F-IL-GEL. The peaks at ~1633 cm−1 in the
DPEPA[19] and PEGMA,[20] and ~3107 cm−1 in the F-IL monomers
were assigned to the stretching bands of C=C and C-H,
respectively.[21] Upon polymerization, FT-IR spectrum of the F-IL-
GEL showed (Figure 2a) a significant decrease in the intensity of
the C=C stretching band pointing to the successful polymerization.
The optimized ratio of the F-IL, DPEPA and PEGMA monomers,
which was determined based on the formation of a gel, was found
to be 2: 4: 1 by weight (Figure S4). In order to determine the
critical gelation concentration, we used 1, 3 and 5 wt% of
monomers in the LE for the polymerization reaction. Whereas 1
wt% monomers in the LE did not lead to the gel formation, we
obtained homogenous translucent gels by using 3 and 5 wt%
monomers (F-IL-GEL-3% and F-IL-GEL-5%) as verified by
inverted (inset of Figure 2b) vial technique. We performed
scanning electron microscopy (SEM) analysis to probe the
morphology of the F-IL-GEL-3% (Figures 2b and S5). The surface
of the F-IL-GEL-3% was found to be uneven and wrinkled, leading
to the efficient uptake of the LE within the network. The
mechanical strength of F-IL-GEL-3% was measured by using a
rheometer in a plate-plate (0.5 mm gap) setting. The sample was
Figure 1. Schematic representation of a) the synthesis of gel polymer electrolyte
containing ionic liquid end groups and the proposed b) Li plating mechanism in
liquid electrolyte (LE) and F-IL-GEL electrolyte.
Results and Discussion
The F-IL was synthesized by reacting 1-vinylimidazole and
1,1,1,2,2-pentafluoro-4-iodobutane at 90 °C for 3 days. F-IL was
isolated following an anion exchange with TFSI− (Figure S1a,
S2).[16] In order to demonstrate the impact of fluorinated side
chains, we also synthesized 3-butyl-1-vinyl-1H-imidazol-3-ium
salt, IL, as a control sample by reacting 1-bromobutane and 1-
vinylimidazole at 40 °C for 24 h and subsequently undergoing an
anion exchange with the TFSI− anion (Figure S1b, S3).[17] The
synthesis of the F-IL-GEL was completed by free radical
polymerization of F-IL, DPEPA and PEGMA monomers in the
liquid
electrolyte
(LE)
of
1
M
lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,3-dioxolane
2
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10.1002/anie.202106237
Angewandte Chemie International Edition
RESEARCH ARTICLE
prepared by putting the monomer solution on the bottom-plate
and in-situ polymerizing between two plates at 70 oC for 2 hours.
The shear modulus of F-IL-GEL-3% was found to be 2500 Pa
(Figure S6). Since thermal stability and volatility of an electrolyte
are crucial for the battery safety, we compared the thermal
stability of the F-IL-GEL-3% and LE using thermogravimetric
analysis (TGA) in the temperature range from 25 °C to 300 °C at
a heating rate of 5 °C min−1 (Figure S7). As expected, while the
LE rapidly evaporated, F-IL-GEL-3% showed a very small mass
loss (4.5 wt%) below 100 °C, thus verifying its superior thermal
stability and efficient uptake of the LE within the cross-linked
polymer network.
The simultaneous realization of high ionic conductivity and tL+i
in CGPEs is highly desirable for their practical use in LMBs, but is
rather challenging to achieve in practice. Accordingly, we
compared (Figure 2c) the ionic conductivities of the prepared
electrolytes at different temperatures. The ionic conductivities of
GEL-5% and IL-GEL-5% were 4.80 × 10−3 and 6.86 × 10−3 S cm−1
at 20 °C, respectively, which were lower than those of F-IL-GEL-
5% (8.01 × 10−3 S cm−1) and F-IL-GEL-3% (9.16 × 10−3 S cm−1) at
the same temperature. Notably, the ionic conductivity of F-IL-
GEL-3% is the highest among all the CGPEs and is even close to
that of the LE (1.08×10−2 Sꢀcm−1). Such high ionic conductivity of
F-IL-GEL-3% could be attributed to the introduction of ionic liquid
segments incorporating fluorinated side chains as verified by the
gradual increase in the ionic conductivity going from GEL-5% to
IL-GEL-5% to F-IL-GEL-3%. Based on the rationale that the
higher t+Li value leads to a longer time for Li dendrite initiation
according to the Sand’s time formula,[22] we also calculated the tL+i
of different CGPEs to demonstrate the effect of F-IL. The t+Li value
of the F-IL-GEL-3% and F-IL-GEL-5% reached 0.69 and 0.41
(Figures 2d and S8a), respectively, which is significantly higher
than those of the IL-GEL-5% (0.35), GEL-5% (0.26) and LE (0.25)
(Figures S8b-d). The marked improvement of t+Li for F-IL-GEL
arises from the inherent Lewis acidity of the polymer backbone to
immobilize the TFSI anion, which promotes the Li cation mobility.
Higher F-IL content, however, does not seem to be beneficial
considering the lower t+Li of F-IL-GEL-5% compared to that of F-IL-
GEL-3%, which suggests that there is an optimum F-IL content
and that higher F-IL content could interfere with the Li+ transport
through Coulombic repulsion. These results point to the fact that
magnification SEM images of the plated Li with the LE, F-IL-GEL-
3% and F-IL-GEL-5% CGPEs are presented in Figure S10a-c. In
order to further evaluate the structural stability of Li metal anode,
we conducted a plating/stripping for 50 cycles with a capacity of
2ꢀmAhꢀcm−2 at 1ꢀmAꢀcm−2 under
a
Li|Li symmetric cell
configuration. We observed the formation of cracks on the surface
of the Li metal in the cell with the LE and F-IL-GEL-5%, resulting
from a nonhomogeneous Li deposition (Figures 3d, 3f, S11a and
S11c). On the contrary, the cell with F-IL-GEL-3% exhibited a
relatively uniform, smooth and dense morphology of Li deposit
(Figures 3e and S11b). The changes in the Li metal anode surface
morphology agreed nicely with the trend in t+Li. The F-IL-GEL-3%
effectively reduced anion mobility, giving rise to high Li+ mobility
and conductivity, thus effectively suppressing Li dendrite growth.
Figure 3. a-c) Top and cross-sectional SEM images of plated Li with a capacity
of 5 mAh cm−2 Li on Cu substrate at 0.5 mA cm−2 with a) LE, b) F-IL-GEL-3%
and c) F-IL-GEL-5%. d-f) The morphology of Li anodes after 50 cycles at 1 mA
cm−2 with a cut-off capacity of 2 mAh cm−2 in d) Li|LE|Li, e) Li|F-IL-GEL-3%|Li,
and f) Li|F-IL-GEL-5%|Li cells.
Linear sweep voltammetry (LSV) analysis was conducted at
a scan rate of 0.1 mV s−1 to probe the electrochemical stability
window of the LE and the prepared CGPEs (Figure S12 and S13).
The LE (DOL/DME) started to become unstable at 4.00 V and
showed a serious decomposition at about 4.20 V.[23] The
instability of LE largely originates from the relatively unstable
cyclic structure of DOL.[24] In the case of GEL, the onset potential
for the decomposition was found to be higher than 4.00 V pointing
to the encapsulation of electrolyte within the GEL structure.
Notably, the introduction of ILs led to a significant improvement in
the electrochemical stability window up to 4.55 V for both IL-GEL-
3% and F-IL-GEL-3%. This enhancement is attributed to the
electrostatic and hydrogen bonding interactions between IL and
TFSI- anion, which in turn retards the decomposition of anion.[25]
These findings were further verified by the cyclic voltammetry
(CV) analyses of F-IL-GEL-3% and LE, which were found to be in
a good agreement with the LSV analysis results (Figure S14). In
the potential range between −1.0 V and 1.0 V versus Li+/Li,
reduction and oxidation peaks were observed, which stemmed
from the reversible plating and stripping of metallic Li.[26] The
stability window up to 4.55 V for the F-IL-GEL would play an
important role in employing high voltage cathode materials in
practical LMBs.
the incorporation of F-IL to the CGPE boosts both the ionic
+
Li
conductivity and t
simultaneously, a challenging task in
developing electrolytes, which is also proven by the deshielding
7
of 7Li peak in the Li solid-state NMR spectrum of F-IL based
polymer electrolyte compared to that of IL based one (Figure
S9).’’ .
Morphology evolutions and structural stability of Li plating on
the Cu foil with the LE, F-IL-GEL-3% and F-IL-GEL-5% were
investigated (Figures 3a-c). The SEM image of Li|LE|Cu cell, with
5 mAhꢀcm−2 Li plating at 0.5 mAꢀcm−2, showed a nonuniform
morphology with massive holes. Moreover, the cross-sectional
SEM analysis revealed the thickness of the Li plating layer is ~46
μm with a loose structure (Figure 3a, inset). In comparison, the
surface of the Li plating turned out to be smooth in the cell with F-
IL-GEL-3% when tested under the same conditions, and the
thickness decreased to ∼28 μm, which is close to the theoretically
expected value of 25 μm for the case of pure metallic Li upon the
same amount of Li plating (Figure 3b, inset). When we tested F-
IL-GEL-5%, the morphology of the Li metal exhibited a thicker Li
deposition layer of ~35 μm (Figure 3c, inset). The high
3
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10.1002/anie.202106237
Angewandte Chemie International Edition
RESEARCH ARTICLE
Electrochemical cycling stability of symmetric Li|Li cells was
investigated at different current densities. Under a current density
of 2 mA cm−2 with an areal capacity of 2 mAh cm−2, the voltage
hysteresis increased sharply at only around 50 h for the cell with
LE, indicating unstable interface caused by the uncontrolled Li-
dendrite growth. On the contrary, the symmetrical cell based on
F-IL-GEL-3% maintained a flat voltage plateau and reduced
overpotential of 26 mV for 400 cycles (860 h). Due to the
decreased ionic conductivity, the symmetric cell with F-IL-GEL-
5% delivered a steady but higher voltage hysteresis of 46 mV
(Figure S15).
F-IL-GEL-5%, F-IL-GEL-3% maintained a low and stable Rf of
2.16 Ω without obvious oscillation (Table S1). These results
indicate that there is an optimum crosslinking density to achieve
stable interface and low polarization on LMA. Electrode |
electrolyte interphases were characterized by X-ray photoelectron
spectroscopy (XPS) analysis after 20 cycles (Figure S18 and S19).
In the C1s spectra, the higher intensity of C-O and C-C
components in the gel-based electrolytes compared to the LE
originates from the remaining polymer on the Li metal surface.
After 1 min of sputtering, most of the remaining polymer was
etched. In the F1s spectra, we observed both TFSI anion and LiF
on the Li surface. The F-IL-GEL-3% showed the highest CF3
signal, owing to the trapping of TFSI anion in the polymer network.
Moreover, the highest LiF content in the case of F-IL-GEL-3%
observed after sputtering explains the stable interphase
information and uniform Li plating. FT-IR and SEM analyses were
conducted to evaluate the stability of F-IL-GEL-3% during cycling
(Figure S20 and S21). The results indicated highly consistent
chemical identity of F-IL-GEL-3% after cycling, implying its
(electro)chemical stability.
Considering the ability of ILs to immobilize anions, the
CGPEs were applied for lithium-sulfur (Li-S) batteries in order to
mitigate the polysulfide shuttling, which is a major cause of
capacity decay. Towards this end, the Li2S8 permeation test was
performed to compare the polysulfide migration in different
electrolytes (Figure S22). After 4 h, F-IL-GEL-3% showed a minor
polysulfide diffusion, thus showing its ability to block polysulfide
migration. On the contrary, LE showed complete diffusion of
polysulfides. In order to evaluate the electrochemical
performance of CGPEs in Li-S batteries, we employed pre-plated
Li on Cu foil as an anode with an areal capacity of 3 mAhꢀcm−2,
and coupled it with a sulfur cathode with ketjen black. Li-S cells
using LE, F-IL-GEL-3% and F-IL-GEL-5% were cycled at a
constant rate of 1 C (1675 mA g−1 or 1.67 mA cm−2) to evaluate
the influence of the gel polymer electrolytes (Figure 4b). After 250
cycles, the cell with F-IL-GEL-3% exhibited a reversible capacity
of 519.3 mAh g−1, while the discharge capacity of the cell with F-
IL-GEL-5% was only 373.1 mAh g−1 with a constant falling of CE,
ascribed to the increased kinetic barrier caused by the lower tL+i
and ionic conductivity. In contrast, the cell based on LE cycled
only for 105 cycles, which exhibited failed charging curves without
achieving the cut-off voltage (Figure 4c). This phenomenon was
caused by the Li dendrite growth and consequent short-circuits,
which is consistent with the sudden voltage fluctuations in the
above symmetric Li|Li cell tests. During the initial 100 cycles, the
cells based on the F-IL-GEL-3% and F-IL-GEL-5% showed higher
capacity retention (92.4% and 83.4%) compared to that of the LE
(79.5%), indicating homogenous Li plating and suppression of
polysulfide shuttling during Li-S full cell cycling by the introduction
of the fluorinated ionic liquids to the gel electrolyte. While the cells
employing gel electrolytes showed superior capacity retention,
their capacity was found to be lower than that of LE. In order to
understand the capacity difference between LE and CGPEs, we
compared the discharge profiles at 1st and 31st cycles (Figure
S23).[28] The first plateau at 2.3 V, which corresponds to the
formation of polysulfides, Li2S8, and sulfur utilization, was clearly
observed for all the electrolytes and found to be extended for F-
IL-GEL-3%. The second plateau originating from the conversion
of soluble Li2S8 to Li2S4 and Li2S2 , however, was shortened for
both F-IL-GEL-3% and -5%, which was attributed to the slower
mass transport of sulfur redox intermediates, thus explaining the
0.3
a
3 mA cm-2 3 h
0.2
0.1
0.0
-0.1
-0.2
-0.3
LE
300 cycles
F-IL-GEL-3%
F-IL-GEL-5%
0
300
600
900
1200
1500
1800
Time (h)
b
c
1000
800
600
400
200
0
2.8
100
80
2.6
2.4
2.2
2.0
1.8
105 th
1C
60
40
cell died
LE
LE
F-IL-GEL-3% 20
F-IL-GEL-3%
F-IL-GEL-5%
F-IL-GEL-5%
0
0
400 800 1200 1600 2000
Specific capacity (mAh g-1
0
50 100 150 200 250
Cycle number
)
Figure 4. a) Voltage-time profiles of symmetric cells with LE, F-IL-GEL-3% and
F-IL-GEL-5% when measured at 3 mA cm−2 with 9 mAh cm−2. b) Cycling
performance and CEs of Li-S cells with LE, F-IL-GEL-3% and F-IL-GEL-5% at
1 C. c) Galvanostatic discharge-charge profiles at 105th cycle for the three Li-S
cells.
We also tested cycling performance for symmetric cells with
an areal capacity of 9 mAh cm−2 under the current density of 3 mA
cm−2 to evaluate the practical viability of CGPEs (Figure 4a).
Smooth Li stripping/plating plateaus were achieved with the F-IL-
GEL-3% protected lithium anode even after 1800 h during 300
cycles. The voltage hysteresis without obvious oscillation kept the
overpotential within 24 mV, which was smaller than the cell with
F-IL-GEL-5% of 52 mV. In contrast, the failure of the cell occurred
in only 100 h with a sharp overpotential increase for LE, due to
the rapid growth of dendrites and thicker SEI layer formation. Li|Li
symmetrical cells with each electrolyte were further evaluated at
different current rates (Figure S16). The symmetrical cell using F-
IL-GEL-3% exhibited the smallest and stable polarization at 5 mA
cm−2 of 5 mAh cm−2 capacity, verifying its ability toward
suppressing Li dendrites at high current densities. Furthermore,
electrochemical impedance spectroscopy (EIS) analyses were
conducted for Li|LE|Li, Li|F-IL-GEL-3%|Li and Li|F-IL-GEL-5%|Li
symmetric cells after cycling at 3 mA cm−2 with a cut-off capacity
of 9 mAh cm−2 at different cycling times to evaluate the interfacial
stability (Figure S17). During the initial 36 h, F-IL-GEL-5% showed
the largest interfacial resistance (Rf) of 10.3 Ω originating from the
lowest ionic conductivity and dense polymer backbone. When the
cycling time was increased to 150 h, the Rf of Li|LE|Li significantly
increased to 19.94 Ω due to the accumulated thick SEI associated
with uncontrolled Li dendrite growth.[27] In stark contrast to LE and
4
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RESEARCH ARTICLE
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We demonstrated a new type of cross-linked gel polymer
electrolyte containing imidazolium ionic liquid end groups bearing
a fluorinated alkyl chain, providing both high ionic conductivity and
high Li ion transference number. Specifically, these exceptional
properties were enabled by the Lewis acidic nature of the polymer
backbone that reduced the mobility of Li salt anion, and played a
crucial role for uniform Li plating and for restraining Li dendrite
growth during cycling. This study highlights the impact of
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A.C. acknowledges the support from the Swiss National Science
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J.W.C. acknowledges the support by the National Research
Foundation
of
Korea
(NRF)
grants
(NRF-
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generous support from the Institute of Engineering Research
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(ISRC) at Seoul National University. We would like to thank Dr
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Keywords: ionic liquid • gel polymer electrolyte • Li metal anode
• ionic conductivity • Li ion transference number
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5
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RESEARCH ARTICLE
Entry for the Table of Contents
Gel polymer electrolyte containing imidazolium ionic liquid end groups with a fluorinated alkyl chain (F-IL) was developed to suppress
Li dendrite growth by realizing both high ionic conductivity and Li ion transference number as Lewis-acidic segments in the polymer
backbone reduced the mobility of Li salt anion and decreased the binding affinity of Li cation towards the glycol chains, enabling a
fast transfer of Li cation within the gel network.
Institute and/or researcher Twitter usernames: @CoskunLab, @unifrChemistry, @Jang_Wook_Choi
6
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